DESIGN OF STABLE SLOPE FOR OPENCAST MINES

DESIGN OF STABLE SLOPE FOR OPENCAST MINES
DESIGN OF STABLE SLOPE FOR OPENCAST MINES
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
Bachelor of Technology
in
Mining Engineering
By
BISLESHANA BRAHMA PRAKASH
10505020
Department of Mining Engineering
National Institute of Technology
Rourkela-769008
2009
DESIGN OF STABLE SLOPE FOR OPENCAST MINES
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
Bachelor of Technology
in
Mining Engineering
By
BISLESHANA BRAHMA PRAKASH
Under the Guidance of
DR. SINGAM JAYANTHU
&
DR. DEBI PRASAD TRIPATHY
Department of Mining Engineering
National Institute of Technology
Rourkela-769008
2009
National Institute of Technology
Rourkela
CERTIFICATE
This is to certify that the thesis entitled “Design of Stable Slope for Opencast Mines” submitted
by Sri Bisleshana Brahma Prakash, Roll No. 10505020 in partial fulfillment of the requirements
for the award of Bachelor of Technology degree in Mining Engineering at the National Institute
of Technology, Rourkela (Deemed University) is an authentic work carried out by him under our
supervision and guidance.
To the best of our knowledge, the matter embodied in the thesis has not been submitted to any
other University/Institute for the award of any Degree or Diploma.
Dr. Singam Jayanthu
Department of Mining Engineering
National Institute of Technology
Dr. Debi Prasad Tripathy
Department of Mining Engineering
National Institute of Technology
Rourkela-769008
DATE:
Rourkela-769008
DATE:
ACKNOWLEDGEMENT
My heart pulsates with the thrill for tendering gratitude to those persons who helped me in
completion of the project.
First and foremost, I express my sincere gratitude and indebtedness to Dr. S. Jayanthu,
Professor and Head of the Department and Dr. Debi Prasad Tripathy, Professor for
allowing me to carry on the present topic “Design of Stable Slope for Opencast Mines” and later
on for their inspiring guidance, constructive criticism and valuable suggestions throughout this
project work. I am very much thankful to them for their able guidance and pain taking effort in
improving my understanding of this project.
I am thankful to mine officials of Jindal Opencast Mine at Raigarh who have extended all sorts
of help for accomplishing this undertaking. I am also thankful to all staff members of
Department of Mining Engineering, NIT Rourkela
An assemblage of this nature could never have been attempted without reference to and
inspiration from the works of others whose details are mentioned in reference section. I
acknowledge my indebtedness to all of them.
At the last, my sincere thanks to all my friends who have patiently extended all sorts of help for
accomplishing this assignment.
Bisleshana Brahma Prakash
DATE:
Dept. of Mining engineering
National Institute of Technology
Rourkela – 769008
i
ABSTRACT
Slope stability analysis forms an integral part of the opencast mining operations during the life
cycle of the project. In Indian mining conditions, slope design guidelines were not yet formulated
for different types of mining practices and there is a growing need to develop such guidelines for
maintaining safety and productivity. Till date, most of the design methods are purely based on
field experience, rules of thumb followed by sound engineering judgment. During the last four
decades, the concepts of slope stability analysis have emerged within the domain of rock
engineering to address the problems of design and stability of excavated slopes. The basic
objective of the project is primarily addressed towards: a) Understanding the different types and
modes of slope failures b) Designs of stable slopes for opencast mines using numerical models.
Analyses were conducted using the finite difference code FLAC/Slope. The work was aimed at
investigating failure mechanisms in more detail, at the same time developing the modeling
technique for pit slopes. The results showed that it was possible to simulate several failure
mechanisms, in particular circular shear and toppling failure, using numerical modeling. The
modeling results enabled description of the different phases of slope failures (initiation and
propagation). Failures initiated in some form at the toe of the slope, but the process leading up to
total collapse was complex, involving successive redistribution of stress and accumulation of
strain. Significant displacements resulted before the failure had been developed fully. Based on
parametric studies it can be concluded that friction angle plays a major role on slope stability in
comparison to Cohesion.
Keywords: Slope stability, open pit mining, numerical modeling, rock mass strength, failure
mechanisms.
ii
ITEM
TITLE
PAGE NO.
1
ACKNOWLEDGEMENT
i
2
ABSTRACT
ii
3
List of Figures
vi
4
List of Tables
vii
Chapter: 01
INTRODUCTION
1
1.1 Overview
2
1.2 Objectives
3
1.3 Research Strategies
3
1.4 Outline of Report
3
Chapter: 02
LITERATURE REVIEW
4
2. Open Pit Slopes — An Introduction
5
2.1 Slope Stability
5
2.2 Types of Slope Failure
11
2.2.1 Plane Failure
11
2.2.2 Wedge Failure
14
2.2.3 Circular Failure
17
2.2.4 Two Block Failure
18
2.2.5 Toppling Failure
19
2.3 Factors To Be Considered In Assessment Of Stability
19
2.3.1 Ground Investigation
19
2.3.2 Most Critical Failure Surface
20
2.3.3 Tension Cracks
21
iii
2.3.4 Submerged Slopes
22
2.3.5 Factor Of Safety
22
2.3.6 Progressive Failure
23
2.3.7 Pre-Existing Failure Surfaces
23
23
2.4 Methods Of Analysis
2.4.1 Wedge Failure Analysis
2.4.1.1 Spherical Projection Solution using Factor of
23
23
Safety
2.4.1.2 Chart Solution
24
2.4.1.3 Spherical Projections Solutions using
24
Probabilistic Approach
2.4.2 Circular Failure Analysis
24
2.4.2.1 Method of Slices
24
2.4.2.2 Modified Method of Slices
25
2.4.2.3 Simplified Method of Slices
25
2.4.2.4 Friction Circle Method
25
2.4.2.5 Taylor’s Stability Number
25
2.4.3 Two Block Failure Analysis
2.4.3.1 Stereographic Solution
Chapter: 03
26
26
2.4.4 Toppling Failure Analysis
26
2.4.5 Other Methods Of Analysis
26
2.4.5.1 Limit Equilibrium Method
26
2.4.5.2 Stress Analysis Method
27
NUMERICAL MODELLING
iv
28
3.1 Introduction
29
3.1.1 Continuum Modelling
30
3.1.2 Discontinuum Modelling
31
3.1.3 Hybrid Techniques
31
3.2 General Approach of FLAC
32
3.3 Overview
38
3.4 Summary of Features
39
3.5 Analysis Procedure
40
Chapter: 04
CASE STUDY
41
4.1 Introduction
42
4.2 Geology
42
4.3 Data Collection
44
4.4 Laboratory Test
44
4.4.1 Sample Preparation
44
4.4.2 Triaxial Testing Apparatus for Determination of
45
Sample Properties
4.4.3 Test Procedure
46
4.5 Parametric Studies
48
4.6 Results and Discussions
53
Chapter: 05
CONCLUSION
54
5.1 Conclusion
55
5.2 Scope for Future Work
56
REFERENCE
v
57
LIST OF FIGURES
SL. NO.
Fig. 2.1
TITLE
Diagram showing bench, ramp, overall slope and their respective
PAGE NO.
