Work Practices to Manage Bump Prone Ground 

Floyd Varley, Acting Branch Chief
 

Jeff Whyatt, Mining Engineer
 

NIOSH - Spokane Research Laboratory
 

Spokane, WA
 

The findings and conclusions in this report are those of the authors and do not necessarily represent the 

views of the National Institute for Occupational Safety and Health. 

ABSTRACT 

In deep and highly-stressed coal mining, advanced design 

practices are used to minimize the potential for damaging events 

to occur. Despite these design efforts, the hazard of coal mine 

bumps can not be completely eliminated. This paper will review 

current and historic work practices used internationally to 

minimize the hazard to miners from mining in conditions which 

could produce a bump. These site specific work practices have a 

common base in monitoring for conditions which could result in a 

bump and application of controls to prevent the occurrence or 

minimize the severity of an event where miners are working. 

Monitoring practices range from purely observational to semi 

quantitative. Control methods attempt to initiate events in a 

controlled manner, absorb the energy of a potential event or 

reduce the potential for or magnitude of an event by altering the 

stress conditions of the mine workings. Building from the 

experiences of other coal fields, an empirical framework to 

manage the potential hazard of coal mine bumps can be 

developed to enhance the level of safety provided by prudent 

mine design. 

INTRODUCTION 

Mining-induced seismicity, when it results in damaging events 

to an operating mine, is also referred to as bumps, bounces, 

outbursts and rockbursts. These terms all describe the sudden, 

violent failure of the coal seam or adjacent strata that has resulted 

in loss of life and property in deep coal mines. Mine design 

practices strive to minimize the risk of injury to persons and 

damage to equipment and mine infrastructure from mining 

induced seismicity. For instance, many Utah mines have replaced 

bump prone pillars with yielding pillars and one uses inter-panel 

barrier pillars. Despite theses efforts, the hazard of mine bumps is 

not completely removed. Local variations in geologic structures 

and properties, mining practices and coal gas content can increase 

the risk of a bump event in an otherwise conservative mine 

design. Controlling this hazard requires a means to identify when 

and where mining conditions have changed and active measures 

are needed to reduce excess hazards. 

While a number of control systems have been proposed and 

applied, none have achieved sufficient success in deep western 

US coal mines to be considered a viable standard practice. For 

instance, Agapito and Goodrich (1) maintain that “experience 

with de-stressing in the Utah mines has been limited to a small 

scale and results have been inconclusive.” Moreover, they point 

out that the “techniques are unpopular because they are expensive 

and the bump triggering potential when drilling into highly 

stressed areas, has caused some injuries in the past.” Iannacchione 

and Zelanko (2) found five reportable bumps that occurred during 

destressing operations, the first in the 1950’s. 

One of the most systematic applications of hazard reduction 

methods in western longwall coal mining was conducted by one 

of the authors at Mid-Continent’s advancing longwall at the 

Dutch Creek No. 1 mine in Colorado (3). While this program was 

successful technically, it was not compatible with the production 

expectations of modern longwall mines. The significant deficit of 

this program was the reliance on a single, labor intensive method 

to identify and control potential bump hazards and the need for 

universal application of the system due to the mine’s design and 

geologic conditions. 

The objective of this paper is to identify means and methods 

which may be adapted to a targeted approach of reducing residual 

bump hazard that has been primarily controlled by mine design. 

While bump hazard management systems must be designed for 

site specific conditions, the experiences gained in other mines can 

assist in the development of these programs. 

MONITORING METHODS 

Key to developing a bump hazard management program is the 

development of means of assessing changes in mining conditions 

that indicate a departure from either the foundation of the mine 

design or the expected response of the ground to mining. 

Monitoring systems can range from purely observational to active 

probing of the coal and adjacent strata. Following are techniques 

which have successfully identified high hazard areas or have 

otherwise revealed information that has been associated 

retrospectively with damaging events. Common to the application 

of these methods is the need to have an understanding of the 

expected mode of failure during a bump event if not the 

mechanism behind the failure. Many mines may have multiple 

potential modes and mechanisms of failure. Mine operators are 

encouraged to explore not only these methods but, armed with an 

understanding of the intent of the method, develop new strategies 

that may be better suited to their mine’s conditions. 
 

