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Rock Mechanics for Natural Resources and Infrastructure 
SBMR 2014 – ISRM Specialized Conference 09-13 September, Goiania, Brazil 
© CBMR/ABMS and ISRM, 2014 
 
SBMR 2014 
Slope Stability Analysis of Open Pit Highwall Mining 
Excavations – A Parametric Study 
 
Shivakumar Karekal 
CSIRO Earth Science and Resource Engineering, Brisbane, Australia, shivakumar.karekal@csiro.au 
 
Srikrishnan Siva Subramanian 
Central Institute of Mining and Fuel Research, Regional Centre, Nagpur, India, srikrishnan16@gmail.com 
 
John Loui Porathur 
Central Institute of Mining and Fuel Research, Regional Centre, Nagpur, India, johnlouip@yahoo.com 
 
SUMMARY: Highwall mining operation involves driving a series of parallel unsupported, 
unmanned and unventilated excavations into a coal seam exposed at the open pit Highwall using a 
remotely operated continuous miner with attached conveying system. These parallel excavations are 
separated by web pillars of pre-designed width which are critical to the Highwall mining operations. 
The Highwall slope must remain stable during Highwall mining operation to ensure safety of 
workers and machinery. In this paper, Highwall slope stability is investigated with respect to 
different Highwall mining parameters using FLAC3D numerical modeling software. The parameters 
included in the study are: (i) single seam and multiple seams Highwall mining excavations with 
different width to height ratios; (ii) different Slope angles; (iii) different excavation heights; and (iv) 
different cover depths. A narrow strip of rock mass is considered by taking a plane of symmetry. 
The modeling results reveal that stability of open pit slopes have profound influence on the 
Highwall mining parameters, and the web pillar design can affect the stability of Highwall slopes. In 
designing Highwall slopes for an open pit, the design must include Highwall mining excavations, 
otherwise, near critical failure slopes could become critical and fail with Highwall excavations. In 
authors’ knowledge, this work is the first attempt at exploring the effect of Highwall mining 
parameters on overall slope stability. 
 
KEYWORDS: Highwall mining, slope stability, 3D numerical modeling, multi-seam modeling 
 
 
1 INTRODUCTION 
 
Highwall mining operation involves driving a 
series of parallel unsupported, unmanned and 
unventilated excavations into a coal seam 
exposed at the open pit Highwall using remotely 
operated continuous miner with attached 
conveying system. These parallel excavations 
are separated by the web pillars of pre-designed 
width which are critical to the Highwall mining 
operations (Porathur et al, 2013). Geological 
settings and the web pillar design affect the 
Highwall stability. During Highwall mining 
operation, the Highwall slope must remain 
stable to ensure safety of workers and 
machinery. In the US, it has been identified that 
slope failures have been triggered by the 
collapse of web pillars (Zipf, 1999; Zipf and 
Bhatt, 2004). For a successful Highwall Mining 
operation, one needs to ensure that the stability 
of both web pillars and Highwall slope are 
maintained. Further, there is hardly any work 
carried out on interdependency of web pillar 
stability and the overall slope stability, and this 
requires more elaborate studies. The main aim 
of this paper is to gain understanding of the 
variation of Factor of safety (FOS) of Highwall 
mining slope with or without Highwall 
excavations, and with different Highwall 
mining parameters such as width to height 
ratios of web pillars, cover depth, seam 
excavation heights and slope angles. In authors’ 
 
 
 
 
SBMR 2014 
knowledge, this work is the first attempt at 
exploring the effect of Highwall mining 
parameters on overall slope stability. 
 
