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eloped with declines at these tonnages at depths of more than about 350 to 500 m 1 100 to 1 600 ft a shaft is probably more economical than a decline except for very high tonnage mines using conveyor belts however many factors must be taken into consideration before making this decision for example if developing from the bottom of a mined out open pit at a depth of 300 to 400 m 980 to 1 300 ft the decline down another 800 m 2 600 ft would probably make more sense than a shaft from the surface down to 1 200 m 3 900 ft likewise if there is only proven ore down to a depth of 400 to 500 m 1 300 to 1 650 ft and the annual tonnage was to be between 1 and 2 mt a 1 1 and 2 2 million tpy then in all probability the decline would have the advantage however there is a strong caveat even to this general statement if the surface material is very much unconsolidated and or is below the water table the cost of developing through this material is much higher for a decline rather than a shaft for example if the ground must be frozen in order to penetrate safely a shaft would be the obvious choice this is one of the reasons that shafts in north america are sometime favored over declines particularly in the plains area where large regional aquifers occur incline shaft development an incline shaft is an opening driven steeper than a decline but not vertical they were often used in the past for maintaining a somewhat constant haul distance between the dipping ore body and the shaft however with today s trackless haulage equipment this advantage is minimized hoisting with incline shafts is less efficient and requires more maintenance than vertical shafts thus they are used very little today multiple stage hoisting when the hoisting depth reaches approximately 2 400 m 8 000 ft in depth a second hoist may be placed underground and continued with multiple stage hoisting usually these facilities are not placed one beneath the other as shown in figure 12 5 1 but rather the lower shaft is placed wherever it makes more engineering sense to optimize both hauling to the shaft and hauling from the lower shaft to the loading facility of the upper shaft also ground conditions could play a role in where to locate the lower shaft many times with today s powerful diesel or electric trucks the choice may be to haul up an incline rather than sink a second deeper shaft types of raise developments raises are needed for many reasons mainly for the transfer of ore workers and materials they also provide for a void space for stope blasting ventilation escapeways and combinations of these there are many ways to excavate a raise conventional raise drilling and blasting here the miner enters a bald raise and drills and blasts the drill round a work floor is created by drilling pin holes in the sides of the raise raise boring by upreaming this system requires two levels between which the raise is bored figure 12 5 4 first

a pilot hole is drilled from the top down then a large reaming bit is put on the drill string and the hole is reamed upward blind hole raise drilling with a box hole drill in this case there is only one level and the blind hole boring machine usually drills a small pilot hole and then reams out the blind raise figure 12 5 5 this is sometimes called a box hole drop raise by predrilling and blasting the raise in stages from the bottom up the drillers use long hole drill machines and very accurately drill small blastholes and larger parallel cut open holes the holes are charged and blasted in sequence from the bottom up making sure that each sequence blasts clear before the next round is blasted raise climber the raise climber is a mechanical machine that climbs on a cog wheel track installed in the raise figure 12 5 6 the miner drills out the raise round charges the round then moves the raise climber back out of the raise before he shoots the round the miner then runs the machine back up the raise scales the loose rock under the protection of a canopy installs another section of track and repeats the cycle boliden raise cage as raise boring technology has become increasingly efficient the boliden raise cage is not used as much as in the past still it is a viable method where there are two levels a pilot hole is first drilled between them and a small hoist is set at the top of the hole a hoist cable is dropped down to the lower level and a two deck drill cage is hoisted to the top of the raise the upper deck has a canopy to protect the miners while scaling and drilling after drilling and charging the cage is lowered and moved from the bottom of the raise for blasting the cycle is then repeated other types of mine developments many other types of developments shown on figure 12 5 1 need some explanation the terms drift and crosscut have unique meanings in a vein type of ore body drift usually means the opening is driven parallel to the strike of the vein whereas a crosscut typically is an opening driven perpendicular to the strike of the ore body however crosscuts can also mean any opening driven between two other openings the term raise is commonly used for an opening driven at a high angle upward from the existing opening and a winze is aan opening driven at a high angle downward from an existing opening however with today s technology it is very common to drill and blast a drop raise from the opening in the downward direction and then blast it from the bottom up this would still be called a raise ramps are similar to declines in that they are driven at a steep grade that can still be navigated by trackless equipment however they are usually driven between mine levels more for the purpose of traveling between production stopes and dumping points or to develop a new stoping entry ramps are better developed in an oval or figure eight shape than in a true circular configuration ramps

are more efficient when they are limited to a 15 18 grade stope developments for the various mining methods for those who are not familiar with the various underground mining methods the following discussion should be studied along with the chapters on mining methods in part 13 of this handbook it is not the intent of this chapter to explain or describe how the mining methods work but rather to describe the differences in the developments that set up the various mining methods hard rock room and pillar mine development the configuration of a room and pillar mine generally follows the shape of the mineralized ore body to be mined therefore the basic shape of the mine should start by determining the path of the main haulageways entries branching from these to reach the lateral extent of the reserve wherever possible a few basic principles should be followed in planning the production developments the central gathering point for hoisting should be as near to the center of the extractable mass as practical that keeps the haulage personnel and material movement distances to a minimum although this may mean a large shaft pillar is left in the center of the mineral reserve of a flat lying deposit it does make the mining operation more efficient at the end of the mining project most of the pillars can probably be extracted providing that proper mine planning has been used in the design and proper pillar removal systems have been established normally room and pillar mines without ground stability problems should be developed as symmetrically as possible to allow the maximum number of working areas to be opened with a minimum of development time in trackless mines it is advisable to cut the main haulage opening as much as 50 larger than the minimum size needed to operate the existing equipment large main haulageways improve productivity ventilation and road conditions and they are much more efficient to drive they also allow productivity to be maintained by permitting efficient operation of larger equipment the haulage drifts in several mississippi valley type room and pillar mines are driven 5 5 9 8 m 18 32 ft and the mines use very large equipment such as 36 t 40 st or larger trucks haulage roads and railroads should be kept as straight as possible with steep grades other than conveyor slopes being kept as short as possible the grade limits depend on the type of haulage equipment being used for main haulageways railroad grades should be less than 2 and for rubber tired diesel equipment it is advisable to keep the grade less than 8 however for very short distances where the equipment is allowed to gain mass momentum 15 18 grades can be negotiated quite efficiently if a 20 decline is to be driven for a conveyor system or service slope rubber tired diesel powered loaders are the best equipment for loading and hauling the material from the development as mentioned previously for such steep

declines very high horsepower engines are required for efficient haulage the radius of a curve that can be negotiated by trackless equipment is somewhat dependent on the size of the equipment in relation to the size of the drift and the turning radius of the equipment normally a radius of 15 m 50 ft is handled easily by any trackless mining equipment a road grader usually requires the greatest turning radius for rail mounted haulage a radius of 24 m 80 ft is adequate for stope development but a limited service main line requires a minimum radius of 38 m 125 ft however a radius of 60 to 150 m 200 to 500 ft would be appropriate for main line haulage depending on the speed of the train the preceding discussion has outlined the need for operating flexibility in relation to the size shape and thickness of the mineral reserve in the room and pillar mine plan for example it is often advantageous to incorporate a lower level of uniform grade below the main ore horizon to create a ventilation circuit providing mine drainage that removes water from the operating headings and roadways to make ore movement more efficient the fletcher brushy creek buick and sweetwater mines all located in the united states of doe run company incorporated multiple level planning in the early years of operations with all of the planning features described these mines have been extremely efficient over their 45 years of operation shrinkage stoping development for production shrinkage stopes for extraction can be developed in many ways several years ago william lyman 1998 wrote on the subject describing thirteen ways to develop the shrinkage stope and variations of the ventilation and service facilities in chapter 13 3 haptonstall describes several of these methods what is described here is a very common method that can be adapted to developing a trackless or a rail haulage shrinkage stope as depicted in figure 12 5 7 marchand et al 2001 the mine described here is cambior s mouska mine in quebec canada an ore drift sill is driven and kept in the ore at about 30 m 100 ft ahead of a footwall haulage drift it is important that the sill drift width stays as near the same width of the ore vein as possible the sill drift and the haulage drift are kept about 10 m 30 ft apart in this case the vertical distance between levels is 60 m 200 ft with a stope width of 60 m 200 ft at 10 m 30 ft intervals crosscuts are driven between the haulage drift and the ore drift to serve as drawpoints to shrink the broken ore from the stopes shrink refers to the process of shrinkage stoping where the amount of broken ore out of the stope is decreased most of it is left temporarily as a working platform and ground support the crosscut size will depend on what type of equipment is used to remove the ore in this case small overshot pneumatic muckers were used and trammed back to the railcars for dumping and the crosscuts were only 2 7

2 9 m 8 9 9 5 ft for load haul dump lhd haulage from a drawpoint to an orepass or dump pocket the crosscuts would have to be sized at least 1 2 m 4 ft larger than the dimensions of the lhd from the lower sill drift a service raise is driven to the haulage drift of the level above this service raise is equipped with a ladderway service pipes and a slide with air tugger all connecting to the level above the upper level also provides the ventilation circuit shown also is a service manway from the bottom of the stope which is in this case timbered as the stope advances upward thus completing the ventilation circuit and maintaining two access points to the stope at all times other mines sometimes leave pillars between the stopes and place the second access in these pillars which would require small doghole drifts between the raises and the stope a doghole is a very small drift driven to serve as an access between a vertical manway and the stope in some cases it may be only 1 2 m 4 ft high where one must crawl on all fours to access the stope these pillars can later be removed and capture the ore in the drawpoint below when the stope is completed sublevel open stope development most sublevel open stope mines are applicable to dipping type deposits but the thickness of the deposit between the hanging wall and footwall normally varies from a few meters to tens of meters wide the development access from the main decline or hoisting shaft is gained by level developments usually in the footwall that vary from 46 to 122 m 150 to 400 ft depending on the vertical extent of the ore body the eventual stope heights and the number of production stopes that may be excavated per level these levels usually become the haulage drifts and approach the bottom of each stoping block running parallel to the block with connecting access crosscuts running over and into the ore between the main levels ramps are usually driven for trackless haulage transport between levels these ramps also give access to the sublevels which are developed at intervals to remove blocks of ore the sublevel drifts are located in the footwall for haulage with crosscuts into the ore for connecting to the drifts in the ore which first become undercut access drifts and later draw drives these sublevel drifts in the footwall can also be used for accurate delineation of the vein reef by drilling prospect holes from the sublevel drifts up above at the next sublevel these drifts driven in ore are used for drilling the long holes that blast the ore for wide ore bodies there will probably be a hanging wall drift and a footwall drift usually at the widest part of the ore for that block there will be a slot crosscut drive and a slot raise drive between the sublevels which opens to the undercut of the lower level this slot raise is then slabbed slashed out to the full width of the ore body and when the broken slot ore falls to the undercut opening

the slot allows room for the vertical slabs of ore to be drilled and blasted from the upper sublevel drill drifts the ore then falls down to the draw drifts which are accessed by drawpoint crosscuts for loading out the ore when the ore is completely drilled and blasted from the first sublevel block the drilling and blasting moves up to the next sublevel and the cycle is repeated until the next main level is reached an ore sill pillar is usually remaining at each main level sometimes the ore body is narrow enough that a horizontal room can be opened and instead of drilling fan holes from drill drives vertical holes can be drilled for the long hole blasting this also applies when vertical crater retreat stoping is used in the sublevel extraction method termed longitudinal mining in chapter 13 4 figure 13 4 21 the sublevel sill drifts serve as both the top cut for the downhole drilling and as a bottom cut for remote mucking ventilation raises can be placed in the ore or close to the stope accessway from the ramp raises are usually opened with raise boring machines raise climbers or drop raises rarely in today s modern mechanized mines are raises drilled and blasted by conventional methods cut and fill stoping because cut and fill mining is so versatile and can be used to mine any shape of an ore body it is impossible to generalize a specific development approach that fits all types of cut and fill systems normally the essential criteria for using cut and fill stoping is that the excavated stope needs support the remaining rock is not competent enough to stand open to allow further mining of the area however where the ore is so high grade that it is planned to extract nearly 100 of the ore and leave no pillars there is a need for immediate backfill even in competent rock consideration must be given to the general types of deposits that are to be mined by one of the many forms of cut and fill mining steeply dipping narrow ore bodies steeply dipping ore bodies are most commonly mined by cut and fill systems such mining occurs throughout the silver valley in idaho s coeur d alene district the stillwater mines and the cannon mine in washington all in the united states to name but a few the major infrastructure developments are normally developed in the footwall but not necessarily since nothing will be allowed to cave even the hanging wall should remain structurally competent for developments a classic example of infrastructure development is seen in figure 12 5 8 of idaho s lucky friday mine illustrating the main shaft the haulage levels coming off the shaft the oval shaped ramp contacting all of the sublevels and the service and ventilation raise because this is an undercut and fill mine there are 15 stope attack ramps to the top of each sublevel stope block and working down slice by slice to the bottom of the sublevel stope block in an overhand cut and fill mine the sublevel attack ramps would s

tart at the bottom of the stope block and work up the ore block in slices establishing a new attack ramp for each slice not shown on this sketch are orepasses from the sublevel ramp down to the next haulage level flat thick and wide ore bodies a few years ago only steep vein deposits were worked by cut and fill mining but with today s technology wide and thick ore bodies are also mined in this manner almost all of the underground mines of the nevada united states gold district use cut and fill mining and more specifically undercut drift and fill or ramp and fill in this type of mining the upper level drifts should be driven and filled at a different direction azimuth than the drifts immediately below this can be seen in chapter 6 5 figure 6 5 6 sublevel caving sublevel caving is an underhand method where all of the blastholes are drilled upward gravity moves the ore down to the extraction and drilling drift the main haulage drifts on the major levels will probably be longer from the hoisting facility to the production stoping than in noncaving mining systems since this mining system allows for the overburden to cave when the angle of the caving surface is taken into consideration the hoisting facilities must be well removed from the zone of potential caving for transverse sublevel development the haulage drift for a particular level is driven down the strike of the ore body probably in the footwall waste with production crosscuts turned off at regular intervals crossing the ore body this divides the ore body into geometric blocks that will correspond to the geometric spacing of the sublevels these drifts are driven to the hanging wall and a slot raise up to the cave above is developed at the end these slot raises must be slabbed out to the full dimension of block and will provide the opening for the initial blasting at the end of the stope there are many development schemes to connect the sublevels but all of them involve ramps that connect to the various sublevel haulage drifts also connecting to the sublevel haulage drifts are major orepasses that transfer the ore to a main haulage level development for a sublevel cave stoping method is extensive this is because so much of the ore must be mined by development to set up the drilling and blasting of the ore so that the surrounding waste rock will cave uniformly in 1990 about 25 of the ore was extracted by drifting in the sublevels now this can be as little as 6 bullock and hustrulid 2001 the reason for the drastic decrease in development is because the better drills and drilling systems today are more efficient drilling larger longer and much more accurate holes for blasting thus the spacing between sublevels has more than doubled as indicated ore is recovered both through drifting and through stoping because the cost per ton for drifting is several times that for stoping it is desirable to maximize stoping and minimize drifting this has meant

that through the years the height of the sublevels has steadily increased until today they are up to 30 m 98 ft the sublevel intervals have changed from 9 m up to nearly 30 m 30 to 98 ft a number of factors determine the design the sublevel drifts typically have dimensions width height of 5 4 m 6 5 m or 7 5 m 16 13 ft 20 16 ft or 23 16 ft to accommodate lhds after the sublevel vertical interval has been decided it is necessary to position horizontal dimensions of the sublevel drifts as an example the drifts might be placed so that the angle drawn from the upper corner of the extraction drift to the bottom center of drifts on the overlying sublevel is 70 this is approximately the minimum angle at which the material in the ring would move to the drawpoint the resulting center tocenter spacing is 22 m 72 ft longitudinal sublevel development is commonly used in ore widths of 18 m 60 ft or less but may also be used to advantage in wider ore bodies for longitudinal sublevel caving developments almost all of the stope development is in ore but for irregular ore bodies there is a greater chance of leaving ore undrilled and blasted mine development for block caving panel caving in this section the term block caving will be used to represent both block and panel caving suggesting the mining of individual blocks and panel caving indicating a laterally expanding extraction the time and cost of developing a block caving mine is very significant compared to other types of mining since block caving methods are associated with extremely high production rates the main haulage developments are likewise designed for large high volume high speed haulage these initial developments must also accommodate very large ventilation capacities to handle all of the equipment needs of a modern massive caving mine at the same time the major permanent development facilities including shafts and permanent ventilation raises must be located far enough from the caving material so as not to be engulfed within a zone of high stress or rock mass deterioration an example of this infrastructure development in relation to the caving ore body is shown in chapter 6 1 figure 6 1 2 the most important development elements of a caving system are the undercut level which removes the support for the overlying ore column the funnel trough or bell through which the ore is transported downward to the extraction level and the extraction or production level in some cases the undercutting can take place on two levels rather than one see chapter 6 5 figure 6 5 24c there are three types of systems for drawing the ore from the caving ore body the lhd system the slusher system and the gravity system in order of importance much of the main development infrastructure is similar for the three methods but the stope developments are somewhat different stope development for lhd caving systems in figure 12 5 9 the undercut leve

l is the upper level where the long drill holes of the undercut ring are shown the production level is the level below where the lhd is loading ore from the bell draw holes note the intake and exhaust drifts and raises that carry the ventilation and the orepasses and load out chutes on the mail haulage level drift for more extensive discussion and illustrations on lhd caving systems development see chapter 13 10 stope development for slusher caving systems in the top level shown in figure 12 5 10 the undercut level longhole drilling will open the troughs that the ore will collapse into and then flow down into the multitude of finger raises where it flows by gravity to the grizzly haulage level where it is slushed to the main haulage level in some cases instead of slushing directly into the ore trains the ore may be slushed into another orepass leading to an even lower main haulage level the ventilation drifts and raises are not shown in the diagram this mining system is labor intensive and would only be used where labor costs are extremely low stope development for gravity caving systems figure 12 5 11 illustrates how the development has much less horizontal development but many more raises for orepasses and draw extraction bells in a 14 7 6 m 76 25 ft grid pattern when the ore is being drawn from a specific area a miner must stand and assist the ore flow with a mining bar or explosive bombs in each of those grizzly chambers as the ore flow becomes blocked with boulders because this method of block caving is so labor intensive it is seldom used in new mines being developed in countries where labor costs are high planning for excavation rapid development incentives very large initial capital expenditure is required to construct a block cave mine and bring it into production the eventual npv of the investment is dominated by the time taken to start production typically 5 8 years the rate of advance is a key factor and cost per meter although important tends to be secondary one very large multinational mining company that is currently operating or developing four large block caving mines plans to develop 1 000 km 600 mi of development and 12 000 drawpoints over the next 11 years moss 2009 over many years and projects this mining company s advance rate has stayed at around 4 m d 13 ft d and the company states this has been a critical factor for bringing any block cave projects in on schedule while rock drilling efficiency has increased by fivefold over the years between 1960 and 2000 the advance rate in meters per day of development has decreased by threefold thus the cost per meter has gone up by tenfold the primary reason for this is the new emphasis on safety at the development face these protocols have now evolved in the mines to a point where most development requires that all ground is at least fully bolted and in most cases has skin support in the form of shotcrete or wire mesh this means

that currently most of the ground control functions must be carried out within the development cycle yet a few mines and many tunneling operations get much higher advance rates some more than 10 15 m d 31 49 ft d how is this accomplished it must be planned equipped and executed as if it were a civil tunnel contract what must be in place from the very beginning there cannot be any compromise of worker safety management must develop procedures whereby no worker is exposed to ground that has been inadequately secured by reviewing many studies of rapid excavation it is apparent that the following steps must be set in place to achieve these advance rates the mining operation must know the best drill round to achieve 95 of the drilled distance for the type of ground at the mine location a thorough geotechnical study must be ongoing to define rock types related to blast design and ground control problems and detailed planning must precede each step of the mine development drill rounds must be designed by qualified engineers and the execution of drilling and blasting must be followed according to the design in a recent survey by the national institute for occupational safety and health niosh 60 of mines allow drillers to decide what type of a drill pattern and how much explosive to use in the round miners think in terms of blasting or bombarding the rock with as much force as possible and every hole is the same blast designers tend to think of cutting the rock particularly in the cut area and the periphery of the drift oxygen balanced emulsions should be used to minimize ventilation time and electronic blasting should probably be used to ensure perfect hole timing and minimize vibration and peak particle velocity where it may be required since 50 of the cost and about 25 40 of the face time are now spent in ground control controlled smooth wall blasting should be used damage to the remaining rock must be limited and overbreak minimized niosh recently conducted a research study showing that the total cost of drifting using controlled blasting is about 4 or 59 m 18 ft less than conventional drilling and blasting no credit was given for less ground control cost camm and miller 2009 the ground control savings could easily be 150 m 45 ft driving the largest faces consistently with geotechnical issues of ground control and ground stress conditions allows for the most drifting efficiency since larger equipment can be used and in some cases there is room for multitasking at the face in one case the development advance went from 1 7 m shift 6 ft shift to 3 4 m shift 11 1 ft shift even though the size of the drift tripled bullock 1961 the most efficient and reliable equipment should be acquired for driving the developments particularly the critical path developments including computerized drilling controls and the use of improved explosives systems designed for controlled blasting

the initial up front cost may be millions of dollars but many millions more will be saved in the npv of the operation the entire mine development operation must focus on getting the men and supply logistics and maintenance planned so there will be little or no time lost at the face between cycles in one rio tinto study moss 2009 39 of the time was spent waiting for the next step in the 15 hour cycle procedures and metrics must be put into place where all parameters of the drilling blasting ground control and loading portions of the drifting cycle can be defined in quantitative forms and are measured for development projects if quality is not measured it cannot be managed to achieve the desired results every project needs to establish a quality control group that has the responsibility and authority to enforce the drilling blasting and ground control procedures that have been put in place this group should report to the mine manager in one project observed by the author where quality control was completely ignored during the controlled blasting of poor ground the result was that the tunnel had a slab of more than 1 5 m 5 ft of overbreak for at least 15 m 50 ft of tunnel which the contractor on a cost plus contract was allowed to simply fill with layers of shotcrete until the correct tunnel dimension was achieved the following are a few recent examples of high speed development norwegian tunneling technology opened a new haulageway for a coal mine which averaged 103 m week 338 ft week driving a 38 5 m2 414 ft2 tunnel 5 630 m 18 471 ft long nilsen 2009 much of the tunnel was in permafrost making the advance more difficult the key is to use the face to advance development to achieve this some norwegian tunneling operations use side tipping loaders trucks sized to the tunnel enabling drivers to perform a three point turn in the tunnel drilling equipment with navigation that is fully instrumented to implement a plan of the drill layout including the survey of the hole collar and hole toe use of 5 8 m 19 ft long drill steels where ground is appropriate logging of the drilling performance blasting using emulsion and rapid setting shotcrete one half hour in freezing conditions and most importantly a dedicated highly skilled work force regularly achieving more than 100 m 328 ft per week of development in openings up to 38 5 m2 414 4 ft2 the best week achieved 150 1 m 492 5 ft of advance the world s best decline advance rate is that currently achieved at the cadia east project in new south wales for newcrest mining limited which was 260 m month 853 ft month for 9 months ending in may 2008 and included a single best month of 311 m 1 020 ft this project also shows a cost of a 4 500 m us 3 894 ft which is 31 lower than the industry average for australia lehany 2008 in general tunnel contractors do a better job of rapid advance rate excavations than min

ing companies do what is the payoff for rapid excavation assume one is developing a 2 copper block caving mine to produce 100 000 t d 91 000 tpd one can also assume a typical cost per ton of copper production a copper sales price of about 0 90 kg 2 00 lb a 360 million capital cost 45 million for 8 years versus 72 million for 5 years a 10 discount rate and a regular development rate of 4 m d 13 1 ft d compared to 8 m d 26 2 ft d based on this data the mine would come into full production in the sixth year rather than the ninth year this would result in a difference in npv in the eighth year of 260 million for the regular development compared to 86 million for the accelerated development or a difference in npv of 154 million favoring the accelerated case however by the 13th year the accelerated development case shows a positive discounted cash flow and npv compared to the regular development case which does not show a positive discounted cash flow until approximately the 35th year this theoretical example illustrates that with a difference of 154 million considerably more capital dollars might be spent to put the proper equipment and practices in place resulting in the doubling of development advance per shift developments driven for obtaining information exploration often developments must be driven out ahead of the rest of the mine infrastructure to identify both the specific location and grade of the ore to be mined there are no set rules as to how these developments must be driven since so much depends on the type of deposit in some cases the development needs to be driven directly into the ore to assess its character and in other geologic settings the developments need to be driven a specific distance from the deposit so a series of diamond drill holes can be used to sample those deposit characteristics the primary objective should be to get these developments completed well ahead of the need to mine the area this means getting the developments done many months before the information will be needed for mining operations information for mine planning obtained by development during the very early stages of evaluation and planning a lot of information is needed to make the correct decisions concerning the property therefore the initial opening might be driven to test a particular aspect of the mineral reserve that is considered critical for successful exploitation more often this is termed a test mine this could be from an adit ramp or shaft see chapter 4 6 in any case permitting is usually required that may delay the project a large amount of the technical information required can be obtained from diamond drill holes particularly where the mineralized zone is fairly shallow continuous and flat unfortunately the true economic potential of many mineral resources cannot be determined until after an exploration development or test mine is driven from the development opening under

