DETECTING PROBLEMS WITH MINE SLOPE STABILITY
 

Jami M. Girard1 and Ed McHugh1 

1

National Institute for Occupational Safety and Health, Spokane Research Laboratory 

ABSTRACT 

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. The purpose of this paper to 

discuss some of this research and to present 

potential new technologies for slope monitoring 

and design. The paper also briefly discusses 

various warning signs of slope instability, 

introduces the most common slope monitoring 

methods, describes the limitations of various 

slope monitoring systems, and presents some 

field results using some of this new technology. 

CONSEQUENCES OF SLOPE FAILURES

 Whether on the surface or underground, 

unanticipated movement of the ground can pose 

hazardous conditions which may lead to 

endangerment of lives, demolition of equipment, 

and the loss of property. In the five years since 

1995, 33 miners have lost their lives as a result 

of surface ground control accidents (see Figure 

1). 

Figure 1. Map of 33 fatalities occurring from surface 

ground control problems January 1995 -June 2000. 

As such, the National Institute for 

Occupational Safety and Health’s (NIOSH) 

Office of Mine Safety and Health Research in 

Spokane, Washington has initiated a research 

program with the goal of reducing the number of 

injuries and fatalities resulting from slope 

failures at mines. 

 There are several ways to reduce the chances 

of surface ground control failures: 1) safe 

geotechnical designs; 2) secondary supports or 

rock fall catchment systems; or 3) monitoring 

devices for advance warning of impending 

failures. While it is important to note that 

geotechnical designs can be improved to increase 

factors of safety, proper bench designs can be 

improved to minimize rock fall hazards, and 
certain support systems may enhance overall 

rock mass strength, diligent monitoring and 

examination of slopes for failure warning signs is 

the most important means of protecting exposed 

mine workers. Even the most carefully designed 

slopes may experience failure from unknown 

geologic structures, unexpected weather patterns, 

or seismic shock (Figure 2). 

Figure 2. Consequences of unexpected slope failures. 

SLOPE MONITORING SYSTEMS 

 Relative displacement measurements are the 

most common type of monitoring, complemented 

by monitoring of groundwater. The most 

important purpose of a slope monitoring program 

is to: 1) maintain safe operational practices; 2) 

provide advance notice of instability; and 3) 

provide additional geotechnical information 

regarding slope behavior (Sjöberg, 1996). The 

following is a list of the most common 

monitoring systems currently in use and is not 

intended to be an all-inclusive list of monitoring 

equipment. Readers interested in a more 

comprehensive list are referred to Szwedzicki, 

1993. 

Surface Measurements 

Survey Network: A survey network consists of 

target prisms placed on and around areas of 

anticipated instability on the slopes, 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 

on the slope. It is extremely important to place 

the permanent control points for the survey 

stations on stable ground. The surveys can be 

done manually by a survey crew or can be 

automated. 

Tension Crack Mapping: The formation of 

cracks at the top of a slope is an obvious sign of 

instability. Measuring and monitoring the 

changes in crack width and direction of crack 

propagation is required to establish the extent of 

the unstable area. Existing cracks should be 

painted or flagged so that new cracks can be 

easily identified on subsequent inspections. 

Measurements of tension cracks may be as 

simple as driving two stakes on either side of the 

crack and using a survey tape or rod to measure 

the separations. 

Another common method for monitoring 

movement across tension cracks is with a 

portable wire-line extensometer (Figure 3). 

 Figure 3. Portable wire-line extensometer for 

monitoring a tension crack.

 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. 

Long lengths of wire can lead to errors due to sag 
or to thermal expansion, so readjustments and 

corrections are often necessary. 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). 

Subsurface Measurements 

Inclinometers: An inclinometer (figure 4) 

consists of a casing that is placed in the ground 

through the area of expected movements. 

Figure 4. Cross-sectional schematic of typical 

traverse-probe inclinometer system. 

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. The information collected from the 

inclinometers is important to slope stability 

studies for the following reasons (Kliche, 1999): 

• to locate shear zone(s); 

• to determine whether the shear along
 

the zone(s) is planar or rotational; 

• to measure the movement along the 

shear zone(s) and determine whether 

the movement is constant, 

accelerating, or decelerating.
 

Time Domain Reflectometry (TDR): Time 

Domain Reflectometry is a technique in which 

electronic pulses are sent down a length of a 

coaxial cable. 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 Wade (1994). 

Borehole Extensometers: An extensometer 

consists of tensioned rods anchored at different 

points in a borehole (figure 5). Changes in the 

distance between the anchor and the rod head 

provide the displacement information for the 

rock mass. 
Figure 5. Multi-point borehole extensometer. 

Piezometer: 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. 

RESEARCH AND TECHNOLOGY 

 

 Stress, gravity loading, rock mass strength, 

geology, pore pressure, the presence of unknown 

underground workings, and many other factors 

contribute to slope failures. Because of the 

enormous surface area of many large 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 near-vertical faces and lack of benching 

limit access to areas along the highwall. 