2 NUMERICAL MODELLING 
 
A three dimentional continuum model was used 
for stress analysis for assessing overal Highwall 
slope stability for different Highwall mining 
parameters. A numerical modelling code called 
FLAC3D developed by Itasca Consulting 
Group, USA (Itasca, 2006) was used for the 
parametric study by varying Highwall mining 
parameters. FLAC3D is a contiuum finite 
difference code widely used for geotechnical 
analyses of rock and soil structures. 
For numerical modelling, a narrow rock 
mass strip was considered by taking the plane of 
symmetry. Gravity stresses were considered. 
Roller boundaries were applied on either sides 
and fixed boundary was considered along the 
bottom. The 3D mesh adopted for the numerical 
modelling for the rock mass strip is shown in 
Figure 1. The open pit excavation was carried 
out in staged manner by progressively 
excavating the benches. Figure 2 shows the 
excavated pit up to a single coal seam horizon. 
For multiple seams Highwall excavation, the pit 
is excavated up to top seam horizon first (Figure 
3) followed by the excavation up to bottom 
seam horizon excavation (Figure 4). Slope 
stability analysis was undertaken without 
Highwall mining excavations and with 
Highwall mining excavations for different 
width to height ratios of web pillars and for 
different cover depths, excavation heights and 
slope angles. Different Highwall excavation 
lengths into the coal seams from the pit surface 
were used and excavation lengths depended on 
the cover depth and slope angles (Table 1). 
 
Table 1. Different Highwall excavation lengths for 
different slope heights and slope angles. 
 Slope Angle 
Slope 
Height(m) 
 45° 60° 70° 
50 Penetration 
(excavation 
lengths into the 
coal seam), (m) 
50 30 19 
100 100 59 37 
150 150 88 55 
 
 
 
Figure 1. Three dimensional numerical grid used for 
numerical modeling which is extruded out of plane across 
the rock mass strip. 
 
 
Figure 2. Excavation of open pit up to a single coal seam 
horizon. 
 
 
Figure 3. Pit after excavating up to top seam horizon. 
 
 
 
Figure 4. Pit after excavating up to bottom seam horizon. 
 
 
 
 
SBMR 2014 
 
2.1 Slope Stability Analysis using Shear 
Strength Reduction Method 
 
The shear strength reduction method is used to 
get the limiting shear strength properties of 
cohesion (c) and friction angle (φ) by gradually 
reducing these properties according to the 
following equations (Lorig et al, 2000). 
 
c 
trial 
= (1/F) (1) 
 
φ trial = arctan {(1/F) tan φ } (2) 
 
where c is cohesion, φ is friction angle and F is 
factor of safety. 
 
In the above technique incorporated in 
FLAC3D, it is not necessary to specify the 
shape of the failure surface in advance, as the 
critical failure surface evolves automatically. 
The anticipated failure surface has been 
determined for the limiting case of overall slope 
stability by reducing the strength parameters of 
the rock mass until hypothetically bringing the 
slope to an unstable state. The ratio between the 
actual shear strength and the model strength at 
an unstable state is the Factor of safety. 
 
2.2 Rock Mass Properties 
 
The rock mass properties used in the 3D 
numerical modeling for slope stability analysis 
are given in Table 2. A Mohr Coulomb material 
model was used instead of strain softening 
model. It may be noted that low and same 
cohesion values were used for both the rock 
types in order to facilitate shear strength 
reduction (SSR) method to reduce cohesion and 
friction equally for both the rock types. The 
assumed lowcohesion value for the coal can 
cause web pillar failure by undergoing 
plasticity. 
 
Table 2. The rock mass parameters used in this study. 
Rock 
Young’s 
Modulus 
 (GPa) 
Poisson's 
Ratio 
Density 
(kN/m3) 
Cohesion 
(KPa) 
Friction 
Angle 
Sand-
stone 
1 0.2 24 250 35° 
Coal 0.5 0.2 15 250 35° 
 
3 RESULTS AND DISCUSSION 
 
Parametric results are discussed in the following 
sections which include variation in Highwall 
Factor of Safety (FOS) for various width to 
height ratios of web pillars, depth of cover 
(slope heights), slope angles, coal seam 
excavation heights and excavation of single and 
multiple coal seams. 
 