ground prospecting can further delineate the materials present a bulk sample can be taken for metallurgical testing and or true mining conditions can be revealed this procedure involves considerable financial exposure and unfortunately the delineated resource may turn out not to be economically viable nevertheless there may be little choice about driving a test development opening if the potential of the resource is to be proven and the risk to larger downstream capital expenditures is to be minimized this is another case where the decision must be made between a decline or a shaft as well as the size of the opening regardless of the reason for driving a preliminary development three basic principles should be observed 1 the development should be planned to obtain as much of the critical information as possible without driving the opening in a location that will interfere with later production for most mining methods it is usually possible to locate at least a portion of the exploration development where it can be used in the developed mine that may come later even if it becomes necessary to enlarge openings to accommodate production equipment at a later date 2 the preliminary development should be restricted to the minimum needed to obtain the necessary information 3 the development should be completed as quickly as possible most exploration drifts for metal mines are only large enough to accommodate small loading and hauling units from 2 4 3 m to 3 4 3 m 8 10 ft to 10 14 ft if geologic information is needed it is often possible to locate the developments in proximity to several of the questionable structures without actually drifting to each one with modern high speed diamond drills having a range of 213 to 305 m 700 to 1 000 ft these structures can usually be cored assayed and mapped at minimum cost after the preliminary development has been completed to satisfy a specific need the development should be used to maximize all other physical and structural information to the extent that will not delay the project as much geotechnical information on the rock mass quality should be obtained as possible and should be just as important to the company as resource information no rules regulate the size of the shaft or the entry for exploration development the size depends on many factors including the probability of the mine being later developed to the production stage if the probability favors follow up with full mine development immediately after exploration development and if the shaft depth is not excessive e g 244 366 m 800 1 200 ft it would be logical to sink a shaft sized for later use in production development this is particularly true of operations normally using small shafts for production using a small shaft of 3 7 to 4 0 m 12 to 13 ft in diameter is not uncommon in hard rock mining in the united states for example most of the doe run company s shafts in the viburnum trend area o

f missouri are of this size as well as many of the shafts in the tennessee zinc district the elmwood mine originally opened by new jersey zinc is a good example of this practice initially a 3 7 m 12 ft diameter conventional shaft was sunk for exploration after the continuity and quality of the ore were proven by drifting and underground prospecting the shaft was used for further development of the mine and was eventually used as the production shaft the savage zinc company s mine at carthage tennessee united states is another example of a 3 8 m shaft originally sunk as a test mine and then later opened as an operating shaft in contrast some companies choose smaller drilled shafts to speed shaft sinking and minimize early expenses such shafts are usually completed considerably faster than conventional shafts but not as fast as a raise bored shaft i e where an opening already exists and the hole is reamed upward and the cuttings fall back into the mine because this section is directed toward initial mine openings removal of cuttings must be upward through the hole and out from the shaft collar various methods involve the use of air or mud with either direct or reverse circulation a large number of blind bored shafts exist in the industry although bored shafts may stand up very well without support most of these shafts contain either a steel or concrete lining developing service facilities shop and storehouse developments the efficiency of a modern mining system depends heavily on the productivity and availability of the equipment used to extract the material most of the energy to move the material horizontally must be provided since the material may be very heavy and or abrasive and because the environment of an underground mine imposes adverse operating conditions underground mining equipment requires a great deal of preventive maintenance and repair one of the most serious and most prevalent errors made in designing a mine is the failure to provide adequate space and equipment for necessary maintenance and repair work the amount of service if any provided underground depends on several factors the degree of difficulty in moving equipment into or out of the mine is a major consideration if the property is a limestone mine with adit entrances and a good shop on the surface it would probably not be advantageous to duplicate the facilities and personnel underground as a generalization most decline mines do not need an underground workshop they require only service and refueling bays maintenance is done on the surface underground shops should provide a safe and clean working environment in mines that are gassy or carry a gassy classification building underground shop facilities may not be practical if the active working area will be totally abandoned after a fairly short life it would not pay to invest in an extensive underground shop area mines such as the punch mines in appalachia united st

ates are typical examples of this situation most underground mines with shaft entries should develop good underground shop facilities in the viburnum trend mines which are developed with shafts 152 396 m 500 1 300 ft deep all of the mines rely on large mining equipment and have well developed underground shop areas the total area usually used as a shop area can range from 465 to 1 394 m2 5 000 to 15 000 ft2 the important criteria for shop design are to provide one or two large bridge cranes over the motor pits easy access from several different directions for access to and around the cranes when they are in service for extended periods of time a motor pit and service area for scheduled lubrications performed as part of the preventive maintenance program a separate area for welding operations a separate area and equipment for tire mounting and repair an enclosed separate area for recharging batteries various work areas equipped with steel worktables or benches close proximity to the main supply house an area for washing and steam cleaning the equipment an office for the shop foreman records manuals catalogs drawings and possibly a drafting table the main pit areas should be visible from the office windows and if rail haulage is used a separate locomotive and car repair shop must be developed with provisions for at least one motor pit the size of the underground supply room also depends on several factors some mines have no supply room whereas others have very large rooms providing as much as 1 115 m2 12 000 ft2 factors influencing the inventory policy include the frequency and ease with which the mine receives supplies whether a supply system is installed at the collar or portal of the mine whether there is a separate access to obtaining repair parts without disrupting the mine production and the dependability of the parts suppliers for the equipment being used sump area and pump station in mines below surface drainage areas must be provided to store the water before it is pumped to the surface and discharged no general rule exists to determine the capacity for water storage the water flow into mines in the united states has varied from 0 to 2 208 l s 0 to 35 000 gpm adequate pumping capacity is the only permanent solution to removing water from a mine however fluctuations in the water inflow and or the time periods when some or all of the pumps are inoperable must be handled by having an adequate sump capacity many mines use dirty water positive displacement pumps the challenge is to keep the mud suspended until it enters the pumps screening out the big lumps this is economical up to about 80 l s 1 320 gpm beyond that settlers with centrifugal pumps will be necessary sumps are also needed in wet trackless room and pillar mines the rubbertired vehicles traveling on the roadways create fine material that eventually collects in the water ditches if the roadways have dri

ps or continuous streams of water on them although this situation should be corrected to keep water off the roadways it invariably occurs or recurs a place must be provided for the fines to settle out of the water so they do not damage the clear water high head pump impellers this can sometimes be done effectively in a small catch basin that is frequently cleaned out however if the water flows are substantial the main sump receives the bulk of this fine material as a result provisions must be made to clean the sump with conventional or special equipment divert the water into another sump while one sump is being cleaned provide a place to put the fines soup removed from the sump and transport the fines to the surface one of the most common methods of cleaning a sump is to use conventional front end loaders to remove the material however a ramp must be provided down to the sumplevel floor if deep well impeller sections are suspended from a pump station down into the sump considerable care must be taken to avoid bumping the impellers with the loader other systems using slurry pumps scrapers or diverting the material into skips have been tried but there still is no single efficient low cost way to clean a sump when cleaning a sump diverting the water flow into another sump can also be a problem unless at least twice the normally required pumping capacity is provided half of the pumps must handle all of the inflowing water while one sump is being pumped down and cleaned mines that make large amounts of water also produce large amounts of fine material that fills the sumps much faster than anticipated the sump of a new mine in the development stage cannot be cleaned too soon if the mine is not producing much water at a particular stage of development the sump should still be kept clean the next shift may bring in more water than can be handled with half the pumps and when that point is reached there will be no way to reach the muck in the bottom of the sump for cleaning two methods are often used to divide the sumps for cleaning one is to locate a concrete wall between the two areas and provide a means of closing off one side and making the water flow to the other side with this method the wall should be anchored properly on the top and bottom with reinforcing steel into the rock this provides a safety factor in preventing the wall from collapsing into the side being cleaned the other system is to develop two physically separate sumps both sumps must be provided with pumps a method of stopping the water inflow and diverting it to the other sump and a means of access for cleaning many mines locate the pump station below the sump chambers using horizontal centrifugal pumps instead of deep well turbine pumps these mines avoid the installation of a vacuum priming system because the overflow feeds the main pump although this system provides easier access to the sumps and makes cleaning easier the risk of lo

sing the pump station by flooding is increased because of its lower elevation if the power for the pumps originates from a source not maintained by the mine i e public power with or without tie lines the mine may be without power for extended periods of time therefore every protective measure should be provided using deep well pumps on one of the upper or middle levels helps protect the facility for the maximum length of time if the pumps are in a room or chamber with the power lines coming in and the water pipes going out it may be impossible to access the pumps for replacement without a small overhead crane on a rail above the pumps if deep well pumps are used sufficient headroom is needed to remove the multistage impellers storage pocket skip pocket size for mines that extend over a large vertical distance there is usually enough storage capacity in the orepasses to accommodate any mismatch between production schedule and hoisting schedule but for mines that are on a single level there is often a need to build a large storage capacity between the production of the mine and the hoisting system other service facility developments a variety of other rooms or drifts must also be developed a lunch area or room is essential around an underground shop in many underground mines the lunchroom doubles as a mine rescue chamber office space should be provided for the privacy of the underground supervisor and the security of records and equipment another underground facility that must be developed is a fuel storage area for supplying all of the diesel powered equipment used in the mines it is usually advisable that a system be developed for centralized fuel storage where the mines are shallow it is common to put the fuel oil underground through boreholes however where the mines are deep and have multiple levels palletized storage containers are common in either case a retaining wall must be maintained that will contain the fluids in case a tank ruptures a new consideration is that of storing biodiesel fuel underground such fuel has the same requirements as diesel fuel in that it is constantly needed in the refueling of the trackless equipment but there is also the added advantage that biodiesel is low temperature sensitive and storing it underground will keep it fluid there must also be a space for explosive storage of course this should be located in a safe location away from main haulageways and high voltage electrical lines equally important is to construct the explosive magazine with complete security and management control in addition a separate storage and secure location must be built for the detonator systems making mine development decisions many ways exist to develop a mine however there is only one least cost method for any specific mine and it is up to the mine engineering staff to develop trade off studies to determine the optimum method to develop the mine a classic example was recently comp

leted for stillwater mining company smc for the development of mining of ore below the 3 200 level down to the 2 000 level the mine production is at a nominal capacity of 4 200 t d 4 600 tpd of combined ore and waste and is projected to increase to 4 900 t d 5 400 tpd over the next 10 years the 3200 haulage system trade off study c jacobs and b lamoure personal communication at the stillwater mine was initiated to evaluate several life of mine muck handling systems each having the potential to provide for long term muck handling requirements for the operation while meeting common operating criteria and long range goals each system evaluated was unique with respect to mine development timing required infrastructure and expenditure profile the objective of the study was to evaluate alternate longterm production from the deeper levels of the mine for ore and waste handling systems of various hauling options in lieu of the proposed belt conveyor system and to make a recommendation for the best haulage option the proposed conveyor system was listed as an option to provide a base case comparison with other options the primary options in this study included a decline with a conveyor option with diesel trucks base case deepening of the existing shaft with rail haulage an internal shaft or winze with rail haulage and secondary hoisting a decline with trucks using only diesel fuel and a decline using electric trucks all options consider a common 20 year mining plan with a maximum total throughput of 4 900 t d 5 400 tpd economic evaluation of the trade off study options considered incremental differences in capital and operating expenditures within smc s gross mining model gmm this precluded a pure incremental analysis of the different options but allowed smc technical services to construct 20 year cash flows for each different haulage option which consider all gross mining costs with respect to each plan the revenue stream generated for each plan based on metals price assumptions and downstream processing and commercial cost combined with applicable credits it is important to evaluate both annual cash flow and cumulative cash flow the option with the highest cumulative cash would logically be the best option but may not be the desired option if annual cash flow is negative for 1 or more years net cash flow ncf from each option was taken directly from option specific cash flow models and compared to perform an economic analysis considering mutually exclusive projects the npv for each cash flow is considered incremental analysis is then applied to derive incremental npv values and ror based on the analysis the following results are relevant ncf in all options are positive values there are no negative years this is due to full consideration of revenues and costs via the gmm applied to what are relatively small capital costs and incremental capital costs of the haulage options using a disco

unt rate of 10 the electric trucks option is the best economic choice based on npv the incremental npv for the cash flow comparison confirms this with a cash positive incremental npv and a positive incremental ror present value ratio is also favorable in the absence of the electric trucks option the shaftdeepening option becomes economically favorable increasing the discount rate in the analysis to 20 indicates that the base case option is the economic choice based on npv where the shaft deepening option has more cost in forward years the diesel truck option was eliminated because of ventilation constraints and the logistics of operating eight trucks in the ramp system mechanical excavation methods of development because of the many advantages of mechanical mining when the physical properties of the rock and the tools are right for cost competitive excavation hard rock mining companies keep trying to push the edge of technology so they too can benefit from mechanical mining while it has been a very slow process of change there have been a few notable successes the principal advantages to mechanical excavation are improved personal safety minimal ground disturbance reduced ground support needed minimum overbreak thus less material to move minimal ground vibration or air blast uniform muck size reduced ventilation requirements continuous operations conducive to automation of system where the system is applicable higher production rates and where the system is applicable reduced tunneling costs the disadvantages to mechanical excavation are in hard rock the system may have a poor penetration rate in hard or abrasive rock the system may have a high cutter cost some mechanical systems are not easily adaptable to changing ground conditions it may lack flexibility of operations tbms can t cut sharp curves and it often has a very high initial cost for the mechanical excavating equipment for designing a tbm for mine developments as compared to civil applications friant 2001 makes the following design recommendations multiple speed cutterhead an infinitely variable drive with high low speed torque is needed for broken ground changing to solid ground a variable frequency drive is available closed or shielded face cutterhead recessed cutters combine with radial bucket openings that extend toward the center of the head cuttings should not have to drop all the way to the invert through the slot between the rock face and cutterhead shield to be picked up back loading cutters should be mandatory there is no excuse for a fatality due to rock falls while changing cutters adequate shielding for rock support shielding sometimes covering a 360 area should extend up nearly to the gauge cutters adequate space must be provided near the cutterhead to install bolts and ring beams shields should not have wide gaps that allow small rocks to rain in shields should be active i e

with hydraulic adjustments so if the machine gets squeezed it can release itself soft touch grippers grippers if applied with too much pressure can break up the tunnel wall and cause massive cleanup jobs after each push putting pressure on the crown should be avoided a small length diameter ratio either the whole machine length or its segment lengths dictate the radius curve through which it can bore often a 30 m 100 ft radius is desirable on a midsize machine a short stroke is an acceptable way to achieve the short turn radius desired continuous boring systems are desirable i e with two sets of grippers but only if they do not compromise other requirements of shielding and grippers mobility a machine needs the ability to back out of its own tunnel break down into manageable size subassemblies and come apart and go together quickly this may be achieved by the machine itself or through accessories for the machine it must be kept in mind that in a mine there will often be many short tunnels as opposed to one continuous drive access for drills in addition to rock bolt drilling the machine must supply access for drilling for grout curtains spiling soil stabilization probe holes and drain holes design simplicity the simplest methods of thrust steering and control must be used not those that are the most clever human engineered controls and safety systems should be incorporated other types of mechanical excavation several other types of tools have been tried in mechanical excavation bullock 1994 impact breakers tunnels in italy machines with disks on rotating drums mobile miners machines with disks on rotating programmable arms continuous mobile miner various microboring machines development cost estimates by drilland blast it is not extremely useful to present cost estimates for developments because the many variables of the rock types rock mass conditions and strengths water condition size inclination and use will affect cost however a few examples are presented in table 12 5 1 in another example the stillwater mine contracted two alimak 3 35 m 11 ft diameter raises driven an average of 521 m 1 705 ft for a cost of 5 612 m 1 700 ft in 2005 and 2006 when escalated to 2009 costs this approximates to 6 600 m 2 000 ft there were two reasons why the raise climber was used for these ventilation raises rather than using upreaming with a raise boring machine 1 there was no access to the top of the mountain in this isolated area and everything would have had to have been transported by helicopter and 2 permitting to build a heliport and raise boring site would have been difficult to obtain in this pristine area in the end designers and consultants must remember that in planning a mine project unlike a civil project the product is not the hole the hole is only a means to get at what you came for the ore versatility capability flexibility and at times raw

power are the features that count development of significant underground infrastructure in the host rock construction of these openings constitutes a major aspect in the development of an underground mine depending on their use development openings will assume various sizes shapes and orientations the openings must be large enough to provide ample space and perform the intended functions to support the underground mining process with allowances for the geologic and geotechnical characteristics of the rock mass mine excavations are categorized into two distinct groups excavations that are used primarily for rock handling and excavations that are used for service development openings that fall in the latter category are considered permanent or semipermanent usually with an operating life equal to or exceeding that of the operating mine thus the locations of these excavations are particularly critical in the overall mine infrastructure plan they must be designed and constructed to meet rigorous performance standards while maintaining their operating role in the development of underground mine openings the design and construction aspects of the excavations must be considered concurrently the design aspect will address the size location and ground support requirements of the openings with respect to the geotechnical characterization of the host rock to ensure the associated equipment can be installed and any unfavorable geologic geotechnical conditions will be accommodated the construction aspect will address the excavation methods and equipment excavation sequencing and integration of the development with the associated construction activities a significant portion of the underground development schedule is normally dedicated to the construction of major mine service facilities such as pump rooms crusher rooms underground maintenance shops ore bins orepasses loading pockets ventilation and haulage drifts and so on in hard rock mines the openings for most of these facilities are normally excavated using drill blast muck cycles vertical and inclined openings including raises and orepasses can be excavated using either drill and blast or semicontinuous methods such as raise boring accessibility requirements are critical as well in order for mobile equipment and workers to gain access and construct the associated facilities each underground facility has a significant role in the continuous operation of the underground mine the decisions that are made from the development of the design such as operational redundancy to the operation and maintenance phase such as implementation of control systems and periodic maintenance are critical in ensuring constant uninterrupted mine operations for instance the pumping system for an underground mine facility with high water inflows would be designed and implemented with enough redundancy to ensure that in the event of a system failure auxiliary equipment is present that

can be commissioned immediately to augment the primary system until such time that repairs are completed and the primary pumping system can be put back into operation the ultimate reasoning behind designing and implementing redundancy in most underground mine service facilities is to ensure with economic considerations for capital expenditure a safe working environment and eliminate or significantly reduce the potential for catastrophes that could result in the eventual loss of revenue underground openings and related infrastructure a number of underground openings and related infrastructure are common to most underground mine operations they include developments such as access drifts ventilation drifts sumps refuge stations battery rooms and so on major infrastructure such as conveyor galleries underground crusher chambers pump rooms sumps and underground maintenance shops normally involve significant capital expenditure but are necessary facilities in underground mining operations sumps and pump rooms sumps and pump rooms are used in underground operations primarily to collect and manage water in and around the mine workings water inflow in a mine can occur from the following primary sources drilling and other associated activities during underground development and production service water groundwater inflows in either case the water must be effectively handled by locating designing selecting constructing and maintaining a dependable storage and pumping system the size and configuration of the pumping system depends largely on the predicted amount of water inflow to the mine depending on the predicted water inflows the water handling system arrangement may consist of a series of drainage ditches channels and pipes at different working levels that direct water to sumps the water is then stored and ultimately pumped to the surface either with a single high efficiency pump or by a series of pumps or drainage galleries or drifts used to transfer water to a central sump or storage facility and then pump it to the surface the first arrangement is best suited for small to medium water inflows up to several hundred cubic meters per hour where large inflows not anticipated in the hydrologic study could overwhelm the pumping system the second arrangement is suited for underground operations with large water inflows several thousand cubic meters per hour where hydrologic studies indicate there is a potential for flows to peak beyond those predicted common characteristics of underground sumps and pump rooms include the following excavation and concrete structures for the sump capacity and pump room size concrete bulkhead s for the sump containment equipment foundations and pipe support structures access platforms and walkways to provide for equipment inspection and maintenance mechanical equipment pumps motors piping and associated appurtenances power electrical instrumentation and cont

rol equipment switchgear mine load center drives etc lifting devices monorails cranes for removing major equipment and reassembling after service figures 12 6 1 through 12 6 3 illustrate three varieties of underground pumping system arrangements that are common in underground mine operations in the majority of cases underground mine pumping systems are designed to handle suspended solids that can be abrasive depending on the size and distribution of the particles in suspension on the pump components that are in direct contact with the carrier liquid loss of pump operating efficiency due to wear of pump components is a major concern in underground mine operations the pumps specifically designed for this type of application are referred to as slurry pumps centrifugal pumps and positive displacement pumps are the two most common types of pumps used in underground mine applications other pumping arrangements such as those employing multiple stage vertical or horizontal pumps have been employed in underground mining for pumping clear water these do not handle suspended solids well and thus require the excavation of settling sumps vertical deepwell pumps are unpopular in conventional mine dewatering because the entire pump column needs to be removed to access the pump for repair or maintenance however they are typically used when dewatering a flooded mine the pumping unit illustrated in figure 12 6 1 is one of several skid mounted packaged units that comprise the entire pumping system this type of pumping system is commonly used during the early development of a mine the pumping arrangement can consist of multiple units of similar design connected in series and located at specific elevations within the mine to collect and transfer water upstream from the lowest extremities of the mine workings each unit can incorporate single or multiple centrifugal pumps with motors driving the pumps integrally or via v belt drive systems typically each unit discharges water upstream into the storage tank of the successive unit in the pumping circuit the setup in figure 12 6 1 can potentially handle flows up to several hundred cubic meters per hour the actual performance is contingent on the total head to overcome the number and size of pumps per unit and the total number of units within the pumping circuit figure 12 6 2 also shown in plan view later in figure 12 6 21 incorporates a vertical sump that is located directly upstream of a pump room a concrete bulkhead separates the sump compartment from the pump room in this type of arrangement particular attention is required to locate design and construct the bulkhead to ensure stability and provide a watertight seal around the rock concrete interface and at penetrations through the concrete bulkhead the sump walls are typically lined with shotcrete or concrete to limit and or eliminate any seepage into the surrounding rock and to maintain the integrity of the rock wall pump

s are usually erected on a concrete pump base that is raised several centimeters above the pump room floor this arrangement prevents equipment from coming into contact with water in the event of a pipe leak or other malfunction an overhead crane or monorail is usually provided to aid initial installation and for removing and replacing pump components the pumping arrangement shown in figures 12 6 2 and later in 12 6 21 consists of multiple banks of pumps each having multiple centrifugal pumps connected in series the number of pump stations that are required to make up the complete pumping system depends on the pressure rating of the pumps contingent on the quantity and specific gravity of suspended solids in the carrier liquid agitators may be installed inside the sump to keep solid particles in suspension at all times this type of layout provides the following advantages can be used to provide redundancy in a mine s pumping system where only one bank of pumps operates at any time while the other is on standby allows for the possibility of installing additional banks of pumps should there be a future demand for more pumping capacity supports the ability to perform equipment maintenance or repair without a complete system shutdown the primary pumping equipment shown in figure 12 6 3 comprises multiple piston diaphragm positive displacement pumps complete with electronics and other accessory features each unit is capable of operating at up to 1 000 m of head at low flow rates and up to about 90 m3 h at lower heads they are efficient in handling high concentrations of solids the figure also illustrates an arrangement where the pump room is located in a separate excavation that is completely independent of the storage reservoir sump cutout see figure 12 6 4 for sump illustration water is fed to the pumps by pipes running from the sump to the main pump room intake unlike figure 12 6 2 allowance is made immediately above the concrete bulkhead to access the agitator the agitator keeps solids in suspension and other equipment in the sump pumping arrangements of this type offer several operating advantages including pumping of dirty water operating under high static heads pumping in a single lift without the need for multiple intermediate pump stations operating at a high efficiency and eliminating the need for large settling sump cutouts crusher rooms underground crushers are used to reduce run of mine ore or waste to a size that is suitable for the materials handling system to transport to the surface crusher chambers can be the largest single excavation and infrastructure development in an underground mine substantial capital expenditure normally goes toward the planning design procurement and construction of these underground facilities an underground gyratory crushing station is illustrated in figure 12 6 5 critical information necessary for making a decision regarding plant suitability and final unde

rground crushing arrangement includes location access mining method ground conditions ore properties and required support facilities the underground ore transport network must be integrated with the crushing plant location and design as the mined ore must be transported and fed into the plant for crushing before being transported to the surface underground crushing operations can be classified as single stage or multistage i e primary crushing or primary secondary crushing the size and complexity of the facility depend on the number of crushing stages because of advancements in belt conveyor equipment and other associated materials handling systems to handle larger and more abrasive materials most modern underground mines only need to install a primary crushing system there is also an added capital and operating cost savings benefit that can be realized through elimination of a stage in the overall crushing circuit some underground mines in the past have implemented multistaged crushing operations in the 1970s ore produced from underground operations at the climax mine in colorado united states currently abandoned was crushed by primary and secondary crushing at their no 4 crusher station in hard rock mines gyratory and jaw crushers are most commonly used because of their proven capability in breaking hard rock sizer technology is advancing to the point where they are now suitable for size reduction of hard rock feeder breakers are more successful in softer ores in coal mines where rock fragmentation activities are not as aggressive other crushing plant alternatives such as roll and impact crushers are more predominant when developing underground crusher rooms the following must be considered location remote or central with respect to ore delivery and conveyance after crushing crusher chamber access primarily for maintenance and supply of spare parts excavation size stability ground control geotechnical instrumentation and monitoring of the rock mass around the chamber overhead cranes of suitable capacity and other localized lifting devices within the chamber dump pocket truck or rail dump and coarse ore surge bin oversize handling arrangement at truck and rail dumps grizzly rock breaker etc crusher drives mountings control room concrete structures supporting the crusher drives and motors crushed ore loading pocket beneath the crusher or remote surge bin pan feeders impact feeder belts screens and belt conveyors access platforms and walkways dust collectors scrubbers and crusher room ventilation power electrical instrumentation and control equipment substation switchgear drives etc underground maintenance shops when no direct ramp access exists between the underground operation and the surface or when the access decline is too long to tram equipment to the surface in a reasonable time frame a shop and maintenance facility may be construc