 

 

 

 

 

 

Face Observations 

One of the most basic means of assessing potential local 

hazards is consistent systematic observations of face conditions 

for a change from nominal experience. While informal 

observations may raise awareness there is a potential for 

rationalization and inexperience to minimize perceived risks. The 

process of systematically recording key observations allows 

correlation of and trending of observations to minor events and 

provides a basis to pursue more active monitoring methods. 

Critical observations should be based on the expected strata 

behaviors identified in the mine design process. Observed 

conditions which have been associated with abnormal loading of 

the face or pillars that should be considered are: 

3⁄4 Lagging of the cave line: Any delay in caving or increase in 

distance from the pillar line to cave has the potential to 

change the behavior of the adjacent face or pillars 

3⁄4 Characteristics of the cave: A cave that changes from small 

debris to larger blockier sizes may indicate a localized 

increase in immediate roof strength that can impact load 

transfer. A change in cave material size can also reduce 

bulking of the cave and increase the span from the face to 

where the gob can support overlying strata. 

3⁄4 Variation in distance between yielding of chain pillars and/ 

or floor heave and the face: A mining area should develop a 

relatively predictable relationship between face position and 

pillar yield as reflected by rib spalling or closure of the 

mine opening such as floor heave. Changes from this norm 

require investigation of the reason for the variation. In 

particular a sudden reduction in magnitude of yield 

behavior may indicate stored energy. 

3⁄4 Change in shield loading cycles: Sudden changes in shield 

loading magnitude or location of cyclic loading may 

indicate changes in the overburden’s behavior. Absent an 

explanation, a reduction in shield loading should be 

considered stored energy. 

3⁄4 Changes in spalling behavior of the face or rib: A decrease 

in spalling of the face or pillar ribs can, absent a structural 

explanation, indicate a localized area of stronger or more 

brittle coal prone to sudden failure. 

3⁄4 The appearance of red dust and/or slickensides at the coal/ 

roof or floor interface: A bump resulting in coal ejected 

into the mine opening is the expression of a loss of cohesion 

within portions of the seam or between the seam and the 

roof or floor. A highly stressed zone will show signs of 

minor failures in the form of red dust or polished rock 

surfaces as the coal resists extrusion in addition to more 

obvious expressions of these features after a major bump. 

Geologic Investigations 

During the design process a stratagraphic model of the geology 

above the coal seam is developed. This model represents nominal 

conditions that the mining area will influence as extraction 

progresses. While the process of developing this model is 

generally not` precise it does make assumptions about the 

thickness and properties of the strata that can influence the 

performance of a mine design. An understanding of the the 

sensitivity of the mine design to variations from this model will 

enable an operator to identify what observations are necessary to 

assure that the mine design remains valid. In the near seam 

geology damaging events have been associated with: 

3⁄4 Sandstone channels (4) 

3⁄4 Faults (5; 6) 

3⁄4 Seam rolls (2) 

3⁄4 Changes in coal composition (7) 

Collecting these observations typically requires the services of 

a geologist with assistance from mine operations personnel. 

Stewart (6) describes the use of information collected during roof 

bolting to develop roof hazard maps that would also provide 

information for this purpose. Current NIOSH research is 

investigating remote monitoring methods for identifying gross 

changes beyond the bolting zone and investigation of other 

potential geologic hazard markers. 