6
angles
Fig. 2.2
Different types of joints and faults
7
Fig. 2.3
Plane failure
11
Fig. 2.4
Geometries of plane slope failure: (a) tension crack in the upper
12
slope; (b) tension crack in the face
Fig. 2.5
Wedge failure
14
Fig. 2.6
Conditions of effective forces in the wedge failure analysis
15
Fig. 2.7
Diagram of the plane normal to the intersection of joint sets 1 and 2
15
Fig. 2.8
The geometry of the sliding wedge
16
Fig. 2.9
Three-dimensional failure geometry of a rotational shear failure
18
Fig. 2.10
Toppling failure
19
Variety of slope failure circles analysed at varying radii from a
20
Fig. 2.11(a)
centre
Variation of factor of safety with radius
21
Fig. 2.12
Effect of tension crack at the head of a slide
21
Fig. 3.1
Spectrum of modeling situations
33
Fig. 3.2
Flow chart for determination of factor of safety using FLAC/Slope
37
Fig. 4.1
A typical triaxial test apparatus
45
Fig. 4.2
Mohr’s circle for determination of cohesion and angle of internal
48
Fig. 2.11(b)
friction
Fig. 4.3
Projected pit slope
49
Fig. 4.4
Some models developed by FLAC/Slope with varying cohesion and
49
friction angle
Fig. 4.5
Variation of factor of safety with friction angle for different
cohesion
vi
53
LIST OF TABLES
SL. NO.
TITLE
Table 2.1
Guidelines for equilibrium of a slope
22
Table 3.1
Numerical methods of analysis
29
Table 3.2
Recommended steps for numerical analysis in geomechanics
33
Table 4.1
The lithology of the seams
43
Table 4.2
Details of the seams
44
Table 4.3
Dimensions of the tested samples
45
Table 4.4
Readings of proving and deviator and dial gauge
47
Table 4.5
Safety factors for various slope angles (Depth= 116m)
49
Table 4.6
Safety factors for various C and Ø values (Depth= 116m)
52
vii
PAGE NO.
CHAPTER: 01
INTRODUCTION
-1-
CHAPTER: 01
INTRODUCTION
1.1 Overview
Slope stability analysis forms an integral part of the opencast mining operations during the life
cycle of the project. In Indian mining conditions, slope design guidelines are yet to be formulated
for different types of mining practices and there is a growing need to develop such guidelines for
maintaining safety and productivity. Till date, most of the design methods are purely based on
field experience, rules of thumb followed by sound engineering judgment. During the last four
decades, the concepts of slope stability analysis have emerged within the domain of rock
engineering to address the problems of design and stability of excavated slopes.
In India, the number of operating opencast mines is steadily increasing as compared to
underground mines. It is due to low gestation period, higher productivity, and quick rate of
investment. On the contrary, opencast mining attracts environmental concerns such as solidwaste management, land degradation and socio-economical problems. In addition to that a large
number of opencast mines, whether large or small, are now days reaching to deeper mining
depths. As a result analysis of stability of operating slopes and ultimate pit slope design are
becoming a major concern. Slope failures cause loss of production, extra stripping cost for
recovery and handling of failed material, dewatering the pits and sometimes lead to mine
abandonment/premature closure.
Maintaining pit slope angles that are as steep as possible is of vital importance to the reduction of
stripping (mining of waste rock), which will in turn have direct consequences on the economy of
the mining operation. Design of the final pit limit is thus governed not only by the ore grade
distribution and the production costs, but also by the overall rock mass strength and stability. The
potential for failure must be assessed for given mining layouts and incorporated into the design of
the ultimate pit.
Against this backdrop, there is a strong need for good practices in slope design and management
so that suitable corrective actions can be taken in a timely manner to minimize the slope failures.
-2-
1.2 Objectives
The prime objectives of the project are addressed towards:
a) Understanding the different types and modes of slope failures; and
b) Designing of stable slopes for opencast mines using numerical models.
1.3 Research Strategies
Extensive literature review has been carried out for understanding the different types and modes
of slope failures. Numerical model FLAC/Slope was critically reviewed for its application to
evaluation of the stability of slopes in opencast mines. Field investigation was conducted in
Jindal Opencast Mine with 116 m ultimate pit depth at Raigarh in Chhattisgarh State.
Laboratory tests were conducted for the rock samples taken during field investigation.
Parametric studies were conducted through numerical models (FLAC/Slope) to study the effect
of cohesion (140-220 kPa) and friction angle (20°-30° at the interval of 2°). Pit slope angle was
varied from 35° to 55° at an interval of 5°.
1.4 Outline of Report
Following the introductory chapter, a general description of the economics of open pit mining,
slope stability, failure modes and failure mechanisms, the assessment of slope stability and
different methods of analysis are discussed in Chapter 2.
In Chapter 3, numerical modelling (FLAC) has been described, starting with FLAC’s overview
followed by summary of its features and finally analysis procedure. Application of numerical
modelling is given through a case study of “Jindal Power OCP, Mand Raigarh Coalfield” in
Chapter 4. Chapter 5 deals with conclusion and scope for future work.
-3-
CHAPTER: 02
LITERATURE REVIEW
-4-
CHAPTER: 02
LITERATURE REVIEW
2. Open Pit Slopes —An Introduction
In open pit mining, mineral deposits are mined from the ground surface and downward.
Consequently, pit slopes are formed as the ore is being extracted. It is seldom, not to say never,
possible to maintain stable vertical slopes or pit walls of substantial height even in very hard and
strong rock. The pit slopes must thus be inclined at some angle to prevent failure of the rock
mass. This angle is governed by the geomechanical conditions at the specific mine and represent
an upper bound to the overall slope angle. The actual slope angles used in the mine depend upon
(i) the presence of haulage roads, or ramps, necessary for the transportation of the blasted ore from
the pit (ii) possible blast damage (iii) ore grades, and (iv)economical constraints.
2.1 Slope Stability
Slope stability problem is greatest problem faced by the open pit mining industry. The scale of
slope stability problem is divided in to two types:
Gross stability problem: It refer to large volumes of materials which come down the
slopes due to large rotational type of shear failure and it involves deeply weathered rock
and soil.
Local stability problem: This problem which refers to much smaller volume of material
and these type of failure effect one or two benches at a time due to shear plane jointing,
slope erosion due to surface drainage.
To study the different types and scales of failure it is essential to know the different types of the
failure, the factors affecting them in details and the slope stability techniques that can be used for
analysis. The different types of the slope failure, factors affecting them, stability analysis
techniques and software available have been described in the following sections:.
-5-
Factors Affecting Slope Stability
Slope failures of different types are affected by the following factors:
2.1.1 Slope Geometry
The basic geometrical slope design parameters are height, overall slope angle and area of failure
surface. With increase in height the slope stability decreases. The overall angle increases the
possible extent of the development of the any failure to the rear of the crests increases and it
should be considered so that the ground deformation at the mine peripheral area can be avoided.
Generally overall slope angle of 45° is considered to be safe by Directorate General of Mines
Safety (DGMS). The curvature of the slope has profound effect on the instability and therefore
convex section slopes should be avoided in the slope design. Steeper and higher the height of
slope less is the stability. Diagram showing bench, ramp, overall slope and their respective
angles is given in Fig. 2.1.
Fig. 2.1 Diagram showing bench, ramp, overall slope and their respective angles (after
Coates, 1977, 1981)
2.1.2 Geological Structure
The main geological structure which affect the stability of the slopes in the open pit mines are:
1. amount and direction of dip
-6-
2. intra-formational shear zones
3. joints and discontinuities
a) reduce shear strength
b) change permeability
c) act as sub surface drain and plains of failure
4. faults
a) weathering and alternation along the faults
b) act as ground water conduits
c) provides a probable plane of failure
Fig. 2.2 Different types of joints and faults (partly after Nordlund and Radberg, 1995)
Instability may occur if the strata dip into the excavations. Faulting provides a lateral or rear
release plane of low strength and such strata plan are highly disturbed. Localized steepening of
strata is critical for the stability of the slopes. If a clay band comes in between the two rock
bands, stability is hampered. Joints and bedding planes also provide surfaces of weakness.