ted underground to service and repair mobile and stationary equipment in mines with only shaft access major equipment is normally transported assembled and maintained underground after commissioning and operating major equipment often only leaves the underground mine for replacement or major rebuilding in a surface or off site shop an underground shop is a major lifeline for the mobile fleets in an underground mine the main advantage of locating a shop facility underground is for close proximity to the operations so scheduled maintenance and emergency repairs can be completed promptly with less equipment downtime whether fleet maintenance is contracted out to a separate entity or performed by the mine s own personnel it is essential to establish an ergonomic layout for the shop plan that is safe and maximizes service to equipment with easy access to supplies spares and consumables so that equipment times in the shop are limited for this reason multiple entry and exit points in the shop must be created to ensure that immobilized equipment undergoing repairs will not hinder the exit or entry of other equipment when an underground mobile fleet is comprised of both rubber tired and track equipment underground maintenance of both types of equipment must be available the maintenance requirements for track equipment such as work on engines braking systems and other critical locomotive components are fundamentally different from those of rubber tired equipment for these reasons it is essential to have a dedicated repair shop for track equipment that is independent of the primary maintenance shop underground shops are often equipped to replace components such as engines transmissions axles buckets and so forth an essential component of any underground shop is the overhead crane and other associated lifting devices see figure 12 6 6 used to move heavy components around the shop floor other items of prime importance in an underground shop include ventilation inside the shop fire protection access in and around the shop number of bays and their function types and level of services to be performed supply and management of consumables management of spent hydrocarbons lube and gray water shop lighting and illumination compressed air supply storage of tires spare parts and tools and equipment cleaning ore bins ore bins are used primarily for temporary storage of ore underground before final transport to the surface depending on the location in the ore circuit and the materials handling methodology ore bins are generally referred to as run of mine or coarse ore bins when used to store ore that has been sized with a grizzly or sizer or that discharges from the primary crusher because of the cyclical nature of ore extraction from the mining face an underground ore bin provides the surge capacity necessary to maintain a constant flow of the ore stream the material to be stored is normally fed

into the bin via trucks rail belt conveyor system typical for remote bins directly from a system of orepasses or directly from the crushing plant if located directly beneath the crusher the content of the bin is later discharged from the bin bottom through a hopper chute arrangement for further ore transport by a belt conveyor rail or truck system ore bin design and construction depend on a variety of conditions but most importantly on the type and condition of the host rock in massive hard rock ore bins are normally excavated in and integrated with the host rock figure 12 6 7 in other rock environments such as medium soft or sedimentary rocks ore bins are predominantly constructed as independent structures in order to limit the effect of any potential ground movement on their ongoing performance see figure 12 6 8 following are other important decision factors to consider with underground ore bins permanent access access during bin development and construction surrounding rock type and ground support requirements location above or beneath crusher remote close to hoisting plant etc size and capacity based on production rate scheduled and or nonscheduled shutdowns refer to the storage pocket skip pocket size section in chapter 12 5 shape circular shape most common can also be rectangular associated materials handling infrastructure feeders chutes conveyors other materials handling equipment to aid in transporting the material effectively bin wall lining and other materials of construction depending on abrasiveness of the stored material and characteristics of the host rock ore and waste passes passes are vertical or subvertical raises that are excavated in the host rock for transporting ore or waste by gravity flow from the upper levels in a mine to a predetermined lower level orepasses can be used to improve underground equipment productivity because they minimize overall travel distances from the ore loading point to the final ore dump point the location and design of the orepasses are therefore critical in mines that primarily haul waste and ore with trackless equipment orepasses commonly run subvertically and may connect production levels by means of finger raises as shown in figure 12 6 9a the top of the pass and finger raises are usually equipped with a rock sizing mechanism such as a grizzly to ensure that oversize materials which can potentially cause hang ups in the pass are reduced in size a load out control mechanism such as a discharge chute chain controls and press frame is usually installed at the lowest point of a drawpoint as illustrated in figure 12 6 9b following are some major items to consider in underground orepasses shape and size of the orepass enough to prevent material hang ups stability of the orepass maximum interval spacing between successive passes orepass inclination daily and total tonnages anticipated type and orien

tation of ore body material characteristics fines moisture content particle size etc load out configuration at bottom static pressures on load out mechanism effect of impact on load out mechanism from release of material hang up or material falling down an empty pass emergency access a secondary means of egress is required in all underground mine operations and normally governed by local mining regulations an emergency access worker access is a secondary means of egress that is provided primarily for miner exit from the underground workings in the event of a fire temporary loss of electric power or any other occurrence warranting worker evacuation it is important that emergency exits are located in an area where the ground is stable and the serviceability of the facility can be guaranteed and will not be hampered if the primary means of egress is blocked a worker access is a vertical or subvertical shaft that is constructed by drill and blast or raise bore and is normally located within the mine s fresh air intake network to ensure the presence of fresh air in the shaft at all times when in use during the development phase of the mine rather than install a separate worker access shaft one of the compartments of a hoisting or ventilation shaft could be dedicated to emergency access after a u s mine is in the operating phase the mine safety and health administration msha requires a separate entry as an escapeway a worker access is usually equipped with one of two evacuation systems illustrated in figures 12 6 10 and 12 6 11 an access ladderway or an emergency hoist and escape pod if an emergency hoist and escape pod system is implemented provision must be made for standby electric power provided the emergency hoist is not powered by an internal combustion engine to operate the hoist in the event of an electric power failure following are some major items to consider in designing and constructing a worker access emergency exit depth of mine operations from the surface type of primary access shaft or decline type of evacuation system mine environment corrosive may require galvanized steel components cost loading pockets loading pockets are used to feed ore or waste into skips in a production shaft system crushed rock or run of mine ore is conveyed and discharged into flasks at the loading pocket by rail belt conveyors or mobile equipment the flasks at the loading pocket serve as a temporary underground repository for rock waiting to be fed into production skips several factors including the following govern the design and construction of loading pockets shaft production rate shaft layout and design type and capacity of the skips method of ore or waste delivery to the loading pocket construction methodology ground support requirements loading pockets consist primarily of steel structures and usually must be integrated with the shaft steelwork at the skip s loading level

it is important that the whole infrastructure around the loading pocket be designed to effectively sustain impact loads from falling rock without inducing undue stresses and movement that could hinder its performance the design of the loading pocket excavation is therefore important to ensure the structure s long term stability and must incorporate adequate ground support ore delivery to the loading pocket can be semiautomated with a belt conveyor system feeding from an ore bin or crushing facility or by means of mobile equipment such as trucks and load haul dumps lhds figures 12 6 12 and 12 6 13 illustrate typical loading pocket arrangements for both methods of ore handling special purpose drifts conveyor ventilation and access drifts are three primary types of major lateral development that belong to the category of special purpose drifts common characteristics among all three underground openings include size normally constructed to the smallest practical cross section to meet the service requirement stability of the final excavations usually over the life of the mine and slope of the drift to maximize equipment productivity in tramming conveyor drifts are normally constructed with a minimum cross section to accommodate the associated equipment while allowing for access maintenance and material spill cleanup they may or may not be inclined depending on the vertical distance the conveyor has to travel the belt conveyor equipment is normally supported from the back of the drift as shown in figure 12 6 14 usually with rock bolted chain supports or on the sill of the drift with steel supports on concrete pedestals as shown in figure 12 6 15 the former method of support is more cost effective and can be installed faster depending on the size capacity and layout of the conveyor equipment the drift may be overexcavated at the head and tail ends or transfer points to accommodate additional equipment such as drives motors take ups transfer tower structure chutes and access platforms see figure 12 6 16 access drifts have similar characteristics to conveyor drifts except that they are sized and constructed to accommodate mobile equipment and utility lines where applicable passing bays to accommodate two way vehicular traffic muck bays and safety bays where personnel and vehicular traffic interact may be incorporated into the construction depending on the use of the drift as in figure 12 6 17 the layout and construction of an access drift must take the following into consideration type and size of haulage equipment to be used trucks and lhds or rails clearance requirements around equipment for passage of mine personnel routing of ventilation ducts pipes electric cables and other utilities as shown in figure 12 6 18 vertical distances between successive mining levels presence of spiral ramps maximum slope of invert to minimize tramming effort and maintain equipment productivity type of sill co

nstruction and drainage requirements ventilation drifts are essential for moving air into and from the underground mine workings these drifts are the primary link between the ventilation shafts or ventilation decline and the underground mine environment and are classified as follows intake drift for taking fresh airflow from a fresh air intake facility to the mine workings exhaust return drift for taking the mine s return air from the underground workings to an air exhaust facility most mine operations normally use the primary and secondary accesses for ventilation however large underground operations with large ventilation airflow requirements may require dedicated ventilation drifts shafts and raises these ventilation routes are usually not accessible to personnel and equipment traffic because of the high velocity of air being moved primary considerations in ventilation drift design and construction include the ventilation fan size and airflow velocity total length of the ventilation circuit excavation crosssectional dimensions roughness of the excavation wall to limit friction losses in airflow and stability of the excavation the fan and associated equipment can be installed underground in the drift at the ventilation shaft collar or at the portal depending on the system design and operating requirements figures 12 6 19 and 12 6 20 illustrate a ventilation decline and an underground ventilation drift respectively the setup in figure 12 6 19 represents a ventilation system used in the interim i e during initial mine development until the required lateral development and permanent ventilation raise is completed fresh air is heated and with the aid of booster fans located downstream of the heaters drawn through heating plants and discharged through vent ducts that extend down the decline to the working face at all times return air is exhausted up the decline and at the portal design construction and maintenance development openings and their associated infrastructure are unique in terms of design approach and design methodologies the interrelationship between various components of the infrastructure usually requires multiple design iterations this can be further complicated by the intrinsic variation of the mechanical properties of the host rock in which the development occurs design methods are established on sound engineering principles combined with empirical concepts research and extensive practical experience because the design process can sometimes be quite involved only the salient points concerning design criteria and design methodologies are addressed in this section underground mine infrastructures are normally designed and constructed with four primary materials of construction steel concrete shotcrete wood and rock in most modern mines wooden structures have been completely replaced by steel and concrete components other types of construction materials such as polymeric materia

ls and carbon fiber reinforced polymer have recently been introduced in the underground mine environment but have not gained widespread acceptance because of limited performance data steel concrete and the host rock will continue to be the primary materials of construction in underground mine development for the foreseeable future because of their proven durability and the ability to accurately predict their behavior under service design aspects the following key aspects must be addressed at various stages of the design of underground openings and related infrastructure scope of work including but not limited to the extent of work to be performed the different engineering disciplines that will be involved and battery limits functional requirements including the intended purpose and performance requirements end results to be achieved effect of the underground environment on performance size of excavation necessary and location within the global infrastructure plan design criteria including the key factors and information that govern the design development size shape capacity and ultimate performance of the completed facility design development the initial and final design of the facility must consider the following duty requirement of the excavation maximum size of excavation needed to satisfy duty requirement geotechnical characteristics of the surrounding rock access limitations constructability materials of construction to be used impact of the facility s construction and operation on the mine environment the underground environment s impact on the facility s serviceability possible methods of assembly of the various equipment supporting structures underground system operating costs planning and engineering including the potential impact of the new facility on existing operations resource requirements e g can engineering be completed inhouse or must it be contracted out to a consultant and schedule for completion of engineering construction aspects the techniques installation methods and equipment normally employed during construction and installation depend on the type and size of the overall facility the construction phase normally starts with planning of the various stages of construction and installation of the complete facility the primary stages during construction and installation are project preplanning procurement construction and installation schedule and resource management and closeout and commissioning construction and installation aspects of the underground development must address the following safety implementation of a comprehensive and practical safety program there are fundamental differences between the protocols of construction project safety and the safety protocols implemented by the mine operation group resource requirements ascertaining whether the work should be outsourced to an independent development contractor the mine may or may n

ot have the equipment and or skilled personnel available to self perform construction contractual preferred method of outsourcing competitive bidding single source alliance etc construction resources materials equipment and services that can be supplied by the mine during construction i e electric power compressed air water concrete shotcrete etc planning emphasis on minimizing interference with other ongoing mine operations infrastructure underground and surface infrastructure necessary to support the construction and installation work safeguards adequate lighting around stationary operating equipment guards around rotating equipment and so on access requirements and interface with existing mine operations construction equipment various pieces of equipment needed for each phase of the development size and capacities of equipment with respect to underground access waste handling removal of rock generated from excavation ventilation ensuring adequate ventilation of the construction area constructability construction methods and approach for each phase of the development installation sequencing consideration of construction equipment limitations construction management determination of whether the mine has the resources to dedicate toward management and procurement of supplies and equipment during construction and installation concrete a versatile component in underground mine construction it is used in almost all facets of underground development from ground support to foundations for structures blasting is a routine activity in underground hard rock mines and the resulting blast induced vibrations if left uncontrolled can have detrimental effects on concrete structures the level of impact of blast vibrations on a concrete structure is dependent on variables such as the distance of the structure from the blast initiation the age of the structure size of the structure and type and amount of explosives used because it is common practice for concrete construction to occur in close proximity or adjacent to underground excavation development blasting around newly constructed concrete structures should be considered during the design and installation phases of underground mine development the hydration and crystallization process in concrete normally occurs within 24 hours of placement this period is very critical in all concrete structures because it is when the hardening and strengthgain process starts to occur normal concrete usually attains about 30 40 of its compressive strength within 3 days and about 60 within 7 days of placement thus blasting in close proximity to major new concrete structures is not advisable before 3 days following placement between 3 to 7 days following the concrete installation blast vibrations adjacent to major concrete structures should be limited to that which produces a peak particle velocity of not more than 25 mm s after 7 days followi

ng concrete placement a peak particle velocity of 50 mm s or less should be used as a guide to limit blast vibrations until the concrete reaches its maximum strength around 28 days project preplanning tasks that are normally accomplished during preplanning include the following after the contract has been awarded become familiar with and understand the contract documents and general conditions of the contract arrange for independent services such as inspections and nondestructive testing training and so forth this is particularly applicable to underground mine development contracts that are outsourced to an independent contractor determine and secure the various construction equipment drill jumbo lhd haul truck access platforms mobile cranes etc required for completing the work understand the ground conditions and plan the excavation muck handling and ground support sequence in accordance with the excavation designs establish procurement protocols and determine the longlead items review their manufacture and delivery schedules and verify when they should be procured in order to avoid schedule disruptions establish availability of services and supply of all construction materials implement subcontracts for various fabrications such as for miscellaneous steel structures pipe spools etc plan the site set up and administrative aspects of the site office office equipment requirements required project personnel and so on depending on the contractual arrangement arrange for transportation and schedule mobilization of resources to site procurement procurement involves soliciting of suppliers and services leasing of equipment and purchasing and marshaling of permanent equipment such as pumps electrical gear overhead cranes crushing equipment lifting devices and any other special item associated with the permanent installations one aspect of the procurement process that must be addressed in the initial stage of planning is the on site management of materials and equipment that are delivered for the construction of mine infrastructure projects planning for such undertakings involving either a single major project or multiple small to medium size construction activities must consider the logistics of transportation sequencing of material delivery and the need for adequate lay down space and warehousing it is advisable to manage construction project related procurement separate from that of mine operations a list of critical spare parts should be developed during the procurement stage the procurement process can become lengthy when dealing with multiple vendor selection whether prequalification based or otherwise mine operations normally establish service agreements with suppliers and contractors that can be capitalized on in order to fast track the procurement process construction and installation excavation methods raises orepasses and other vertical or inclined openings can be exca

vated by either drilling and blasting or semicontinuous methods such as raise boring raise climbing or drop raising ground conditions and rock type ultimately influence the excavation method chosen the most commonly employed excavation method for underground service facilities in hard rock mines is drilling and blasting each drill and blast cycle follows a repetitive pattern consisting of blasthole drilling charging and blasting ventilation and dust removal scaling muck handling and ground support as described in the following paragraphs work at the face is normally started by the mine survey crew that sets controls for the excavation and demarcates the excavation limits and layout of the blasthole drill pattern the blasthole drilling sequence is normally arranged in a manner that maximizes drilling productivity blasthole lengths are selected to complement the length of each round which in turn are dictated by ground conditions and the maximum unsupported roof span allowed short rounds require short drilling depths accomplished with a singleboom feed during fully mechanized drilling all subsequent activities face advancement etc can be completed quickly because there are no equipment related delays for handling drill rod extensions in contrast longer rounds require longer drilling depths accomplished with multiple boom feeds longer rounds can result in blasthole drilling inaccuracies which can lead to increased drilling times increased charging and blasting times longer muck handling times due to larger muck volumes and larger roof spans to be supported after drilling is complete the holes are flushed loaded with explosives and prepared for the blast initiation upon completion of charging and blasting the blastholes are flushed with water or other flushing agent depending on the underground requirements out of reach blastholes are normally accessed with a work platform or basket all equipment at the face is receded to a safe location prior to initiating the blast after the charging and blasting phase is complete the face is ventilated to remove dust and fumes depending on the distance to the new working face the vent duct and fan system may need to be extended to ensure that adequate fresh air reaches the working face any loose rock on the excavation walls are removed by scaling this process can be completed manually by a worker in a basket with a scaling bar or mechanically with a mechanized scaler manual scaling can be time consuming and difficult mechanized scaling on the other hand is quicker and does not expose mine personnel to difficult working positions the muck handling phase commences after it has been determined that no potential rockfall hazards are imminent muck removal can be accomplished by one of the pieces of muck handling equipment listed in table 12 6 1 or a combination thereof for short rounds high mucking rates may be attained with only lhds depending on the muck vol

ume and haul distance for long rounds or where a large volume of muck is involved other equipment combinations such as trucks and lhds may be necessary in order to attain the required mucking speeds hauling the muck away as soon as possible frees up the face for advancement and ultimately improves work cycles as the distance to the face increases the haulage distance and time increase as well to free up the face for subsequent rounds muck can be temporarily moved into muck bays excavated at regular intervals from the working face the muck can later be hauled from the muck bay to the final dump location as a separate operation off the critical path of other face development activities while the face is being further developed ground support normally commences after the muck has been hauled away from the face the type of required ground support depends on the excavation size and ground conditions in poor ground conditions it may be necessary to perform multiple ground support activities initial and final instead of a single activity at the end of each round if the ground is fractured and roof stability becomes critical initial ground support such as with shotcrete and mesh can be installed immediately after scaling to stabilize the excavation before mucking out completely final ground support bolting final shotcrete etc can be installed after the muck has been completely removed and the excavation fully exposed for the next round consideration should be given to the benefits of using the same drill jumbo for drifting and ground support installation however it may prove to be more cost effective overall to obtain a separate jumbo for the ground support exercise with a single jumbo the bolt holes could be drilled while the muck is being hauled away in large excavations and where the excavation completion schedule is critical separate jumbos for blasthole drilling and ground support installation may be warranted excavation sequence ideally underground openings are excavated full face depending on the excavation layout ground conditions and operating range of available equipment in openings where the cross sectional dimensions exceed the operating reach of available equipment particularly in fractured rock the face may be advanced by multiple headings in order for the following to occur limit the unsupported roof span of the excavations prevent collapse maximize equipment operating range per round reach the back and promptly install ground support large excavations for pump rooms crushers and similar facilities can be developed by driving a top heading followed by a single bench or multiple benches several rounds behind the top heading development other excavation techniques for large regular shaped openings include driving a pilot drift and slashing the back of the excavation on retreat factors that influence the excavation sequence include rock properties ground conditions equipment ope

rating range limits and imposed limits on blast vibrations major construction equipment several types of major equipment are used for the various work phases examples of the major equipment used in the construction of underground a tunnel boring machine tbm is a device used to excavate tunnels in a circular cross section through a variety of soil and rock strata tbms can bore through hard rock sand and almost anything in between tunnel diameters can range from 1 to about 16 m 3 3 to 52 5 ft tunnels smaller than that are typically created by horizontal directional drilling rather than by tbm this chapter describes mechanical excavations in soft moderate to very hard rock and in rock mass conditions ranging from marginally self supporting to massive tbm types discussed include state of the art open shielded and other types of machines that incorporate specific features to suit particular applications all using disc cutters mechanical tunnel excavators that create noncircular cross sections such as mobile miners mobile tunneling machines roadheaders and drum miners are not discussed in this chapter although they also serve important roles in civil and mining applications these machines are discussed elsewhere in this handbook tbm types tbms are normally classified into three groups 1 open type machines main beam kelly drive and open gripper 2 shielded type machines single shield and double shield 3 soft ground machines hybrid earth pressure balance mixed shield and slurry a feature common to all tbm types is a rotating cutterhead that does the following 1 presses forward and against the rock face under high pressure 2 uses individual disc cutters placed strategically on the cutterhead 3 performs muck removal by means of buckets and 4 is supported by a main bearing with the structure being moved forward by a thrust system composed of cylinders all tbms have a backup system of power units and muck removal systems shielded type machines are seldom used in mine applications because of the difficulty in removing the shields a third shielded type machine the gripper shield was designed to actively support the ground from the cutterhead to behind the tbm where rock support can be installed and has been used in a wide range of geological conditions however it is no longer available because of limited acceptance in the industry because most mining applications take place in moderate to hard rock soft ground machines are not discussed in this chapter general tbm applications compared to their use in civil engineering applications tbm use in mining has been relatively modest since the first successful hard rock tbm was introduced by james s robbins in 1957 tbms in mining have ranged in diameter from 1 7 to 7 6 m 7 to 25 ft they have been used predominantly in soft to moderately hard formations mine applications at times place severe restrictions on component size and weight so that equipmen

t can be lowered down existing shafts transported to a designated assembly and launching area and after tunnel completion removed tbm systems in some mines must include explosion proof equipment such equipment has been used for access to coal mining in germany england and canada detailed study is required to select the proper tbm system for a project such study involves reviewing geological design reports geological profile drawings and baseline design or equivalent reports prepared by the mine owner or a consultant depicting anticipated conditions along the drive and required design parameters important factors to consider include the following rock types rock strength parameters from a testing laboratory rock mass characteristics including stand up time at the actual tunnel depth profile and alignment of the proposed tunnel tunnel diameter temporary and permanent tunnel support requirements water and gas geological profile mucking requirements tbm design features important design features to consider for achieving efficient rock chipping include effective thrust load on the cutterhead and cutters layout and spacing of cutters and diameter and tip width of the cutter rings the nominal load capacity of a single disc cutter has evolved from 89 kn 20 000 lb for the 300 mm 12 in cutter of the 1950s to 312 kn 70 000 lb for the 500 mm 20 in largest size cutter of today today s state of the art hard rock tbm typically has a flat face cutterhead with single tracking center face and gauge cutters each of which provides concentric kerfs on the rock face during excavation kerf spacing center to center spacing of cutter tracks is nominally 90 mm 3 5 in for the face cutters decreasing into continuously tighter spacing for the gauge cutter positions a flattype cutterhead normally consists of nine cutters in the curved gauge area a typical 4 6 m 15 ft tbm hard rock cutterhead with 432 mm 17 in back loading cutters is shown in figure 12 7 2 single disc cutters 483 mm or 19 in in wedgelock housings are shown in figure 7 1 4 in chapter 7 1 selection of cutter diameter and cutter characteristics is determined by the application study and or specific requirements of the application state of the art cutter rings for hard rock applications are made of tool steel specifically heat treated so as to reduce the number of cutter changes and downtime for the anticipated ground conditions during the drive the cutter hub assembly includes high capacity tapered roller bearings and seals capable of supporting multiple cutterring changes early during tbm design it must be decided whether the cutterhead should have front or back loading cutters frontloading cutters are faster to change but require miners to enter and work beneath unsupported ground in front of the machine back loading cutters can be changed from behind the cutterhead and under the protection of a shielded structure the largest indivi

dual cutters 500 mm 20 in weigh up to 190 kg 425 lb cutters are moved and installed by means of a cutterhandling system normally designed for the specific tbm other design features vary by tbm type for further information on tbm cutters see chapter 7 1 main beam tbm the main beam tbm figure 12 7 3 is suited for highstrength rock and short faulted zones ground support methods can include ring beams bolting and shotcreting a main beam tbm consists of four main elements 1 cutterhead 2 cutterhead support and main beam 3 gripper and thrust assembly and 4 conveyor assembly the gripper assembly with one set of grippers forms the stationary anchoring section of the tbm it transmits thrust and torque to the tunnel wall during boring and carries part of the machine weight the anchoring force is approximately 2 to 3 times the total forward thrust force the machine is forced forward a stroke at a time by hydraulic cylinders connected to the anchoring section and main beam as the cutterhead rotates against the rock face at the end of a stroke and after the cutterhead stops rotating the rear support is lowered to the invert the grippers are retracted from the tunnel wall and moved forward equal to the stroke of the thrust cylinders the grippers are again energized against the tunnel wall and rear support is retracted so that boring can start again the front supports support shoe and extendable side and roof supports provide ground contact immediately behind the rotating cutterhead to stabilize the tbm during tunneling the cutterhead is typically driven by water cooled electric motors located on the cutterhead support each directly mounted to a gear reducer with output pinions drive pinions engage with the large ring gear which is connected to the cutterhead the cutterhead itself is supported by the main bearing the main beam tbm has essentially become the industry standard open tbm mainly because of its ease of operation and ample room around the cutterhead area for ground support kelly drive tbm the kelly drive tbm figure 12 7 4 is suited for highstrength rock and short fault zones ground support methods can include ring beams bolting and shotcreting the kelly style tbm consists of two main elements the anchoring section and the working or moving section the gripper assembly with two sets of grippers forms the stationary anchoring section it transmits thrust and torque to the rock during boring and supports the weight of the machine four grippers in the radial direction are arranged horizontally in pairs and controlled individually the working section includes the cutterhead front bearing housing torque tube rear bearing housing and drive train cutterhead rotation is controlled by motors at the rear of the machine coupled to gear reducers and pinions a common ring gear turns the drive shaft through the center of the torque tube to transmit rotational power to the cutterhead hydraulic pulling cy