Seismic Monitoring 

The use of seismic monitoring systems has been applied to 

coal mining in a number of forms, from local micro seismic to 

regional systems, and to varying degrees of success. Currently, 

systems rely on trained specialists to process waveform data and 

provide accurate locations and magnitudes of events. Some 

system providers offer data processing services to reduce the 

burden. While not a predictive tool, once a pattern of normal 

seismic behavior is established these systems can be used to 

identify changes from that norm that would warrant additional 

monitoring of the hazard potential of mining areas (8). Seismic 

monitoring has also been a resource for post event analysis to 

provide input into future mine designs. The use of mining induced 

seismicity as an input to tomographic analysis of hazard 

potentials surrounding the mining face is an area of current 

applied and basic research (9). 

Holub (10) describes the use of seismic monitoring to evaluate 

hazard potential of mining areas in deep Czech coal mines and the 

development of a quantifiable threshold of seismic energy, above 

which proactive control measures are mandated. The 

establishment of thresholds, whether empirically or theoretically 

derived, is the foundation of a formal hazard management system. 

Probe drilling 

A method of identifying highly stressed zones in the coal face 

and pillars that has seen success in softer coals is probe drilling 

using small diameter (<50mm) bores. The concept behind this 

method is to develop a micro excavation in the coal seam and 

observe the seam’s reaction to the excavation. Yielded coal 

normally surrounds deep mine openings. The peak ground 

pressure is carried in a stress abutment well behind the rib 

surface. Since yielding increases coal permeability, the yield zone 

is degassed as well as de-stressed forming a gas pressure 

abutment. For example, Varley (3) discusses using drill holes to 

locate gas and stress abutments (figure 1). Bräuner (7) provides a 

more extensive discussion, based on experience with softer Ruhr 

coal of Germany. 

Under purely load driven conditions, the coal will close around 

the auger rod producing more cuttings per unit of advance than 

would be produced by drilling an unstressed block. The volume 

of cuttings produced will increase substantially when a highly 

stressed abutment is probed. If the gas content of the coal is 

sufficient, a precursor indicator of gas discharge from the hole 

can be observed as drilling offers a means for pressurized gas 

contained in the coal pores to escape. A third indicator of location 

of the stress abutment is the production of audible noise in the 

form of micro-seismic events. The distance from the face or rib to 
 

 

 

 

 

 

 

the high stress zone determines the relative hazard. An empirical 

relationship has been established (3; 7) to identify the minimum 

distance to the highly stressed zone: 

S=XM+A (1) 

Where S is the critical distance to the high stress zone, X is a 

site specific variable typically between 2 for pillars and 3 for 

faces, M is the mining height, and A is the expected advance in 

the direction of the bore before retesting. In application of this 

method, the drilling observations that would reflect the location of 

the high stressed zone are site specific and should be developed 

by systematic testing of stable and at risk areas. 

Though probe drilling has been successfully used in a number 

of mines to define bump hazards or evaluate the post development 

yield of chain pillars (11) there is no implicit guarantee of 

universally applicability. Pillar and face loading which is not 

progressive but relatively rapid and cyclic may be absent during 

probe drilling but appear between test intervals. Bump 

mechanisms originating outside the seam, often referred to as 

shock bumps (12; 5), may have no predictive expression within 

the seam prior to the event. 

Figure 1. Stress and gas abutments located by drilling (3) 

HAZARD CONTROL METHODS 

Once an area with an increased bump hazard has been 

identified through a well designed monitoring program, a suitable 

reaction to identified hazards must be employed to protect 

workers and property. Internationally a variety of control 

measures have been deployed to respond to bump hazards with 

varying degrees of success and economic impact. Locations 

where intact, highly stressed coal with gas at high pressure is at or 

just behind the face can generate an outburst of coal and gas. 

Control methods are designed to initiate yield in such locations 

or, insulate the miner from the hazard. 

A number of methods have been developed for pursuing this 

goal. These include drilling, volley firing, hydro-fracturing, and 

water infusion (before or during mining). As with monitoring 

strategies, application of bump control methods requires a basic 

understanding of the potential modes of failure and mechanisms 

driving the hazard(s). Mine operators can build from these 

experiences to develop their own site specific strategies to reduce 

the risk of a damaging bump event. 