Stability of the slope is dependent on the shear strength available along such surface, on their
orientations in relation to the slope and water pressure action on the surface. These shear strength
that can be mobilized along joint surface depending on the functional properties of the surface
and the effective stress which are transmitted normal to the surface. Joints can create a situation
where a combination of joint sets provides a cross over surface.
-7-
2.1.3 Lithology
The rock materials forming a pit slope determines the rock mass strength modified by
discontinuities, faulting, folding, old workings and weathering. Low rock mass strength is
characterized by circular; raveling and rock fall instability like the formation of slope in massive
sandstone restrict stability. Pit slopes having alluvium or weathered rocks at the surface have low
shearing strength and the strength gets further reduced if water seepage takes place through
them. These types of slopes must be flatter.
2.1.4 Ground Water
It causes the following:
a) alters the cohesion and frictional parameters and
b) reduce the normal effective stress
Ground water causes increased up thrust and driving water forces and has adverse effect on the
stability of the slopes. Physical and chemical effect of pure water pressure in joints filling
material can thus alter the cohesion and friction of the discontinuity surface. Physical effects of
providing uplift on the joint surface, reduces the frictional resistances. This will reduce the
shearing resistance along the potential failure plane by reducing the effective normal stress
acting on it. Physical and the chemical effect of the water pressure in the pores of the rock cause
a decrease in the compressive strength particularly where confining stress has been reduced.
2.1.5 Mining Method and Equipment
Generally there are four methods of advance in open cast mines. They are:
(a) strike cut- advancing down the dip
(b) strike cut- advancing up the dip
(c) dip cut- along the strike
(d) open pit working
The use of dip cuts with advance on the strike reduces the length and time that a face is exposed
during excavation. Dip cuts with advance oblique to strike may often used to reduce the strata
dip in to the excavation. Dip cut generally offer the most stable method of working but suffer
-8-
from restricted production potential. Open pit method are used in steeply dipping seams, due to
the increased slope height are more prone to large slab/buckling modes of failure. Mining
equipment which piles on the benches of the open pit mine gives rise to the increase in surcharge
which in turn increases the force which tends to pull the slope face downward and thus instability
occurs. Cases of circular failure in spoil dumps are more pronounced.
2.1.6 Dynamic Forces
Due to effect of blasting and vibration, shear stresses are momentarily increased and as result
dynamic acceleration of material and thus increases the stability problem in the slope face. It
causes the ground motion and fracturing of rocks.
Blasting is a primary factor governing the maximum achievable bench face angles. The effects of
careless or poorly designed blasting can be very significant for bench stability, as noted by Sage
(1976) and Bauer and Calder (1971). Besides blast damage and back break which both reduce
the bench face angle, vibrations from blasting could potentially cause failure of the rock mass. For
small scale slopes, various types of smooth blasting have been proposed to reduce these effects
and the experiences are quite good (e.g. Hoek and Bray, 1981). For large scale slopes, however,
blasting becomes less of problem since back break and blast damage of benches have negligible
effects on the stable overall slope angle. Furthermore, the high frequency of the blast acceleration
waves prohibit them from displacing large rock masses uniformly, as pointed out by Bauer and
Calder (1971). Blasting-induced failures are thus a marginal problem for large scale slopes.
Seismic events, i.e., low frequency vibrations, could be more dangerous for large scale slopes and
several seismic-induced failures of natural slopes have been observed in mountainous areas.
Together with all these causes external loading can also plays an important role when they are
present as in case of surcharge due to dumps on the crest of the benches. In high altitude areas,
freezing of water on slope faces can results in the build up of ground water pressure behind the
face which again adds up to instability of the slope.
2.1.7 Cohesion
It is the characteristic property of a rock or soil that measures how well it resists being deformed
or broken by forces such as gravity. In soils/rocks true cohesion is caused by electrostatic forces
-9-
in stiff overconsolidated clays, cementing by Fe2O3, CaCO3, NaCl, etc and root cohesion.
However the apparent cohesion is caused by negative capillary pressure and pore pressure
response during undrained loading. Slopes having rocks/soils with less cohesion tend to be less
stable. The factors that strengthen cohesive force are as follows:
a) Friction
b) Stickiness of particles can hold the soil grains together. However, being too wet or too
dry can reduce cohesive strength.
c) Cementation of grains by calcite or silica deposition can solidify earth materials into
strong rocks.
d) Man-made reinforcements can prevent some movement of material.
The factors that weaken cohesive strength are as follows:
a) High water content can weaken cohesion because abundant water both lubricates
(overcoming friction) and adds weight to a mass.
b) Alternating expansion by wetting and contraction by drying of water reduces strength of
cohesion, just like alternating expansion by freezing and contraction by thawing. This
repeated expansion is perpendicular to the surface and contraction vertically by gravity
overcomes cohesion resulting with the rock and sediment moving slowly downhill.
c) Undercutting in slopes
d) Vibrations from earthquakes, sonic booms, blasting that create vibrations which
overcome cohesion and cause mass movement.
2.1.8 Angle of Internal Friction
Angle of internal friction is the angle ( φ ), measured between the normal force (N) and resultant
force (R), that is attained when failure just occurs in response to a shearing stress (S). Its tangent
(S/N) is the coefficient of sliding friction. It is a measure of the ability of a unit of rock or soil to
withstand a shear stress. This is affected by particle roundness and particle size. Lower
roundness or larger median particle size results in larger friction angle. It is also affected by
quartz content. The sands with less quartz contained greater amounts of potassium-feldspar,
plagioclase, calcite, and/or dolomite and these minerals generally have higher sliding frictional
resistance compared to that of quartz.
- 10 -
2.2 Types of Slope Failure
2.2.1 Plane Failure
Simple plane failure is the easiest form of rock slope failure to analyze. It occurs when a
discontinuity striking approximately parallel to the slope face and dipping at a lower angle
intersects the slope face, enabling the material above the discontinuity to slide. Variations on this
simple failure mode can occur when the sliding plane is a combination of joint sets which form a
straight path.
This means that the solution is never any thing more than the analysis of equilibrium of a single
block resting on a plane and acted upon by a number of external forces (water pressure, earth
quake, etc.) deterministic and probabilistic solution in which parameters are considered as being
precisely known may be readily obtained by hand calculation if effect of moment is neglected.
Fig. 2.3 Plane failure (after Coates, 1977; Call and Savely, 1990).
For a plane failure analysis, the geometry of the slope is very critically studied. In this
connection two cases must be considered:(a) A slope having tension crack in the upper face.
(b) A slope with tension crack in the slope face.
When the upper surface is horizontal ( ψ s =0 ), the transition from one condition to another occurs
when the tension crack coincides with the slope crest, that is when
z
= (1 -co tψ f ta n ψ p )
H
- 11 -
(1)
Where ‘z’ is the depth of the tension crack, ‘H’ is the slope height, ‘ ψ f ’is the slope angle and
‘ ψ p ’ is the dip of the sliding plane.
Fig. 2.4 Geometries of plane slope failure: (a) tension crack in the upper slope; (b) tension
crack in the face
For the analysis, the following assumptions are to be made:a) Both the sliding surface and tension crack must strike parallel to the face.
b) The tension crack is vertical and is filled with water to a depth ‘ z w ’.
c) Water enters the sliding surface along the base of the tension cracks and seeps along the
sliding surface, escaping at atmospheric pressure where the sliding surface daylight in the
slope faces.
d) The forces ‘W’ (weight of sliding block), ‘U’ (uplift force due to water pressure on the
sliding surface) and ‘V’ (force due to water pressure in the tension crack) all acts through
the centroid of the sliding mass.
e) The shear strength of the sliding surface is defined by cohesion ‘c’ and the friction angle
‘ φ ’ that are related by the equation
τ=c+σ tanφ
- 12 -
(2)
f) A slice of unit thickness is considered and it is assumed that the release surfaces are
present so that there is no resistance to the sliding at the lateral boundaries of the failure.
