ASSESSING AND MONITORING OPEN PIT MINE HIGHWALLS 

Jami M. Girard1, P.E., C.M.S.P. 

1

Mining Engineer, National Institute for Occupational Safety & Health, Spokane Research 

Laboratory, 315 E. Montgomery Ave., Spokane, WA 99207

INTRODUCTION 

 Slope stability accidents are one of the 

leading causes of fatalities at U.S. surface 

mining operations. The Spokane Research 

Laboratory of the National Institute for 

Occupational Safety and Health (NIOSH) is 

currently conducting research to reduce the 

fatalities associated with slope failures and other 

unexpected failures of ground. The purpose of 

this paper is to introduce various warning signs 

of slope instability so operators are better able to 

recognize hazards. The most common slope 

monitoring equipment and practical methods of 

installation are discussed as well as the 

limitations of these systems. 

CONSEQUENCES OF SLOPE FAILURES 

 Unexpected movement of ground causes the 

potential to endanger lives, demolish equipment, 

or destroy property. Between 1995 and 2001 

(2nd quarter), 34 fatalities were caused by 

ground instability. This accounts for 

approximately 15% of all surface mine fatalities 

(Figure 1). These figures include fatalities 

classified by the Mine Safety and Health 

Administration (MSHA) as “fall of face or 

highwall,” “powered haulage,” “equipment,” 

and other MSHA accident categories, where the 

primary cause of the accident was unanticipated 

movement of the ground. This includes 

fatalities resulting from bench and highwall 

failures, rock falls, waste dump and stockpile 

failures, and the collapse of unknown 

underground workings. 

0%

5%

10%

15%

20%

25%

30%

1995 1996 1997 1998 1999 2000 2001

Figure 1. Percentage of surface mine fatalities that 

were caused by unstable ground conditions. 1999 

was the highest year in this period with 28%. 

(NOTE: 2001 data is only current thru July.) 
 The fatalities occur most often at stone mines 

followed by coal, sand & gravel, and metal 

operations. As can be seen in figure 2, the 

problem is pervasive in all four commodities. 

Due to the significance of the problem, 

NIOSH’s Spokane Research Laboratory 

maintains a research program dedicated to 

reducing the number of injuries and fatalities 

resulting from ground instability at surface 

mines. 

SAND & 

GRAVEL

STONE

COAL

METAL

Figure 2. Percent distribution of 34 fatalities from 1995 

through the 2nd quarter of 2001 at surface operations 

(grouped by commodity).

 There are several ways to reduce the hazards 

associated with slope failures: 1) safe 

geotechnical designs; 2) secondary supports or 

rock fall catchment systems; 3) monitoring 

devices for adequate advance warning of 

impending failures; and 4) proper and sufficient 

scaling of loose/dangerous material from 

highwalls. At any surface operation, some 

instability can be expected – from minor bench 

raveling to massive slope failures (Figs. 3 & 4). 

(source: Wharf Resources, South Dakota)

Figure 3. Minor raveling along a bench face 

Figure 4. Massive slope failure at an open pit gold 

mine did not cause any injuries or accidents, but did 

cause significant production problems and high costs.

 Diligent monitoring and examination of 

slopes for warning signs is imperative for 

protecting workers and equipment. 

Geotechnical designs can be improved to 

increase factors of safety and proper bench 

designs can be improved to minimize rock fall 

hazards. However, even slopes with 

conservative slope designs may experience 

unexpected failure due to the presence of 

unknown geologic structures, abnormal weather 

patterns, or seismic shock. Unanticipated 

movement of any amount of rock may cause 

severe disruptions to mining operations, pose 

major safety concerns, or contribute to large 

financial losses for companies (Figure 5). 