In addition to methods to reduce or eliminate the likelihood a 

damaging event will occur, supplemental protective measures can 

be utilized to provide a greater amount of protection should the 

control measures not completely remove the hazard. These 

protective measures include efforts to protect miners from ejected 

debris and operational adjustments within a cutting cycle. Caution 

must be exercised to not rely solely on these protective measures 

to ensure the safety of miners. 

PROTECTIVE MEASURES 

Protection from Ejected Debris 

The use of personal protective measures to prevent mine 

workers from being struck by ejected debris during a bump has 

become a standard component of U.S. ground control plans where 

there is a risk of bumps. These measures vary by location but in 

general consist of administrative controls, protective equipment 

worn by miners, energy absorbing barriers on the longwall face 

and barriers wrapping gate pillars. 

3⁄4 Administrative controls focus on limiting the number of 

persons in the face area during high potential phases of 

mining such as cutting the gates. While this has been 

recognized as reducing the exposures it is prone to human 

error with tragic consequences (13). 

3⁄4 The use of personnel protective equipment such as helmets 

with face shields, protective vests, knee and shin guards 

have become an accepted part of deep longwall mining. 

While these protections have no doubt reduced the number 

of injuries from smaller events, they have limited ability to 

afford protection from much larger events. 

3⁄4 Barriers to absorb the energy of ejected material on the 

longwall face include shields erected on the shearing 

machine to deflect material ejected from the face above the 

shearing machine frame and guards, often constructed of 

belting and/or steel mesh, connected from the shield 
 

 

 

 

 

 

 

 

canopies to the face conveyor via chains. In development 

headings, starting a cut with the continuous miner drum 

high on the face and cutting down can take advantage of the 

mass of the head for protection from ejected coal as the face 

is relieved. These barriers offer protection from most debris 

ejected during small events but are likely incapable of 

withstanding the energies of the largest bump events. (13). 

3⁄4 The intent of wrapping gate road pillars is to contain the 

debris ejected from these pillars during the sudden failure of 

a bump event. These measures currently used in U.S. coal 

mines involve bolting screening material to the coal rib with 

vertical supports between the roof and floor on close 

spacing (~0.6m centers) to contain the pillar rib mesh 

should the bolts fail. The vertical supports used vary from 

timbers to yieldable steel posts. Should the bump include 

significant floor heave or large lateral movement of the 

pillar there is significant risk the vertical supports could be 

dislodged. While containment of pillars is widely used in 

rock burst prone mines internationally, there is a distinct 

difference in approach. Pillar containment systems are 

designed to withstand the estimated energy and movement 

of a large event. These designed systems provide a means to 

yield in all directions while developing an ultimate strength 

capable of withstanding a severe event (14) without failure. 

The most significant difference between these systems and 

those currently used in U.S. mines is the use of wire rope 

lacing, designed to allow the ribs to expand without rupture 

as reinforcement to rib mesh. While some mines have 

experimented with wrapping wire rope around a pillar, it is 

not clear a similar level of engineering design was used. 

Cutting Cycle Adjustments 

The execution of any design can influence the potential to 

bump. Adjustments within the mining cycle can avoid sudden 

loading of mine structures through control excavation speed, 

depth and timing. 

Room and pillar mining offers the greatest potential for 

variations in cutting sequence to result in a bump. Turning a 

crosscut and thus creating a pillar one or more cuts behind the 

face can reduce the potential for bumps during the crosscut. 

During retreat operations, attention should be paid to sustaining a 

cave line, avoiding steps or overly large remnant pillars. The use 

of remote control of shuttle car conveyors, which allow the 

operator to be outby the machine during loading, in addition to 

remote control continuous miners can remove all miners from 

proximity to the face during cutting. 

Longwall mining offers little flexibility and provides some 

assurance that mining sequence and geometry will produce a 

sustainable cave line. Within longwall mining the cutting cycle 

can be altered to minimize personnel exposures and attempt to 

relieve energies in smaller increments. 