3.1 Effect of Slope Angle on Slope Stability 
Prior to Highwall Mining Excavations 
 
Slope stability was evaluated for the rock mass 
properties (Table 2) before the Highwall mining 
excavations were made (Pre Highwall Mining 
stage) for different slope angles, namely, 45°, 
60° and 70° and for different depths of cover of 
50m, 100m and 150m. An example of the 
evolved failure surface using the stress 
reduction method is shown in Figure 5 prior to 
Highwall mining excavations for 100m cover 
depth (slope height). The variation in safety 
factor of slopes for different slope angles at 
different cover depths can be seen in Figure 6. 
As expected, the safety factor decreases with the 
increase in the slope angle. Further, it can be 
seen in the Figure that the values of factor of 
safety reduce with the increase in the slope 
heights or the cover depths. The factor of safety 
takes a value of 1.0 at a slope angle of 70° when 
the depth of cover (i.e., slope height) is about 
150m from the toe of the slope for the assumed 
rock mass properties. The decrease in factor of 
safety was about 26% when cover depth 
increased from 50 to 100m and about 40% 
when the cover depth increased from 50 to 
150m for pre Highwall mining for the 
considered slope configuration and the model 
properties. 
 
 
 
 
 
SBMR 2014 
 
Figure 5. Evolved failure surface for 100m cover depth at 
45° slope angle, prior to Highwall excavation. 
 
 
Figure 6. Variation in factor of safety of slope for 
different slope angles and for different depth of cover 
(slope height) prior to Highwall mining excavation. 
 
3.2 Effect of w/h Ratios of Web Pillars on 
Slope Stability after Highwall Mining 
Excavations 
 
The factor of safety of slopes after Highwall 
mining excavations was analysed for different 
w/h ratios of web pillars left between the 
parallel excavations. Figures 7, 9 and 11 show 
the variation in factor of safety of slope against 
different w/h ratio for different slope angles and 
for different depth of covers. As can be seen in 
Figure 7 for 50m cover depth, with increased 
w/h ratio of web pillars, the factor of safety 
improves in general. This is due to the fact that 
as w/h ratio increases the web pillar strength 
between the parallel excavations increases, 
reducing the pillar deformation and hence 
improves the factor of safety of the overall 
slope. In the same figure (Figure 7), factor of 
safety of slopes prior to Highwall mining 
excavations (Pre HWM) are also plotted for 
comparison with factor of safety of slope after 
excavations (Post HWM) for different slope 
angles. With excavations, the slope factor of 
safety decreased quite remarkably as can be 
seen in the figure especially for lower slope 
angles. Further, as slope angle increases, the 
factor of safety decreases. However, with 
increase in slope angle, the factor of safety 
value decreased more for non excavated slope 
compared to excavated slopes. The excavations 
do play a predominant role in decreasing the 
factor of safety of slopes at lower slope angles. 
The evolved failure surface for at w/h ratio of 
0.63 at 50m cover depth is shown in Figure 8. 
This case represents excessive pillar 
deformation due to pillars failure at low w/h 
ratio. 
 
 
Figure 7. Variation in factor of safety of slope against 
different w/h ratios for different slope angles - 50m depth 
of cover. 
 
 
Figure 8. Evolved failure surface using SSR method for 
50m cover depth at 45° slope angle for Highwall mining 
excavation with web pillar w/h ratio of 0.63. Pillar failure 
induced slope failure can be seen. 
 
 
 
 
 