(source: Mine Safety & Health Administration, 2000)

 

 Even the smallest of failures can be 

problematic if benches fail that support main 

haul roads, or if facilities are threatened by 

displacement of the rock mass. Failure to 

adequately scale highwalls at quarries can also 

have devastating consequences as can be seen 

(source: Twin Creeks Mine, Nevada)

Figure 5. Fatality and loss of equipment caused 

by unexpected slope failure.
from the excerpts of these recent MSHA fatal 

accident investigation reports: 

“a rock found near the victim...measured 

about 4 by 4 by 3 inches and weighed 2 

pounds, 13 ounces” (MSHA, 1999a) 

“A rock fell from the quarry wall striking 

the victim, causing fatal injuries. ... Death 

was attributed to head trauma. ... The rock 

or rocks that struck the victim could not be 

identified, nor could it be determined from 

what height they fell from the highwall.”

(MSHA, 1999b)

DETERMINATION OF POTENTIAL 

FAILURE GEOMETRY 

 To determine which failure modes are 

possible at a particular operation the geologic 

parameters in various sectors of the mine need 

to be quantified. Collecting information such as 

orientation, spacing, trace length, and shear 

strength with respect to major structures and 

other geologic features is an important key to 

determining failure potential. The basic failure 

modes which may occur are: 

Plane Failure: Plane failures occur when a 

geologic discontinuity, such 

as a bedding plane, strikes 

parallel to the 

slope face and 

dips into the 

excavation at an angle steeper 

than the angle of friction. 

Wedge Failure: Wedge failures occur when 

two discontinuities intersect 

and their line 

of intersection 

daylights in 

the face.

Step-path Failure: Step path failure is similar 

to plane shear failure, but 

the sliding is due to the 

combined mechanisms of 

multiple discontinuities or 

the tensile failure of the 

intact rock connecting 

members of the master 

joint set.

Raveling: Weathering of material and 

expansion and contraction 

associated with freeze-thaw 

cycles are principle causes 

of raveling. This type of 

failure generally produces 

small rockfalls, not massive 

failures. 

Toppling Failure: Toppling can occur when 

vertical or near-vertical 

structures dip toward the 

pit. If this type of 

structure is present, the 

bench face height should 

be limited to a distance 

approximately equal to 

the bench width. This 

will help catch any 

toppling material and decrease the chances of 

impacting equipment working on the pit floor 

below. 

[Note: Several other complex failure modes are possible 

or have been proposed by others. The reader is referred 

to Sjoberg, 1996 for descriptions and literature reviews, 

but it is not within the scope of this paper to describe all 

of these in great detail.] 

 The first step in determining whether these 

basic failure modes are present is to thoroughly 

map the geology (descriptions of data collection 

methods are found in Piteau, 1970; Call, et al., 

1976; Miller, 1983; Nicholas and Sims, 2001). 

By plotting the orientation of the discontinuities 

and the cut face of the slope on a stereonet, 
potential slope stability problems can be 

recognized. Graphical representations of the 

data are also useful for eliminating structures 

which are unlikely to cause slope stability 

problems. (Readers are referred to Hoek & 

Bray, 1981; Panet, 1969; and John, 1969 for the 

background and methodology.) 

RECOGNIZING HAZARDS 

 As previously mentioned, even the most 

carefully designed slopes may be subject to 

instability. Acknowledging that slope failures 

may occur and knowing what the warning signs 

are will contribute to the safety of the operation. 

Some of the more common warning signs of 

slope instability follow: 

Tension Cracks : 

 The formation of cracks at the top of a slope 

is an obvious sign of instability (figure 6). 

Cracks form when slope material has moved 

toward the pit. Since 

this displacement cannot 

be detected from the pit 

floor, it is extremely 

important to frequently 

inspect the crests of 

highwalls above active 

work sites. Safe access 

should be maintained at 

all times to the regions 

immediately above the 

active mining. Frequent 

inspections may be necessary during periods of 

heavy precipitation or spring run-off and after 

large blasts. 

Scarps: 

 Scarps (figure 7) occur where material has 

moved down in a vertical or nearly vertical 

fashion. Both the material that has moved 

vertically and the face of the scarp may be 

unstable and should be monitored accordingly. 

Abnormal Water flows: 

 Sudden changes in precipitation levels or 

water flow may also precede slope failures. 

Figure 8 illustrates a saturated plane in a failing 

slope. 