Calculation of factor of safety
The factor of safety for the general case of the plane failure is the ratio of the forces acting to
keep the failure mass in place (the cohesion times the area of the failure surface plus the
frictional shear strength determined using the effective normal stress on the failure plane) to the
forces attempting to drive the failure mass down the failure surface (the sum of the component of
the weight, water forces, and all other external forces acting along the failure surface). This is
determined by resolving all forces acting on the on the potential failure mass in to directions
parallel and normal to the potential failure surface. The general factor of safety which results is:
FS=
FS=
Resisting force
Driving force
cA+ ∑ Ntan φ
∑S
(3)
(4)
where ‘c’ is the cohesion and ‘A’ is the area of the sliding plane.
The factor of safety for the slope configurations in Fig. 2.4 is given by
FS =
cA + (W cosψ p -U -V sin ψ p )tan φ
W sin ψ p +V cosψ p
(5)
Where ‘A’ is given by
A = (H + b tan ψ s - z)cosecψ p
(6)
The slope height ‘H’, the tension crack depth is ‘z’ and is located a distance ‘b’ behind the slope
crest. The dip above the crest is ‘ ψ s ’. When the depth of water in the tension crack is ‘ z w ’, the
water forces acting on the sliding plane ‘U’ and in the tension crack ‘V’ are given by
U=
V=
1
γ w z w (H + btanψ s - z)cosecψ p
2
1
γwzw
2
2
(7)
(8)
Where ‘ γ w ’ is the unit weight of water.
- 13 -
The weights of the sliding block for the two geometries shown in Fig. 2.4 are given by the
equations (9) and (10). For the tension crack in the inclined upper slope surface
1
1
W=γ r [(1-cotψ f tanψ p )(bH+ H 2cotψ f )+ b 2 (tanψ s -tanψ p )]
2
2
(9)
And, for the tension crack in the slope face
1
z
W= γ r H 2 [(1- )2cotψ p ×(cotψ p tanψ f -1)]
2
H
(10)
Where ‘ γ r ’ is the unit weight of the rock.
2.2.2 Wedge Failure
The three dimensional wedge failures occur when two discontinuities intersects in such a way
that the wedge of material, formed above the discontinuities, can slide out in a direction parallel
to the line of intersection of the two discontinuities. It is particularly common in the individual
bench scale but can also provide the failure mechanism for a large slope where structures are
very continuous and extensive.
Fig. 2.5 Wedge failure (after Hoek and Bray, 1981)
When two discontinuities strike obliquely across the slope face and their line of intersection
‘daylights’ in the slope, the wedge of the rock resting over these discontinuities will slide down
along the line of intersection provided the inclination of these line is significantly greater than
the angle of friction and the shearing component of the plane of the discontinuities is less than
- 14 -
the total downward force. The total downward force is the downward component of the weight
of the wedge and the external forces (surcharges) acting over the wedge.
The wedge failure analysis is based on satisfying the equilibrium conditions of the wedge. If ‘w’
be the weight of the wedge, the vector ‘w’ can be divided into two components in the parallel
and normal directions to the joint intersection, Fig. 2.6.
N = w cos θ, P = w sin θ
(11)
The vector ‘N’ in the Fig. 2.7 is divided into two components ‘N1’ and ‘N2’, normal to the joint
set surfaces 1 and 2, respectively as follows:
In Fig. 2.6 the equilibrium conditions in the directions x and y are as follows:
N1x = N2x, N1y + N2y = N
(12)
N1x = N1 sin α1, N2x = N2 sin α2
(13)
N1y = N1 cos α1, N2y = N2 cos α2
(14)
Fig. 2.6 Conditions of effective forces in the wedge failure analysis
Fig. 2.7 Diagram of the plane normal to the intersection of joint sets 1 and 2
- 15 -
The forces ‘N1’ and ‘N2’ can be obtained from the Equations. (12), (13), and (14) as follows:
N1sinα1 =N 2sinα 2
(15)
N1cosα1 +N 2cosα 2 = N = Wcosθ
Where
N1 =
Nsinα 2
Nsinα1
, N2 =
sin(α1 +α 2 )
sin(α1 +α 2 )
(16)
Calculation of the angles α1 and α2
In Fig. 2.8 the line CC’ is the intersection line of two joint surfaces 1 and 2. The segment OH is
drawn vertically in the normal plane passing through the line of intersection CC’. Fig. 2.7 is
drawn in the three-dimensional view as the triangle ABH’. From the point O the segment OH’
normal to the intersection is drawn. The plane ABH’ is the plane normal to the intersection CC’
at point H’. From the points H and A on plane 1, two lines are drawn so that the first one is
parallel to the strike and the second one is in the direction of dip line.
Fig. 2.8 The geometry of the sliding wedge
These two lines intersect at point E. EO’ is drawn parallel and with the same size as HO. The
quadrilateral OO’EH is rectangle. Using the geometric and trigonometric relationships in the
triangles H’OA, OO’A, and O’AE, the angles α1 and α2 are obtained from the following equation.
cosθ cos γ 1tan d1 =
H'O AO' EO' HO'
*
*
=
=tanα1
HO AO AO' AO
(17)
It can be shown in the same way that tan α2 = cosθ cos γ2 tan d2, where HO = EO`, ∠EAO` =
∠d1, ∠OAH` = ∠α1, ∠OBH` = ∠α2, ∠HOH` = ∠θ, and ∠OAO` = ∠γ1 in which ‘d1’ and ‘d2’ are
- 16 -
the slope angles of the joint set 1 and 2, respectively. The angles ‘γ1’ and ‘γ2’ are the angle
between the dip directions of joint sets 1 and 2 and the strike of the plane normal to intersection
line, respectively.
The factor of safety can be calculated from the equation (18) given below:
FS=
T1 + T 2
w sin θ
(18)
where
T1 =N1tan(φj1 +i1 )(1-a1 )+C j1 (1-a1 )S1 +N1a1tanφr1 +Cr1a1S1
(19)
T2 =N 2tan(φj2 +i1 )(1-a 2 )+C j2 (1-a 2 )S1 +N 2a2tanφr2 +Cr 2a 2S 2
(20)
The internal frictions of the intact rock ‘Ør1’ and ‘Ør2’ and the cohesion coefficients of the intact
rock ‘Cr1’ and ‘Cr2’ are determined from the triaxial compressive tests and using the Mohr–
Colomb criterion. The correction factor for the effect of intact rock specimen diameter on the
cohesion coefficients could also be included. The internal friction angles of the joint sets 1 and 2
surfaces ‘φj1’ and ‘φj2’ are obtained from the shear tests on the polished rock joint specimens.
The irregularity angles ‘i1’ and ‘i2’ are determined from the direct measurements on the rock
outcrops using the stereographic projections of the joint sets 1 and 2.
2.2.3 Circular Failure
The pioneering work, in the beginning of the century, in Sweden confirmed that the surface of
the failure in spoil dumps or soil slopes resembles the shape of a circular arc. This failure can
occurs in soil slopes, the circular method occurs when the joint sets are not very well defined.
When the material of the spoil dump slopes are weak such as soil, heavily jointed or broken rock
mass, the failure is defined by a single discontinuity surface but will tend to follow a circular
path.
The conditions under which circular failure occurs are follows:
1. When the individual particles of soil or rock mass, comprising the slopes are
small as compared to the slope.