SBMR 2014 
Figure 9 shows the safety factor of slopes 
for non-excavated slope decreases with the 
increased depth of cover to 100m for various 
slope angles compared to 50m depth of cover in 
Figure 7. The decrease in factor of safety was 
about 26% when cover depth increased from 50 
to 100m. The safety factor of the slope in Pre 
HWM stage when depth of cover is 100m at 45° 
slope angle is 1.86 as opposed to 2.49 for 50m 
depth of cover at the same slope angle. With the 
Highwall Mining extractions, the safety factor 
of the slope reduces to 1.57, 1.60, 1.61 and 1.68 
corresponding to w/h ratios 0.79, 0.945, 1.0 and 
1.5 respectively. An example of evolved failure 
surface for 100m cover depth at 45° slope with 
w/h ratio of web pillar of 0.79 is shown in 
Figure 10, indicating excessive pillar 
deformation at this low w/h ratio. The safety 
factor of slope when depth of cover is 100m at 
60° slope angle in Pre HWM stage is 1.47. With 
the Highwall Mining extractions, the safety 
factor of the slope reduces to 1.36, 1.37, 1.37 
and 1.40 corresponding to w/h ratios 0.79, 
0.945, 1.0 and 1.5 respectively. The safety 
factor of slope when depth of cover is 100m at 
70° slope angle in Pre HWM stage is 1.26. With 
the Highwall Mining extractions the safety 
factor of the slope reduces to 1.18, 1.19, 1.19 
and 1.21 corresponding to w/h ratios 0.79, 
0.945, 1.0 and 1.5 respectively. 
 
 
Figure 9. Variation in factor of safety of slope against 
different w/h ratios for different slope angles - 100m 
depth of cover. 
 
 
Figure 10. An example of evolved failure surface using 
SSR method for 100m cover depth at 45° slope angle for 
Highwall mining excavation with web pillar w/h ratio of 
0.79. Pillar failure induced failure can be seen. 
 
Figure 11 shows similar trend as that of 
previous figures. The safety factor of slope in 
Pre HWM stage when depth of cover is 150m at 
45° slope angle is 1.55. With the Highwall 
Mining extractions the safety factor of the slope 
reduces to 1.43, 1.44, 1.44 and 1.49 
corresponding to w/h ratios 0.885, 1.03, 1.085 
and 2.0 respectively. The safety factor of slope 
in Pre HWM stage when depth of cover is 150m 
at 60° slope angle is 1.20. With the Highwall 
Mining extractions the safety factor of the slope 
reduces to 1.14, 1.15, 1.15 and 1.16 
corresponding to w/h ratios 0.885, 1.03, 1.085 
and 2.0 respectively. The safety factor of Pre 
HWM slope when depth of cover is 150m at 
70° slope angle is 1.00. With the Highwall 
Mining extractions in three dimensions the 
safety factor of the slope reduces to 0.96, 0.96, 
0.97 and 0.98 corresponding to w/h ratios 
0.885, 1.03, 1.085 and 2.0 respectively. 
 
 
Figure 11. Variation in factor of safety of slope against 
different w/h ratios for different slope angles - 150m 
depth of cover. 
 
 
 
 
SBMR 2014 
However, the interesting aspect is that at 
larger depth of cover (150m in this case), the 
safety factor for excavated slope approaches the 
safety factor of non-excavated slope at larger 
w/h ratios of web pillars. It can be inferred that 
as w/h ratio of web pillar increases, the stiffness 
of the pillars also increases and this inturn 
contributes to the stability of overall slope. 
Another inference that can be made is that at 
larger depth of cover, the near critical slopes 
(close to factor of safety of 1.0) without 
Highwall excavations can become critical and 
fail with Highwall excavations. The criticality 
or risk of slope failure increases at lower w/h 
ratios of web pillars. 
 
3.3 Percentage Reduction in Safety Factor of 
Slopes for Various Extraction Ratios 
 
Figure 12 depicts the percentage reduction in 
safety factor of slope with increased extraction 
ratio of coal. As can be seen in the figure that 
the % reduction in slope safety factor is 
associated with the extraction ratios of Highwall 
mining of coal, and extraction ratio depends on 
the span of the excavation, height of excavation 
mining seam and the thickness of the web pillar. 
The percentage reduction in the factor of safety 
refers to the % decrease in the safety factor 
from un-excavated or pre Highwall mining 
slope of a fixed angle to the excavated slope 
with a particular extraction ratio of coal for that 
corresponding slope angle. The reduction in 
safety factor is quite remarkable for shallow 
slopes compared to higher slope angles for a 
given extraction ratio. 
 
 
Figure 12. Percentage reduction in safety factor of slope 
against extraction ratios for depth of cover 50m. 
 