 Spring run-off from snow melt is one of the 

most obvious examples of increased water flow 

that may have adverse affects on slopes. 

However, changes in steady flow from 

dewatering wells or unexplained changes in 

piezometer readings may also indicate 

subsurface movement that has cut through a 

perched water table or intersected a water 

bearing structure. Changes in water pressure 

resulting from the blockage of drain channels 

can also trigger slope failures. Water can also 

penetrate fractures and accelerate weathering 

processes. Freeze-thaw cycles cause expansion 

of water filled joints and loosen highwall 

material. Increased scaling may be necessary 

during cold weather. 

Figure 6. Example of 

tension cracks. 

Figure 8. Seep in failing slope.

Figure 7. Example of a scarp 
Bulges or Creep: 

 Bulging material or “cattle tracks” appearing 

on a slope indicate creep or slow subsurface 

movement of the slope. Other indicators of 

creep can be determined by looking at 

vegetation in the area (Figure 9). While most 

mines do not have vegetation on the slope faces, 

movement of trees at the crest of a slope can be 

an indicator of instability. 

Rubble at the Toe: 

 Fresh rubble at the toe or on the pit floor is a 

very obvious indicator that instability has 

occurred. An effort must be made to determine 

which portion of the slope failed, and whether 

more material may fail. One of the most 

dangerous situations that can occur is an 

overhang. If workers are not aware that a 

portion of the material below them has failed, 

they may unwittingly venture out onto an 

unsupported ledge (Figure 10.) 

 Remedial measures such as scaling, 

supporting, or blasting the overhang or other 

hazardous rock may be necessary (Figure 11). 

MONITORING EQUIPMENT 

The type of instruments selected for a slope 

monitoring program depends on the particular 

problems to be monitored. A comprehensive 

 

monitoring system may include instruments 

capable of measuring rock mass displacement, 

ground water parameters, and blast vibration 

levels. Excellent overviews of equipment and 

specification exists in CANMET, 1977; and 

Dead trees 

will point 

downhill 

Live 

trees on 

unstable 

slopes 

will grow 

with an 

abnormal 

curve 

Figure 9. Other potential signs of instability 

Figure 10. The overhang indicated by the arrow is

a hazard to trucks traveling along the haul road

because the drivers cannot see the portion of the

slope that has failed below the road. The weight of

the trucks on the partially failed material could

cause the rest of the rock mass to fail. 

Haul Road 

Figure 11. 

A worker 

suspended by 

mountain 

climbing gear 

is supporting 

a potentially 

unstable 

overhang with 

cable bolts. 
Szwedzicki, 1993. Some of the most common 

monitoring equipment is profiled below. (For 

information on blast vibration monitoring and 

damage control techniques see Hustrulid, 1999; 

Oriard, 1972; Scott, 1996; and, Cunningham, 

2001.) 

Survey Network: 

 The use of EDM (electronic distance 

measurement) equipment is a very common and 

effective method for monitoring slopes. The 

survey network consists of target prisms placed 

on and around areas of anticipated instability 

and one or more non-moving control points for 

survey stations. The angles and distances from 

the survey station to the prisms are measured on 

a regular basis to establish a history of 

movement. The surveys can be done manually 

by a survey crew or can be automated. 

 Manufacturers generally publish the 

accuracy and error limits of their equipment. 

Index of refraction errors may occur as a result 

of atmospheric variations in temperature or 

pressure, and human error can be a factor with 

manual systems. Surveying instruments need to 

be carefully adjusted and correctly calibrated 

according to manufacturers’ instructions to 

ensure equipment accuracy and reliability. It is 

extremely important that permanent control 

points for the survey stations are placed on 

stable ground and that the target prisms are 

securely anchored. Errors can cause a serious 

discrepancy in data, and steps need to be taken 

to ensure these errors remain negligible. The 

source of all errors for the surveying method 

must be less than the minimum required 

accuracy of the displacement measurements. 

Tapes, Crack Meters, Pins, etc.