2. When the particles are not locked as a result of their shape and tend to behave as
soil.
- 17 -
Fig. 2.9 Three-dimensional failure geometry of a rotational shear failure (after Hoek and
Bray, 1981).
Types of circular failure
Circular failure is classified in three types depending on the area that is affected by the failure
surface. They are:(a) Slope failure: In this type of failure, the arc of the rupture surface meets the slope above
the toe of the slope. This happens when the slope angle is very high and the soil close to
the toe posses the high strength.
(b) Toe failure: In this type of failure, the arc of the rupture surface meets the slope at the
toe.
(c) Base failure: In this type of failure, the arc of the failure passes below the toe and in to
base of the slope. This happens when the slope angle is low and the soil below the base is
softer and more plastic than the soil above the base.
2.2.4 Two Block Failure
Two block failures are much less common mode of rock slope failure than single block failures
such as the planes and the 3D wedge and, consequently, are only briefly considered here.
Several methods of solution exist and each may be appropriate at some level of investigation.
- 18 -
2.2.5 Toppling Failure
Toppling or overturning has been recognized by several investigators as being a mechanism of
rock slope failure and has been postulated as the cause of several failures ranging from small to
large ones.
Fig. 2.10 Toppling failure
It occurs in slopes having near vertical joint sets very often the stability depends on the stability
of one or two key blocks. Once they are disturbed the system may collapse or this failure has
been postulated as the cause of several failures ranging from small to large size. This type of
failure involves rotation of blocks of rocks about some fixed base. This type of failure generally
occurred when the hill slopes are very steep.
2.3 Factors to be Considered in Assessment of Stability
2.3.1 Ground Investigation
Before any further examination of an existing slope, or the ground on which a slope is to be built,
essential borehole information must be obtained. This information will give details of the strata,
moisture content and the standing water level and shear planes. Piezometer tubes are installed
into the ground to measure changes in water level over a period of time. Ground investigations
also include:
in-situ and laboratory tests,
aerial photographs,
study of geological maps and memoirs to indicate probable soil conditions,
visiting and observing the slope.
- 19 -
2.3.2 Most Critical Failure Surface
In homogeneous soils relatively unaffected by faults or bedding, deep seated shear failure
surfaces tend to form in a circular, rotational manner. The aim is to find the most critical surface
using "trial circles".
The method is as follows:
A series of slip circles of different radii is to be considered but with same centre of
rotation. Factor of Safety (FOS) for each of these circles is plotted against radius, and the
minimum FOS is found.
This should be repeated for several circles, each investigated from an array of centers.
The simplest way to do this is to form a rectangular grid from the centers.
Each centre will have a minimum FOS and the overall lowest FOS from all the centre
shows that FOS for the whole slope. This assumes that enough circles, with a large
spread of radii, and a large grid of centers have been investigated.
An overall failure surface is found.
Fig. 2.11(a) & (b) shows variety of slope failure circles analysed at varying radii from a single
centre and variation of factor of safety with critical circle radius respectively.
Fig. 2.11(a) Variety of slope failure circles analysed at varying radii from a single centre
- 20 -
Fig. 2.12(b) Variation of factor of safety with critical circle radius
2.3.3 Tension Cracks
A tension crack at the head of a slide suggests strongly that instability is imminent. Tension
cracks are sometimes used in slope stability calculations, and sometimes they are considered to
be full of water. If this is the case, then hydrostatic forces develop as shown in Fig. 2.12.
Fig. 2.12 Effect of tension crack at the head of a slide
- 21 -
Tension cracks are not usually important in stability analysis, but can become so in some special
cases. Therefore assume that the cracks don't occur, but take account of them in analyzing a
slope which has already cracked.
2.3.4 Submerged Slopes
When an external water load is applied to a slope, the pressure it exerts tends to have a
stabilizing effect on the slope. The vertical and horizontal forces due to the water must be taken
into account in analysis of the slope. Thus, allowing for the external water forces by using
submerged densities in the slope, and by ignoring water externally.
2.3.5 Factor of Safety (FOS)
The FOS is chosen as a ratio of the available shear strength to that required to keep the slope
stable.
Table 2.1 Guidelines for equilibrium of a slope
Factor of Safety
<1.0
1.0-1.25
Details of Slope
Unsafe
Questionable safety
Satisfactory for routine cuts and fills,
1.25-1.4
Questionable for dams, or where
failure would be catastrophic
>1.4
Satisfactory for dams
For highly unlikely loading conditions, factors of safety can be as low as 1.2-1.25, even for
dams. e.g. situations based on seismic effects, or where there is rapid drawdown of the water
level in a reservoir.
- 22 -
2.3.6 Progressive Failure
This is the term describing the condition when different parts of a failure surface reach failure at
different times. This often occurs if a potential failure surface passes through a foundation
material which is fissured or has joints or pre-existing failure surfaces. Where these fissures
occur there will be large strain values, so the peak shear strength is reached before other places.
2.3.7 Pre-Existing Failure Surfaces
If the foundation on which a slope sits contains pre-existing failure surfaces, there is a large
possibility that progressive failure will take place if another failure surface were to cut through
them. The way to deal with this situation is to assume that sufficient movement has previously
taken place for the ultimate state to develop in the soil and then using the ultimate state
parameters. If failure has not taken place, then a decision has to be made on which parameters to
be used.
2.4 Methods of Analysis
2.4.1 Wedge Failure Analysis
The 3D nature of the wedge failure analysis complicates the analysis. The different methods of
analysis are given as follows:
2.4.1.1 Spherical Projection Solution using Factor of Safety
The 3D wedge problem can be very easily analyzed using spherical projection techniques.
When the shear strength of the shear surface is entirely frictional and there is no external force,
the problem becomes dimensionless and can be analyzed very simply by the means of a stereo
net analysis alone. The introduction of water pressure or the external forces requires the use of
side calculations to determine the orientation of the resultant forces acting on the wedge.
- 23 -
Use of spherical projection rapidly establishes a zone of orientations for the resultant force for
which the wedge will remain stable. The orientation of the line of intersection of the wedge is
defined by the intersection of the great circles which defines the joints.
To determine the factor of safety against sliding, the great circle containing both the resultant
force acting on the wedge and the resultant shear force is drawn. The intersection of this great
circle with and through both the normal and both the reactions on the shear planes define the
position of the resultant of these normal and reactions. The factor of safety can be defined as the
ratio of the resultant shear force acting along the line of intersection of the wedge to the resultant
shear strength available to resist sliding in the same direction.
2.4.1.2 Chart Solution
Hoek and Bray (1980) produced a series of charts which can be used to rapidly access the
stability of rock wedges for which there is know cohesion or external forces. Under these
condition and for a given friction angle, the factor of safety is a function only of the dip and
direction of the shear plane. These charts are convenient to use for use simple wedge problem
but suffer from the disadvantage that it does not give the feel of the problem.
2.4.1.3 Spherical Projections Solutions using Probabilistic Approach
Monte Carlo analysis of the wedge failure gives, with a specified confidence level, the
uncertainty in the orientations of the shear planes. When the orientations of the shear planes are
known then the spherical projection technique can be used to find out the orientation of the
failure plane.
2.4.2 Circular Failure Analysis
The stability of the slopes of finite extent like that in the case of circular is analyzed by the
method of dividing the whole suspected failure area in to slices and further analyzing the
sequence of events that may follow thereafter. There are several methods of slices in their new
advancement together with friction circle method and tailors stability number method.