linders develop thrust that is transmitted from the main body through the rear bearing housing and drive shaft use of this type of tbm has declined mostly because of the limited access around the tbm for installing ground support open gripper tbm the open gripper tbm figure 12 7 5 is suited for highstrength rock and short fault zones ground support methods can include ring beams bolting and shotcreting the tbm has expanding supports close to the excavation face and a simpler gripper design no main beam or kelly drive structures than do the other types of open type tbms described it typically uses an arrangement of latticetype diagonal pattern thrust cylinders to react to cutterhead torque and correct roll advantages can include low cost short overall length and tight turning radius the machine consists of a cutterhead forward expanding shield and gripper assembly the cutterhead and cutterhead drive are the same as typically seen on main beam and grippershield tbms this configuration provides cyclic advance of the machine in the following sequence bore regrip bore a bridge structure spans from the tbm cutterhead support to the first platform car the bridge carries the grippers and also provides space to install muck haulage rail and a mounting for the forward conveyor material handling equipment exhaust duct and tunnel support installation equipment single shield tbm a single shield tbm figure 12 7 6 performs boring and lining installation in sequence lining typically concrete segments is used as an anchoring station to propel the machine during boring the machine can be used in hard as well as soft ground and is well suited for tunnels in mixed ground and loose formations with low stand up time the main section consists of a cutterhead cutterhead support with integrated ring gear main bearing and drive units thrust cylinders are located in the rear part of the machine and act against the tunnel lining complete lining rings are installed inside the shield by a segment erector steering is done by individual control of the oil volume supplied to separate groups of thrust cylinders steering can be enhanced by the articulation cylinders which bias cutterhead support in relation to the shield double shield tbm the double shield tbm also called a telescopic shield tbm figure 12 7 7 can be used in hard as well as soft formations it is used when there is a risk of unstable ground conditions normally the tunnel walls are used to anchor the machine which allows installation of segmental lining during boring if the walls cannot take the gripper pressure the machine can push off the lining instead the working section at the front of the machine has a cutterhead cutterhead support with integrated ring gear main bearing and drive units a telescoping section between the anchoring and working sections includes hydraulic cylinders for thrust and steering the anchoring section at the rear of the machi

ne has two horizontal grippers that transmit thrust and torque auxiliary cylinders transmit these reactions to and propel the machine from the lining a segment erector is installed inside the tail shield steering is done by individual control of the oil volume supplied to separate groups of thrust cylinders steering can be enhanced by the articulation cylinders which can off angle the working section in relation to the anchoring section to bias the cutterhead in the proper direction if segment lining is being placed an electronic guidance system is mandatory to ensure correct line and grade for installation of the segment ring within the tail shield tbm mining applications underground openings in mines can be associated with exploration access to and development of ore bodies haulage by train conveyor or rubber tire ventilation drainage dewatering movement of supplies and labor safety or a combination of these figures 12 7 8 through 12 7 13 show examples of some underground openings in mines a key consideration for tbm mining applications is the type of muck removal system tunnels under consideration for tbm application are generally close to mine boundaries because only long distance tunnels could justify capital investments in tbm equipment tunnel cross sections are generally relatively small because muck must be transported and hoisted without interrupting full time mining haulage of muck has historically been done by rail rail haulage is often the lowest option in terms of capital and operating costs and still reasonably flexible but conveyor haulage is also common today especially for longer tunnel drives however conveyor systems are not always suitable for tbm applications because their minimum turning radius is 200 m 660 ft while a custom designed tbm may be able to bore a curve radius of 100 m 330 ft haulage by rubber tire vehicles may be considered for larger tunnels with diameters of 7 m 23 ft or more the invert is then typically backfilled with concrete or goodquality rock materials to provide a roadbed vehicle passing stations may be required at intervals to maintain a reasonable tbm utilization another consideration for tbm mining applications is incline and decline declines have been limited to a maximum grade of 15 however because a typical muck removal system uses conveyor belts inclines can be quite steep since mucking can take place as safely contained gravity flow along the invert in civil works inclines have exceeded 45 alignments are further determined by the end use of the excavation geologic conditions and other management or owner constraints such as property boundaries tbm versus drill and blast excavation methods tbm excavation methods have both advantages and disadvantages compared with conventional drill and blast methods tbm advantages high rate of advance instantaneous excavation rates of 1 6 m machine hour 3 20 ft machine hour are regularly achieved by t

bms with diameters of 3 5 5 5 m 11 5 18 ft overall advance rates per day week and month depend on the overall tbm system utilization and work schedules faster ore body development shortens the time period of capital use and capital cost with tbm use fewer headings are operated at one time making supervision and planning easier and management effort more concentrated and focused some types of geological environment can be more effectively supported if support is installed relatively quickly after excavation no blast vibration or blast fumes tbms enable work to be carried out continuously in the heading without delays for clearing gases from blasting from the heading or ventilation system no workers need to be removed from other headings or work places to safety refuges for a blasting cycle however other drill and blast work in the mine can cause interruption in the tbm heading reduced ground disturbance and support mechanical excavation is well known to be less destructive to the structural integrity of material surrounding the excavated opening than is drill and blast excavation less ground support is required with significant reduction in material costs and installation time roof bolts and steel ribs are the two predominant support systems used with tbm excavation in mines yieldable steel sets have commonly been employed in deep german coal mines for tbm drifts installation of ground support in larger tunnel cross sections can largely be automated reduced lining costs where a completed excavation must be lined with shotcrete or concrete the smooth uniform surface created by tbm excavation reduces costs of the lining material and raises the possibility that lining installation can be automated uniform muck size mechanically excavated materials essentially all pass through a 100 mm 4 in grizzly used for coarse screening suitable for automation the uniform size of mechanically excavated materials makes material handling ideal for automation less wear and tear occurs on transport equipment such as rail cars trucks or conveyors in the tunnel vertical transport by means of loading pockets and hoist conveyances can be automated minimizing maintenance costs ore that is encountered in a heading can be partially segregated and sent to the concentrator eliminating primary crushing costs reduced labor costs a rapid rate of advance reduces the unit cost of excavation in most western countries both crew size and rate of advance are larger for tbm than for drill and blast operation consider the following typical average progress for a tunnel of small cross section 10 m2 108 ft2 each with two 10 hour work shifts per day tbm operation crew size 4 to 7 assume 6 for the following calculation rate of advance 140 m 460 ft per week productivity 140 m week 72 shifts week 1 94 m 6 4 ft per shift drill and blast operation crew size 2 to 3 assume 3 for the following calculation rate of advance

38 m 125 ft per week productivity 38 m week 36 shifts week 1 06 m 3 5 ft per shift good safety record tbm excavations are considered relatively safe compared with drill and blast excavations because the latter incur greater risk to workers from falls or ground related accidents even in supported ground most tbm systems have protective canopies for workers in ground support installation work zones both excavation methods involve some heavy lifting which if not properly performed can lead to back injuries a typical 432 mm 17 in single disc cutter assembly weighs about 136 kg 300 lb back loading cutters normally required in mining applications are best changed with slings and hoists using proven cutter handling systems backloading cutters can normally be used for tbms with diameters of 4 m 13 ft lower manning s factor tbm excavation in hard rock leaves a smooth circular profile with a roughness coefficient that is lower than that for a typical drill and blast operation the cross section of an unlined tbm tunnel therefore needs to be only about two thirds of the area of an unlined drill and blast tunnel to carry the same water flow circular cross section a circular profile typically gives the most stable cross section tbm excavation requires substantially less ground support in fairly massive to massive hard rock conditions improved ventilation characteristics the smoother rock surface of a tbm excavation means reduced friction and improved possibility of excavating longer drives from a single heading a tbm tunnel has substantially less overbreak than does a blasted tunnel large savings in concrete and concrete placement may be realized no muck bays or passing bays no muck bays or passing bays are needed for tbm tunnels with diameters of 3 5 m 11 5 ft because all advance is centerline advance tbm disadvantages high capital cost tbm costs vary widely with size supply and power requirements the cost of a typical open type tbm excluding roof drills ring erectors and other ground support or probing equipment is about us 1 7 million per meter of diameter 2009 price levels a simple backup system in a heading with a diameter of 3 4 8 m 10 16 ft varies from us 1 2 to 2 million depending on facilities storage muck transport systems and so on a system with a diameter of 3 6 m 12 ft with spares and cutters thus costs about us 8 million a used fully reconditioned technically comparable system costs about 65 75 of this amount repair and maintenance work are necessary during the life of a machine the life of a tbm system has been suggested at times to be 15 km 50 000 ft of tunnel excavation although this varies widely depending on technical obsolescence operating and maintenance practices and application conditions in the marketplace external to the mine can have a large impact on equipment life and salvage value a number of tbms have bored more than 25 km 80 000 ft of tunnel generally

at least 2 5 km 8 000 ft of tunnel are necessary to justify the use of a tbm system this includes equipment salvage value large value of spares and cutter inventory the recommended inventory of major spare parts including main bearing ring gear drive unit motor clutch gear reducer and pinion propel cylinder and gripper cylinder are in the range us 600 000 to 800 000 2009 price levels inventory of minor spare parts adds another us 60 000 to 100 000 of capital cost us 300 000 can be spent for initial cutter mounting cutter spares and cutter repair tools and fixtures long lead time for delivery design and manufacturing time for a new tbm with a diameter of 3 6 5 5 m 12 18 ft is typically a minimum of 10 months the time for a smaller tbm with a diameter of 2 6 3 0 m 8 10 ft may be as low as 8 months delivery time exworks varies with actual supply clarity of product definition and subsuppliers workloads reconditioning and modification times for existing equipment is 65 95 of these values depending on the particular equipment its location and the scope of work required large components that are difficult to transport to reach the high thrust and torque levels required for tbm excavation components must be large and heavy and are therefore difficult to move through existing mine shafts or mine drifts tbms are also often difficult to remove from completed tunnels boring of tbm drifts from shafts can be challenging transport from the surface and assembly underground as well as removal from the mine require careful planning and preparation at times transport and assembly must take place without interrupting full time mining plans and methods for slinging and lowering heavy components down the shaft must be devised early during machine design machine components and support systems must be designed to be small and light enough for accommodation in the shaft and by the hoist the existing hoist configuration controls or brakes may also require modification and special transport or handling devices may be required to lower large components down the shaft or move them along the level limited range of application in varying ground conditions tbm design is determined by the requirements of the particular application the more uniform or consistent the geologic environment the easier and more reliable the design competent moderate hard rock requiring moderate ground support and with no major water inflows is ideal however mines are normally associated with geologic nonconformities that result in mineral deposits and it is to be expected that all or part of an underground excavation will require some type of rock support machine design can within limits accommodate hard rock to caving or squeezing ground for mining applications a high degree of flexibility is generally desirable today s machines can operate in varying ground conditions because of their variable frequency drive motors for control

ling cutterhead rpm and torque they can be equipped with effective systems for probing pre excavation grouting shotcreting near the cutterhead rock bolting ring beam installation and application of mesh and lagging in addition they can be furnished with the mcnally system a longitudinal roof support system for operation in bad ground conditions mcnally 2010 consumable costs can be high in hard abrasive rock cutter costs can be a very significant part of the overall tbm costs in soft to medium hard sedimentary rocks such as are associated with limestone and other nonabrasive rock types cutter costs are typically as low as us 1 00 m3 0 75 yd3 or less however in hard massive quartzites they can reach us 20 m3 15 yd3 or more due to the high abrasiveness of the rock combined with low instantaneous penetration when cutter costs are high machine utilization drops because of downtime for cutter changes and for cutterhead maintenance and repairs several laboratories can test rock samples needed for tbm application studies most tbm manufacturers and some tbm consultants can provide estimates on instantaneous penetration and cutter consumption based on these lab test data anticipated rock mass characteristics and proposed machine and cutter specifications limited to circular cross section when a heading is intended for haulage the circular cross section produced by a tbm may limit the choice of haulage method and equipment this is particularly true when rubbertired haulage trucks or load haul dump units are to be used the diameter of the excavation must be increased and the invert may require a concrete cast floor as a roadbed in some rock burst conditions a tbm produced circular cross section although usually advantageous for ground support can also be a detriment in highly stressed and brittle rocks the surface stress of the circular opening can be quite high compared to that in rock that has been deeply fractured and stress relieved by use of drill and blast methods limited turning radius a typical tbm has a minimum turning radius of 200 m 660 ft although custom designed tbms may be able to bore a radius of 100 m 330 ft backup equipment conveyors belts mining cars mucking solutions and so on must all be able to negotiate a curve so created in contrast a drill and blast operation has basically no limitations regarding curve radius substantial preparation at work site site preparation for most mine projects that involve adits is not unusually difficult however for a tbm application there must be ample room at the portal site cranes for lifting large pieces and adequate supply of power other requirements are basically similar to those for drill and blast headings when an underground work station is planned adequate hoist capacity and ample length for assembling the trailing gear are needed a typical overall length for a tbm with a diameter of 3 5 m 10 16 ft can be 100 140 m 320 450 ft th

e underground assembly chamber for a 3 66 m 12 ft tbm is usually sized as follows figure 12 7 14 width and height 1 5 times the tbm diameter depending on the type of hoist length length of the tbm 20 m 65 ft for safety and cost reasons it is desirable that the assembly chamber be constructed in good ground close to the launch point for the drive requires substantial crew training and high mechanical and electrical skill levels a tbm system is essentially a crude moving processing plant or factory material is removed and sized in a cutting process from its in situ state the process from that point on involves several electrical mechanical hydraulic lubrication compressed air cooling water wastewater ventilation material handling and storage systems along with monitoring and control of these systems these systems must be maintained in a hostile environment of water dust vibration and varying temperature some of these systems may not be in common use with other machines or systems in the mine operation mine operating personnel must be trained in the use and maintenance of these systems and how they relate to the overall tbm production process training is largely accomplished in the tbm manufacturer s shop during floor testing and machine disassembly and at the work site during final assembly and full load testing tbm operators receive training by visiting or temporarily working at other sites and by working with the tbm supplier s field service personnel instead of training mining crews a better solution can be to use contractors who specialize in tbm work a tbm contractor must be carefully selected and must understand mine priorities and operations requirements requires large power supply most tbm systems today use total power in the range 1 000 2 600 kva or more for tbms with diameters of 3 5 m 10 16 ft total power includes power for cutterhead hydraulics ventilation conveyor lighting pumping cooling and other systems the power required to operate a tbm system may not have been planned in the mine s initial design stage as many mines are more than 25 years old even today mine infrastructure planning does not usually include provision for the use of mechanical excavation equipment in major headings the large power requirement usually necessitates addition of high voltage distribution cables in the shaft and addition of underground substations vary excavation diameter normally it is not practical to change tunnel diameter during a drive because of costly downtime the tbm design itself may allow a change of 1 m 3 3 ft or more from a minimum established diameter possibilities with regard to diameter increases should be based on the original tbm design criteria and the upcoming project application including desired diameter length of drive and rock mass and rock strength characteristics design and operating problems and solutions historically design and operating problems have ar

isen with tbm applications in the mining industry samples of past problems and their solutions are given in table 12 7 1 since 2000 several important improvements have been implemented in tbms and auxiliary equipment that decrease downtime and increase tbm utilization water cooled motors and hydraulics larger diameter cutters higher capacity cutter bearings and cutter ring with improved abrasive resistance characteristics high efficiency variable frequency electric drives improved hydraulic probe drill schemes and preexcavation grouting solutions faster setting grout operational and management problems and solutions the following typical challenges can arise in a tbm mining application failure on the part of mine managers due to categorization of the tbm as a mine research project to commit fully to the decision to use a tbm failure on the part of direct supervisors and miners to commit fully to the success of the tbm system failure to obtain acceptance of new technology due to unrealistic expectations failure on the part of machine suppliers to fully understand the unique mine operating conditions and priorities related to design challenges operating costs performance and the high skill levels required of personnel lack of jobsite preparation and coordination with other mine activities lack of training of operating and maintenance personnel several tbm projects have been considered failures when original schedules costs and full desired end benefits were not achieved however most had acceptable results when compared to realistic alternates the solution to these problems is to keep expectations realistic fully identify mine requirements and possess adequate design experience a summary of mine projects in the western world is presented in table 12 7 2 by date and original tbm manufacturer the summary may not be complete but the large number of projects listed should indicate that there is enough experience in the industry to know what to do and what not to do when applying tbms to mining applications many market studies in attempting to identify the worldwide need for mines have estimated the total annual drive lengths of horizontal openings the total number is very large of this total much is not really suitable for tbm excavation however when drive lengths are sufficiently long and when mine owners operators equipment suppliers and contractors focus on a common result that benefits each of them they can commit to their mutual success and achieve it a good example of such commitment is the case history that follows tbm mining case study the magma copper company s san manuel mine in arizona and the stillwater mining company s platinum mine in montana both in the united states are examples of successful tbm use in mining many comprehensive reports concerning these projects are in the public domain tbm use in the san manuel mine is discussed here in detail mine location and layout the

san manuel mine is located about 72 km 45 mi north of tucson arizona in order to secure future production it became necessary to develop the lower kalamazoo ore body 1 050 m 3 450 ft below surface for a modern block caving operation the base of the mass to be caved would be accessed by drifts on two levels from the existing no 5 shaft based on a feasibility study it was decided to use a tbm rather than traditional drill and blast methods because of the very tight schedule for starting production in the new ore body because of a lack of qualified labor and the short construction time it was determined that an experienced tbm contractor was needed the development layout was designed specifically for tbm use figure 12 7 15 the tbm tunnel route consisted of 9 826 m 32 235 ft of tunnel circling the ore body on three levels 1 the 3 440 grizzly level 2 the 3 570 haulage level and 3 the 3 600 conveyor gathering drift level the grizzly level consisted of 3 963 m 13 000 ft of tbm tunnel with slight grade changes to provide two permanent dewatering sumps the level has five curves with radii in the range 152 107 m 500 350 ft and a total curve length of 1 031 m 3 383 ft the haulage and conveyor gathering drift levels are accessed from the grizzly level by means of two parallel decline tunnels with grades of 5 7 which accounts for the remaining 5 863 m 19 235 ft of the tbm tunnel route these decline tunnels have two curves with radii in the same range as for the grizzly level and a total curve length of 1 255 m 4 119 ft mine geology the geology of the san manuel mine is quite complex with rock structure ranging from very weak to reasonably strong the area geology is described briefly as follows atlas copco robbins 1996 the ore bodies resulted from porphyritic intrusion of granodiorite at the end of the cretaceous period into precambrian quartz monzonite mineralization along the contact zone was shaped originally like a hollow ellipse which later was split and displaced by faults and intersected by numerous dikes the tbm s excavation route included the stable quartz monzonite of 150 180 mpa 22 000 26 000 psi unconfined compressive strength two mineralized haloes and a mineralized core the bore path also crossed the san manuel fault six times and the virgin fault five times the san manuel fault was flat dipping in an 1 m 3 ft wide clay zone that followed the bore some 30 m 100 ft at a time but it influenced the drive for a total of about 190 m 600 ft the virgin fault dipped steeply but a series of related minor faults produced poor rock conditions for about 500 m 1 650 ft of the tunnel in addition where dacitic andesitic and rhyolitic dikes contacted the granodiorite and quartz monzonite weak zones 0 2 0 6 m 0 5 2 ft wide affected the bore for approximately 180 m 600 ft along most of the tbm s path hydrothermal metamorphosis had weakened the rock further by veining fracturin

g and jointing unsupported wall stability ranged from 30 minutes to months and the critical zone with the shortest stand up time was anticipated to be 170 m 560 ft in length although none of the rock masses could be considered an excellent tbm environment comprehensive studies and the 1992 geotechnical report janzon 1995 concluded that the rock conditions would permit mechanical excavation and that stability even in the weakest sections would be adequate for tbm passage and ground support installation the report actually favored the tbm approach because the tbm machine s low level vibrations would destabilize hydrothermally altered rock far less than would shock waves from blasting the report concluded that tbm tunneling would be substantially faster than drill and blast excavation mine equipment selection tbm equipment design for the project was based on three parameters geology logistics and tunnel geometry it was decided to use a hard rock main beam tbm the tbm was ordered in 1993 before the tbm contractor was selected in order to meet the construction schedule mine management conducted detail studies and held discussions with outside consultants machine manufacturers and all potential contractors then placed the tbm order with the robbins company the machine shown assembled at the factory in figure 12 7 2 is relatively short to allow boring of tight curves it has back loading cutters roof drill fixtures a ring beam erector for installing w6 20 ring beams invert thrust system finger shield soundproof operator station on the backup including a closed circuit tv for monitoring selected areas along the tbm and backup specifications for the tbm are listed in table 12 7 3 the tbm for the san manuel mine was custom designed to be lowered down the mine s access shaft piece by piece and assembled underground the cutterhead and main beam were built in two pieces to meet the space and weight limitations of the access shaft and its 24 t 26 5 st hoist figure 12 7 16 components were limited in physical dimension to 2 4 12 m 6 75 13 39 ft more than 65 components were lowered down the shaft large components were suspended beneath the service cage other parts fit inside the cage a further requirement was that all transport and assembly operations had to take place without interrupting full time mining assembly of the machine was essentially complete in 114 shift hours which was proof of excellent planning and a dedicated work crew the tbm contract was awarded to frontier kemper constructors inc daniel haniel gmbh fk dh the contractor designed and manufactured the trailing gear and support equipment to fit the same logistical criteria and requirements as specified for the tbm the 143 m 470 ft long backup equipment consisted of 16 frame cars to house all electrical hydraulic ventilation cooling and muck handling equipment because of the many short radius curves the contractor selec

ted rail haulage using m hlh user 10 m3 13 yd3 muck cars and brookville 27 2 t 30 st locomotives a california switch was selected and moved permanent switches were installed two on the grizzly level and one on the 3570 haulage level mining setbacks and tbm modifications tbm progress in the initial 1 830 m 6 000 ft tunnel was not as expected a number of problems plagued the operation for example clay plugged the cutterhead a large cavity h w l 9 6 5 m 30 20 16 ft developed above the tunnel profile loose material fell through the finger shield steering problems developed because of failure to maintain proper alignment in the curves and some machine problems developed in addition stand up times in highly fractured areas were not as predicted because rock often turned to rubble immediately or within minutes in the crown or along the tunnel sidewalls to achieve the planned rate of advance the tbm needed to be modified quickly an evaluation team consisting of magma copper the tbm manufacturer the contractor and two outside consultants recommended the following modifications replace two of the gauge cutters with two additional muck buckets and close the peripheral side buckets off to reduce ingestion of rock behind the gauge cutters extend the roof support forward by a canopy to limit cutterhead exposure add fingers to the side supports to increase the surface area of the support improve starting and breakout torque by reducing cutterhead rpm from 12 2 to 9 3 replace the two 1 300 kva transformers with two 1 600 kva transformers some of these modifications are shown in figure 12 7 17 early on because of the highly variable ground conditions the use of rock bolting had been abandoned roof drill fixtures had been removed and the operation had moved to exclusive use of ring beams to support the rock doing so not only saved labor but also provided more room in the congested work area immediately behind the cutterhead support the modifications greatly improved tbm performance in all areas cutterhead starting problems and ingestion of muck from above the cutterhead were eliminated fallout of material from the sidewall was reduced to acceptable levels and bucket plugging occurred only in areas of high clay content a curve with a radius of 109 m 350 ft was accomplished by means of crab steering without plowing the sidewalls a beneficial side effect of lowering the cutterhead speed was more efficient transfer of muck from the cutterhead buckets to the muck chute and machine conveyor belt the results of the modifications measured by comparing the lengths of advance before and after modifications are shown in table 12 7 4 the magma copper company concluded the following van der pas and allum 1995 advance rates achieved after completion of the modifications have proven that the decision to utilize a tbm for the initial development of the lower kalamazoo was the correct one even tho

ugh the tbm was designed to the best interpretation of conditions likely to be encountered as so often happens in underground construction work the conditions encountered did not exactly match the assumptions that were the initial basis for the project the correct analysis of the problems by tbm operations personnel the tbm evaluation team the robbins company and the consultants resulted in the successful modifications careful planning scheduling and teamwork by operations personnel allowed the modifications to be completed on schedule and as planned many individuals and individual organizations spent a great deal of time and effort to determine what was required to make the project a success it was the ability of those groups of people to focus on solutions to the problems instead of who to blame that built the foundation to success the formation of a joint management team enabled both magma and fk dh to function side by side while solving the daily coordination problems as created by both organizations commitment to a successful project by the owner the contractor and the equipment manufacturer created a true working team that team collectively was able to generate the results needed to prove that tbm technology in a deep underground copper mine can be a success context within the underground mine the ore handling strategy has considerable impact on the value of any underground mining operation the definition of ore handling is any process that moves or transforms ore from one state in the overall value chain to the next more valuable state in this context every major process in the mine can be considered ore handling including those that do not physically move ore these include resource drilling mine planning mine development production drilling production blasting and backfilling as such although this chapter mainly considers the physical movement of ore it is only one component of the total ore movement it is therefore important to consider a more holistic and integrated view when developing the physical ore handling strategy because this strategy is most likely to be influenced by and complementary to all value adding processes within the mine whether physical or not poor ore handling decisions can lead to underperformance of the mine high production variability declining performance over time and unexpected costs in addition opportunities could be lost if little consideration has been given to future changes in mining strategy or expansion options value of the business could also be reduced if the orehandling system is not flexible enough to respond to external influences such as changes in metal prices technology and license to operate conditions a successful ore handling system will demonstrate planned variability within limits over the expected life of the operation practical and effective maintenance programs to maintain high equipment availability without creating long term throughp