 Measuring and monitoring the changes in 

crack width and direction of crack propagation 

is required to establish the extent of the unstable 

area. The simplest method for monitoring 

tension cracks is to spray paint or flag the ends 

so that new cracks or propagation along existing 

cracks can be easily identified on subsequent 

inspections. Measurements of tension cracks 

may also be as simple as driving two stakes on 

either side of the crack and using a survey tape 

or rod to measure the separations. 

 Stakes can become loose over time and cause 

inaccurate measurements. Multiple stakes can 

be installed to help maintain some reliability in 

measurements. Commercial crack gages with 

electrical readout are also available, but often in 

the case of mine slope problems, the cracks 

exceed the measurement limits of the 

instruments. 

 No matter what method is selected for 

measuring crack displacement, the devices 

should be marked with the dates of installation 

and show the magnitude and direction of 

movement. Monitoring at regular intervals is 

important. Care should be taken to keep 

personnel off the unstable portion of the slide 

when installing . equipment or taking readings. 

Wireline Extensometers: 

 Another common method for monitoring 

movement across tension cracks is with a 

portable wire-line extensometer (Fig. 12). The 

most common 

setup is comprised 

of a wire anchored 

in the unstable 

portion of the 

ground, with the 

monitor and pulley 

station located on a 

stable portion of 

the ground behind 

the last tension 

crack. The wire 

runs over the top of 

a pulley and is 

tensioned by a weight suspended from the other 

end. As the unstable portion of the ground 

moves away from the pulley stand, the weight 

will move and the displacements can be 

recorded either electronically or manually. 

Electronic monitoring equipment can be 

programmed to set off alarms if the 

displacement reaches certain threshold limits. 

Figure 12.
 The length of the extensometer wire should 

be limited to approximately 60 m (197 ft) to 

keep the errors due to line sag at a minimum 

(Call and Savely, 1990). Long lengths of wire 

can lead to errors due to sag so readjustments 

and corrections are often necessary. Some 

extensometers are sensitive to movements of 1 

mm so simultaneous temperature readings 

should be taken to adjust for thermal expansion 

of the wire. Also, while it may sound foolish, 

birds often land on the wires of extensometers. 

This can contribute to a large number of false 

alarms and wildly inaccurate readings. 

Provisions for keeping wildlife away from the 

instrumentation should be made at operations 

where this may be an issue. 

Inclinometers: 

 

 An inclinometer (Figure 13) consists of a 

casing that is placed in the ground through the 

area of expected movements. The end of the 

casing is assumed to be fixed so that the lateral 

profile of displacement can be calculated. The 

casing has grooves cut on the sides that serve as 

tracks for the sensing unit. The deflection of the 

casing, and hence the surrounding rock mass, 

are measured by determining the inclination of 

the sensing unit at various points along the 

length of the installations. Information 

collected from inclinometers can be used to 

(Kliche, 1999): 

• Locate shear zones. 

• Determine whether shearing is planar 

or rotational. 

• Determine whether movement along a 

shear zone is constant, accelerating, 

or decelerating. 

Figure 14 illustrates the proper placement for 

inclinometers to monitor both shallow and deepseated failure planes. If the bottom of the 

inclinometer is not in stable ground, the 

instrument may “float” in the failure zone and 

give erroneous readings. Excessive horizontal 

movement may deflect the casing so much that 

the sensing torpedo will not be able to pass the 

bend to take readings. Manufacturers will 

provide tables of instrument accuracy. 

CANMET (1977) recommends the use of small 

diameter, highly sensitive (1:10,000) 

inclinometers in rock slopes. If taking manual 

readings, two measurements (with the probe 

Figure 13. Cross-sectional schematic of typical 

traverse-probe inclinometer system. (adapted from 

Abramson, et al., 1996.) 

Figure 14. Proper inclinometer placement in 

a slope (adapted from Abramson et al., 1996)

Good for shallow 

failure, but not 

deep. 

Good for deep 

failure, but not 

shallow. 

Good for both 

shallow and deep 

failure planes. 
rotated 180 degrees between measurements) 

should be taken to reduce errors. 