- 24 -
2.4.2.1 Method of Slices
This method was advanced by the Swedish geotechnical commission and developed by
W.Fellienius (1936). By dividing the mass above an assumed rupture surface of failure in to
vertical slices and assuming that the forces on the opposite sides of each slice are equal and
opposite, a statistically determinate problem is obtained and semi graphical method have been
devised by which the stability of the mass may be analyzed for any given circle. The main
objection of this method is that the most dangerous of infinite number of circles are to be found
out for which graphical method is to be used for a number of time.
2.4.2.2 Modified Method of Slices
When there are several dangerous circles to be analyzed usual procedure by the slice method is
quite tedious. N.C.Coutrney of U.S.A. has developed simple graphical solutions by which the
forces that are inherent in the method of slices such as the forces acting on the vertical sides of
the slices.
2.4.2.3 Simplified Method of Slices
This method takes in to account the forces acting on the vertical sides of the slices in the
development of an equation for determining the factor of safety. However, the simplified
equation proposed by Bishop (1955) does not contain the forces acting on the vertical sides and
there by simplifies the computation.
2.4.2.4 Friction Circle Method
It is a very convenient method which takes in to account the total forces acting on the whole
mass lying above the assumed circular surface of failure. This method eliminates the
indeterminate forces that are inherent in the method of slices such as acting on the vertical sides
of the slices.
2.4.2.5 Taylor’s Stability Number
Taylor (1937) made a mathematical trial method using the friction circle method. Charts as
formulated by Taylor give the relationship between stability number and the slope angle for
- 25 -
various angle of friction. This method is applicable to homogeneous simple slopes without
seepage.
2.4.3 Two Block Failure Analysis
2.4.3.1 Stereographic Solution
A stereographic analysis is convenient way of determining weather or not a two block
configuration will stable (Goodman, 1975 and Kuykendall and Goodman, 1976). Any form of
shear strength envelope can be accounted for by use of the secant angle of friction.
2.4.4 Toppling Failure Analysis
Base friction models can be useful insight in to the mechanism of failure. They can also be used
to provide a quantitative assessment of the effect of possible slope stabilization procedure such
as reducing the slope angle or installing horizontal reinforcements. The difference conditions are
taken in to account to ascertain sliding and toppling of block in inclined plane.
2.4.5 Other Methods of Analysis
2.4.5.1 Limit Equilibrium Method
In limit equilibrium method of analysis, static force is applied to analyze the stability of the rock
mass or soil above the failure surface. If failure has already occurred, the geometry of the failure
surface can be determined and the analysis of the failure can be done and is known as back
analysis. If it is a design situation, however the failure surface is potential rather than actual,
many potential surface may have to be analyzed to find the critical geometry before an
acceptable slope geometry can be accounted for.
In the case of plane failure, 3D wedge failure, circular failure, the material above the failure
surface will be on the point of slipping when the disturbing forces due to gravity are just
counterbalanced by the forces tending to restore equilibrium. The ratio of the two forces defines
the factor of safety of the failure surface.
- 26 -
2.4.5.2 Stress Analysis Method
Failure does not necessarily occur along a well defined failure surfaces. The situation where the
structural condition does not permit sliding along the discontinuity surface, crushing of the rock
occurs at the points of the highest stress. Progressive failure of the rock mass can subsequently
deform the slope and may cause the catastrophic failure.
The objectives of the stress analysis method are to represent the rock mass by a series of
structural elements (finite element method) or cells of constraint of materials (one finite different
method) and perform an analysis to determine to stresses at points within the slope. The stress
distribution can be examined to determine where rock failure is likely to occur, rock failure
occurs when the stresses to which the rock is subjected more than its strength.
- 27 -
CHAPTER: 03
NUMERICAL MODELLING
- 28 -
CHAPTER: 03
NUMERICAL MODELLING
3.1 Introduction
Many rock slope stability problems involve complexities relating to geometry, material
anisotropy, non-linear behaviour, in situ stresses and the presence of several coupled processes
(e.g. pore pressures, seismic loading, etc.). Advances in computing power and the availability of
relatively inexpensive commercial numerical modelling codes means that the simulation of
potential rock slope failure mechanisms could, and in many cases should, form a standard
component of a rock slope investigation.
Numerical methods of analysis used for rock slope stability may be conveniently divided into
three approaches: continuum, discontinuum and hybrid modelling. Table 2 provides a summary
of existing numerical techniques.
Table 3.1 Numerical methods of analysis
Analysis
method
Continuum
Modelling
(e.g. Finite
Element,
Finite
Difference
Method)
Critical input
parameters
Representative slope
geometry;constitutive
criteria (e.g. elastic,
elasto-plastic, creep
etc.);
groundwater
characteristics; shear
strength of surfaces;
in situ stress state.
Advantages
Limitations
Allows
for
material
deformation and failure.
Can
model
complex
behaviour
and
mechanisms. Capability of
3-D modelling. Can model
effects of groundwater
and pore pressures. Able
to assess effects of
parameter variations on
instability.
Recent
advances in computing
hardware allow complex
models to be solved on
PC’s with reasonable run
times. Can incorporate
creep deformation. Can
incorporate
dynamic
Users must be well
trained,
experienced
and
observe
good
modelling
practice.
Need to be aware of
model/software
limitations
(e.g.
boundary effects, mesh
aspect
ratios,
symmetry,
hardware
memory restrictions).
Availability of input
data generally poor.
Required
input
parameters
not
routinely
measured.
Inability to
model
effects of highly jointed
- 29 -
analysis.
Representative slope
and
discontinuity
geometry;
intact
constitutive criteria;
discontinuity stiffness
and shear strength;
groundwater
characteristics; in situ
stress state.
Allows
for
block
deformation
and
movement
of
blocks
relative to each other. Can
model complex behaviour
and
mechanisms
(combined material and
discontinuity behaviour
coupled
with
hydromechanical and dynamic
analysis). Able to assess
effects
of
parameter
variations on instability.
Hybrid/Coupled Combination of input
parameters
listed
Modelling
above for stand-alone
models.
Coupled
finiteelement/distinct element
models able to simulate
intact fracture propagation
and fragmentation of
jointed and bedded media.
Discontinuum
Modelling
(e.g. Distinct
Element,
Discrete
Element
Method)
rock. Can be difficult to
perform
sensitivity
analysis due to run time
constraints.
As above, experienced
user required to observe
good
modelling
practice.
General
limitations similar to
those listed above.
Need to be aware of
scale effects. Need to
simulate representative
discontinuity geometry
(spacing, persistence,
etc.). Limited data on
joint
properties
available.
Complex
problems
require high memory
capacity.
Comparatively
little
practical experience in
use. Requires ongoing
calibration
and
constraints.
3.1.1 Continuum Modelling
Continuum modelling is best suited for the analysis of slopes that are comprised of massive,
intact rock, weak rocks, and soil-like or heavily fractured rock masses. Most continuum codes
incorporate a facility for including discrete fractures such as faults and bedding planes but are
inappropriate for the analysis of blocky mediums. The continuum approaches used in rock slope
stability include the finite-difference and finite-element methods. In recent years the vast
majority of published continuum rock slope analyses have used the 2-D finite-difference code,
FLAC. This code allows a wide choice of constitutive models to characterize the rock mass and
incorporates time dependent behaviour, coupled hydro-mechanical and dynamic modelling.
- 30 -
Two-dimensional continuum codes assume plane strain conditions, which are frequently not
valid in inhomogeneous rock slopes with varying structure, lithology and topography. The recent
advent of 3-D continuum codes such as FLAC3D and VISAGE enables the engineer to
undertake 3-D analyses of rock slopes on a desktop computer. Although 2-D and 3-D continuum
codes are extremely useful in characterizing rock slope failure mechanisms it is the responsibility
of the engineer to verify whether they are representative of the rock mass under consideration.