Additionally, as mining progresses, it is 

necessary to monitor different sections of the pit 

walls. Continually relocating devices is not only 

a costly and time consuming operation, but can 

also be dangerous -- especially with an unstable 

slope. 

 In an effort to make up for the shortcomings 

of point monitoring systems, NIOSH is 

examining new technology for slope monitoring 

that will look at the entire surface of the mine 

highwall for rock mass displacement and rock 

mass composition (Girard, 1998). Additionally, 

software has been created under a NIOSH 

contract to assist geotechnical engineers with 

bench designs to minimize rock fall hazards. A 

discussion of each technology follows. 

Highwall Monitoring Using Radar Systems 

 Synthetic aperture radar (SAR) is a type of 

ground-mapping radar originally designed to be 

used from aircraft and satellites. SAR can be 

used to generate high quality digital elevation 

maps (DEM’s) and to detect disturbances of the 

earth’s surface. A variation of SAR – 

Interferometric Synthetic Aperture Radar 

(IFSAR) – uses differences in time-lapsed SAR 

images to generate maps of displacements 

(Fruneau and Achache, 1996). This technique 

has been successfully applied to produce 

displacement maps of ground movement caused 

by earthquakes, volcanic activity, and mine 

subsidence (Massonet, 1997; Canec, 1996). 

IFSAR can also be used to monitor displacement 

of unstable slopes or landslides (Reeves et al., 

1997; Sabine et al., 1999). 

 IFSAR’s have many advantages over current 

types of monitoring systems. Able to work in 

nearly all weather, an IFSAR can acquire 

imagery through fog, mist, rain, haze, or cloud 

cover, and can operate day or night. Also, an 

IFSAR can sample a large area for ground 
 

displacements, which gives them a tremendous 

advantage over survey networks, extensometers, 

and other instruments which sample movement 

on a discrete set of points. Recent developments 

in instruments such as prismless laser range 

finders partially address the problem of under 

sampling large areas for movement. However, 

the range and accuracy of these units can vary 

greatly depending on the reflectivity of the rock, 

the angle of the rock face, weather, and other 

factors. Manufacturers of prismless range 

finders generally claim a range of 500 meters or 

less. 

The Microwave Earth Remote Sensing 

Laboratory at Brigham Young University (BYU) 

has recently designed and built a small synthetic 

aperture radar system capable of operating from 

light aircraft or from stationary ground-based 

stations (Thompson, 1998). The first field tests 

related to geohazards monitoring using this 

instrument were performed by BYU for the 

Canadian National Railways (CN). The railways 

were interested in finding a method to accurately 

detect rockfalls and washouts on railroad tracks 

before trains approached those dangerous areas. 

The initial results from those tests were positive, 

and BYU researchers are confident that their 

system can be adapted for NIOSH to monitor 

highwalls at mines. Field tests are anticipated to 

begin in the fall of 2000. 

Imaging Spectroscopy 

 A contributing factor in many highwall 

failures is the presence of mechanically 

incompetent, argillically-altered rock. Major 

structures are generally well mapped, but weak 

rock units may be much more difficult to 

identify. Mine maps can vary greatly in quality 

and detail due to the subjectivity of various 

geologists and the extreme geologic complexity 

of many deposits. In addition, there are financial 

and practical limits to the number of samples that 

can be taken for geochemical or engineering 

analyses. Oftentimes, a large percentage of the 

data shown on geologic maps is an estimation by 

a geologist or mathematical interpolations of 

geotechnical results. 

 

 In order to help identify weak rock structures 

and remove ambiguity from geologic mapping, 

NIOSH researchers are testing applications of 

imaging spectrometers (Sabine, et al., 1999). An 

imaging spectrometer is a device that can 

determine the composition of minerals from a 

distance by analyzing the diagnostic spectral 

absorption signatures (unique reflectance 

patterns of light that uniquely define each 

mineral). Like IFSAR, imaging spectrometers 

have been used from satellites and aircraft for 

geologic mapping for quite some time, but recent 

advances in technology have led to the 

development of smaller, portable, units that can 

also work from ground level. 

 One such instrument has been developed by a 

team of researchers at Carnegie-Mellon Research 

Institute (Denes et al., 1997). The instrument 

(figure 6), known as a spectro-polarimetric 

imager (SPI), utilizes an acousto-optic tunable 

filter (AOTF) to detect the interaction of light 

and acoustic waves in certain crystals. 

Figure 6. Carnegie-Mellon Research Institute’s 

Spectro-polarimetric Imager.

 The 

optical absorption or reflectance spectrum in the 

presence of ambient light then reveals the 

composition of objects in the image. 