Time Domain Reflectometry (TDR):

 Time Domain Reflectometry is a technique 

in which electronic pulses are sent down a 

length of a coaxial cable which has been grouted 

in a drillhole. When deformation or a break in 

the cable is encountered, a signal is reflected 

giving information on the subsurface rock mass 

deformation. While inclinometers are more 

common for monitoring subsurface 

displacements, TDR cables are gaining 

popularity and have several advantages over 

traditional inclinometers (Kane, 1998): 

• Lower cost of installation. 

• Deeper hole depths possible. 

• Rapid and remote monitoring 

possible. 

• Immediate deformation 

determinations. 

• Complex installations possible. 

 Recent advances have also been made in the 

use of TDR for monitoring ground water levels 

and piezometric pressures (Dowding, et al. 

1996). A summary of applications of TDR in 

the mining industry is provided by O’Connor 

and Dowding (1984). 

Piezometers: 

 Piezometers are used to measure pore 

pressures and are valuable tools for evaluating 

the effectiveness of mine dewatering programs 

and the effects of seasonal variations. Excessive 

pore pressures, especially water infiltration at 

geologic boundaries, are responsible for many 

slope failures. Data on water pressure is 

essential for maintaining safe slopes since water 

behind a rock slope will decrease the resisting 

forces and will increase the driving forces on 

potentially unstable rock masses. Highwalls 

should be visually examined for new seeps or 

changes in flow rates as these are sometimes 

precursors to highwall failure. Additionally, pit 

slopes should be thoroughly examined for new 

zones of movement after heavy rains or 

snowmelts. 

Borehole Extensometers: 

 A tensioned rod extensometer is used to 

detect and monitor changes in distance between 

one or more anchors in a borehole and the 

borehole collar (figure 15). Changes in the 

distance between the anchor and the rod head 

provide the displacement information for the 

rock mass. 

 

 Figure 15. Multi-point borehole extensometer.

 Extensometers of this type are best used to 

monitor known structural features which will 

have a major influence on slope stability. These 

instruments are fairly expensive when compared 

to other instrumentation options, and are 

therefore, not suitable for surveillance of large 

areas of the pit. 

IMPLEMENTING A MONITORING PLAN 

 Sufficient, suitable monitoring must be 

provided to detect instability at an early, noncritical stage to allow for the initiation of safety 

measures. Monitoring “after the fact” does little 

to undo the damage caused by unexpected 

failures. Determining the objective of a 

monitoring plan is a seemingly simple, but 

crucial step in the instrumentation process. The 
purpose of a monitoring plan (adapted from Call 

& Savely, 1990) is to: 

• Maintain safe operational practices for 

the protection of personnel, equipment, 

and facilities. 

• Provide warning of instability so 

action can be taken to minimize the 

impact of slope displacement. 

• Provide crucial geotechnical 

information to analyze the slope 

failure mechanism and design the 

appropriate corrective measures. 

 The following steps should be taken when 

planning the instrumentation portion of the 

monitoring program: 

1. Understand the mechanisms that may 

cause instability. 

2. Define and prioritize the geotechnical 

information required. 

3. Establish monitoring locations. 

4. Predict the magnitudes of movement 

and other parameters at these 

locations. 

5. Establish an instrumentation budget. 

6. Select instrumentation based on steps 

1-5 above. 

When selecting instrumentation, incorporate 

some level of redundancy in the system to 

cross-check instrument performance and 

eliminate errors. Redundant or over-lapping 

measurements will also provide a back-up in 

the case of instrument failure. Automated 

equipment is generally more accurate than 

manual equipment since some “human” error is 

removed. Automated systems also provide 

added flexibility in the sampling rate, and 

therefore can monitor more frequently than 

manual readings. Another distinct advantage 

of the automated systems is their ability to 

trigger alarms if certain threshold limits are 

reached. However, these systems are generally 

more expensive than manual systems, and the 

electronics may be more sensitive. 

