Geomaterials, 2011, 1, 1-13
doi:10.4236/gm.2011.11001 Published Online April 2011 (
Copyright © 2011 SciRes. GM
A Coal Mine Dump Stability Analysis
A Case Study
Ashutosh Kainthola*, Dhananjai Verma, S. S. Gupte, T. N.Singh
Department of Earth Sciences, Indian Institute of Technology Bombay , Mumbai, India
Received April 12, 2011; revised April 14, 2011; accepted April 18, 2011
The present paper mainly deals with a case study of failed dump slope in western coalfield limited, Nagpur,
India. A huge mass of debris flow had happened during the routine the activity of mining. The failed dump
had a height of 75 m with 43˚ slope angle which had slipped forward by 18m. Representative loose dump
material samples were collected from the site and tested to determine the physico-mechanical properties of
dump material. The dump material consisted of loose fragments and lumps of friable sandstone, shale, clay
and carbonaceous shale. To evaluate the condition of failure, a well known, shear strength reduction tech-
nique has been applied to achieve the desired factor of safety using two dimensional finite element code. Fi-
nally, a economical, sustainable and stable dump angle and height has been suggested for smooth and safe
disposal of the dump.
Keywords: Dump Stability, Factor of Safety, Shear Strength Reduction, Wardha Valley
1. Introduction
With the increasing size of opencast mines and the large
stripping ratio associated with these mines, the amount of
overburden removal will also increase substantially.
Only Coal India Limited (CIL) has removed overburden
21, 160, 462 and 695 million cubic metres during 1976,
1986 - 87, 1999 - 2000 and 2009 - 2010 respectively.
Considering the major opencast projects XII Five Year
plan, the amount of overburden to be handled in near
future will be more than 20 000 million cubic metres for
these projects alone. The maximum overburden disposal
from an individual mine in the coal sector is likely to be
50 million cubic metres per year (WCL).
Overburden dumps can be external dumps created at a
site away from the coal bearing area or it can be internal-
dumps created by in-pit dumping (IPD) concurrent to the
creation of voids by extraction of coal. Practice of
dumping overburden in the external dumps have some
serious problems [1] foremost amongst them are re-
quirement of additional land, involves very high trans-
port and rehandling cost which will increase the cost of
coal production subs tantially, stability an d reclamation at
the site. It is not possible to eliminate the option of the
external dumps concept completely, even if we adopt
IPD practice. The internal dump concept is very well
utilized by various local produ cing countries like Austra-
lia, Canada and USA, then there is no fear to adopt this
technique to avoid further requirement of land for
dumping and aggravate various associated problems.
However, the combination of external dumps and inter-
nal dumps shall substantially reduce the required land.
As a result, it shall reduce the surface land requirement
significantly which is very difficu lt task to arrang e in an y
area due to growth of population forest cover and associ-
ated problem. In this decade few destabilization of in ter-
nal dumps have taken place in coal mines. It is necessary
to study such cases and find out the cause of destabiliza-
2. Destabilization of Internal Dumps
Failure of internal dumps is a complex problem. In addi-
tion to environmental considerations, it directly affects
the resource recovery, mine safety and mining cost.
Overburden has been traditionally disposed off in the
most economical way throughout the world. However,
some massive spoil pile failures around the world attract
the attention of geo-scien tists and engineers to this prob-
lem [2-4]. The stability of dump is now reco gnized to be
an important aspect of designing large open pit mines.
The majority of slope stability analyses performed in
practice still use traditional limit equilibrium approaches
involving methods of slices that have remained essen-
tially unchanged for decades. The finite element method
represents a powerful alternative approach for slope sta-
bility analysis which is use to accurate, versatile and re-
quires fewer a priori assumptions, especially, regarding
the failure mechanism. Slope failure in the finite element
model occurs ‘naturally’ through the zones in which the
shear strength of the dump material is insufficient to re-
sist the shear stresses. It is argued that the finite element
method of slope stability analysis is a more powerful
alternative to traditional limit equilibrium methods and
its wide-spread use should be standard in geotechnical
practice [5].
Elasto-plastic analysis of geotechnical problems using
the finite element method (FEM) has been widely ac-
cepted in the research area for many years; however, its
routine use in geotechnical practice for slope stability
analysis still remains limited. The challenge for an ex-
perienced engineer is to know which kind of problem
would benefit from a FEM treatment and which would
not. In general, linear problems such as the predictions of
settlements and deformations, the calculation of flow
quantities due to steady seepage or th e study of transient
effects due to consolidation are all highly amenable to
solution by finite element.