ut bottlenecks good cost management and optionality to respond to changes in business strategy and external influences because an ore handling system can be the most inflexible component of the overall mining system a mine s ability to adapt to future business conditions is linked directly to its orehandling system s ability to respond to such changes planning for the future should therefore be a significant consideration in any ore handling system design process waste handling also needs to be considered as an integrated part of the ore handling strategy because business value impacts e g costs destination downstream processing associated with handling ore and waste will be different the two material streams cannot be considered in isolation for example if the financial cost of disposing waste separately exceeds the cost of including it in the ore stream then the waste should be included in the ore stream strategy with reference to figure 12 8 1 each step in a mining system value chain consists of an upstream buffer stockpile of ore an ore handling process that upgrades the ore to another value state and a downstream buffer the downstream buffer then becomes the upstream buffer to the next ore handling process a real system may be more complex in some parts than the simple series value chain shown in figure 12 8 1 but the linkage between buffers and ore handling processes remains unchanged bottlenecks based on the theory of constraints goldratt and cox 1992 any manufacturing process contains a single bottleneck or process that controls the output rate by definition the bottleneck has the lowest rate of production controlled systems have a clearly identified bottleneck that is stable meaning it does not migrate from one process to another over time because of their predictability of performance controlled systems are more amenable to cost efficiency and continuous improvement whereas uncontrolled systems have high throughput variability and significant unplanned interruptions the process of developing a controlled ore handling system is to identify where the bottleneck is or should be and establish capacities and rates for upstream and downstream buffers and ore handling processes respectively in order to stabilize the bottleneck at the designed location in the value chain when this is achieved the overall performance of the underground mine can be managed to achieve a desired outcome if throughput is fixed then cost efficiencies can be achieved by ensuring that only the minimum work required to maintain the buffers at the desired levels is undertaken if throughput is to be increased efforts should be focused on maximizing the rate of ore movement through the bottleneck because this is the rate limiting step in the overall value chain resources allocated elsewhere in the mine will not impact overall throughput as the rate through the bottleneck increases its location may migrate to an

other ore handling process at this stage the exercise is repeated at this new location with the establishment of new buffer capacities and so forth and an ongoing drive to increase the rate through the new bottleneck this is continued until a new throughput level is achieved for an operation to run effectively there must be a single well defined bottleneck attempting to match all rates of the process will only lead to poor control of the overall operation as the bottleneck will shift in response to short term variability in the performance of each individual process the choice of the most appropriate bottleneck location depends on many factors foremost is whether the mining operation is currently in an operating state or the design phase for a new operation there is an opportunity to optimally locate the bottleneck from the start of the project ultimately the constraint on mine throughput is the resource footprint and the rate at which this resource can be exploited however from a valuation perspective e g net present value sensitivity analyses for various operational configurations should be undertaken to establish the best location for the bottleneck as a guide the process in which the capital cost per ton of incremental ore is the greatest should be considered as optimal for locating the bottleneck this is because over time it is likely that the bottleneck will naturally migrate to this location through continuous improvement of lower capital cost processes so the sooner this is established the greater the opportunity to bring forward in time positive cash flows to the business this will not always be the case however because a good business case may be made for installing latent capacity initially in high capital items if expansion is planned in the short to medium term and the cost in present value terms of expanding these capital items is far greater at a later date essentially the appropriate choice of bottleneck needs to factor in both initial and future plans for the operation if the operation is currently in production an initial choice for the bottleneck is the most practical location to stabilize production in the short term this allows control to be established and an effective process of debottlenecking to commence without this initial control frequent shifts in the bottleneck will make any attempt at continuous improvement difficult to establish and progress at the same time a vision of the bottleneck s long term location should be established together with a plan for gradually migrating the bottleneck to this location once again sensitivity analysis of various configurations should be undertaken to establish this optimal position with a likely outcome being the process that has the highest capital cost per ton of incremental ore by definition because the bottleneck is the only process that is 100 utilized at any given time nonbottleneck processes cannot be 100 utilized unless buf

fer capacity is unlimited and high cost stock buildup was not being effectively managed therefore nonbottleneck processes must cycle through periods of high and low utilization according to the buffers capacities this is a difficult concept to manage in a mining operation where high utilization is demanded for all equipment and people one option is to manage buffer levels between high and low trigger values by deliberately under resourcing with permanent equipment and people and employing contractors for fixed periods of time when required in that way utilization of the mobilized resources is always high this concept is described in figure 12 8 2 bloss 2009 describes a practical example where the theory of constraints was applied to the underground mine at olympic dam in south australia to increase throughput by 18 buffers as described in figure 12 8 1 an effective ore handling strategy needs to include the application of buffers between all processes in the ore handling stream buffers exist in a number of forms for example as stockpiles storage bins orepasses broken ore stocks within stopes drilled and developed ore stocks ore reserve and mineral resource they exist between two processes in the value chain in order to provide surge capacity so that variability in performance of either process can exist with minimal impact on the other process buffers are particularly important in order to protect the bottleneck as by definition lost production through the bottleneck is lost to the system and cannot be recovered as a default starting point it is good practice to assume that a buffer is required between every pair of processes and that any decision not to provide a buffer is justified explicitly as part of the mining strategy in that way inadvertent omissions of critical buffers are minimized some key aspects that need to be addressed during designing any buffer include the following what is the size of buffer required what is the required life what options are available to provide a buffer of the required size and life span what are the capital and ongoing operating costs of the first three options how can it be designed to minimize operational downtime e g blockages how can it be effectively maintained in a serviceable state including replacement options without adversely impacting future mine throughput how effectively does the buffer integrate into upstream and downstream processes what optionality exists for expanding the capacity of the buffer enlarging duplicating to meet future strategic scenarios are there requirements for separating the ore into high low waste grade or according to other specifications maintenance contingency and optionality plans in order to plan for future uncertainties management strategies for each buffer and ore handling process should include maintenance contingency and optionality plans maintenance plans protect throughput during ongoi

ng operations and discrete and planned periods of major maintenance work contingency plans manage the risk of reduced performance as a result of unforeseen events optionality plans consider the potential for either a change in mining strategy or future capacity expansions in many cases these management plans are complementary for example provision of a redundant orepass can be an effective strategy to manage both maintenance and operational issues but can also be built into an optionality strategy in which the additional pass can be used for example to increase throughput in the mine or to segregate ores of different grades table 12 8 1 describes some examples of maintenance contingency and optionality plans that could be employed to various components of an ore handling system selection process materials handling is generally a big investment and can be the largest single investment in any new mine the selection process needs to balance the knowns capital cost current operating conditions against the unknowns future operating conditions this is not a trivial exercise as the future operating condition has many dimensions most of which are difficult to quantify with certainty these include the following safety is the ore handling system capable of adopting ongoing improvements in safety systems processes and technology e g automation revenue will changing metal prices affect business decisions that could impact the choice of ore handling cost will the cost of handling ore increase excessively throughput options does the ore handling system provide the flexibility to increase throughput if required without significant financial hurdles including business interruptions geographic footprint will the changing mining footprint over time adversely impact production cost and or throughput ground conditions what will be the impact of deteriorating ground conditions directly relating to ore handling on production cost and or throughput license to operate company strategy and regulations government regulations etc will mandated changes to operating practices e g health safety environmental impact the ore handling system such that production costs and or throughput are adversely affected availability and reliability will maintenance performance decline excessively and will the cost of sustaining performance be excessive can major maintenance works be carried out without adversely affecting mine performance maintenance support will the maintenance support be adequate this can be an issue if new and or specialized technology has been employed where maintenance support can be problematic in terms of either speed of response or quality of service a risk reward evaluation is recommended to determine whether the benefits of the technology are outweighed by the reduction in equipment availability as a result of substandard maintenance support replacement and or major maintenance

is there scope for replacing maintaining key infrastructure if required without significant business interruptions new technologies can the system take advantage of new technologies to add value e g reduce costs increase throughput system robustness is the system capable of effectively responding to unplanned variability in key input characteristics and maintain planned throughput some examples are particle size changes presence of undesirable material e g rock bolts cables presence of backfill material clay or other sticky waste material e g fine high moisture content potentially causing significant blockages in chutes and transfer points water content changes major unplanned failures of internal upstream or downstream processes and changes in spatial distribution of ore supply change in mining method will potential future changes in the mining method be adequately serviced by the orehandling system change in ore handling system can possible additions changes or enhancements to the ore handling system over time be adequately integrated into the existing system and implemented without adversely affecting system availability continuous improvement is the ore handling system capable of facilitating ongoing improvement in order to increase operational efficiency short term operational variability can the system adequately manage short term operational variability through effective use of upstream and downstream buffers this is particularly important if the ore handling system is the business bottleneck a common selection process described in table 12 8 2 is based on the development of a matrix in which the various ore handling options are assessed and ranked against key design and operational criteria each criterion is assigned a weighting based on the importance of that criterion to the selection for each ore handling option a performance score is then assigned to each criterion the weighted scores are then totalled and the highest score is used to determine the preferred option although this process is relatively simple and efficient and can achieve good stakeholder buy in it has a number of potential shortcomings the ore handling options will be based on experience and may not include a broad enough choice all relevant selection criteria particularly those that relate to future uncertainty may not be considered the weighting assigned to each selection criterion may be subjective the score assigned to some selection criterion for each ore handling option may also be subjective the summing of weighted scores for each ore handling option to determine an overall score may not reflect the true relative impact of each selection criterion there is a risk that the selection outcome will be biased toward those options that prioritize currently known aspects of the design e g capital and operating costs over future unknown aspects if for example two options

have similar appeal in the current time frame but the less attractive has greater optionality in the future this option should be the preferred option for this reason it is recommended that this process be used as only one component of an overall selection study comparison between ore handling system options the following discussion compares the three most common ore handling systems shaft conveyor and truck these systems usually contain other supplementary ore handling components including loaders orepasses and crushers in this comparison it is assumed that a significant vertical lift is required to bring the ore to the surface therefore limiting the application of rail as a primary ore handling system in assessing the relativity of these systems notably other issues not related to materials handling may often affect the choice of system for example if a decline system is required for access and provision of routine services to the underground the incremental cost to also utilize this access for trucking for example may be much lower than the total cost hence the decline option may be cost effective relative to other ore handling options cost shaft systems usually attract the highest capital cost however as depth increases the alternatives become progressively more expensive and the capital cost difference relative to shaft systems decreases once installed however shafts attract a relatively low operating cost conveyor systems have recently been demonstrated to be cost effective relative to shaft systems pratt and ellen 2005 demonstrated negligible capital and operating cost differential between the two systems during a selection study at the telfer operation in northern western australia the shaft option was preferred due to other factors such as physical impact on the orebody footprint required and flexibility to service future economic ore zones trucks are usually lower in capital cost but have higher operating costs recent improvements in trucking technology have increased the depth to which trucking costs remain economically attractive pratt and ellen 2005 suggest that trucking is not economically viable over a range of mining rates from 1 5 mt a million metric tons per annum to 6 mt a below a depth of 650 m figure 12 8 3 describes the capability of established underground mine haulage systems at the telfer and ridgeway gold mines in australia the choice of system is largely based on the cost efficiency as a function of depth and throughput safety all options have safety risks that need to be managed for shafts risks are associated with working at heights during inspection and maintenance work and interactions with personnel during operations for conveyors fires and personnel interaction with the moving belts are of particular concern for trucks fires and vehicle to vehicle and vehicle to pedestrian interactions are the highest risk issues all of which can be managed effectively thro

ugh the application of adequate design and appropriate procedures some examples are presented in more detail later in the chapter timing truck haulage via a decline offers a particular advantage in the ability to access ore progressively as the decline is sunk in this way positive cash flow can be generated earlier in the mine life compared to alternative systems where installation must be achieved to full depth prior to ore production in cases in which a surface access decline is being established in a mine where the ore will ultimately be hoisted using a shaft it may be advantageous to utilize truck haulage up the decline early in the mine life prior to commissioning of the shaft examples of this application include the bhp billiton cannington mine in northwest queensland australia and newmont mining s leeville mine in northern nevada united states optionality because mines typically evolve over time the ability of a mine to vary its operating strategy in order to optimize production in response to changes can often be a function of the optionality embedded within the ore handling system although shaft and conveyor systems can be inflexible because of the limited number of fixed feed points trucking systems are flexible because generally trucks can travel to most locations in the underground mine where fixed infrastructure ore handling systems are employed the tendency is to introduce hybrid systems e g trucks feeding shaft as the mine evolves in order to respond to changes in situations where the underground mine capacity needs to expand to increase business value the potential is often related to the configuration and current utilization of the orehandling system if the existing system is based on trucks the expansion can typically be achieved incrementally by adding trucks as required until the capacity e g ventilation traffic congestion of the decline system is reached following this additional access would be required at significant cost increasing throughput in fixed systems such as shaft hoisting and conveyors is relatively cheap up to a point where the systems utilization is at the industry benchmark following this higher utilization will probably require duplication of the existing system at significant cost this is likely to be justified financially only if there is a step increase in throughput or the geologic footprint has evolved substantially enough that a duplicate system is justified to maintain or create a step change in expected efficiencies and associated operating costs loaders loaders move ore and waste from the development face stope pass or stockpile to the next step in the ore or wastehandling system the types of loaders include rubber tired rail mounted track mounted and shuttle loaders loaders are most commonly a single function unit but in certain applications can be part of a multifunction unit the most common form of the latter is a continuous miner which ca

n cut coal from a coal face load the coal into a transport vehicle and also install roof support when equipped to do so rubber tired loaders in metalliferous mines are referred to by many different names around the world according to local preference these include loader load haul dump lhd scooptram scoop bogger digger mucker and underground loader an underground loader is shown in figure 12 8 4 underground loaders are typically the first component of the ore handling system loaders extract ore from the stope and either dump tip directly into an orepass or load into a truck that hauls the ore to an orepass or surface stockpile the size of the loader fleet will be determined according to the following factors mining method geometry of the stope ore working face size of underground excavation geometry of the ore handling system production rate based on planned strategic objectives productive operating time or availability and utilization per shift loader productivity geometry and condition of the tram haul route capital and operating costs the mining method will determine the relative proportions of stope ore development ore and waste the geometry of the stope or ore working face may limit the size of loader that can be used ground conditions will determine underground excavation dimensions which in turn will impact fleet size and equipment selection the geometry of the ore handling system will determine the proportion of material required to be rehandled sweigard 1992 provides a good overview of the equipment selection and the fleet size estimation process this information allows mapping of loader tram segments to determine quantities of ore movement required to achieve the nominated production rate typical units of measurement are metric tons per operating hour t h and ton kilometer per operating hour t km h which is based on the one way length of the tram preliminary estimates of total operating hours per year or total ton kilometers per year for various sizes of loaders allow for an initial estimate of loader fleet size further details on loader productivity are available in sweigard 1992 and the various underground equipment manufacturers specifications e g available on the caterpillar sandvik and atlas copco web sites gradeability speed rimpull charts similar to figure 12 8 5 and equipment dimensions caterpillar 2004 provide useful specifications for determining loader performance in differing conditions the calculation for estimating truck fleet performance is performed using a similar methodology loaders tend to be as large as development allows and tram distances are minimized to maximize productivity per loader loader trams greater than 250 m become less effective to allow production to continue when the stope brow becomes open the loader requires line of sight remote or tele remote capacity the loader bucket may also use different tooth configurations to assis

t digging in the stope drawpoint safety considerations safety issues that need to be considered during the design phase and implemented during the operations phase include the following light vehicle heavy vehicle and pedestrian interaction some strategies that can be employed to manage this issue include separating heavy vehicles from light vehicles and pedestrians detection and collision avoidance systems traffic rules and visual and audible signalling systems remote loading at the stope drawpoint operator interaction the most effective method is to isolate the operator from the machine by using tele remoting systems loading at a stope brow or orepass and initiating a mud rush or air blast to manage this situation a good understanding of the stope s material characteristics is required together with strict controls and procedures regarding the methods of extraction from the drawpoint to ensure predictable behavior of material flow from the drawpoint fire on the loader minimizing risks and consequences of loader fires requires quality equipment design effective preventive maintenance programs well trained operators reliable automated onboard fire prevention and firefighting systems and sound procedures for managing fires equipment failure during operation minimizing risks and consequences of equipment failure requires quality equipment design an effective preventive maintenance system well trained operators and controls that minimize personnel exposure to loader operations open holes including tipples effective hard and soft controls are required to minimize risks associated with operating around open holes including solid barriers good lighting and signage and procedures operator ergonomics the well being of personnel during loader operation and maintenance is paramount to sustainable operations high quality fit for purpose design both within the operator s cabin and wherever personnel work on the loader is critical to achieving this objective design and construction issues underground access development needs to be purposely designed to accommodate selected loaders and trucks as a general rule all rubber tired equipment should operate in development with gradients of at least 1 50 to minimize water ponding and to provide good road conditions key development components are drawpoints tramming drifts drives tipple accesses stockpile bays and truck loading bays these components are described in detail in the following sections drawpoints drawpoints provide access to the stope for loaders to extract ore from the stope the design of a drawpoint needs to consider horizontal and vertical clearances relative to the access walls and back and gradient of the access ideally the drawpoint should be angled relative to the tramming drive e g 60 to facilitate ease of entering and exiting it also needs to be of sufficient length to accommodate the loader while digging the mu

ckpile without being too long to trigger poor ventilation conditions at mines where remote loading from the stope is required a safe operating position needs to be designed into the drawpoint area to protect the loader operator figure 12 8 6 shows a tele remote setup in a canadian mining operation tramming drifts drives similar to drawpoints the main design considerations for tramming accesses are clearance gradient and orientation geometry tipple accesses once again clearance gradient and orientation of tipple accesses are the main design considerations back heights above the orepass and in the approach to the orepass are critical to facilitate the dumping tipping of the loader bucket and truck tray if back height becomes problematic trucks equipped with hydraulic ejector beds and plates should be considered a steep upward gradient into the orepass reduces the potential for mine water to accumulate and flow into the orepass orientation of the tipple access to the main tramming drift drive can be perpendicular or angled a perpendicular access allows loaders to approach from both directions without the requirement for a turnaround bay stockpile bays stockpile bays provide surge capacity and can improve overall ore handling system utilization they allow loaders to keep producing while waiting for trucks or full orepasses to clear stockpile bays need enough capacity to minimize delays to other parts of the development or production cycle truck loading bays truck loading bays need to consider back height for loading and localized gradient and geometry constraints detailed access development design criteria can be obtained from the different manufacturers equipment handbooks examples of the information available in the equipment handbooks are illustrated in figure 12 8 5 generally maximum efficiency and minimum cost are achieved if the loader is sized to load the truck on the same level rather than from an elevated loading ramp operational aspects stope mucking the productivity of a loader is dependent on a number of factors that facilitate the removal of ore from the drawpoint ore size distribution should be relatively homogeneous with minimal oversize or high percentage of fines oversized rocks slow the production rate because they must be resized either in the drawpoint or in nearby bomb bays processing oversized material in the drawpoint also introduces safety issues which need to be managed a high percentage of fine material e g from the failure of a backfill mass can create difficult operating conditions throughout the ore handling system particularly if water is present both oversize and fines issues can be better controlled through drill and blast design geotechnical design and the placement of quality engineered backfill good physical operating conditions include good ventilation for visibility and water sprays to help suppress dust drawpoint brows need to be supported to minimize wear over the

ir anticipated lives brow wear can lead to a reduction in effective drawpoint length making loading difficult and increases the tonnage of ore that needs to be mucked by remote control the ventilation system needs to be monitored because changes in airflow can occur when the stope brow opens additional consideration is required in uranium mines where air transit time becomes a factor in limiting radon residence time a ventilation quantity of 0 06 m3 s kw of engine power is a good standard to meet acceptable environmental conditions within typical mining operations tramming loader operators generally prefer to tram with the loader bucket at the rear when loaded and the front when empty this reduces the occurrence of loaders driving over rocks that fall from the bucket while tramming the following key aspects need to be considered and implemented in a practical manner to achieve the best cycle time results road maintenance well maintained roadways are critical for achieving maximum loader speeds and reducing equipment maintenance issues associated with continuous vibration of critical loader components drainage the establishment of well drained roadways will also increase achievable tramming speeds reduce road and vehicle maintenance and reduce collision with submerged objects that are not visible ventilation a well ventilated roadway will provide good visibility of the tramming route allowing higher loader speeds and reducing the potential for vehicle collision lighting the installation of lighting on the tramming route can allow higher loader speeds however lights should only be considered on a tramming route that is in use for a significant time period tram geometry this includes the drift drive size or available clearance the gradient uphill or downhill loaded and length of tram and the number of corners or bends in the tram loader costs budget capital cost estimates can be provided from equipment manufacturers the maintenance change out program will determine ongoing sustaining capital requirements depending on the frequency of rebuilds and major component changeouts loader operating costs are dependent on labor fuel and lubricants tires bucket wear maintenance consumables and equipment damage a remote loading operation typically has a higher operating cost resulting from reduced productivity the ability to estimate the cost of maintenance depends on the relative proportion of predictive and preventive maintenance to breakdown maintenance equipment suppliers can provide budget estimates for the majority of consumable operating costs equipment damage is difficult to estimate because it is related to a number of factors including road conditions visibility operator experience equipment clearance dimensions mining method tram geometry and stope drawpoint conditions a loader performing remote operation into an open stope will have a higher exposure to rockfall damage from unsupported ar

eas of the stope loader productivity and utilization will depend on the activity for example development loaders 4 000 h yr sublevel open stoping production loaders 4 500 h yr and block cave and sublevel cave production loaders 5 000 h yr technology production management systems a number of commercially available production management systems are used in underground mining operations including micromine s pitram and modular mining s dispatch numerous operations have also developed their own in house systems which capture data and allow managers to monitor performance to determine whether operational targets are being achieved the data are collected using radio communication or tag readers located throughout the mine automation automation and remote control technology have been available for underground loaders for more than 20 years numerous automation systems are being developed by equipment manufacturers mining companies research organizations technology companies and various partnerships between these groups the level of advancement with these automation systems and their ability to operate practically and cost effectively in the challenging underground environment are still under debate substantial progress has been made and cost effective practical automation solutions will be an important part of the future for improved loader productivity and operator safety maintenance all maintenance programs can be broken down into preventive and breakdown components both are required to minimize the risk of fatalities injuries and incidents arising from the use of mobile equipment underground and to maximize equipment productivity an effective predictive and preventive maintenance program that is prioritized over breakdown maintenance will reduce the potential for personnel injury and increase equipment utilization as a result of the following aspects replacement of components before they fail and while the equipment is not operating prevents potentially serious consequences including injury to personnel and damage to associated components more predictable scheduling of maintenance activities allows for better matching between supply of and demand for maintenance resources breakdown maintenance tends to create boom and bust work cycles that cannot be effectively managed using a fixed level of resources more predictable availability of equipment allows for more efficient utilization as a result of better matching between availability of equipment and supply of operators best practice equipment availabilities are generally achieved by reducing the frequency of breakdown maintenance and increasing mean time between equipment failures to maintain high availability throughout the useful life of equipment a well planned component change out program is essential the operating life and timing of major equipment rebuilds will have an impact on capital and operating cost profiles for the mine costs of preven

tive maintenance and major component change outs need to be consolidated with other operating costs to determine an overall cost per ton kilometer this measure can be used to determine the effectiveness of the maintenance program trucks trucks are used for long or inclined hauls to orepasses or directly to the surface commonly available trucking options include diesel or electric articulated dump trucks or adts figure 12 8 7 diesel or electric rigid dump trucks diesel road trains figure 12 8 8 and diesel electric and battery shuttle cars figure 12 8 9 trucks can be side loaded near the ore source e g stope drawpoint or side loaded or chute loaded at the bottom of an orepass safety considerations safety issues that need to be considered during the design phase and implemented during the operations phase include light vehicle heavy vehicle and pedestrian interaction chute loading the key to minimizing risks is to ensure that the design of the chute fully accounts for the material flow characteristics and the loads that the chute will be subjected to so there are no unexpected failures in the chute equipment failure fire on the truck tipping into open holes and operator ergonomics selection criteria selection of the appropriate size and type of truck fleet depends on the following key factors ore reserve tonnage production rate haulage distance depth below surface lateral extent of the mine mining method used integration with upstream and downstream materialshandling processes truck haulage is generally the preferred option where the ore reserve is not large enough to justify the capital for a shaft or conveyor hoisting system for hoisting shaft or inclined conveyor systems the lateral extent of the mine distribution of ore and throughput requirements determine the most effective solution for upstream delivery to a centralized hoist from a choice of trucking rail haulage or conveyor adts have traditionally been used because of their flexibility and ability to maneuver throughout the ore body in variable conditions road trains electric articulated and rigid body trucks have been utilized with varying degrees of success depending on road conditions grade vehicle clearance and road maintenance and environmental conditions temperature and visibility underground coal and potash mines have a unique application for trucks given the typically narrow seams that are mined specialized shuttle cars are used to transport the coal or potash these are low profile ac electric trucks designed to interact effectively with high capacity continuous miners or roadheaders they remove material that has been cut by the mining machine and transfer it directly to a main conveyor which creates an efficient ore handling system shuttle cars can operate in seam heights down to 1 2 m with a capacity of 8 t e g figure 12 8 10 design and construction issues access design considerations