 In July 1999, NIOSH and Carnegie-Mellon 

Research Institute (CMRI) researchers conducted 

field-tests of the SPI for mine highwall imaging 

at Molycorp’s Mountain Pass Mine near the 
California/Nevada border. This particular mine 

site was selected for field tests because the 

diagnostic spectral signatures of the ore 

(lanthanide series rare earths) are within the 

current spectral range of the prototype 

instrument. 

 Multispectral images were collected at twelve 

scenes at the Mountain Pass mine, including pit 

highwalls, outcrops, drill core, and hand samples. 

Figure 7a shows an area of interest 

(approximately 30 x 40 ft) on an exposed 

highwall. After the images were collected, 

substantial image processing, filtering, and 

computer analyses were performed. Figure 7b 

shows the results of the image analyses. 

Figure 7a. 

Region of 

interest for field 

test of SPI. 

Figure 7b. 

Results of SPI 

image analyses. 

Light areas 

correspond to 

bastnaesite ore, 

darker areas 

waste or 

unclassified 

spectra. 

The 

light areas are ore and the darker areas are waste 

rock. The field tests were successful in that the 

SPI images clearly illustrate the capabilities of 

the instrument to collect a multispectral image 

and discriminate materials within the image that 

may or may not be distinguishable by the human 

eye. 

 There are many advantages to using spectral 

analyses for geologic mapping. First of all, 

spectral identification of minerals would remove 

the human error and subjectivity of trying to 

visually determine the degree of alteration in a 

rock mass. Secondly, workers would be able to 

map mine highwalls, or other inaccessible or 

unsafe areas, from a safe distance. However, this 

instrument will only be useful for imaging weak, 

altered zones in pit highwalls if the spectral 

range is extended further into the infrared region. 

At this time, funding for this advancement in 

technology is not available at NIOSH or at 

CMRI. 

Bench Design Software

 Thorough engineering analyses of large slope 

cut in discontinuous rock masses often include 

investigations of bench stability. If kinematically 

viable rock failure modes are present in benches, 

it is unlikely that actual bench widths will match 

the original slope geometry plan. Consequently, 

rock fall hazard assessment and related slope 

stability safety issues must consider the 

operational (“as-built”) catch-bench geometry 

and not the idealized (“as-designed”) geometry. 

 Minor bench instabilities and rock falls 

adversely impact mine safety in two key areas. 

First, as failures break back along the top of a 

bench, storage capacity for holding rock fall 

debris is significantly reduced and falling rock 

from above may not be caught and retained on 

the bench (see figure 8). Secondly, large 

amounts of rock fall debris on benches may even 

trigger multiple-bench failures. Therefore, the 

ability to predict the volumes of bench-scale 

failures is essential for properly designing wide 

enough catch benches. 

 In order to assist mine operators with bench 

design, computer software has been developed 

for NIOSH’s Spokane Research Laboratory 

(Miller, 2000). The software computes the 

probability of bench stability for plane shear or 

wedge failures using basic geological 

engineering input on bench dimensions, rock 
mass characteristics, and fracture data. After all 

of the potential failure geometries are computed, 

the probability of sliding is computed using a 

stochastic shear strength model (Miller, 1988; 

Miller and Girard, 2000.) 

 Results from a bench stability study can be 

used to help select inter-ramp slope angles and 

overall slope angles. Bench geometry has a 

direct influence on the overall slope angle as 

expressed by: 

tan(A)=1/[(W/H) + (1/tanB)] (1) 

Where: A = overall (average) slope angle; 

 B = bench face angle 

 H = vertical height of bench; and 

 W = horizontal width of bench. 

 Using this relationship and output from the 

software, geotechnical engineers can design 

overall slope angles and bench configurations to 

minimize extensive loss of catch bench width 

and thus minimize rockfall hazards. 

Figure 8. Small, unexpected rock falls may indeed be 

more hazardous than a massive failures that involve 

slow displacement of material over a longer period of 

time. 

CONCLUSIONS AND 

RECOMMENDATIONS

 Steps need to be taken to reduce the 

number of mining deaths resulting from slope 

instability. Diligent monitoring and safe design 

by qualified geotechnical engineers at mine sites 

is crucial. Additionally, proper catch bench 

design, blasting patterns that minimize 

overbreak, effective highwall scaling (where 

appropriate), and dewatering of potentially 

unstable zone are also important to minimize 

hazards related to highwall failures. 

 There are many new technologies being 

explored, but remote sensing, at the present time, 

cannot replace conventional geotechnical 

methods of investigation. A great deal of 

research is still needed to design and test new 

systems in order to make certain they are 

scientifically valid and economically viable 

options. 

 The NIOSH Slope Stability Hazard 

Recognition Team is pursuing a variety of 

research options to minimize the dangers 

associated with surface ground control problems. 

Results of this research may also benefit others 

involved in studies of landslides, rock falls, 

avalanches, volcanic activity, and other 

geohazards. 

 For more information about this or any other 

NIOSH project call 1-800-35-NIOSH. Or visit 

the website at: http://www.cdc.gov/niosh 

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