3⁄4 The web and speed of the cut can be reduced with some 

mines taking as little as 0.5 m per pass in bi-directional 

cuts. 

3⁄4 The face can be cut in one direction only, allowing more 

time between passes for stresses to equilibrate. 

3⁄4 Only a portion of the seam can be cut on a pass, typically 

the center in the first pass toward the tailgate. The pass is 

completed by taking the roof and floor coal on the return 

pass to the head gate. The concept, attributed to the 

Sunnyside mine in Utah, is the center cut pass can be made 

without operators near the shearing machine and the large 

kerf created by the center cut reduces the hazard for the 

return pass where operator proximity and attention to the 

cutting drums is greater. 

3⁄4 To reduce the potential for a lagging cave to contribute to 

the hazard, some mines use a cutting sequence where one or 

both of the gates are snaked in advance of the face line. 

3⁄4 Any combination of the above as conditions warrant. 

HAZARD REDUCTION MEASURES 

Water Infusion/Fracturing 

The distinction between hydro-fracturing and water infusion is 

somewhat confused in reported trials. In some cases, both terms 

have been applied to the same operation. Hydro-fracturing is 

distinguished by a few large fractures created over a relatively 

short time span. Water infusion is a more general softening 

through an increase in pore pressure, moisture content and 

generalized fracturing. The increase in moisture content affects 

coal physical properties. Bräuner (7) comments on the distinction 

as follows: 

“If the situation is critical already, high pressures are 

applied for hydraulic fracturing. If the stresses allow [drilling 

of] deep holes, the fluid pressure is kept low and acts for a 

longer time. This is to make the coal break successively 

instead of violently.” 

Bräuner also emphasizes that de-stressing operations should 

begin at the sides of the stressed zone and progress into more 

highly stressed material rather than start in the middle. Finally, he 

reports that burst control methods have beep applied in the Ruhr 

Coalfield (Germany) since the 1960’s and found to be highly 

effective. Bräuner also reports that increasing the moisture 

content of Ruhr coal from 1 to 5% resulted in a 60-70% reduction 

in uni-axial compressive strength and a 40-70% reduction in 

elastic modulus. In addition, Bräuner observed a transition from 

violent (dry) to ductile (saturated) failure mode. Borehole-bursts, 

in the laboratory or field, are mechanically similarly to rib bursts. 

In addition, Bräuner reports on laboratory borehole-burst tests 

that found: 

1. “Borehole-bursts occurred in almost all types of hard coal 

2. having more than 10% volatile matter. 

3. No borehole-bursts were observed when such coal had 

been 

4. saturated with water.” 

Yuguang (15) discusses similar practices in China. Bump 

potential is evaluated using drill cutting volume, water content 

and micro-seismic monitoring. Two protective measures are 

described. First, the extraction of a protective seam, preferably an 

overlying seam with low bump potential. Second, water infusion, 

both before mining and with face advance. In both cases, the 

water reduces seam strength and elasticity which moves the 

pressure abutment deeper into the coal rib. 

Water infusion has also been applied to dust and methane 

control. For instance, Jackson and Merritts (16) describe water 

infusion of coal pillars for dust control at the Kenilworth Mine in 

Utah. Infusion reduces dust during mining and proved useful to 

extinguish a burning pillar, apparently ignited by frictional 

heating along a shear plane during a pillar bump. 
 

 

 

 

 

 

 

 

 

 

Varley (3) describes favorable results obtained by the infusion 

of 28 MPa water through 30m deep holes drilled from the tailgate 

on 30m centers (figure 2). Application times range from 3 to 20 

days, ceasing when flows rise to 400lph. This method was 

frustrated by ground conditions, particularly floor heave near the 

face of the advancing longwall, limiting access to the panel ahead 

of the face. Drilling of large diameter (100mm) holes was the 

least successful method, leaving intact sections of face capable of 

outburst. 

Figure 2. Water infusion holes in the rib of an advancing 

longwall (17). 