discussed in the loaders section can also be applied to trucks equipment clearance gradient and ground conditions additional considerations include side loading bays orepass dump point tipple configuration chute loading systems and decline haulage access side loading bays side loading bays facilitate the efficient loading of a truck side loading of trucks can be performed on level ground figure 12 8 7 or with the loader elevated above the truck utilizing a perpendicularly configured side loading bay the latter configuration can be used when the loading point is fixed over an extended time frame and truck spotting times have a significant impact on truck productivity to facilitate efficient truck loading and reduced spillage the loader can be elevated 1 5 m above the truck orepass dump points tipples orepass tipple configuration for trucks requires additional height and length of back stripping compared with a loader configuration it is also necessary to be able to reverse into the tipple additional development may be required to achieve efficient truck tipping road trains dump tip to the side and therefore require a significantly different layout for an underground tipple to accommodate side dumping tipping a truck loop is generally required consideration for a turning radius and a straight section at the dumping point can add a significant amount of underground development as compared with other truck dumping layouts chute loading systems truck loops and truck reversing bays are two commonly used configurations for loading trucks from chutes when sufficient tonnage is available e g 3 to 4 million t a truck loop configuration can achieve higher productivity for lower tonnages a chute that requires trucks to reverse provides an option with less capital to further reduce capital costs in low tonnage short term situations ore can be side loaded into trucks at the bottom of orepasses to facilitate chute loading of multiple trailers road trains require a truck loop configuration decline haulage access decline or ramp haulage provides access for trucks to haul ore from underground to a surface stockpile design considerations include clearance gradient geometry and frequency and geometry of passing bays declines can be designed as a series of long straight sections or as a circular spiral passing bays can be designed parallel with or perpendicular to the decline depending on ground conditions in most australian decline trucking operations stockpile bays are developed every 150 m during construction and then utilized for passing bays block light systems are also commonly used to control traffic flow adts large rigid chassis trucks and road trains with power trailer are effectively used to haul ore long distances to surface stockpiles figure 12 8 8 shows a road train exiting a decline portal and figure 12 8 11 shows design information for drift drive size and road train turning req

uirements robertson et al 2005 provides a good overview of the future direction of trucking because decline haulage trucks usually have to share the decline with other vehicles numerous traffic control systems are available to reduce the hazard of vehicle interaction trucking costs many factors that influence trucking costs are similar to those for loaders see the loaders section the differences are primarily due to the specific functions of the equipment loaders are generally more influenced by loading conditions whereas trucks are more influenced by haulage conditions road conditions and haul distances the two technologies described under technology in the loaders section have similar application to underground trucks and should be considered if justified cost effectively underground ore movement 1281 underground coal and potash mines have a unique application for trucks given the typically narrow seams that are mined specialized shuttle cars are used to transport the coal or potash these are low profile ac electric trucks designed to interact effectively with high capacity continuous miners or roadheaders they remove material that has been cut by the mining machine and transfer it directly to a main conveyor which creates an efficient ore handling system shuttle cars can operate in seam heights down to 1 2 m with a capacity of 8 t e g figure 12 8 10 design and construction issues access design considerations discussed in the loaders section can also be applied to trucks equipment clearance gradient and ground conditions additional considerations include side loading bays orepass dump point tipple configuration chute loading systems and decline haulage access side loading bays side loading bays facilitate the efficient loading of a truck side loading of trucks can be performed on level ground figure 12 8 7 or with the loader elevated above the truck utilizing a perpendicularly configured side loading bay the latter configuration can be used when the loading point is fixed over an extended time frame and truck spotting times have a significant impact on truck productivity to facilitate efficient truck loading and reduced spillage the loader can be elevated 1 5 m above the truck orepass dump points tipples orepass tipple configuration for trucks requires additional height and length of back stripping compared with a loader configuration it is also necessary to be able to reverse into the tipple additional development may be required to achieve efficient truck tipping road trains dump tip to the side and therefore require a significantly different layout for an underground tipple to accommodate side dumping tipping a truck loop is generally required consideration for a turning radius and a straight section at the dumping point can add a significant amount of underground development as compared with other truck dumping layouts chute loading systems truck loops and truck reve

rsing bays are two commonly used configurations for loading trucks from chutes when sufficient tonnage is available e g 3 to 4 million t a truck loop configuration can achieve higher productivity for lower tonnages a chute that requires trucks to reverse provides an option with less capital to further reduce capital costs in low tonnage short term situations ore can be side loaded into trucks at the bottom of orepasses to facilitate chute loading of multiple trailers road trains require a truck loop configuration decline haulage access decline or ramp haulage provides access for trucks to haul ore from underground to a surface stockpile design considerations include clearance gradient geometry and frequency and geometry of passing bays declines can be designed as a series of long straight sections or as a circular spiral passing bays can be designed parallel with or perpendicular to the decline depending on ground conditions in most australian decline trucking operations stockpile bays are developed every 150 m during construction and then utilized for passing bays block light systems are also commonly used to control traffic flow adts large rigid chassis trucks and road trains with power trailer are effectively used to haul ore long distances to surface stockpiles figure 12 8 8 shows a road train exiting a decline portal and figure 12 8 11 shows design information for drift drive size and road train turning requirements robertson et al 2005 provides a good overview of the future direction of trucking because decline haulage trucks usually have to share the decline with other vehicles numerous traffic control systems are available to reduce the hazard of vehicle interaction trucking costs many factors that influence trucking costs are similar to those for loaders see the loaders section the differences are primarily due to the specific functions of the equipment loaders are generally more influenced by loading conditions whereas trucks are more influenced by haulage conditions road conditions and haul distances the two technologies described under technology in the loaders section have similar application to underground trucks and should be considered if justified cost effectively orepasses orepasses provide a low cost method to move ore and waste downward between operational horizons in a mine initial capital is the dominant cost associated with an orepass system with lower ongoing sustaining capital and operating costs orepasses can be constructed as one long section or as multiple shorter sections depending on the geometry and or timing of access to the top and bottom of the orepass and the relative operational and safety aspects during construction flow of material in the orepass is critical to its success a number of factors need to be considered to maintain continuous flow conditions early origins of material flow were analyzed by janssen 1895 who determined that the weight of

the material in the bin or orepass was actually transferred to the walls rather than the bottom of the orepass this produces the potential for arching where broken material stabilizes in the pass and prevents material flow strategies for avoiding this are discussed in later sections for example when considering orepass width safety considerations the safety issues that need to be considered during the design phase and implemented during construction and operation include the following risks during construction the main risks are people working below vertical openings from which material can fall and people falling into open holes thorough risk assessments need to be undertaken to ensure that key risks are identified and managed clearing of blockages clearance in orepasses involves drilling hang up holes into the immediate vicinity of the blockage and feeding explosives into the pass to free the blockage fall prevention stop bumper blocks orepass covers signs barricades and flashing lights are used to prevent people from falling into or driving equipment into orepass open holes mud rushes these hazards occur when material containing a high proportion of fines is dumped tipped into an orepass and the orepass becomes blocked when mine water or natural groundwater mixes with the fines in the orepass there is the potential for a mud rush the severity of a mud rush depends on the amount of fines in the orepass and the proximity of personnel and equipment to the bottom discharge point of the orepass during a mud rush management of mud rushes includes understanding material flow properties and developing procedures for managing undesirable combinations of materials that report to the drawpoint typically pedestrians are isolated from any orepass loading activity and in cases where mud rushes are likely remote loader operations are common design and construction issues orepass design is largely dependent on the geometry of the ore body and the strength of the rock mass in the mine hadjigeorgiou et al 2005 provide a good review of orepass practice in canadian mines inclination angle orepass inclination angles are generally greater than 60 and should have at least a small amount of inclination from vertical to allow the dumped material s energy to be dissipated on the orepass footwall and therefore prevent free fall of the material to the orepass bottom orepass fingers that connect to chutes can be as low as 50 in order to reduce the amount of energy transfer from the ore in the orepass onto the chute length the length of an orepass is dependent on the vertical distance between stope extraction level s and the common orehandling level orepasses are sometimes extended upward to allow more cost effective dumping tipping of development material from upper levels of the stope the orepass system can be constructed using one or two longer sections or a larger number of shorter sections depending on access a

vailable for construction the length of each orepass section generally determines the excavation method typically alimak raising and raise borers can construct longer orepasses whereas drilled and blasted drop raises and conventional raises are used for shorter length holes orientation the orientation of an orepass can be a critical design consideration in rock masses with dominant structures the most favorable orientation is as close to perpendicular to the dominant structure as practical in situations where the orepass needs to be oriented parallel with the dominant structure additional support liners or a redundant orepass should be considered shape the shape of an orepass is dependent on the excavation method raise boring will ream a circular shape while alimak drop raising and conventional raising typically excavate a square or rectangular shape if high stress is believed to be a stability issue for the orepass circular shapes can be excavated with drill and blast methods width the width of an orepass is dependent on size distribution of the broken material characteristics of the wall rock and capacity requirements of the orepass it generally needs to be no greater than five times the dimension of the largest particle that enters the orepass as described in table 12 8 3 recent work by hustrulid and changshou 2004 indicates that the ratios in table 12 8 3 are conservative their work recommends considering size distribution and orepass wall conditions in the design which will usually lower d d ratios being practically achieved rock mass strength of the orepass wall relative to the predicted induced stress levels and orientation also needs to be considered when sizing the orepass quality of the rock mass in orepass walls will partially determine what orepass size is achievable empirical design methods can be used to determine stable spans and if high stress fields exist in the mine numerical modeling should be considered to understand possible future states of stress magnitude and orientation in the orepass walls surge capacity requirements of the ore handling system need to be considered when designing each individual orepass when the ore handling system downstream of the orepasses is taken out for maintenance the loader and truck operation will stop when the orepasses are filled effective surge capacity in the ore handling system allows the components of the system to be less dependent on each other surge capacity is generated by expanding the dimensions of the orepass and or developing strategically located stockpile bays enough surge capacity needs to be designed into the orepasses to increase operational flexibility of the total ore handling system how much surge capacity can be installed has practical and cost limitations and the amount of surge required will depend on the benefit realized by smoothing out interruptions in ore flow due to downstream process variability surge capacity from severa

l hours to a week should benefit any mining operation if this surge supports the operational bottleneck where downstream process variability is significant consideration should be given to installing the maximum possible surge capacity material may become captured in certain areas of the orepass and this reduces the effective capacity of the orepass sections of the orepass that have changes in direction or angle are more likely to create areas for captured ore and are more susceptible to blockages construction orepasses can be constructed using a raise boring machine figure 12 8 12 drop raising long hole winzing alimak raising figure 12 8 13 or conventional handheld raising the most common methods for longer passes are a raise bore machine or alimak raising the preferred construction method depends on the specific requirements of the orepass the geometrical layout of the ore handling system and local preference and experience in the past alimak raises were more popular in north america but are becoming less common because the alimak platform operators are exposed to vertical openings alimak raises have the advantage of providing access to install ground support during construction which can provide long term orepass stability the alimak raise climber shown in figure 12 8 13 runs on a special guide rail using the alimak rack and pinion system the guide rail contains three pipes providing compressed air and water for ventilation scaling and drilling drop raises are commonly used for short orepasses 60 m or less as blasthole deviation becomes greater at 60 m conventional handheld raises are generally limited to short finger raises that connect chutes to the orepass support systems ground support requirements for orepasses depend on rock mass and induced stress conditions orientation of the orepass relative to geological structures size and inclination of the orepass planned mode of operation choke fed or open and planned life span of the orepass the type and method of ground support depends on the excavation method for example if an alimak raise is used it will need to be supported with rock bolts and mesh to protect operators during the construction phase ground support systems include metal cans or rings concrete liners specialized wear resistant sprayed on shotcrete mesh and rock reinforcement systems such as rock bolts and cable bolts in some cases the cost of ground support can exceed excavation and therefore the decision can be made to leave the orepass unsupported this option is justified if life expectancy of the orepass in an unsupported state is sufficient a replacement plan and monitoring program needs to be well defined and communicated to management rehabilitation of an orepass sometimes considered in order to extend its life can be a time consuming and costly exercise which can be avoided by installing ground support at the construction stage or by planning to construct replacement

orepasses hadjigeorgiou et al 2004 provides additional details of the different orepass support systems ventilation good ventilation at orepass tipples and the extraction horizon is essential to maintaining a productive ore handling system exhaust air from the orepass needs to be removed quickly from operating areas good ventilation can be provided by designing and constructing a parallel return air system which provides effective exhaust capacity at each tipple and the extraction horizon spray bars can also be used to suppress dust a number of ventilation design systems for orepasses are reviewed in calizaya and mutama 2004 operational aspects key considerations for safe and efficient operation of an orepass are controlling material size entering the orepass controlling material flow continuously monitoring the orepass for wear and stability and maintaining good ventilation over time wear of the sidewalls can make an orepass inoperable with good condition monitoring orepass refurbishment or replacement can be planned in a proactive and controlled manner if wear in an orepass goes undetected and makes it unserviceable the ore handling system can be disabled for an unacceptable period of time orepass monitoring is an effective tool that can be used to understand the condition and potential life of an orepass a number of monitoring techniques are available including quantitative physical surveying of the orepass using probe hole drilling to determine the orepass profile costly and timeconsuming a cavity monitoring system e g optech s cavity autoscanning laser system e g measurement devices ltd s or other laser scanning devices other qualitative methods include orepass camera surveying to obtain a video of the orepass production data to determine a tonnage reconciliation of material movements and other data captured by operations i e number of hang up blasts per time period and type of material blasted down during hang up blasts orepass wear is largely caused by structural failure stress failure material impact and abrasion and blast damage hadjigeorgiou et al 2005 the orientation of an orepass relative to dominant structures can have a significant impact on its long term stability the control of material size in an orepass is facilitated using grizzlies scalpers mantles and orepass rings at the entry point material flow in the orepass can be controlled by chutes with chains or gates or a plate feeder at the orepass exit point when the orepass bottom has no control a loader is used to control material flow when oversize material producing interlocking arches and or fines producing cohesive arches are introduced to the orepass the probability of hang ups increases and can cause delays this either reduces throughput or increases the cost to maintain throughput by implementing an alternate ore handling option oversize can be introduced to the orepass by loaders dumping tippin

g elongated shaped material through the size controls at the entry point of the orepass or from internal falloff within the orepass reestablishment of material flow in the orepass usually requires some form of blasting to provide vibration or sometimes water flow to initiate material flow a comprehensive review of methods for releasing blockages is discussed in hadjigeorgiou et al 2004 depending on the severity of the hang up it can be a long process to reestablish material flow and redundant or alternate orepasses should be considered at the design stage choke feeding of material in the orepass can limit internal fall off by providing confinement to reduce the effect of high velocity material impacting the orepass walls in choke fed orepasses the material should be continually drawn to reduce the risk of hang ups fine material can come from backfill falloff within the stope raise borer cuttings or highly fragmented development material the cohesive nature of fine material can cause blockages especially when the material is not periodically moved in the orepass the issues with fine material can be magnified if water is simultaneously introduced into the orepass which increases the potential for a mud rush at the orepass exit point cost considerations cost estimations need to consider the expected life of an orepass and future cost of replacement budget capital cost estimates can be obtained from mining contractors or in house project teams that specialize in raise boring or alimak raising initial capital cost of orepass controls chutes feeder and chain controls and material sizing systems grizzlies orepass rings rock breakers etc can also be estimated by specialized mining contractors ongoing sustaining capital costs for replacement of chutes plate feeders grizzlies orepass rings liners and rock support systems need to be factored into overall cost estimates orepass operating costs include mechanical and electrical maintenance on control systems and rock breakers and drilling and blasting costs associated with clearing orepass blockages a good orepass design potentially at a higher capital cost and followed up with disciplined operating practice will minimize future operating costs and increase productivity of the orepass system advantages and disadvantages advantages of utilizing orepasses include a high capacity system for vertical transport of material low operating cost and buffer capacity disadvantages of utilizing orepasses include high capital cost good technical design required subject to ongoing operational challenges for example hang ups performance subject to ongoing ground conditions and difficulty to maintain if ground conditions deteriorate crushers material size reduction in many operations is often required for productive and reliable materials handling by crushing ore underground an operation can reduce wear on equipment reduce clearances required in materia

ls handling processes increase hoist skip or other conveyance capacity reduce material void ratio reduce oversize handling issues such as hang ups or blockages and increase secondary crushing and processing productivity crushers are typically required ahead of conveyors and shafts where the material is transported to the surface by trucks underground crushing is generally less justified because a greater size range is capable of being transported in this manner although the material needs to be crushed prior to being processed in most cases the crushing process is more cost efficient when undertaken on the surface in operations where underground crushing occurs uncrushed material is usually delivered to the primary crusher by dump trucks wheeled loaders trains or orepasses a feeder device such as a plate feeder or apron feeder can be used to control the rate at which this material enters the crusher the feeder often contains a preliminary grizzly or screening device which ensures an appropriate material feed size is received by the crusher safety considerations the modern crushing plant includes the following safety features safety guards around all moving equipment boyd 2002 emergency stop facilities on components where personnel access is required boyd 2002 dust abatement suppression or collection equipment dust emissions must comply with the latest regulations for the jurisdiction designers must make provisions for the installation of equipment to control these emissions spillage prevention and collection facilities spillages from feeders chutes and conveyors should also be minimized noise abatement devices and controls because crushers screens and dust collection fans all contribute to high noise levels strategies need to minimize such exposure to personnel e g engineering solutions restricted access or personal protective equipment fire protection systems fires are major hazards in any underground mine and the potential for fires on conveyors is high protection through the use of automatic sprinklers or deluge systems is important to managing this risk design and construction issues crushing plant design can be split into three main components process design equipment selection and layout the first two are often dictated by production requirements and design parameters the layout can reflect input preferences and operational experience of a number of parties in order to provide a balanced workable safe and economic plant design because many underground crushing facilities are located within purposely built rock excavations it is vital that access and maintenance of all components is sufficiently allocated during the design phase this should include suitable lifting equipment for major component maintenance or replacement as large mobile crane access in many underground locations is not possible the fundamental goal for a crushing plant design is to provide an installa

tion that meets the required production rates and material sizing can be constructed within capital constraints operates at competitive cost and complies with safety and environmental regulations the following critical design parameters should be considered ore characteristics geographical location expected operational life expansion potential safety and environment operability and maintainability figure 12 8 14 describes the ore handling system at the olympic dam mine including the primary cone crusher underground crusher location in underground operations the excavations required for the installation of crushers and associated infrastructure can be substantial and are often the largest unfilled excavations within the mine when considering an underground crusher layout and location consideration should be given to local and regional ground conditions proximity to planned and potential future production areas current and future stress conditions within the excavation and proximity to planned and potential future mine infrastructure good geotechnical information is essential in the selection of a crushing plant location and the extent of ground support required life of mine expansion plans planning for expansion should be considered in the design stage for underground crushing facilities expansion plans for most crushing plants can be incorporated in the early planning stages at much lower cost than waiting until the mine is up and running particularly if the cost includes an interruption to normal production activities crusher selection crusher selection is dependent on many operation specific variables including throughput rates planned utilization ore characteristics material feed size and required discharge product size many of the factors influencing underground crusher selection are common to surface crusher selection and are not covered in detail in this section as a guideline the following can be applied for a hardrock mine application below 600 t h feed rate into the crusher a jaw crusher is the preferred option as the primary crusher for feed rates more than 1 000 t h a gyratory crusher is the preferred option between these capacities the choice will be dependent on other factors ottergren 2003 an example of a primary cone crusher is presented in figure 12 8 15 equipment overview the major equipment in a primary crushing circuit usually includes a crusher feeder and conveyor secondary and tertiary crushing circuits have the same basic equipment items along with screens and surge storage bins grizzlies a grizzly is a sizing device used to ensure a maximum passing size of material and to protect the crusher square mesh grizzlies get a more accurate maximum size of material than other shaped grizzlies but can be prone to plugging rectangular and nongrided grizzlies are less prone to plugging but can allow material of larger size to pass many grizzlies are rigidly mount

ed so that material must flow through them or be pushed through with equipment some grizzlies vibrate to encourage material on the grid to reorient and pass through grizzlies can be fed using a loader truck or conveyor grizzlies also act as a metal scavenger to trap rock bolts wire mesh drill steels and cables removing them from the ore stream periodically by a loader from the top of the grizzly rock breakers fixed or mobile rock breakers are often used in conjunction with a grizzly a modern rock breaker uses a hydraulic arm with a pneumatic impact tip to move material around on the grizzly and impact oversize until it passes fixed rock breakers can be operated from a remote operator s cabin or even from the surface to reduce the installation size and ventilation needed underground before the advent of the modern rock breaker oversize material feeding the primary crusher was a greater problem and primary crushers were designed largely on the basis of gape minimum dimension of feed opening rather than capacity additional and optional equipment other equipment items in crushing circuits can include the following overhead crane freight elevator service air compressor sump pumps air vacuum cleanup systems rock grapple conveyor belt magnets conveyor belt metal detectors belt monitoring systems belt feeders screw feeders bin ventilators apron feeder to the primary crusher dust collection suppression system eccentric trolley removal cart personnel elevator air cannons water booster pumps service trolleys conveyor gravity take up service winch conveyor belt rip detector conveyor belt weigh scales vibratory feeders sampling station underground crushing installations are high capital cost items due to fixed equipment costs and large scale excavations required to house the system crusher installation capital cost estimates should include equipment costs plus the following direct and indirect construction costs excavation and ground support mechanicals concrete electrical structural steel instrumentation indirect costs can fall within a range of 40 to 60 of the direct costs integration aspects as mentioned in the introduction each step in a mining system value chain consists of an upstream buffer an orehandling process that upgrades the ore to another value state and a downstream buffer the upstream buffer in most operations will be the storage capacity of uncrushed material available for crusher feed within typical feed crushing configurations this may be in the form of one or a number of stockpiles or live capacity within one or a number of orepasses downstream buffer is the capacity of crushed material storage between the crushing discharge and the next step in the materials handling system e g truck conveyor or skip load and transport where such upstream and or downstream buffer is of limited capacity or nonexistent operational variability around the crush

er may adversely affect crusher uptime because of the high capital costs of large underground storage facilities for uncrushed or crushed material it may not be possible to install large buffer capacity at this step of the materials handling process therefore crushing should not be the operation bottleneck by design and the crushing and immediate upstream and downstream processes should have capacity in excess of the bottleneck process technology for the most part advances in crusher design have moved slowly jaw crushers have remained virtually unchanged since about 1950 more reliability and higher production have been added to basic cone crusher designs that have also remained largely unchanged increases in rotating speed have provided the largest variation production improvements have come from speed increases and better crushing chamber designs the largest advance in cone crusher reliability has been in the use of hydraulics to protect crushers from being damaged when uncrushable objects enter the crushing chamber foreign objects such as steel can cause extensive damage to a cone crusher and additional costs in lost production advancement in hydraulic relief systems has greatly reduced downtime and improved the life of these machines conveyors mine conveyor systems have been utilized in underground mass mining for many years conveyor systems are commonly found in underground continuous coal mining operations which use longwall and room and pillar methods more recently underground conveyor systems have been employed in large scale long life metalliferous mines conveyors are more commonly used in operations where mine geometry and production flow allow conveyor haulage to be more efficient or cost effective than other methods inclined troughed belt conveyors are more frequently being selected from a range of alternatives which include shafts and trucks for ore haulage in underground mass mining projects belt conveyor haulage systems are being operated with lifts exceeding those normally associated with truck haulage systems and approaching the limits of shaft haulage systems spreadborough and pratt 2008 continued development of conveyor technology has resulted in increased confidence in and reliability of these systems reliability of conveyor based haulage systems as a whole is impacted by the complexity of larger configurations constructed of multiple units or lifts modern conveyor installations have proven high availabilities 85 are achievable in underground operations which has added confidence to their use in recent times figure 12 8 16 shows a typical conveyor in an underground metalliferous mine operation safety considerations in the selection of conveying options for an underground operation several risk areas must be addressed including belt fires managed by self extinguishing belt covers and auto detection and suppression systems belt failures addressed by belt rip detection systems monthl

y belt scans and concrete bulkheads guarding requirements to prevent injury to personnel audible start up alarms where personnel work near the belt emergency stop provisions in areas where access by personnel is required and dust management selection criteria the selection of haulage systems for underground mass mining has historically focused on shaft haulage trucks and belt conveyors the application of these alternatives in the australian mining industry is summarized in figure 12 8 3 troughed belt conveyors are shown in this figure to be applied in the range up to 8 mt a production rate and 1 200 m lift selection criteria for underground conveyor haulage include inherent safety capacity steady state and surge flow simplicity in design and operation dimensions length height and width maneuverability adaptability to various layouts cost capital and operating reliability availability operating life size of product handled spillage and carry back dust and noise generation and automatic remote operations the advantages of conveying material include high automation minimal operational labor requirements and lower operating costs and high reliability availability the disadvantages of conveying material include high capital cost therefore lower probability of duplication a large footprint and limited flexibility belt conveyors for underground service usually have a more rugged design and operate at slower speeds than a comparable overland conveyor hard rock mine belt conveyors normally require the ore to be crushed before it is conveyed or at least sized through a grizzly reasons for this include increased belt life due to reduced impact from lumps and elimination of tramp material including rock bolts rebar drill steels and scaling bars which can seriously affect the integrity of the belt system belt conveyor systems are less flexible than truck haulage and require a high initial investment this generally means that belt conveyors are the economical choice when the production rate is relatively high and the transport distance is significant in certain applications belt conveyor systems are selected for other reasons for example short conveyors are employed underground to optimize feed control to a loading pocket and prevent a run of fines from reaching the shaft elements in a conveyor haulage subsystem can include feed chute of dimensions appropriate for the material received belt to support the material and to transfer tractive forces to the material idlers to support the belt pulleys to resist belt tensions at changes of direction drives to provide driving and braking force take up rollers to provide belt tension for no slip at the drives and for belt sag control and discharge or transfer chute maintenance equipment at the haulage subsystem will typically include monorails and or cranes at drives and pulleys for changeout or repair belt cl