Where a rock slope comprises multiple joint sets, which control the mechanism of failure, then a
discontinuum modelling approach may be considered more appropriate.
3.1.2 Discontinuum Modelling
Discontinuum methods treat the rock slope as a discontinuous rock mass by considering it as an
assemblage of rigid or deformable blocks. The analysis includes sliding along and
opening/closure of rock discontinuities controlled principally by the joint normal and joint shear
stiffness. Discontinuum modelling constitutes the most commonly applied numerical approach to
rock slope analysis, the most popular method being the distinct-element method. Distinctelement codes such as UDEC use a force-displacement law specifying interaction between the
deformable joint bounded blocks and Newton’s second law of motion, providing displacements
induced within the rock slope.
UDEC is particularly well suited to problems involving jointed media and has been used
extensively in the investigation of both landslides and surface mine slopes. The influence of
external factors such as underground mining, earthquakes and groundwater pressure on block
sliding and deformation can also be simulated.
3.1.3 Hybrid Techniques
Hybrid approaches are increasingly being adopted in rock slope analysis. This may include
combined analyses using limit equilibrium stability analysis and finite-element groundwater flow
and stress analysis such as adopted in the GEO-SLOPE suite of software. Hybrid numerical
models have been used for a considerable time in underground rock engineering including
coupled boundary-/finite-element and coupled boundary-/distinct-element solutions. Recent
advances include coupled particle flow and finite-difference analyses using FLAC3D and
PFC3D. These hybrid techniques already show significant potential in the investigation of such
- 31 -
phenomena as piping slope failures, and the influence of high groundwater pressures on the
failure of weak rock slopes. Coupled finite-/distinct-element codes are now available which
incorporate adaptive remeshing. These methods use a finite-element mesh to represent either the
rock slope or joint bounded block. This is coupled with a discrete -element model able to model
deformation involving joints. If the stresses within the rock slope exceed the failure criteria
within the finite-element model a crack is initiated. Remeshing allows the propagation of the
cracks through the finite-element mesh to be simulated. Hybrid codes with adaptive remeshing
routines, such as ELFEN, have been successfully applied to the simulation of intense fracturing
associated with surface mine blasting, mineral grinding, retaining wall failure and underground
rock caving.
3.2 General Approach of FLAC
The modeling of geo-engineering processes involves special considerations and a design
philosophy different from that followed for design with fabricated materials. Analyses and
designs for structures and excavations in or on rocks and soils must be achieved with relatively
little site-specific data, and an awareness that deformability and strength properties may vary
considerably. It is impossible to obtain complete field data at a rock or soil site.
Since the input data necessary for design predictions are limited, a numerical model in
geomechanics should be used primarily to understand the dominant mechanisms affecting the
behavior of the system. Once the behavior of the system is understood, it is then appropriate to
develop simple calculations for a design process.
It is possible to use FLAC directly in design if sufficient data, as well as an understanding of
material behavior, are available. The results produced in a FLAC analysis will be accurate when
the program is supplied with appropriate data. Modelers should recognize that there is a
continuous spectrum of situations, as illustrated in Figure 3.1, below.
- 32 -
Complicated geology;
Simple geology; Lots
inaccessible; no testing
of money spent on site
budget
investigation
Data
None
Complete
Approach
Investigation of
Typical Situation
mechanisms
Bracket field behaviour
Predictive (direct use
by parameter studies
in design)
Fig. 3.1 Spectrum of modeling situations
FLAC may be used either in a fully predictive mode (right-hand side of Fig. 3.1) or as a
“numerical laboratory” to test ideas (left-hand side). It is the field situation (and budget), rather
than the program, that determine the types of use. If enough data of a high quality are available,
FLAC can give good predictions.
The model should never be considered as a “black box” that accepts data input at one end and
produces a prediction of behavior at the other. The numerical “sample” must be prepared
carefully, and several samples tested, to gain an understanding of the problem. Table 3.2 lists the
steps recommended to perform a successful numerical experiment; each step is discussed
separately.
Table 3.2 Recommended steps for numerical analysis in geomechanics
Step 1
Define the objectives for the model analysis
Step 2
Create a conceptual picture of the physical system
Step 3
Construct and run simple idealized models
Step 4
Assemble problem-specific data
Step 5
Prepare a series of detailed model runs
Step 6
Perform the model calculations
Step 7
Present results for interpretation
- 33 -
3.2.1 Define the Objectives for the Model Analysis
The level of detail to be included in a model often depends on the purpose of the analysis. For
example, if the objective is to decide between two conflicting mechanisms that are proposed to
explain the behavior of a system, then a crude model may be constructed, provided that it allows
the mechanisms to occur. It is tempting to include complexity in a model just because it exists in
reality. However, complicating features should be omitted if they are likely to have little
influence on the response of the model, or if they are irrelevant to the model’s purpose. Start
with a global view and add refinement if necessary.
3.2.2 Create a Conceptual Picture of the Physical System
It is important to have a conceptual picture of the problem to provide an initial estimate of the
expected behavior under the imposed conditions. Several questions should be asked when
preparing this picture. For example, is it anticipated that the system could become unstable? Is
the predominant mechanical response linear or nonlinear? Are movements expected to be large
or small in comparison with the sizes of objects within the problem region? Are there welldefined discontinuities that may affect the behavior, or does the material behave essentially as a
continuum? Is there an influence from groundwater interaction? Is the system bounded by
physical structures, or do its boundaries extend to infinity? Is there any geometric symmetry in
the physical structure of the system?
These considerations will dictate the gross characteristics of the numerical model, such as the
design of the model geometry, the types of material models, the boundary conditions, and the
initial equilibrium state for the analysis. They will determine whether a three-dimensional model
is required, or if a two-dimensional model can be used to take advantage of geometric conditions
in the physical system.
3.2.3 Construct and Run Simple Idealized Models
When idealizing a physical system for numerical analysis, it is more efficient to construct and
run simple test models first, before building the detailed model. Simple models should be created
at the earliest possible stage in a project to generate both data and understanding. The results can
- 34 -
provide further insight into the conceptual picture of the system; Step 2 may need to be repeated
after simple models are run.
Simple models can reveal shortcomings that can be remedied before any significant effort is
invested in the analysis. For example, do the selected material models sufficiently represent the
expected behavior? Are the boundary conditions influencing the model response? The results
from the simple models can also help guide the plan for data collection by identifying which
parameters have the most influence on the analysis.
3.2.4 Assemble Problem-Specific Data
The types of data required for a model analysis include:
details of the geometry (e.g., profile of underground openings, surface topography, dam
profile, rock/soil structure);
locations of geologic structure (e.g., faults, bedding planes, joint sets);
material behavior (e.g., elastic/plastic properties, post-failure behavior);
initial conditions (e.g., in-situ state of stress, pore pressures, saturation); and
external loading (e.g., explosive loading, pressurized cavern).
Since, typically, there are large uncertainties associated with specific conditions (in particular,
state of stress, deformability and strength properties), a reasonable range of parameters must be
selected for the investigation. The results from the simple model runs (in Step 3) can often prove
helpful in determining this range, and in providing insight for the design of laboratory and field
experiments to collect the needed data.
3.2.5 Prepare a Series of Detailed Model Runs
Most often, the numerical analysis will involve a series of computer simulations that include the
different mechanisms under investigation and span the range of parameters derived from the
assembled database. When preparing a set of model runs for calculation, several aspects, such as
those listed below, should be considered.
I.
How much time is required to perform each model calculation? It can be difficult
to obtain sufficient information to arrive at a useful conclusion if model runtimes
- 35 -
are excessive. Consideration should be given to performing parameter variations
on multiple computers to shorten the total computation time.