Haramy et al. (17) echo Varley’s conclusions, and discuss 

many of the same trials. However, they refer to Varley’s water 

infusion as hydraulic fracturing of the seam and caution that 

“fracturing a very large and highly stressed area causes loads to 

be redistributed and may create bursts conditions elsewhere in the 

mine.” 

DeMarco et al. (18) discusses face de-stressing near the 

tailgate of the 9th East Panel at the Castle Gate No. 3 Mine with 

“face and tailgate high-pressure hydro-fracture holes.” This 

method was discontinued as the mine progressed from 600 m 

(2000 ft) of overburden to areas with shallower overburden – that 

did not include the Castlegate Sandstone. That is, the Castlegate 

was eroded over some mining areas. However, “the complicated 

nature of panel loading” led DeMarco et al. to state that any 

judgment of de-stressing program success was “speculation.” 

However, bumps, including a severe bump that halted mining for 

several days, did occur as mining re-entered deep overburden 

without de-stressing. The mine was idled in April 1989 after 

occurrence of several additional bump events (19). 

Routine gate road pillar infusion has been used at the Elk 

Creek Mine, operating under 760m of overburden in Colorado’s 

North Fork Valley. Randy Litwiller, Elk Creek mine manager, 

describes drilling infusion holes in gate road pillars 100 meters or 

more ahead of the longwall face to a depth less than half the 

width of the pillar. (20) Once a pillar begins to yield the infusion 

holes are pressurized using the face support hydraulic supply. The 

infusion increases the yielding of the pillar, reducing the 

occurrence of damaging bumps. The location and direction of 

bumps are controlled by infusing only one side of the pillar. 

Cave Management 

Controlled yielding of strata around deep longwall panels can 

also be important. Haramy et al. (21) review depths and geologic 

characteristics of Utah and Colorado mines operating under 

strong roofs. Haramy et al. show that a stronger roof correlated 

with more periodic loading of shields and suggest the need for 

induced caving by hydraulic fracturing, large-hole blasting, and 

water infusion. 

Dubinski and Konopko (22) describe bump control practices in 

deep Polish mines including the weakening of strong strata above, 

but not adjacent to, the coal seam by “torpedo” blasting. In this 

method large charges of explosives, up to 150 kg, are detonated in 

the target strata above longwall panels and development headings 

(30 kg). Holub (2007) describes similar practices in deep Czech 

coal mines. 

Dvorsky and Konicek (23) describe preconditioning of 

massive sandstone and conglomerate strata above a “hard coal” 

longwall panel in the Czech Republic. Blast holes are positioned 

to create a break line above a barrier pillar separating the panel 

from a previously mined pillar to the south, and to break up strata 

above the longwall to ease gob formation. A tectonic fault north 

of the east-west trending panel suggests that these blasts would 

relieve high north-south stresses as well. 

Dubinski and Konopko (22) further describe the use of 

directional hydraulic fracturing to both establish a break line for 

the cave and to alter a massive strata into a layered member better 

able to cave progressively rather than cyclically. The authors also 

describe the use of water infusion to soften sandstone members in 

the roof by up to 60% when clay is present. 

Hayes (24) and Mills et al. (25) discuss hydro-fracturing of a 

30m thick conglomerate roof at Moonee Colliery, Australia. The 

conglomerate is prone to sudden failure over wide areas, creating 

windblast hazards. The panel design intends the conglomerate 

remain intact, bridging the 100m panel by resting on 35m wide 

chain pillars. Caving is induced by hydraulic fracturing via holes 

drilled from the shields, and pressurized after the longwall has 

advanced beyond the holes. These fractures initiate smaller 

caving events, limiting the length of immediate roof behind the 

shields. 

Water infusion of strong strata might also have promise since 

the strength of some sandstone has been shown to depend on 

moisture. McCarter and Wilson (26) report the softening and 

weakening of a variety of sandstones with increasing moisture 

content. Saturation effects on uni-axial strength effects vary from 

none to a roughly 30% reduction, depending on sample 

characteristics. They also describe how vibration characteristics 

can be used to assess moisture content. 