With increased depth of cover (Figures 13 
and Figure 14), the % reduction in safety factor 
reduces at corresponding extraction ratios. This 
is because as the depth of cover increases, the 
factor of safety of non-excavated (Pre HWM) 
slope decreases. The larger extraction ratio 
would indicate excessive pillar deformation. 
 
 
Figure 13. Percentage reduction in safety factor of slope 
against extraction ratios for depth of cover 100m. 
 
 
Figure 14. Percentage reduction in safety factor of slope 
against extraction ratios for depth of cover 150m. 
 
3.4 Effect of Excavation Seam Height 
 
To analyse the effect of excavation height of 
coal seam, three different coal seam heights 
(2.0m, 4.0m and 6.0m) were considered. The 
web pillar widths were adjusted corresponding 
to the seam heights so as to maintain constant 
w/h ratios of web pillars. Three different w/h 
ratios of web pillars were considered (viz., 0.79, 
0.945 and 1.0) for each 2m, 4m and 6m 
excavation heights of coal seam. Table 3 
summarizes the web pillar widths adopted for 
2m, 4m and 6m excavation heights and 
corresponding w/h ratios of web pillars. Figure 
15 shows the variation of slope safety factor 
with increase in excavation height of coal seam 
 
 
 
 
SBMR 2014 
for different w/h ratios of web pillars. It can be 
seen in the figure that excavation height in the 
coal seam has remarkable effect on overall 
slope stability for all the w/h ratios considered 
in the modeling. At higher mining heights, the 
slope safety factor drastically reduces, even to 
the level of instability. 
 
Table 3. Web Pillar thicknesses adopted for different 
mining heights. 
 
Seam Height 
2m 
Seam Height 
4m 
Seam Height 
6m 
Pre 
Highwall 
Safety 
Factor 
1.86 1.85 1.81 
w/h ratio 
of web 
pillars 
Pillar 
width 
FOS 
Pillar 
width 
FOS 
Pillar 
width 
FOS 
0.79 1.58 1.57 3.16 1.1 4.74 1.08 
0.945 1.89 1.6 3.78 1.21 5.67 1.17 
1.0 2.0 1.61 4.0 1.24 6.0 1.2 
 
 
Figure 15. Effect of excavation seam height on factor of 
safety of slope for different web pillar w/h ratios. 
 
3.5 Effect of Depth of Cover 
 
Figure 16 shows the variation of slope safety 
factor with different cover depth for different 
slope angles for a given mining height. It may 
be noted that for a given mining height, the 
width of the web pillar can increase with depth 
so as to maintain constant web pillar factor of 
safety. For a given mining height it is evident 
that at a shallow depth of cover the drop in 
slope safety factor in post Highwall Mining is 
more prominent, but with increased depth the 
drop in slope safety factor diminishes. This is 
because with increased depth of cover, the pillar 
width also increases to compensate the 
overburden stress and this in turn increases the 
stiffness and confinement of the pillars and 
thereby reducing the gap in the safety factor 
between Pre and Post Highwall mining. 
 
 
Figure 16. Safety factor of slopes for different depth of 
cover and slope angles. 
 
3.6 Effect of Multiple Seams 
 
The effect of multiple seam excavations on the 
overall safety factor of slope is analysed. Two 
seams were considered in order to see their 
effects on slope stability. The geometry 
configuration discussed earlier on (Figure 3 and 
Figure 4) is used for numerical modeling. An 
open pit slope with 100m cover depth with an 
angle of 45° was simulated using the stage by 
stage excavation until the ultimate pit limit is 
reached. Figure 17 shows the variation in the 
slope safety factor prior to Highwall mining 
excavation, single seam excavation and two 
seam (multiple) excavations. The drop in the 
factor of safety was about 9% between the pre 
Highwall mining and single seam excavation 
and the drop was about 2% between single seam 
and two seams excavations for the assumed 
model configuration and properties. 
 