 Other items to keep in mind when selecting 

equipment is the amount of personnel training 

that is needed and the time requirements for 

data collection. Personnel may require highly 

technical training to calibrate and maintain 

complex electronic systems. Sometimes 

installing a greater number of cheaper, reliable 

instruments is more useful than installing a few 

expensive, highly sensitive instruments. 

Instruments should be placed where they will 

be the most effective. Estimating the 

movement expected in a particular area should 

help ensure that the limits of the instrument are 

not exceeded. There may also be 

environmental limitations (extreme heat or 

cold, etc.) that determine whether a particular 

instrument will work at an operation. All of 

these factors need to be evaluated against the 

primary objectives of the monitoring program. 

DATA REDUCTION & ANALYSIS 

 All slopes will deform in response to mining. 

The deformation will vary depending on the 

slope geometry, the geology, the rockmass 

properties, and the ground water conditions. 

Monitoring instruments are useful for collecting 

a large amount of data, but knowing what data is 

pertinent is will guide the necessary course of 

action. 

 Measure simple, obvious movements first -- 

surface displacements are very useful for 

determining the mechanism responsible for the 

instability and the extent of the failure surface. 

Plotting the rate of movement is the most 

important variable to track. If the rate of 

movement decreases, the slope may have 

temporarily stabilized. If the rate of movement 

increases, slope failure may be pending and 

more frequent readings of the site should be 

taken.
SLOPE STABILIZATION METHODS & 

PRE-PLANNED RESPONSE TO 

MOVEMENT 

 Even with diligent geologic mapping, careful 

geotechnical designs, and adequate monitoring 

programs, the chances for instability still exist. 

With today’s instruments it is neither feasible 

nor practical to monitor every possible failure at 

an open pit mine. If material does fail, the mine 

should have a pre-planned response to the 

movement. If a slope failure is eminent, 

personnel must immediately be pulled out of the 

hazardous area. Operating procedures should be 

in place to establish what the threshold values of 

movement are, and how an evacuation scenario 

would be communicated to the workers. 

Standard operating procedures should also 

define those employees responsible for doing 

pre-shift inspections of highwalls, and define 

which personnel are responsible for collecting 

and compiling data from the monitoring 

instruments. Slope failures very rarely occur 

without some warning, and all workers need to 

be able to recognize potential hazards and act 

accordingly. 

 If the failure is not immediately threatening 

to personnel, a variety of other actions can be 

taken in response to the movement. The 

selection of remedial measures taken depends 

on the nature of the instability and the 

operational impact. Each case should be 

evaluated individually with respect to safety, 

mine plans, and cost-benefit analyses. 

Let the material fail.

 If the failure is in a non-critical area of the 

pit, the easiest response may be to leave the 

material in place. Mining can continue at a 

controlled rate if the velocity of the failure is 

low and predictable and the mechanism of the 

failure is well understood. However, if there is 

any question about the subsequent stability, an 

effort should be made to remove the material. 

Large-scale failures can be difficult and costly 

to clean up. Often, a mining company will 

choose to leave a step-out in the mine design to 

contain the failed material and continue mining 

beneath the step-out. The value of the ore that 

is lost needs to be evaluated against the costs of 

clean-up to determine if this is a feasible 

solution. The size of the blasts may also need to 

be reduced to minimize impacts on the unstable 

zone. 

 To prevent small-scale failures from reaching 

the bottom of the pit, both the number of catch 

benches and the width of catch benches can be 

increased. Catch fences (figure 16) have also 

been installed at some operations to contain 

falling material. 

Support the material:

 If allowing the instability to fail is not an 

option, artificially supporting the failure may be 

a solution. Some operations have successfully 

used reinforcement such as bolts, cables, mesh, 

and shotcrete to support the rock mass. The use 

of such supports can be very expensive. 

However if the overall angle of the highwall can 

be steepened and clean-up costs are reduced, the 

added expense of reinforcement may be 

justified. A careful study of the geologic 

structures must be performed to select the 

proper reinforcement (i.e. length of bolts or 

cables, thickness of shotcrete, etc.) Bolts that 

are too short will do little to prevent slope 

stability problems from continuing. In some 

cases, reinforcement has only served to tie 

several small failures together, creating a larger 

failure. 