3. Factors Affecting Dump Failure
In dump materials, failure is likely to form as a shallow,
large radius surface extending from a tension crack close
behind the crest to the toe of the slope (Figure 1). It fails
in a circular manner as the slope dimensions are substan-
tially greater than the dimensions of the rock fragments.
The actual shape of the “circular” slide surface is influ-
enced by the geological conditions in the slope mass.
The slopes with loose, weak and soft rock masses can
basically have three different failure geometries viz.
slope failure, toe failure and base failure (Figure 2). In
the slope failure, the arc of rupture surface meets the
slope above the toe. This is possible only when slope
angle steep and the soil close to the toe possess high
strength. Toe failure occurs when the soil mass of the
slope above the base and below the base is mainly ho-
There are a number of factors, which affect the dump
stability [6-8]. These factors are broadly classified as:
Geometry and strength of the dump material,
Hydro geological and rain water condition of
dumping area,
Load bearing capacity of dumping ground, and
External loading conditions
The slope geometry and the geo-mechanical strength
of the dump material always control the stability of the
dump [2,9,10]. Dump material is anisotropic in their be-
havior and its stress-strain behavior is quite erratic, ow-
ing to the presence of clay mineral. The visco-elastic
behavior due to the presence of water poses serious
threat during the rainy season. The shear strength reduc-
tion due to rise in pore water pressure leads to the failure.
Consolidation and compaction is another key factor be-
cause of the uneven size distribution of the dump mate-
Bearing capacity of the ground has a direct influence
on the stability of the dump slope. A sloping ground with
low bearing capacity lead destabilizing of the dump
slope stability due to foundation failures [11].
High variation in temperature can cause dump material
to spall due to the accompanying dilation. Water freezing
in voids may causes damage by further loosening the
slope material. Repeated freeze/thaw cycles may result in
gradual loss of strength. Except for periodic maintenance
requirements, temperature effects are a surface phe-
nomenon and are most likely of little concern for final
waste dump slopes. However, in a few cases, surface
weakening could activate dump slope instability in large
scale [12].
Figure 1. View of the failed dump slope.
Copyright © 2011 SciRes. GM
Figure 2. Different type s of failure in waste dump slopes.
Ground water and surface water flow condition plays a
critical role on the stability of dump slope. Frictional
strength is reduced as the height of the dump is raised,
due to the presence of water. The pore water pressure
reduces the normal stress acting on the material. The
cohesive strength of weak geo-material will be further
reduced due to presence of pore water pressure. Waste
dump placed in a loose state, shear failure is often fol-
lowed by static liquefaction which is complete loss of
strength [13]. The flow of water may enhance the seep-
age force, leading to the formation and migration of ten-
sion cracks.
Erosion also plays an important role in affecting the
stability by surface weathering [14], groundwater or sur-
face-water seepage [15,16], toe erosion and slope modi-
fication [17], primary is massive scale erosion, such as
river erosion at the toe of the river bed. The other one is
comparatively localized erosion caused due to ground-
water or surface runoff. Erosion changes the waste dump
slope geometry and morphological characters. The
weathering and attrition of material at the toe of a poten-
tial slide reduces the restraining force that may destabi-
lize the slope. Erosion of void filling material, or zones
of percolation, can efficiently decrease cohesion among
grain boundaries. The decease of cohesive strength nota-
bly reduces the rock mass shear strength. The decrease in
shear strength may allow movement in dump slope along
weak plane. In addition, lo calized erosion may also result
in increased permeability and ground-water flow either
the slope. This may lead to failure due to formation of
gully and deep flow channels in dump.
4. Failure Criterion and Geo- Mechanical
Strength Parameters of the Dump
Mohr- Coulomb’s failure criterion was used for the nu-
merical analysis of the dump material. The soft and loose
mine dumps are mostly fine grained material. The shear
failure in dumps is by slippage of particles shear failure.
The failure is caused by a critical combination of normal
and shear stresses. The dump material fails when the
shear stress on the failure plane at failure is an exclusive
function of the normal stress acting on that plane.