Shimada et al. (27) describe hydraulic fracturing of strong roof 

over a longwall panel in the Miike Colliery, Japan to control air 

blasts from sudden caves that have injured miners. The panel is 

off shore, 500 m below sea level, where barrier pillars between 

panels were introduced to control subsidence. Shimada et al. also 

report on significant softening and weakening of coal and coal 

measure rock with introduction of wetting (water infusion), 

including reductions of strength in Miike sandstone of 30 – 50% 

(Figure 3). 

Vasarhelyi (28) analyzes moisture versus strength relationships 

for 35 British sandstones from 21 locations and found a best-fit 

equation of USCsaturated = 0.759 UCSdry with an R2

 of 0.906. Chen 

and Hu (29) report substantially larger strength reductions for 

weak sandstones in Taiwan. These two studies identify the 
 

 

 

potential to utilize water infusion to reduce the strength of strong 

bridging strata where it presents a local hazard to a portion of a 

mine. The other significant result is the potential for localized 

natural changes in moisture content of strata, such as may be 

found near prominent joints or faults, to alter the caving behavior 

of a normally strong member. 

Figure 3. Change of Miike sandstone properties (27) 

Ray et al. (30) reports softening of Indian coal measure rock, 

particularly sandstones, with water infusion, in the laboratory. 

Fine-grained sandstones with a clay content of 13.6% showed the 

least response after 3 days of saturation, with a strength reduction 

of 10% over a dry sample. Increasing grain size and/or clay 

content showed an increased impact (50% reduction for mediumgrained sandstone and 60% reduction for fine-grained sandstone 

with 27.4% clay). A reduction in strength of nearly 80% was 

attained for coarse-grained sandstone with 23.1% clay content. 

Further reduction in strength was noted in some sandstones, as the 

saturation time was extended from 3 to 7 days. Based on these 

results, Ray et al. argue that water infusion of sandstones could be 

used to improve caving over longwall panels. 

The impact of water infusion might be heightened by addition 

of small concentrations of various chemicals that affect the zeta 

potential (the electric potential across the solid-liquid interface) 

which may further decrease tensile strength (31). This 

phenomenon has been studied mainly for drilling applications, but 

may also apply to natural and induced changes in the caving 

characteristics of massive strata. The phenomenon may also 

explain failure of apparently strong bridging strata that is exposed 

to ground water. 

De-stress Drilling 

De-stress drilling is simply piercing highly-stressed coal with 

drill holes that, ideally, induce yielding of the surrounding coal, 

moving highly-stressed zones away from the mining face. The 

process must remove enough material to induce yield in a highly 

stressed zone that is then mined and the process repeated. 

Verification that the hazard is removed requires subsequent probe 

drilling. Experience in Europe and elsewhere indicates holes 100 

mm and smaller in diameter are unlikely to initiate dangerous 

bumps. 

Iannacchione (2) reports on the use of de-stress drilling using 

very large (61 cm) holes at the Gary mine in West Virginia. This 

practice was discontinued due to initiation of large damaging 

bumps. Iannacchione also reports the Olga mine, also in West 

Virginia, found success in using 100 mm holes to relieve stress in 

coal ribs. Olga used the 100 mm holes to probe for pressure 

abutments and, when encountered de-stress the zone through 

removal of coal by holding the drill depth at the abutment. 

Varley (3) reports the drilling of large diameter (100 mm) 

holes was the least successful method for control of the hazard of 

bumps on a longwall face, leaving intact sections of face capable 

of outburst. Mid Continent Resources did use large diameter 

drilling to de-stress development faces as an alternative to volley 

firing but experienced several injuries when drilling did not fully 

remove the hazard (32). 