amps to resist belt tensions when installing or maintaining the belt spring applied hydraulic release tested for holding capacity belt reel handling facilities and belt splicing repair facilities integration aspects when considering the conveyor components of the materialshandling system consideration should be given to the upstream and downstream buffers the upstream buffer in most operations will be the storage capacity of crushed material available for conveyor feed in typical crushing conveying configurations this may be in the form of fine ore stocks within dedicated stockpiles or storage bins the downstream buffer may be underground storage at transfer points to the next step in the materials handling system or surface stockpile storage capacity technology advances in belt technology and particularly the development of stronger belt carcasses support the potential for further increases in capacity of inclined conveyor haulage systems vertical conveyors and hydraulic hoisting represent two potential technologies for haulage from underground mines the attraction of vertical conveying is due to the combination of a small footprint continuous process and overall energy efficiency limiting factors for vertical conveyors include tensile strength of the steel cored belts safety of belt splices and required production rate pratt 2008 current precedence for the application of vertical or high angle conveyors in a mining context is limited to a few hundred meters other special types of belt conveyors include extendable systems cable belts and high angle belts to date these special conveyor types have had few applications in hard rock mines conveyor costs conveyor haulage systems are considered high in capital and low in operating cost compared with truck haulage generally shaft haulage options attract higher capital costs and marginally lower operating costs for a similar scale operation operating costs will rise for conveyors as the complexity of the system increases with depth as a result of the addition of lifts and transfer points this does not necessarily favor selection of shaft haulage over a conveyor system because other factors must be considered examples of aspects to be considered are horizontal transportation distances confidence in geotechnical prediction for shaft and conveyor decline options and other functional requirements of the mine the actual selection of a haulage strategy for a large deep mine will require evaluation of the trade offs in risks and opportunities identified by the various options available limits of application the slope of a high lift conveyor group is limited by the slope of the decline incline development in which the conveyor is installed this is normally in the range of 1 5 3 to 1 5 4 10 5 to 10 7 where the development is carried out using conventional rubber tired equipment length and lift are limited by the belt carcass construction and the strength of the bel

t splice the choice of belt construction is also limited by the troughability of the belt troughability is the ratio of the crossbelt sag to belt width and generally decreases with increasing carcass strength future applications advances in future designs will be fundamentally linked to the load carrying capacity of available belt constructions and splice designs advances in conveyor technology continue to ensure success of conveyor systems in mass underground mining applications including improved drive control equipment belt condition monitoring improved splice designs for higher strength and longer life and improved chute designs rail haulage systems although belt conveying remains the dominant underground haulage system in coal mines rail haulage continues to be favored in large tonnage 5 mt a long life underground metalliferous operations haulage by rail can be limited to ore and or waste haulage or can include integrated rail systems for transportation of personnel materials and supplies throughout the mine although the following discussion focuses only on the haulage of ore and waste from production areas to the hoisting system it includes the consideration of interactions with other rail system users where required prior to the emergence of trackless or rubber tired mobile equipment in underground mining in the 1960s and 1970s rail or tracked haulage systems were predominant many of today s rail haulage mining operations remain a legacy of that period with improved productivity and performance of diesel mobile equipment and belt conveyors into the 1980s this equipment has grown in popularity and application as significant methods of underground haulage however rail haulage continues to offer competitive operating cost productivity and throughput benefits over a range of applications in many underground mining settings rail haulage systems consist of three primary types defined by their power supply diesel battery electric and overhead trolley electric a diverse range of systems are used around the world from small scale selective mines producing less than 100 ktpa to the largest bulk mining operations producing greater than 20 mt a health and safety considerations a fundamental consideration in haulage method selection and design is to ensure a safe and healthy workplace some of the key potential hazards that require consideration in rail haulage design and potential mitigating actions within a health and safety management system follow high speed equipment traffic management plan system access control adequate lighting speed limits comprehensive equipment operating procedures interaction between people and equipment equipment control systems physical barriers and guarding design for visibility unmanned automated equipment access control fail to safe control systems communication systems overhead exposed conductors permit to work process isolation systems electrical

protection systems entry height barriers water and dust management program maintenance hazards custom designed and fit for purpose maintenance facility regular inspection and maintenance schedules high standard maintenance equipment isolation systems regular track cleaning program mud rushes from chutes and orepasses water controls at all orepass entry points orepass monitoring regular chute inspections and or continuous remote closed circuit television monitoring fire automatic fire suppression systems emergency response teams and training good housekeeping common and paramount in all the these circumstances are the underlying skilled workplace supervision customized training programs skills assurance process good safety signage and proactive maintenance programs that meet oem original equipment manufacturer requirements at a minimum selection criteria selection of the preferred haulage system for a mine must consider a wide range of factors including health and safety economic technological reliability operability and flexibility rail haulage can exist either as a stand alone lateral haulage system or as part of an integrated system in the case of the former for example rail provides for all lateral movement of personnel materials ore and waste in the deep level south african gold mines and northern idaho united states silver mines in the case of the latter rail haulage can operate with mobile equipment as at the olympic dam mine ultimate selection needs to match the shape size nature and productive capacity of the ore body within the framework of an economically feasible operation the optimal haulage system for a mining operation is dependent on a variety of factors and their interrelationships the more significant of which are discussed in the following paragraphs health and safety of fundamental importance are the design installation and operation of a safe rail system as discussed previously capital cost rail haulage systems typically have a high capital cost and long construction time compared with alternative haulage systems this is because economically competitive rail systems are often large scale by nature and require long distances of tunnel development which takes years to excavate orepasses and chute systems for train loading long term ground support systems high upfront equipment capital purchase costs for locomotives rail rolling stock tipplers trolley line system and power reticulation equipment where installed and large construction labor and installation costs operating cost converse to capital cost rail haulage operating costs are among the most efficient per ton of material moved this is driven by comparatively lower maintenance costs due to a more highly engineered and thus reliable system compared with diesel mobile equipment requirement for a low number of operators and better energy efficiency spare parts inventories may also

be reduced reliability and operability well engineered rail haulage systems can achieve high system availability greater than 85 compared to diesel truck fleets which range from 50 to 75 of total available clock time the fixed nature of rail systems and repeatability of processes makes it highly attractive for automation with the potential to deliver increased utilization state of the art systems such as lkab s 26 mt a rail system at the kiruna mine in sweden codelco s 48 mt a teniente 8 railway at the el teniente mine in chile and bhp billiton s 8 mt a rail system at olympic dam in south australia are highly automated requiring high levels of technological support while delivering substantial unit operating cost and productivity benefits figure 12 8 17 due to the fixed nature of rail systems maintenance can be readily planned and scheduled spare locomotives can limit production interruptions while a smaller work force is typically required per ton hauled rail haulage systems require specific skill sets to maintain locomotives high voltage reticulation systems communication systems automation and control systems production rate and scale of operations given the higher capital cost of rail haulage systems they are typically used in operations with high production rates or long lives in order to pay back the high upfront costs likewise the high productivities and low unit operating costs offered by rail haulage favor their selection in mines with high throughput rates and or long lives however smaller scale rail system are also competitive in mines where the reduced flexibility of a fixed horizon haulage system can be managed this is particularly relevant in large flat lying ore bodies where long haul distances may exist where larger openings for other forms of haulage cannot be considered or where diesel mobile haulage systems are avoided because of environmental conditions e g heat gases and fire risk ore body geometry rail haulage systems are by nature horizontal installations and their selection must consider the lower flexibility of their fixed nature in general larger ore bodies are more suited to rail haulage as production is sourced from one or a few albeit large areas as opposed to disseminated narrow vein or dispersed ore bodies where production comes from numerous locations in differing amounts at different elevations however rail haulage can be a competitive haulage option in such mines if the fixed nature of rail systems can be managed and matched to other aspects of the operations design production growth although diesel mobile equipment has the flexibility to travel wherever tunnel access is provided flat or inclined rail haulage is a constrained system in order to support growth of operations to new areas or greater depths rail haulages need to be extended or replicated to support mine growth or extension or to sustain production from new areas the 25 mt a rail haulage sys

tem of lkab s kiruna sublevel cave iron ore mine is a good case study where major rail haulage horizons have been replicated at increasing depth in 300 m vertical increments on 10 to 15 year intervals as the mine has progressively moved its production areas downdip consideration at the design stage can enable rail systems to increase their productive capacity if required through measures such as adding more rolling stock upgrading locomotive capacity or increasing haul speeds system life the operational life of haulage systems and equipment is a key criterion in selection of haulage systems rail systems generally have a longer equipment life than mobile haulage equipment which requires replacement nominally every 5 to 7 years high throughput rail systems require longer operational life in order to provide an acceptable return on capital invested mine plan selection of haulage systems must be aligned and complementary to the mine plan rail haulage is more typically suited to deeper shaft hoisting operations where ore and waste need to be collected centrally for crushing transport to and removal from a single elevation typically at depth development of ore bodies in areas of high topographic relief offers the potential for adit access and efficient rail haulage directly from the mine to the surface processing facility such as freeport mcmoran copper and gold s deep ore zone mine at grasberg in west papua indonesia and codelco s el teniente mine design issues inclusive in the economic analysis of selecting a cost effective haulage system are four key aspects for rail haulage design inherent to this analysis are capital and operating costs the following factors need to be evaluated in a fully integrated design approach capacity system layout and geometry materials properties and materials handling equipment capacity the required ore production rate is the primary driver of rail haulage system design this will determine such key design parameters as tunnel layouts and size equipment capacity operating philosophy the number of units in the fleet integration with upstream and downstream production process capabilities and haul speed consideration also needs to be given to whether the system is dedicated to ore haulage or is to be shared with movement of personnel and or materials and haulage of waste as well as ore due to the fixed nature of rail haulage system infrastructure consideration should be given at the design phase for the potential and likelihood of future increased production or extension of mining operations to new areas in such circumstances engineering designs should consider such key items as the adequacy of tunnel size and extent maintenance facilities power supply systems and infrastructure track quality and the number and configuration of orepasses and dumping tipping points system layout and geometry the layout and extent of a rail haulage system requires economic trade off studies o

f numerous factors that impact the geometric design including physical dimensions and depth of the ore body degree of coverage required by the haulage system beneath the ore body footprint primary mining excavation costs including ground support final preparation and drainage trade off between optimal rail haulage design and operation and optimal production mining systems in the mine workings above single or dual direction of travel haulage speed a key driver of productive capacity radii of curves single or double rail tunnel dimensions gradient maximization of straights and super elevation of curves number and location of ore and or waste delivery points into the rail system number and location of ore and or waste dumping tipping points from the rail system future operating cost impacts and benefits and suitability for future expansion with an inherent need for wide radius curves to accommodate rail equipment and a preference for straight sections rail haulage horizons can lend themselves to potential mechanical excavation techniques such as tunnel boring machines these are commonly applied in civil engineering rail projects and warrant consideration in the preliminary phase of mine rail haulage system design the schematic presented in figure 12 8 18 demonstrates the preference for geometric rail loop layouts wide radius curves and long straights material properties and materials handling rail haulage systems must be designed to manage the physical attributes of the material s to be hauled for example the physical properties of the material will determine the strength of mechanical equipment against wear and tear and flow properties will impact orepass and chute design and determine the potential need for lining of passes bins or chutes for longevity the key physical properties to be considered include moisture content size distribution proportion of fines mud rush hazard and maximum lump size specific gravity material density abrasiveness dust generation presence of deleterious materials such as tramp steel cemented backfill and waste dilution and changes in material properties over time careful management of these properties has a significant downstream impact on system operability through aspects such as pass and chute maintenance spillage which can cause safety hazards derailments and significant downtime or equipment damage train hopper wear and tear downtime due to blockages hang ups fines rushes etc and maintainability and maintenance cost equipment the selection of rail haulage equipment is a key component that must be fully integrated with all the previously discussed aspects of system design locomotive prime movers are typically powered by either battery electric diesel or overhead electric energy sources the majority of large scale high production underground rail haulage systems today are overhead dc electric inverted t

o ac drive locomotives examples of such systems are shown in figure 12 8 17 ore waste hoppers and tipplers are available in various configurations with capacities ranging from 1 t to more than 70 t per hopper hoppers can be either side or floor dumping tipping with a range of chassis side door and floor opening designs available depending on various parameters train combinations can be sized to match production and rail system capability with rakes of up to 30 hoppers and total train payloads in excess of 1 300 t rail haulage systems lend themselves to significant levels of automation design of remote control systems is critical to operating productivity and reliability and needs to consider locally available skill sets technical support spare parts availability mine communications systems and environmental aspects such as water humidity dust radiation and gases train control systems must be fully integrated with the proposed operating philosophy for the haulage system with the aim of preventing downtime and production delays and minimizing maintenance other equipment aspects to consider in system design include rail gauge and installation quality control and communications systems switch design train loading points passes and chute design tippler design drainage and lighting and signaling the maintainability and operability of all equipment requires close attention this includes access to all parts of the rail system for maintenance personnel and equipment in the event of derailment or breakdown specialized equipment for rail maintenance standby equipment including spare locomotives and hoppers ore cars critical insurance spares and regular spare parts management hydraulic haulage systems hydraulic ore transport is an often cited but rarely implemented ore transportation system in underground mining applications while slurry pumping engineering is mature in a multitude of diverse surface applications its serious consideration in underground environments is hampered by risks associated with the need to pretreat and prepare suitable material underground beyond typical crushing and sizing arrangements pump and column wear considerations capital costs and equipment technologies for anything more than very small tonnage rates and lack of courage to lead change one well established application of underground hydraulic transportation and hoisting is at the mcarthur river uranium mine in saskatchewan canada operated by cameco h goetz personal communication mcarthur river produces approximately 47 000 tpa of high grade average 17 u3o8 triuranium oxtoxide uranium bearing ore from a depth of 640 m below the surface the operation includes full ore preparation underground through a 600 t d semiautogenous grinding mill hydrocycloning and 2 13 m diameter thickeners the thickener underflow feeds into a 200 m3 thickener underflow holding tank that supplies the wirth triplex p

iston diaphragm positive displacement pumps 13 million pa discharge pressure for pumping up 2 100 mm diameter columns in a single stage to the surface the physical characteristics and health and safety aspects associated with handling relatively low quantities of very high grade uranium ore with attendant high radiation levels were a key driver in the selection of hydraulic transportation ahead of more conventional methods hydraulic systems for transporting backfill are described in a separate chapter other haulage systems a small number of less common haulage systems are used in underground mines these are typically selected because of highly specific geological environments particular material properties or unique operating systems and processes these include the following rope scrapers slushers commonly used for ore haulage from operating stopes to orepasses in south african longwall gold and platinum mining operations slushers are also used for ore collection haulage and delivery into orepasses beneath established block caving operations monorails traditionally a materials handling system monorail systems can be either electrohydraulic friction or rack and pinion drive and have been used in moderately dipping applications up to 14 in south african gold mines a shaft hoisting system consists of the following major sections starting from the bottom of the shaft a loading station for ore or a service station for workers and material shaft conveyances called skips for ore transport and cages for transporting workers and material ropes that suspend the conveyances a shaft that connects the underground to the surface and that is equipped with a system to guide the conveyances as they move in the shaft a headframe located on the surface that supports either the hoist itself or supports head sheaves that direct the headropes from the top of the conveyance to a groundmounted hoist and that provides the tipping arrangements for the rock a hoist and hoist room the headframe also typically includes an ore storage area into which ore and waste are temporarily discharged before being transferred to some other surface facility hoists two basic types of hoists are commonly used today drum hoists in which the hoist rope is stored on a drum and friction hoists in which the rope passes over the wheel during the hoisting cycle within each category are several variations drum hoists figure 12 9 1 shows the various arrangements of drum hoists drum hoists are usually located some distance from the shaft and require a headframe and sheaves to center the hoisting ropes in the shaft compartment and maintain a rope fleeting angle of less than 2 single drum hoist the single drum hoist may be used for balanced or unbalanced operation when used for unbalanced hoisting the cost of the electric drive may become quite high for long hoisting distances and high tonnages this is because the motor must be large enough to p

roduce sufficient torque to handle the combined weight of the rope conveyance and payload in the absence of a counterweight these costs are however offset by the reduced size of the installation because there is one less compartment conveyance head sheave and head rope to accommodate as well as a smaller hoist foundation and building there is virtually no difference in the power costs for balanced versus unbalanced operation because power costs are for the energy consumed which is derived from the number of tons hoisted as the shaft gets deeper the difference in the size of the electrical drive for balanced versus unbalanced operation becomes less significant because the mass of the unbalanced rope which is the same whether there are one or two conveyances begins to dominate the total unbalance experienced by the hoist single drum production hoists are unusual because their hoisting efficiency is halved for the same skip size but they are fairly common when used for service duty in which the hoisting cycle time is not as significant in a balanced hoisting system one rope winds off the drum as the other winds on when used with a skip or cage in balance with a counterweight a single drum hoist can service one or more levels because the location of the counterweight is not important when used with two skips in balance the singledrum hoist is only suitable for single level hoisting any rope adjustments to locate the conveyance must be done manually for shallow shafts with one layer of rope no dividing of the drum is required for deeper shafts the drum must be divided to wind several layers of rope divided single drum hoist this type of hoist is used for deeper shafts with balanced hoisting when several layers of rope must be stored on the drum peak horsepower is less than with unbalanced hoisting because the skip weights are balanced because the payload and weight of the rope is not balanced the maximum unbalanced load occurs when the loaded conveyance is at the bottom of the shaft split differential diameter drum a third though uncommon type of single drum hoist is the split differential diameter drum in this arrangement each of the drum compartments is of a different diameter this type of hoist is used with a conveyance and counterweight in balance or with a skip and cage in balance if for example the counterweight rope is wound on the smaller diameter drum it moves less than the main conveyance and rope adjustment problems are reduced double drum hoist one drum clutched although more expensive than a single drum hoist the doubledrum hoist with one drum clutched has certain advantages with this type of hoist it is possible to make quick adjustments to the ropes due to initial stretch as a service hoist with cage and counterweight this type of hoist can serve several levels efficiently as a production hoist with two skips the ropes can be adjusted to maintain balanced hoisting at any level in a mu

ltilevel operation double drum hoist both drums clutched the double drum hoist with both drums clutched has the added feature of allowing hoisting to continue in one compartment should something happen to the other compartment this is an excellent feature if there is only one hoist available this type of hoist is also favored during shaft sinking operations multiple rope hoist blair type with this type of hoist each conveyance is suspended from two hoist ropes that are each coiled onto the same drum the advantage of this is that by doubling the number of ropes per conveyance the conveyance payload can be increased this type of hoist was developed in south africa for deep mines typically more than 1 800 m 5 900 ft another advantage of this arrangement is its ability to dispense with safety dogs on the cage which are only required for single rope hoists two such hoists are now operating in canada and several more are scheduled to be installed in the near future bicylindrical conical drum hoists this type of hoist is no longer manufactured but there are still a few in operation in north america the drums on these hoists both single and double drum configurations are made in two steps with a conical section in between the hoists were developed before multilayer winding was practical prior to multilayer winding drum diameters simply increased to match depth this practice leads to very large diameters 10 m 33 ft or more resulting in very high starting torque requirements by beginning the acceleration on a small diameter the high starting torque requirement could be avoided friction hoists figure 12 9 2 shows the two possible arrangements of friction hoists the koepe or friction hoist was developed by frederick koepe in 1877 it consists of a wheel with grooved liner or liners made from a friction material to resist slippage the hoist rope is not attached or stored on the wheel in early installations the hoist was mounted on the ground and a single rope was wound around the drum and over the head sheaves to the two conveyances in a balanced arrangement in addition a tail rope of the same weight per unit length as the headrope was suspended in the shaft below each conveyance thus the only out of balance load was simply the payload friction hoists can be either tower mounted or ground mounted tower mounted the main reason for selecting tower mounting is the reduction in cost of the structure because the rope loads are confined internally this arrangement also provides a very small footprint that allows the structure to be supported by the headframe s collar concrete in the slip formed design or by localized piled foundations in the structural steel design of the headframe because the koepe wheel diameter is generally larger than the conveyance compartment centers deflection sheaves must also be installed in the headframe below the hoist to move the ropes over to match these centers tower mounting is also p

referred in cold climates because the conveyance ropes are isolated from the weather ground mounted a headframe for a ground mounted friction hoist must be designed to resist the overturning loads produced by many headropes these buildings are generally of a structural steel design and require the use of a pair of cluster sheaves to locate the rope centers over the conveyance compartments because they must support the entire suspended rope load rather than simply deflecting the ropes by 10 to 15 the cluster sheaves are much more robust and are therefore significantly more costly than the deflection sheaves used in tower mounted applications comparison of friction and drum hoists a friction hoist system differs from a drum hoist system in performance as well as components therefore when deciding which type of hoist to use it is necessary to compare the two complete systems rather than the two hoists alone in addition to comparing the total capital costs of the hoist headframe ropes conveyances and shaft it is necessary to consider operating costs maintenance costs reliability power supply system local custom and individual preference the following general statements help distinguish between these two hoisting systems double drum hoists are the preferred hoist for shaft sinking double drum hoists are the best choice for hoisting in two compartments from several levels drum type hoists are best suited for high payloads from shallow depths the limitation on a drum hoist employing a single rope is the ultimate strength of the rope because large ropes are difficult to manufacture and handle the depth capacity of drum hoists can be extended by using two ropes per conveyance blair type hoist and with this arrangement blair hoists can be used for depths exceeding those of either single rope drum hoists or friction hoists friction hoists with multiple ropes can carry a higher payload and have a higher output in tons per hour than drum hoists within a range of depths from 460 to 1 520 m 1 500 to 5 000 ft friction hoist mechanical operation is very simple has a low rotational inertia and is less costly than a drum hoist friction hoists have a lower peak power demand than drum hoists with the same output the friction hoist can operate on a relatively light power supply rope maintenance is more intensive and difficult for friction hoists hoist components the major components that make up a hoist are the drum or wheel bearings gearing brakes clutch motor power conversion and controls drum the drum of a drum hoist must be designed to store the amount of rope required for the shaft depth and accommodate the stresses caused by the rope tension loads the prediction of these rope loads is well documented in the literature although originally provided with no machined grooves i e they were smooth drums today are generally supplied with parallel grooving which includes a half pitch crossov

er twice per revolution this grooving pattern was developed by lebus international longview texas to achieve stable spooling in multilayer winding drums can also be supplied grooved in a helical pattern which is a continuous spiral from one end of the drum to the other this pattern is limited to three layers of rope because beyond this number of layers the rope winding pattern becomes unpredictable lebus international s grooved drums in contrast have been used to wind as many as 15 layers of rope without a problem rope resonance must be considered in the selection of the grooving pattern this is discussed in a later section of this chapter wheel the wheel of a friction hoist is not subjected to crushing forces as a result the drums are much lighter and are made wide enough to accommodate the rope end attachment dimensions as well as the space for the deflection sheaves the linings for the rope treads on koepe hoists were originally made of wood or leather which were then replaced by modern plastic bearings as shown in figures 12 9 3 and 12 9 4 the hoist drum or wheel is supported on a shaft that rides in two or more bearings bearings provide the mechanism that supports the shaft loads and bearings allow it to rotate at the same time with very little friction there are two types of bearings sleeve bearings which use a film of oil to keep the shaft out of contact with the fixed part of the bearing and roller bearings sleeve bearings are being supplanted by spherical roller bearings that have the following advantages low coefficient of friction economy of space simplicity of lubrication practical elimination of wear due to roller point contact maintenance of accurate alignment consistency in design and manufacture the main disadvantages of the latter are the fact that they are not split which complicates their replacement and long order times are involved when bearings larger than about 500 mm 19 7 in in diameter are required gearing hoists can be driven either directly by large diameter relatively low speed ac or dc motors 90 rpm or less or through a gear reduction system by small diameter high speed 400 to 1 800 rpm motors with the change in power conversion to variable frequency drives the motors have altered to ac machines operating in the 900 to 1 800 rpm range gear reduction was commonly a single stage system that incorporated a large gear fixed to the drum shaft and driven by one or more pinion gears this cannot be used practically for the sealed self lubricating multistage reducers now for high speed ac motors reducers are now fully enclosed multistage and equipped with oil coolers as well as bearing vibration monitoring systems these utilize flexible couplings between the motors and high speed input shafts as well as between the low speed output shaft and the hoist drum shaft brakes a hoist must be equipped with a mechanical braking system to decelerate stop and hold the loa