This failure criterion is concerned with the sh ear stress
at failure plane at failure. The curve defined by this is
known as the failure envelope. The shear strength of a
material at a point on a particular plane was expressed by
Coulomb as a linear function of the normal stress on that
plane, τ = c + tan(φ), where τ is the shear strength, σ is
Copyright © 2011 SciRes. GM
normal stress, φ is angle of internal friction and C is the
cohesive strength of the material [18].
Shear potency of the waste dump material is the criti-
cally important parameter in stability analysis. The loose
broken material usually has low shear strength but its
strength increases with time as it becomes more and
more compact. Therefore, the evaluation of shear strength
with rational exactness is a condition for the stability
analysis of the dump slopes [19]. The various laboratory
techniques have been used by different researchers to
determine the shear strength of rock and soil [20-22].
The strength of the material depends on the grain size, as
well as the interlocking of the material. The deform-
ability has been found to be associated with arrangement
of the granular m at e rial as well its compaction [23].
In the studied dump slope the material consists of
fragments of friable sandstone, clay, shale, carbonaceous
shale. The fragment dump samples from five different
zones from the slope were collected for determination of
the geo-mechanical parameters of the failure criterion.
The fragments range in size from 1 / 256 mm to 1m. The
tests were carried out as per the specification of stan-
dards [20,21]. The samples were tested for the measure-
ment of their cohesive strength, angle of internal friction,
elastic modulus and p oisso ns rat i o.
The average an nual rainfall in the stud ied area is 1200
mm, to take into account the effect of water on the dump
material so test were conducted under 30% saturation
level. The mean value of the five tested sample were
taken for the numerical solution of the dump stability
problem (Table 1).
5. Simulation of the Dump
The dump slope was numerically analyzed using a finite
element code. The finite element code (FEM) is a con-
tinuum model which can be used for analysis of complex
geometries, stress modelling and material behaviour. In
the FEM, the continuum structural system is modelled by
a set of appropriate finite elements interconnected at
points called nodes. Elements may have physical as well
as elastic properties such as thickness, density, Young’s
modulus, shear modulus and Poisson's ratio. The ele-
ments are interconnected only at the exterior nodes, and
altogether they cover the entire domain as accurately as
possible. Nodes have nodal (vector) displacements or
degrees of freedom which may include translations, rota-
tions, and for special applications, higher order deriva-
tives of displacements. When the nodes displace, they
drag the elements along in a certain manner dictated by
the element formulation. In other words, displacements
of any points in the element will be in terpolated from the
nodal displacements, and this is the main reason for the
approximate nature of the solutio n. A uniform mesh with
6 noded trianglular elements was used for the analysis
mine dump. A major advantage of the finite element-
SSR method is that it does not demand any earlier as-
sumptions on the nature of failure mechanisms. The
Shear Strength Reduction (SSR) technique in the finite
element method involves successive reduction (by some
factors) in the shear strengths of the slope forming mate-
rial until it fails, which is indicated by the non conver-
gence to a solution of the finite element model [25-27].
For Mohr-Coulomb material shear strength reduction
factor (factor of safety) F can be determined from the
where τ is the shear strength of the material and F is the
strength reduction factor (SRF)or the factor of safety
Table 1. Strength parameters for the dump material.
Dump material Sr. no. Unit weight
(MN/m3) Elastic modulus
(MPa) Poissons’s
ratio Peak cohesion
Peak angle of
internal friction
cohesion (KPa)
Residual angle
of internal
friction (˚)
1 0.0240 68 0.31 95 23.5 39 19.5
2 0.0245 73 0.33 88 24 37 20.4
3 0.0243 70 0.31 93 25.5 38.4 22.3
4 0.0248 77 0.34 83 24.6 34 21.9
5 0.0245 75 0.33 88 25 35 22
6 0.0247 71 0.34 85 25.2 35.7 23
Compacted fragments
of friable sandstone,
shale, clay and
carbonaceous shale
Value 0.0244 72.3 0.326 88.6 24.6 36.5 21.5
Copyright © 2011 SciRes. GM
Copyright © 2011 SciRes. GM
6. Numerical Analysis of the Dump Slope
The failed dump slope having an initial height of 75 m
and 43˚ slope angle was simulated with the determined
strength parameters in the laboratory to optimize the
slope. The back analysis of the failed slope was done for
validation of the simulation which gave a SRF of 0.8
(Figure 3). The simulation provides a displacement of
1.3 m for the critical SRF of 0.8, which will increase to
68 m for the critical SRF value of 1. The slope angles
were reduced successively at an interval of 2˚, keeping
rest of the parameters constant. When the safety factor of
1.3 was achieved, the slope height was raised keeping the
rest of the input parameters constant, till the safety factor
reduced to 1.2. An effective shear stress of 0.29 MPa had
developed inside the dump above the critical SRF of 0.8
which lead to the failure, when at an angle of 43˚ (Figure
3 & 4). With the dump slope angle of 41˚ and slope height
of 75 m, the SRF achieved was 0.82 generating a maxi-
mum shear stress 0.075 MP a a t t he t oe dump (Figure 13).