Osamu & Katsuhiko (33) report using numerous small 

diameter (50 mm) holes to gradually yield a highly stressed zone 

in a longwall panel gate at the Miike mine in Japan. Probe drilling 

on progressively closer spacing was continued until results 

indicated the zone of interest had been relieved. While labor 

intensive, this method may have merit for mines with infrequent 

exposures to bump hazards. 

De-stress Blasting 

De-stress blasting or volley firing is used to trigger failure at a 

known and safe time. This method uses smaller holes and small 

charges that shake and fracture the coal while leaving it in place 

(figure 4). A number of charges are fired simultaneously (a 

volley), increasing the likelihood that a dynamic failure will be 

initiated in a relatively controlled setting. 

Figure 4. Volley firing scheme for an advancing longwall (17). 

Varley (3) describes stress relief efforts at the Dutch Creek #1 

Mine (Colorado) advancing longwall operation. The efforts focus 

on areas identified as posing an outburst threat, based on response 

to drilling. Varley finds the most effective means of distressing a 

mining face is drilling and blasting of the face (volley firing). The 

downside of this approach is the necessary halting of production 

and evacuating the face for a shot. The method does protect 

miners from induced outbursts. Success depends on two 

considerations. First, the blast must be designed to fracture in 

place, rather than pull the coal using relatively small charges. 

Second, care must be taken to avoid mining beyond the destressed zone. 

Agapito et al. (34) discuss burst control methods in the Book 

Cliff and Wasatch Front coal fields in Utah. They mention destressing as a method for minimizing stresses at the face induced 

by mining of adjacent panels. However, they assert that this 

method has “not yet demonstrated satisfactory results under 

continuous operating conditions” and “can be hazardous if not 
 

 

 

 

properly implemented.” Moreover, “use of de-stressing in the 

Price area mines has been limited to the Castlegate No. 3 Mine 

where it was applied unsuccessfully.” Agapito et al. report that “it 

was deemed hazardous by miners who tried it, due to bursts 

induced by drilling.” 

IMPACT ON OPERATIONS 

The implementation of bump hazard management practices 

will have an impact on mine operational budgets and staffing. A 

common element in programs described by the various authors is 

the need for an organizational structure, often including a site 

champion, to coordinate the design expectations, monitor results 

and control activities of the effort to manage bump hazards. Little 

information is available on the cost implications of bump 

management systems. Anecdotal evidence suggests that US mines 

which have implemented universal systems, to every face or 

panel, have not been viable economically over the long term. 

Information on reportable incidents and production available from 

MSHA (32) indicate that implementation of universal 

management systems did not impact the production rates of mines 

(Dutch Creek #1, Castlegate #3) as compared to periods of 

uncontrolled bumps. The production interruptions, costs and 

injuries avoided by implementation of universal bump 

management systems are not measurable. Anecdotal evidence 

suggests application of control methods to a limited high hazard 

area, such as at the Elk Creek Mine, has the potential for success 

in U.S. mines. 

SUMMARY 

Prudent management of mining in bump prone ground requires 

a systematic approach including an understanding of the 

mechanisms, monitoring for conditions indicating an increase in 

potential for a bump to occur and active means to control the 

hazard. Control efforts should be directed by monitoring to reduce 

the hazard with protective measures and hazard reduction 

measures as described. Internationally, a wide array of means of 

identifying and assessing hazard potentials and implementation of 

active controls has been developed. Though similar in approach, 

these systems are essentially site specific and can only be 

transported to another coal district or mine on careful examination 

of the relative properties or mechanisms the system evaluates or 

attempts to control. Control methods must be carefully selected to 

ensure a greater hazard is not created. Despite the site specific 

nature of these systems they do offer mines a basis for developing 

a method to improve the safety and perhaps productivity of an 

operation at risk of damaging bumps. Lessons learned include the 

need to be systematic in the application of a control program with 

clearly defined action levels. Controls measures, when indicated, 

should be initiated outby the suspected hazard area and seek to 

move the hazard inby. Finally, active control measures require 

verification that the bump hazard has been removed through 

monitoring or assurance the potential energy has been relieved. 

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