 
Figure 17. Comparison of safety factor of slope with web 
pillar w/h ratio for single and multiple seam extractions. 
 
 
 
 
SBMR 2014 
 
4 CONCLUSIONS 
 
A hyptothetical case was considered to gain 
understanding of the overall slope stability of 
Highwall mining with and without excavations 
by using shear strength reduction method. A 
parametric study was undertaken using the 
finite difference numerical code, FLAC3D by 
varying Highwall mining parameters such as 
width to height ratios of web pillars, mining 
excavation height of coal seam, depth of cover, 
slope angles and excavation of single and 
multiple coal seams. For the assumed material 
properties and model configuration, the 
modeling results revealed that stability of open 
pit slopes have profound influence on the 
Highwall mining parameters. In designing 
Highwall slopes for an open pit, the design must 
include Highwall mining excavations, otherwise 
near critical failure slopes would become 
critical and fail with Highwall excavations. The 
other inferences include: 
 
 The Highwall mining excavations can affect 
the overall slope stability, especially at 
lower width to height ratios of web pillars 
and with excessive web pillar deformation 
 In general, as the width to height ratio of 
web pillars increases, the factor of safety of 
the slope also increases. 
 At a larger depth of cover, the safety factor 
for the excavated slope tends to approach 
the safety factor of a non-excavated slope at 
larger w/h ratios of web pillars considered 
in the modelling. As the w/h ratio of web 
pillar increases, the stiffness of the pillars 
also increases and this in turn contributes to 
the stability of overall slope. 
 The near critical slopes (close to factor of 
safety of 1.0) without Highwall excavations 
can become critical and fail with Highwall 
mining excavations. The risk of slope 
failure can increase at shallower depths with 
Highwall excavations. 
 The percentage reduction in slope safety 
factor is associated with the extraction ratios 
of Highwall mining of coal. Increased 
extraction ratios can affect the slope 
stability as it decreases the width of the 
pillar resulting higherpillar deformation. 
 The excavation height in the coal seam has a 
dominant effect on overall slope stability, 
and the increased mining height of 
excavation has a profound effect on the 
slope safety factors with the parameters 
considered in the modeling. 
 For a given mining height at a shallow depth 
of cover, the drop in slope safety factor in 
post Highwall mining is more prominent 
compared to pre Highwall mining, but this 
drop appears to diminish with increased 
depth of cover. 
 Multiple seam extractions have shown to 
affect the overall slope stability for the 
considered Highwall configuration and 
material properties. 
 Strain softening material model instead of 
elasto-plastic material model will be 
considered in future study with higher 
cohesion values. 
 
 
ACKNOWLEDGEMENTS 
 
The authors express their gratitude to Australia 
India Strategic Research Fund (AISRF) for 
funding this research work. Views expressed are 
of the authors and not necessarily of the 
institutes to which they belong. 
 
 
REFERENCES 
 
Itasca Consulting Group, Inc. (2006). Fast Lagrangian 
Analysis of Continua in 3Dimensions FLAC 3D 
Manual, Itasca, Minneapolis, USA. 
Lorig, L. and Varona, P. (2000). Practical slope-stability 
analysis using finite-difference codes. Slope stability 
in surface mining. Littleton, Colorado: SME, Ch. 12, 
p. 115-124. 
Porathur, J.L., Karekal. S. and Palroy, P. (2013). Web 
pillar design approach for Highwall Mining 
extraction, International Journal of Rock Mechanics 
and Mining Sciences, Volume 64, December 2013, 
Pages 73-83, ISSN 1365-1609 
Zipf, R.K. (1999). Catastrophic collapse of Highwall web 
pillars and preventative design measures, Proceedings 
of 18th Conference on Ground Control in Mining, 
 
 
 
 
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West Virginia University, pp. 18-28. 
Zipf, R.K. and Bhatt, S.K. (2004). Analysis of Practical 
Ground Control Issues in Highwall Mining, 
Proceedings of 23rd Conference on Ground Control 
in Mining, West Virginia University, pp. 210-219.