d although the electrical motor drive and controls can initially decelerate and stop the hoist this equipment cannot hold the load because of thermal limitations inherent in the drive electrical systems also require a source of energy to function for these reasons hoists must be provided with mechanical braking systems that ultimately rely on either gravity or spring force to operate drum brakes and disc brakes are the two main types of braking systems until the mid 1990s drum hoists were exclusively equipped with drum brakes in jaw or parallel motion configuration mainly because they were part of the hoist speed control system rather than solely for holding or emergency braking functions most new hoists both drum and friction are equipped with disc brakes figures 12 9 5 12 9 6 and 12 9 7 illustrate the designs of jaw parallel motion and disc braking systems respectively brake control systems are now required to control deceleration rates to very high accuracy to protect passengers from injury limit the dynamic stresses that arise in the hoisting ropes during emergency stops and prevent rope slip on koepe hoists this level of accuracy is readily achievable with the higher operating pressures and lower inertias associated with disc brakes in addition to accurate control of deceleration rates braking systems must be capable of dissipating the energy absorbed during emergency stops disc brakes have lower mass than drum brakes and run at higher surface pressures therefore they tend to heat up more quickly by manufacturing brake discs in several segments and bolting the segments to the hoist drum or wheel a disc brake can be designed to expand freely under thermal load and therefore avoid distortion or failure clutch double drum hoists used for unbalanced hoisting such as when sinking or when operating from several levels must have at least one of the drums clutch connected to the drum shaft this assembly consists of a clutched or floating drum that is supported on the shaft through a sleeve bushing and a clutch arm which is used to transfer the torque to the drum the clutch arm consists of a spider or gear that slides axially along the shaft supported by a hub driven by keys or splines or driven by a hexagonal section of the shaft the drum also incorporates a clutch ring which has teeth matching the clutch arm on its entire periphery bolted to the side of the drum as the spider moves axially these two sets of teeth engage or disengage the tooth pitch determines how accurately the rope lengths on each drum can be matched driving torque is then transmitted from the clutch spider to the drum through the bolts used to attach the clutch ring to the drum as the drum must be prevented from rotating when it is unclutched it is necessary that the clutch s operating mechanism and the brake holding the drum be interlocked both electrically and mechanically drive motor and power conversion hoists have used energy s

ources such as steam compressed air and internal combustion engines to drive them these have largely been replaced with electrical power electrical drives for hoists originally employed ac wound rotor induction motors using variable resistance in the secondary for speed control these systems were then supplanted by dc machines that utilized motor generator sets to provide dc power the motor generator sets were replaced by so called static drives in the late 1960s the name static drive referred to the fact that the power was converted from ac to dc using thyristors which are made from silicon wafers and involve no moving parts the dc technology began to disappear in the late 1980s when it was replaced with ac technology that utilized synchronous motors and variable frequency sources called cycloconverters the variable frequency sources have now been replaced with pulse width modulated digitally controlled equipment that utilize transistors injection enhanced gate resistors or isolated gate bipolar transistors for power conversion the ac motors are now a mixture of synchronous and squirrel cage designs with the squirrel cage design being the preferred option because of its simplicity and ruggedness this new generation of variable frequency static drives also provides ac power with low harmonic content and ensures operation at unity power factor these drives consist of two inverters back to back the first converts line frequency ac to dc the second dc to variable frequency ac hoist control systems the hoist control system consists of two components active and passive the active component starts and stops the hoist controls its speed throughout the hoisting cycle and displays the state of the hoist to the operator the passive component monitors the speed and position of the hoist itself as well as the state of its subsystems such as the brakes hydraulic power units motor ventilation and similar auxiliary equipment the passive component also signals the active component when the system is operating outside of its normal range and thereby initiates a stoppage of the system until the fault that raised the alarm is cleared all of the required functions are now fulfilled by digital devices call programmable logic controllers these devices utilize redundancy to a large degree to ensure that they are reliable the operator s interface has also evolved to what is called a human machine interface which is a graphical display provided on a computer monitor hoist selection the capacity of a hoist depends on the drum or wheel dimensions number and size of ropes speed and horsepower the mechanical components such as drum shaft bearings gearing and brakes are svized to match the rope loading the electrical components are sized to match the torque required to handle the loads and the speed at which the hoist will run the rope is selected to handle the required payload while operating at a reasonable or legislated safet

y factor selection procedure for production drum hoists sizing the hoist runs through an iterative process which requires the following initial information hoisting depth required annual production hours available for hoisting the process of selection is as follows 1 using the depth calculate the hoisting cycle time the cycle time will depend on the maximum speed acceleration and deceleration rates creep speeds and distances arriving at and departing the loading pocket and dump and the loading dumping time when the hoist is stopped 2 varying these items will enable the designer to optimize the selection good guidelines are a maximum speed of 5 m s 1 000 fpm per 300 m 1 000 ft of depth up to a maximum of 20 m s 4 200 fpm acceleration and deceleration times of 15 seconds a creep speed of 1 5 m s and a creep time of 5 seconds and load and dump times of 10 to 15 seconds depending on density of the load 3 use the cycle time to calculate the cycles per hour room and pillar r p mining is a mining method whereby a series of rooms horizontal openings is extracted leaving pillars of ore rock or coal in place between the rooms figure 13 1 1 shows a plan view of an ideal r p mine the square blocks are pillars the spaces between the pillars are rooms in hard rock pillars are usually much smaller horizontally than rooms in soft rock and coal mines they are usually much larger the r p method has been used widely and often for example in lead zinc copper ore bodies of mines in southeast missouri united states for 150 years in mines in charcas mexico for more than 100 years in copper mines in poland for nearly 50 years and in underground rock quarries and salt mines worldwide for more than 100 years the proliferation of this method for such a long period of time suggests that it is low cost versatile and safe ore body characteristics of r p mining the r p method is normally employed on fairly flat seams or ore bodies but may in some cases be used in seams that angle up to 45 ideally the ore body or seam to be mined should be very large laterally and fairly uniform in thickness however many r p mines in hard rock ores have nonuniform ore horizons for example in some of the r p mines in the old lead belt of missouri the ore horizon varies in thickness from 3 to 91 m 10 to 300 ft in the viburnum trend mines of missouri ore varies in thickness from 3 5 to 30 m 12 to 100 ft the strength of both the back roof and the ore are important obviously roof strength must be sufficient to stand open for the width of the room when the room is opened similarly ore strength determines the size of the pillar required to support the open span of the back thus pillar size is not fixed but rather is determined by the sum of 1 stress in the pillar due to the weight of the overburden above the opening 2 additional stress in the pillar due to removal of the ore that originally supported the c

urrent back and 3 tectonic stressrelated forces that may still remain in the rock the ore in the pillar must be strong enough to compensate for these collective stresses farmer 1992 and zipf 2001 described the design of stope pillars and barrier pillars normally used for a new r p mine in a new mining district the caveat new mine in a new district is important since in opening a mine in an alreadymined district decisions regarding pillar size and room size for a given stope depth and ore strength have already been made yet it is nevertheless important to understand how to design stope panel pillars and barrier pillars for a new mine traditional strength based pillar design methods this section is adapted from zipf 2001 significant research has been done on pillar design for both r p and longwall mining many ground control and pillar design experts have contributed e g holland 1964 salamon and munro 1967 hardy and agapito 1977 mark 1999 their studies include verification of pillar design formulas and formula constants by statistical verification with a large database of both failed and nonfailed coal mine incidents from australia and south africa galvin et al 1999 gale and hebblewhite 2005 however with the exception of the work of hardy and agapito these studies concern coal and there has been no similar verification for hard rock or even other softer minerals such as trona salt oil shales or borates the purpose of the following discussion is to extend this work to the design of pillars for all minerals not just coal r p mines may cover only a hundred square meters and contain just a few pillars as is typical for small zinc deposits or they may cover many square kilometers as is typical for coal trona and limestone mines in an r p mine large arrays of pillars are typically grouped into panels which in turn are surrounded by barrier pillars the small pillars within a panel are sometimes called panel pillars in developing layouts the mining engineer must develop appropriate dimensions for room spans panel pillar widths panel sizes and barrier pillar widths developing these dimensions requires evaluation of not just pillar strength but also the consequences of pillar failure which can happen anywhere in the layout at any time traditional strength based pillar design requires estimates of pillar stress and pillar strength the safety factor for the pillar is then calculated by dividing pillar strength by pillar stress what constitutes an acceptable safety factor depends on the tolerable risk of failure safety factors of 2 are typical for pillars in main development headings or panels during advance mining safety factors of 1 1 to 1 3 are typical for panel pillars after retreat mining safety factors much less than 1 0 are possible for panels where pillar failure is the eventual intent summary of traditional strength based pillar design methods for r p coal mining the analysis of retrea

t mining pillar stability armps method and program developed by mark and chase 1997 applies this method incorporates the tributary area method and empirical strength formulas for sizing coal pillars during advance and retreat mining and includes adjustments to pillar stress for factors such as side loading from previously mined panels it can also determine barrier pillar size again an equivalent program does not appear to exist for noncoal r p mining but the armps method should apply with suitable adjustments to the input parameters with traditional strength based pillar design methods the mining engineer can determine panel pillar size and barrier pillar size but not panel dimensions operational considerations such as equipment and productivity determine the panel width and usually set it as large as possible based on strength considerations alone a narrow panel requires a narrow barrier pillar and a wide panel requires a wide barrier pillar and rock mechanics factors do not affect the panel width determination maximum panel width can be determined by the size of air blast that an operation can withstand but this is not a rock mechanics factor other rock mechanics considerations are needed to determine maximum panel width as well as panel pillar and barrier pillar sizes typical r p sizes for metal mines within the bounds of roof and pillar safety larger rooms enable use of larger equipment and increase the productivity of the stoping operation case studies completed many years ago showed that the average productivity of three mines with average room size of 41 m2 440 ft2 was 21 7 t or metric tons 23 9 st or short tons per worker shift in contrast the average productivity of three mines with average room size of 158 m2 1 700 ft2 was 71 5 t 78 8 st per employee shift bullock 1998a while safety should never be compromised larger rooms are more productive providing that they do not get too high to safely maintain the back and the loose rock on high pillars and ribs many limestone producers typically have rooms 12 2 m 40 ft high in a single pass examples of typical r p mines with very high rockquality designations rqd 85 are the mines of the viburnum trend carmack et al 2001 room widths are held to 11 m 36 ft but are typically 9 8 m 32 ft final heights after multiple passes are preferred to be 15 m 50 ft roof bolting is done typically in every intersection and elsewhere as needed historically pillar sizes were 8 5 8 5 m 28 28 ft on 20 m 60 ft centers then panel pillar sizes of 11 6 23 m 38 76 ft were adopted for wider areas of mining where pillar extraction is planned upon retreat from the area relative pillar strengths in the mines are monitored using a rating system of 1 to 6 where 1 denotes no visual stress and 6 denotes pillar failure figure 13 1 2 this classification system is important not only for pillar recovery but also for pillar maintenance pillars whose

rating reaches 3 must be reinforced with grouted rebar bolts to halt further deterioration if they are to maintain their integrity for a long period the rock strength of samples in this area depends considerably on mineral content but varies from 70 to 200 mpa 10 000 to 30 000 psi a similar but less defined and seemingly more conservative pillar rating system was developed by westmin resources ltd lunder and pakalnis 1997 some resources exist for which mineral continuity is so consistently inconsistent that no amount of surface prospecting can accurately identify ore continuity and thus pillar and stope layouts cannot be preplanned the mississippi valley type zinc deposits of central and east tennessee united states are of this type if this is the case and it is the first mine in a new mining district an underground test mine is required so that better geotechnical information can be obtained on which to base plans for room widths and pillar widths and height all of which greatly affect mine operating cost structural information can then be seen and accurately mapped data needed to determine joint and fracture information can be accurately measured in situ stress measurements can be taken and large core samples can be gathered for laboratory analysis orientation of rooms and pillars pillar orientation is affected by in situ stress the orientations of both rooms and pillars are affected by the extent of dip pillar orientation due to in situ stress as in all mining methods mining engineers who are planning r p operations must be aware of probable in situ stress within the rock before mining any significant horizontal stress in a particular direction must be taken into account by orienting the room advance and the direction of rectangular pillars to give the most support in the direction of stress in the early phase of development research should be done to determine the magnitude and direction of inherent stress however very shallow r p operations may have very little horizontal stress in which case the orientation of rectangular or barrier pillars is not a concern particularly if a mine has been opened from a hillside with adits where nature could have relieved horizontal stresses eons ago in sharp contrast to this condition some deep r p mines have tremendous problems not only with horizontal stress but also with rock that absorbs a large amount of energy and then fails violently in such operations not only is pillar orientation important but the sequence of extraction and how it takes place is also important such an operation is documented as a case study by korzeniowski and stankiewicz 2001 r p orientation due to dip r p mining is usually done on fairly flat strata but need not be limited to that shallow dips of up to 8 have little effect on the layout of the r p stope if the mine is developed from a shaft rooms should be laid out so that haulage of the load goes downgrade if the mine is develop

ed from an adit or decline of course this would not be the case as to the orientation of rectangular pillars it probably makes sense to orient the long side of the pillar in the direction of the dip steep dips of 35 to 40 20 are best mined in a series of parallel slices in steps each following a level contour of the dipping ore body as each round is blasted much of the rock cascades down to the next level where it can be loaded this so called cascade mining method was first reported for the mufulira copper mine in zambia anon 1966 however in this case the pillars were removed almost immediately in a second cycle of mining leaving the hanging wall to cave as mining retreats along the strike very steep dips of 20 do not allow operation of trackless equipment manufacturers have proposed back mounted cogwheel driven jumbos that can drill stope rounds under these conditions but the ore so mined does not flow by gravity and must be removed by scraper conditions being too steep for any other type of loading equipment thus many operators over the years have resorted to drilling with handheld jackleg drills and using rope scrapers slushers to move ore from steep slopes to the haulage equipment christiansen et al 1959 r p mine haulage development for hard rock r p mines the production shaft is normally developed somewhere near the centroid of the ore body unless the production opening is by adit or decline or the ground will be allowed to cave the objective of production development is to minimize the haul cost for the ore to the shaft rail haulage should be kept as straight as possible with long haul grades of 2 although within the rooms themselves grades of up to 5 are permissible in a trackless haulage operation however these numbers may differ and the following apply keep grades as low as possible and long hauls at 8 0 keep haul roads as straight as possible locate pillars in such a way that roads need not deviate around them lay out all main haul roads before mining and lay pillars out from this plan keep haul roads in excellent condition use adequate crushed stone and keep them well graded and dry water not only causes potholes but also lubricates rock to the point that it can cut tires in laying out a mine development plan spearing s rules of thumb should be kept in mind spearing 1995 bullock and hustrulid 2001 keep intersections off of the main drift six widths apart avoid acute angle turnouts that create sharp bullnose pillars separate near parallel ramps and declines from the main drift by at least three times the diagonal width of the main drift in planning a trackless rubber tire haulage system for an r p mine consider the various methods of moving rock from the working face to the crushing hoisting facility the versatility and flexibility of modern hydraulic excavators and powerful rubber tired equipment can make it difficult to choose the optimal method the f

ollowing guidelines can help 1 when the mine starts operation from the bottom of a decline or shaft the haul distance is short unless hauling is up a decline to the surface a rubber tired load hauldump lhd unit is probably optimal 2 as the mine works out from the dump point the lhd unit should eventually start loading a truck just one truck suffices as long as the loader can load the truck then load itself and follow the truck to the dump point by the socalled load and follow laf method 3 as the distance between the mine and the dump point continues to increase eventually the lhd unit should stay in the heading and load multiple trucks enough to keep itself busy at this point the lhd unit is no longer optimal and a front end loader fel may be required as distance or haulage grade increases larger trucks and fels may be also required examples exist bullock 1975 where true wheeled loaders have been successfully modified and adapted for both lhd and fel operation and perform efficiently at all three stages figure 13 1 3 shows a graph generated by a materialsmoving simulation program used in the late 1970s in the viburnum trend lead zinc copper mines the program optimizes equipment selection based on least cost at that time the equipment manufacturer charged less capital cost per ton of capacity for the 25 4 t 28 st truck than for the 36 2 t 40 st truck when it took this into account the simulation found it cheaper to run a fleet of smaller trucks at the greater distance and grade this is not the normal expected result and thus the simulation proved its effectiveness if a mining zone has more than one level and the main haulage is on the bottom level then various combinations of lhd units to the orepass or fels with trucks hauling to the orepass are possible and at the bottom of the orepass automatic truck loading feeders can transfer the ore to trucks or rail mounted trains if the upper ore body is small and an automatic ore chute cannot be justified then the ore can be allowed to fall to the ground and an lhd unit or fel can load it into another truck figure 13 1 4 shows all of the possibilities for moving material from an r p face to the final ore pocket at the shaft for a complete explanation of how to optimize these variable conditions see bullock 1975 or bullock 1998b clearly then in an expanding r p mine the optimal method or combination of methods for moving ore constantly changes but for every condition and distance there is one optimal least cost method which can be determined by computer simulation of the underground mine environment bullock 1975 gignac 1978 another r p concept is worth noting in some older tennessee mines the haulage ways along which ore is transported are somewhat narrow as haul distance increased it became desirable to use larger haulage trucks but such trucks could not be accommodated on the narrow haulage ways to solve the problem savage zinc in their

gordonville mine hooked side dump powered truck trailers built by gulf transport of australia together into a mini truck train thus increasing the payload per trip and optimizing production for load haul conditions if a conveyor system of haulage is used for the main distance haulage then the preceding advice for mine layout still applies to the general layout of the mine except conveyance starts from the decline of the mine entrance where the main conveyor carries mine production to the surface in these cases a hard rock mine usually has a semimoveable crusher or breaker underground before the feeder to the conveyor system the system of haulage either lhd unit or fel truck hauls broken rock from the faces to the central receiving point at the crusher the crusher then feeds the conveyor the crusher must be moved periodically about 6 to 12 months depending on how fast mine production moves the faces away from the central hauling point at one time rail haulage was the principal method of gathering ore from the faces to the production shafts in the old lead belt of missouri more than 556 km 300 mi of interconnected railroad was used to bring ore into two main shafts from what was originally about 15 mines today in the united states very few r p metal mines use rail haulage but many coal mines still do and in other countries many metal mines still do as well single pass or multiple pass extraction two approaches exist to mining ore body or valuable rock 1 taking the entire ore body thickness in a single pass or slice or 2 removing the ore body by multiple passes or slices the choice of approach of course is related to the overall thickness of the ore body to be extracted and to concerns for safety and efficiency normally in mining bedded deposits for aggregate the total thickness of the desired horizon is known and the decision concerning slice thickness can be made in advance however in metal ore deposits particularly in mississippi valley or collapsed breccia type deposits most of the time the thickness of the total mining horizon is not known except where the formation has been penetrated by diamond drill holes and identified although a few meters away from the hole the formation may very well be different for both aggregate producer and metalore producer the best approach is first to mine what is thought to be the top slice through the ore body the thickness of this first mining step depends on the available equipment and on the height of ground that can be mined and maintained safely and efficiently the advantage of first mining what is thought to be the top slice is that whatever back and rib pillar scaling and reinforcement is required can be reached easily and safely probably at distances not more than 6 1 m 20 ft however some aggregate producers mine 12 2 m 40 ft or more in one pass by using high mass jumbo and extendable boom roofscaling equipment r p rock breaking for single pass stopi

ng in most hard rock r p mines extraction is done by drilling and blasting the face the initial pass of drilling and blasting is usually done by drilling either burn cut or v cut drill patterns however only about 40 of the rock should be broken with drilled swing patterns rounds breaking to only one free face the other 60 of the rock should be broken by slabbing that is by drilling holes parallel to the second free face as it is exposed figure 13 1 5 which minimizes the cost per ton of rock broken and maximizes productivity particularly in the r p mining of aggregates and metals extraction is often done by mechanical means in many trona and potash mines and some salt mines all extraction is done mechanically r p rock breaking for multiple pass stoping after the first pass is completed in a metal mine for a given stoping area the back and floor should be sludge sampled with either pneumatic or all hydraulic drills some operations do this with a stope rock breaking jumbo and some with an allhydraulic jumbo designed for this role the purpose of this sludge sampling prospecting is to determine what ore remaining in the back and floor needs to be removed by additional slices if ore is found in the back and floor then the ore in the back should be removed first after it is removed and additional back prospecting reveals no more ore the back should again be made secure and safe and ore in the floor can be taken methods of mining ore in the back vary depending on the thickness of the ore yet to be mined and the height of the original stope if the original stope height was no more than 7 6 m 25 ft and the back slice thickness was no more than 2 m 7 ft most extendable boom jumbos can reach high enough to drill the brow with breast horizontal drilling drilling horizontal holes increases the chance of leaving a smooth back which requires less maintenance than when holes break to the free face of the brow this is especially true for smooth wall blasting or flat seam deposits a jumbo feed should not be tilted up for drilling near vertical holes and for drilling uppers to break a brow particularly in bedded deposits that exist in so many r p mines if ore thickness in the back is greater than the conditions cited previously or if room height is already at the maximum that can be safely maintained then the back slice should be mined by first cutting a slot in the back at the edge of the entrance to the stope figure 13 1 6 this slot should be between the pillars and reach the height of the next slice or the top of the ore body care must be taken not to damage the rock that will form the top of the new pillars smooth wall blasting can be used to advantage in this area a mine dozer usually a small dozer of d 4 or d 6 size begins by pushing rock up and making a roadway for the jumbo to travel on the top of the rock pile if a dozer is unavailable an fel can also do a reasonable job of building the roadway the ju

mbo then drills breast horizontal holes in the brow and follows the ore zone wherever it goes throughout the stoped area even if it goes into the solid beyond the original stoped area this is not a problem except that a loader now must load the ore as it is broken about 75 of the ore must remain in the stope rock pile until back mining is complete which can be a disadvantage if the mine must produce immediately when the top of the ore is finally reached the back should be made safe since miners are still working close to the back this type of mining has been practiced in the tennessee zinc and viburnum trend lead zinc districts it is not uncommon for several passes of ore to be mined from broken rock piles in these cases either ore must be loaded from the bottom edges of the rock piles to make room for new broken rock or a loader must go up on the rock pile and load out the excess rock if after the first slice is taken through the stope it is then discovered that a thick continuous ore zone lies above this first slice a zone of perhaps 15 to 20 m 50 to 65 ft or more then an entirely different approach is required it may be preferable to create a development ramp to what is now the top of the ore mine the top slice from this new ramp and then put through a slot raise from the bottom to the top level that can be slabbed out to make room for long hole drilling and blasting the ore body down to the level below again the pillars should be presplit or smooth wall blasted to protect them if the eventual height of a pillar can be anticipated the size of the pillar at its base should be determined taking this additional height into account if it is not taken into account and safety factors in the design are insufficient then the pillars may need reinforcement with grouted rebar bolts to give them the required strength and stiffness when all back ore is removed from the stope and the final back is made completely secure bottom ore can be removed from the floor this is best done by first cutting a ditch or short decline in the floor at the entrance to the stope to the depth of the desired bluff or bench bluffs can be carried very thick and are limited only by the height required for the loading equipment to work safely it is common practice to carry bluffs up to 9 m 30 ft in the lead mines of the viburnum trend area using 7 11 t 8 12 st loaders beyond this height the safety of the loader operator may become an issue bluffs at least 4 m 13 ft thick are usually drilled and blasted with downholes by small surface quarry type drills bluffs of less than this thickness can be drilled out with face jumbos drilling breasting or horizontal holes sometimes called lifters and splitters these procedures can be repeated over and over until the bottom of the ore is reached when it is known beforehand that bluff mining will be done for the initial pass the pillar width must be left large enough to accommodate considerable

height should it be needed if additional ore is not found then the pillars can be slabbed down to smaller size however again the precaution in both removal of back ore and taking up of the bottom the pillars most be protected with presplitting or smooth wall blasting pillar design is discussed in more detail in the following section the point here is that unless the original mine plan specifically allows for taking down the back and taking up the floor or specifies a very large safety factor in pillar design the width to height ratio required for safe operation will be exceeded by multiple pass mining it cannot be overemphasized that everything must be done to protect pillar integrity during the first of what may become multiple passes unless caution during presplitting or smooth wall blasting is observed blasting fractures may extend into the pillar as far as 2 m 6 6 ft or more as more slices are removed these fractures begin to open reducing pillar size and subjecting the pillar to failure methods of pillar extraction in hard rock mines the mining engineer should anticipate some percentage of pillar removal beyond what is cited in the original mine plan and stope layouts if future pillar extraction is planned whether by partial slabbing removal of a few high grade pillars or complete removal incorporating some system of backfilling then what is left after the first pass greatly influences what can be done in the future too often too much is extracted on the first pass so that the entire area is weakened and must be reinforced just to keep the mine open much less enabling the selective robbing of pillars the first and most important thing to do is to install a complete network of convergence stations throughout the affected area experience shows that regular monitoring of such a network over time can reveal what rate of back floor convergence is acceptable and what rate will lead to massive failure convergence in some mines can be as high as 1 25 mm per month without the back breaking up and failing parker 1973 convergence in other mines such as those of the old lead belt of missouri and the viburnum trend can be as low as 0 1 mm per month and still cause problems however the following are good general guidelines convergence of 0 0254 mm 0 001 in per month is not significant convergence of 0 0762 mm 0 003 in per month indicates a serious problem but is controllable with immediate action convergence of 0 1778 mm 0 007 in per month indicates that acceleration is getting out of control and the area may be lost several methods exist for pillar removal of r p stoping slab some ore off of each pillar containing the high grade portion of the ore during retreat from the area completely remove a few of the most valuable pillars but leave enough pillars untouched to support the back in narrow stopes completely remove all pillars in a controlled retreat use massive backfill methods to