The safety factor increased to 0.87 when the mine
dump angle was reduced to 39˚, the maximum effective
stress at this critical SRF was 0.094 MPa. The plot shows
the maximum shear strain concentration for the SRF
above the critical value to get a better picture of the shear
strain focus (Figure 5).
Figure 3. Back analysis of the failed dump slope at 43˚ inclination showing maximum strain concentration and the deformed
Figure 4. Effective stress variation along the distance A-B, for the failed dump slope.
Copyright © 2011 SciRes. GM
A safety factor of 0.92 was yielded by the model at a
slope angle of 37˚, the maximum shear stress developed
was 0.14MPa at the toe of the dump slope. Most of the
mining companies go for the dump angle ranging be-
tween 37˚ - 39˚, but the FOS yielded for these dump an-
gles are less than 1 and at very critical state. Dump angle
was further reduced to reach the optimum FOS.
The safety factor further increased to 0.94 when the
dump angle was reduced by 2˚ to 35˚ (Figure 6). The
critical SRF of 1 was achieved with the dump angle of
33˚. The aim was to reach the safety factor of 1.3 for the
mine dump as with FOS value of 1, the dump is only
theoretically stable as the driving and resisting forces for
the failure are in equilibrium with each other [28].
With additional reduction in dump angle by 2˚, keep-
ing the dump angle at 31˚, the FOS achieved was 1.07
(Figure 7). For the dump angle of 29˚ the analysis gave a
critical SRF of 1.13 with maximum shear stress of 0.26
Diminution of dump angle to 27˚ yielded the safety
factor of 1.2 and further lessening of the dump angle to
25˚ gave the desired safety fact or of 1.3 (Figures 8 and 9).
Once the FOS reached up to 1.3, the slope height was
raised from 75 m to 95 m at an interval of 5m to see the
effect of increase in height on the FOS as well as to op-
timize the dump for safe and stable accommodation of
the mine waste. The material properties were kept con-
stant as like in their earlier case.
Figure 5. Maximum shear strain plot for the dump with a slope angle of 39˚.
Figure 6. Maximum shear strain plot for the dump with a slope angle of 35˚.
Figure 7. Total displacement plot with displacement vectors with for 31˚ slope angle.
Figure 8. Total displacement plot with displacement vectors with for 27˚ slope angle.
Figure 9. Maximum shear strain plot shear and tension points for dump slope angle of 25˚.
Copyright © 2011 SciRes. GM
Copyright © 2011 SciRes. GM
The FOS reduced by 3% when the dump height was
raised by 5 m (Figure 10). Effective shear stress of 0.045
MPa was developed at the toe of the dump with a height
of 80 m.
A critical safety factor of 1.22 was attained when the
dump height was raised to 85m (Figure 11). It is it in-
teresting to observe that, even changing the height from
85 m to 90 m does not affect FOS.
When the height of the dump was raised further more
with slope angle of 25˚, the FOS reduced up to 1.2 (Fig-
ure 12). The simulation was stopped at 95 m slope
height as raising more height would have given a much
lesser value of FOS, which is undesired and not appro-
7. Discussion
A 75 m high failed dump slope consisting of low
strength material was numerically solved and analyzed
for slope angle optimization. The investigated mine
dump having weak material strength gave a FOS value of
1 when it was at angle of 33˚ i.e. 10˚ less the original
value at which it has already failed. Keeping the Dump
height constant at 75 m the FOS varied logarithmically
with th e dump slope ang le (Figure 13). FOS can be cor-
related with dump angle for the 75 m high studied dump
Figure 10. Maximum shear strain plot with tension and shear points for slope angle 25˚ and dump height of 80 m.
Figure 11. Maximum shear strain plot with tension and shear points for slope angle 25˚ and dump height of 85 m.