Figure 16. Catch fence installed to prevent loose 

rocks from traveling to the pit floor. 
 Another potential solution to stop or slow 

down a slope failure is to build a buttress at the 

toe. The buttress offsets or counters the driving 

forces of the slope by increasing the resisting 

force. Short hauls of waste-rock often make this 

an attrative and economical alternative for 

stabilizing slope failures. 

Remove the hazard:

 If a slope continues to fail, and supporting 

the slope is not a feasible alternative, steps need 

to be taken to remove the hazard. Often, 

flattening the slope to a more favorable angle 

with respect to the local geology will solve the 

problem (Fig. 17). When catchment systems are 

not available, proper and sufficient scaling 

methods should be employed on a regular basis 

to remove hazards associated with small rock 

falls. 

 Removing, or unweighting, the top portion of 

a slide may also decrease the driving forces and 

stabilize the area. However, Call & Savely 

(1990) warn that this option is generally 

“unsuccessful” and cite situations involving 

high water pressure where unloading actually 

decreased the stability of the remaining 

material. 

 Since water pressure creates slope stability 

problems, dewatering using horizontal or 

vertical wells is a powerful means of controlling 

slope behavior and minimizing hazards. 

Surface drainage and diversions should also be 

used to keep surface runoff away from tension 

cracks and open rock mass discontinuities near 

the slope face. 

FUTURE MONITORING 

TECHNOLOGIES

 Because of the enormous surface area of 

many open-pit mines, several varieties and 

scales of instabilities can occur. Complete 

vigilance to monitor each and every potential 

failure block is neither feasible, nor economical, 

and is certainly not attainable using today’s 

most common point displacement monitoring 

techniques. Many of the current monitoring 

methods are also difficult to implement at 

quarries and surface coal mines, where steep 

highwalls and lack of benching limit access to 

areas above the working floor. Additionally, as 

mining progresses, it is necessary to monitor 

different sections of the pit walls. Continually 

relocating devices is not only costly and time 

consuming, but can also be dangerous -- 

especially on unstable slopes. 

 In an effort to make up for the shortcomings 

of point monitoring systems, NIOSH is testing 

several new technologies that will monitor the 

entire slope for rock mass displacement and 

rock mass composition (Girard and McHugh, 

2001; McHugh et al., 2000; Sabine, Mayerle, 

and others, 1999; Sabine, Denes, and others, 

1999; Girard et al., 1998). These technologies 

include imaging spectroscopy, interferometric 

synthetic aperture radar, and digital image 

change detection. Additionally, software has 

been created under a NIOSH contract to assist 

geotechnical engineers with bench designs to 

minimize rock fall hazards (Miller, 2000; 

Miller, et al., 2000.) 

CONCLUSIONS 

 Slope failures and ground instability at 

surface mining operations contribute to nearly 

15% of surface mining fatalities. Knowing how 

to properly design slopes with respect to 

geologic structures will help minimize slope 

Material to be 

removed 

Figure 17. Flattening slope to avoid 

slope failures. 
failures. Carefully designed monitoring 

programs are very useful for supplementing safe 

operational practices. A properly designed 

monitoring program will also send warnings of 

impending instability and provide the necessary 

geotechnical information for designing 

appropriate corrective measures. Future 

technologies may overcome the limitations of 

current monitoring equipment, but until that 

time, diligent inspections of the highwalls above 

workers is crucial. Understanding and 

recognizing warning signs of impending ground 

instabilities will hopefully reduce the injury and 

fatality rates at surface mining operations. 

ACKNOWLEDGEMENTS 

This paper would not have been possible without 

technical assistance from numerous mining 

companies and tours of several mine properties. 

The Mine Safety and Health Administration data 

and photos were invaluable for evaluating the 

significance of ground stability problems at 

surface mines. Special thanks for timely reviews 

by my co-workers Ed McHugh and Brandi 

Neagle, and for graphics support by Ken Strunk. 

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