Figure 12. Stress trajectories for dump slope angle 25˚ and dump height of 95 m.
Figure 13. Correlation between dump slope angle Vs. FOS with constant slope he ight of 75 m.
by the equation
FOS= –0.91 ln (slope angle) + 4.215 (2)
The aim was to reach a FOS of 1.3 for the optimum
stability of the dump which was attained at an angle of
25˚ (Table 2). The effective shear stress along the dump
slope length has been calcu lated to increase with FOS of
the dump. This increase in stress can be attributed to the
fact that, with higher SRF value for the model, the shear
strength of the material will reduce significantly above
the critical SRF value (Figure 14).
The effective shear stresses have a similar trend for the
different dump angles and FOS or SRF values along the
dump slope length (Figure 15).
The effective shear stress variation for different
FOS/SRF values has been depicted against slope length
which has been divided into 200 equal divisions for each
lope length (Figures 14 and 15). s
Copyright © 2011 SciRes. GM
Table 2. Variation of FOS with respect to slope angle with constant height of 75 m.
Slope angle FOS or SRF Percentage Increase in FOS
430 0.8 0
41 0.82 2.5
390 0.87 .09
370 0.92 5.7
350 0.94 2.1
330 1.0 6.3
310 1.07 7
290 1.13 5.6
270 1.2 6.2
250 1.3 8.3
Figure 14. Effective shear strain plot along the slope length for different dump angles.
Figure 15. Effective shear strain plot along the slope length for different dump height.
Copyright © 2011 SciRes. GM
For better efficiency of the dump, managements the
height was raised to keeping the dump angle constant at
25˚, till the FOS was reduced to 1.2 (Table 3). The FOS
again reduced logarithmically with respect to the raise in
dump height, Similar observation was expressed previ-
ously by an author [30] (Figure 16). For the inve stigated
dump with slope angle of 25˚ the FOS can be estimated
with respect to the dump height using the empirical equa-
FOS= –0.41 ln (dump height) + 3.059 (3)
Effective shear stress has increased with rise in dump
height reaching a maximum value of 0.16 MPa for the 95
m dump (Figure 15). The results have only minor incon-
sistencies, as there was no significant reduction in FOS
when the dump height was raised from 85 m to 90 m.
But the maximum effectiv
ngth was raised
The generation of tension and shear zones on the
dump slope model when in the critical state gives us a
great deal of information about the critical zones which
can be used for the stabilization. The rear vicinity and the
slope surface of the dump are undergoing tension avoid
of failure while the maximum shear strain concentration
is at depth of 14 m from the dump surface (Figure 9).
Generally, waste dumps are designed for factor of
safety of 1.10 to 1.15 have only a minor risk of failure
under saturated conditions. Waste dumps with a FOS less
than 1.1 are subject to always used great risk, even with
accurate data, it is due to the anomalous conditions re-
lating to height and strength of dump material or the un-
derlay which are likely to be present withthe dump.
Such anomalous conditions may result in loal fluctua-
e heig
e shear stress along the slope
from 0.08 MPa to 0.1 MPa (Figure tion in the FOS by 10%.
Table 3. Variation of FOS w.r.t slop
Dump Height (m) FOS or SR
ht with constant 25˚ slope angle.
Percentage Reduction in FOS
75 1.3
80 1.26 3.07
85 1.22 3.17
90 1.22
95 1.2
Figure 16. Variation of FOS with respect to the dump height (constant dump angle 25˚) .
Copyright © 2011 SciRes. GM
8. Conclusion
In pit dumping , particularly in Wes tern coal field Ltd.,
is very crucial due to the none availability of land as well
as weak rock cond ition as well as heavy rain. In the pre-
sent study a failed dump slope problem from India was
numerically examined and based on back analysis, an
optimum dump angle and height have been suggested for
safe and stable dumping. The factor of safety was found
to increase logarithmically with red uction in dump angle
while keeping the dump height constant at 75 m. The
dump slope gave an FOS of 1.3 when at an angle of 25˚
and height of 85 m and hence owing to the weak geo-
mechanical strength of the dump it was suggested to
keep the flatter slope of 25˚ with a height of 75 m. The
FOS also reduced logarithmically when the dump height
was raised up till 25 m. The numerical study provides a
comprehensive understanding about the slope mecha-
nism failure in weak material.
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