II.
The state of the model should be saved at several intermediate stages so that the
entire run does not have to be repeated for each parameter variation. For example,
if the analysis involves several loading/unloading stages, the user should be able
to return to any stage, change a parameter and continue the analysis from that
stage.
III.
Are there a sufficient number of monitoring locations in the model to provide for
a clear interpretation of model results and for comparison with physical data? It is
helpful to locate several points in the model at which a record of the change of a
parameter (such as displacement) can be monitored during the calculation.
3.2.6 Perform the Model Calculations
It is best to first make one or two model runs split into separate sections before launching a series
of complete runs. The runs should be checked at each stage to ensure that the response is as
expected. Once there is assurance that the model is performing correctly, several data files can be
linked together to run a complete calculation sequence. At any time during a sequence of runs, it
should be possible to interrupt the calculation, view the results, and then continue or modify the
model as appropriate.
3.2.7 Present Results for Interpretation
The final stage of problem solving is the presentation of the results for a clear interpretation of
the analysis. This is best accomplished by displaying the results graphically, either directly on
the computer screen, or as output to a hardcopy plotting device. The graphical output should be
presented in a format that can be directly compared to field measurements and observations.
Plots should clearly identify regions of interest from the analysis, such as locations of calculated
stress concentrations, or areas of stable movement versus unstable movement in the model. The
numeric values of any variable in the model should also be readily available for more detailed
interpretation by the modeler.
The above seven steps are to be followed to solve geo-engineering problems efficiently.
- 36 -
Start
MODEL SETUP
1. Generate grid, deform to desired shape
2. Define constitutive and material properties
3. Specify boundary and initial conditions
Step to equilibrium state
Results unsatisfactory
Examine the model
response
Model makes sense
Perform Alternations
For Example: 1.Excavate Materials
2. Change boundary conditions
Step to solutions
More tests needed
Examine the model
response
Acceptable Result
Yes
Parameter Study
Needed
No
EndNo
Fig. 3.2 Flow chart for determination of factor of safety using FLAC/Slope
- 37 -
3.3 Overview
FLAC/Slope is a mini-version of FLAC that is designed specifically to perform factor-of-safety
calculations for slope stability analysis. This version is operated entirely from FLAC’s graphical
interface (the GIIC) which provides for rapid creation of models for soil and/or rock slopes and
solution of their stability condition.
FLAC/Slope provides an alternative to traditional “limit equilibrium” programs to determine
factor of safety. Limit equilibrium codes use an approximate scheme — typically based on the
method of slices — in which a number of assumptions are made (e.g., the location and angle of
interslice forces). Several assumed failure surfaces are tested, and the one giving the lowest
factor of safety is chosen. Equilibrium is only satisfied on an idealized set of surfaces. In
contrast, it provides a full solution of the coupled stress/displacement, equilibrium and
constitutive equations. Given a set of properties, the system is determined to be stable or
unstable. By automatically performing a series of simulations while changing the strength
properties, the factor of safety can be found to correspond to the point of stability, and the critical
failure (slip) surface can be located.
FLAC/Slope does take longer to determine a factor of safety than a limit equilibrium program.
However, with the advancement of computer processing speeds (e.g., 1 GHz and faster chips),
solutions can now be obtained in a reasonable amount of time. This makes FLAC/Slope a
practical alternative to a limit equilibrium program, and provides advantages over a limit
equilibrium solution:
1. Any failure mode develops naturally; there is no need to specify a range of trial surfaces in
advance.
2. No artificial parameters (e.g., functions for interslice force angles) need to be given as input.
3. Multiple failure surfaces (or complex internal yielding) evolve naturally, if the conditions give
rise to them.
4. Structural interaction (e.g., rock bolt, soil nail or geogrid) is modeled realistically as fully
coupled deforming elements, not simply as equivalent forces.
5. The solution consists of mechanisms that are kinematically feasible. (The limit equilibrium
method only considers forces, not kinematics.)
- 38 -
3.4 Summary of Features
FLAC/Slope can be applied to a wide variety of conditions to evaluate the stability of slopes and
embankments. Each condition is defined in a separate graphical tool.
1. The creation of the slope boundary geometry allows for rapid generation of linear, nonlinear
and benched slopes and embankments. The Bound tool provides separate generation modes for
both simple slope shapes and more complicated non-linear slope surfaces. A bitmap or DXF
image can also be imported as a background image to assist boundary creation.
2. Multiple layers of materials can be defined in the model at arbitrary orientations and nonuniform thicknesses. Layers are defined simply by clicking and dragging the mouse to locate
layer boundaries in the Layers tool.
3. Materials and properties can be specified manually or from a database in the Material tool. At
present, all materials obey the Mohr-Coulomb yield model, and heterogeneous properties can be
assigned. Material properties are entered via material dialog boxes that can be edited and cloned
to create multiple materials rapidly.
4. With the Interface tool, a planar or non-planar interface, representing a joint, fault or weak
plane, can be positioned at an arbitrary location and orientation in the model. The interface
strength properties are entered in a properties dialog; the properties can be specified to vary
during the factor-of-safety calculation, or remain constant.
FLAC/Slope is limited to slope configurations with no more than one interface. For analyses
which involve multiple (and intersecting) interfaces or weak planes, full FLAC should be used.
5. An Apply tool is used to apply surface loading to the model in the form of either a real
pressure (surface load) or a point load.
6. A water table can be located at an arbitrary location by using the Water tool; the water table
defines the phreatic surface and pore pressure distribution for incorporation of effective stresses
and the assignment of wet and dry densities in the factor-of-safety calculation.
7. Structural reinforcement, such as soil nails, rock bolts or geotextiles, can be installed at any
location within the model using the Reinforce tool. Structural properties can be assigned
individually for different elements, or groups of elements, through a properties dialog.
8. Selected regions of a FLAC/Slope model can be excluded from the factor-of-safety
calculation.
- 39 -
3.5 Analysis Procedure
FLAC/Slope is specifically designed to perform multiple analyses and parametric studies for
slope stability projects. The structure of the program allows different models in a project to be
easily created, stored and accessed for direct comparison of model results. A FLAC/Slope
analysis project is divided into four stages which is described below.
a) Models Stage
Each model in a project is named and listed in a tabbed bar in the Models stage. This allows easy
access to any model and results in a project. New models can be added to the tabbed bar or
deleted from it at any time in the project study. Models can also be restored (loaded) from
previous projects and added to the current project. The slope boundary is also defined for each
model at this stage.
b) Build Stage
For a specific model, the slope conditions are defined in the Build stage. This includes: changes
to the slope geometry, addition of layers, specification of materials and weak plane, application
of surface loading, positioning of a water table and installation of reinforcement. Also, spatial
regions of the model can be excluded from the factor-of-safety calculation. The build-stage
conditions can be added, deleted and modified at any time during this stage.
c) Solve Stage
In the Solve stage, the factor of safety is calculated. The resolution of the numerical mesh is
selected first (coarse, medium and fine), and then the factor-of-safety calculation is performed.
Different strength parameters can be selected for inclusion in the strength reduction approach to
calculate the safety factor. By default, the material cohesion and friction angle are used.
d) Plot Stage
After the solution is complete, several output selections are available in the Plot stage for
displaying the failure surface and recording the results. Model results are available for
subsequent access and comparison to other models in the project. All models created within a
project, along with their solutions, can be saved, the project files can be easily restored and
results viewed at a later time.
- 40 -
CHAPTER: 04
CASE STUDY
- 41 -
CHAPTER: 04
CASE STUDY
JINDAL POWER OCP, MAND RAIGARH COALFIELD
4.1 Introduction
Jindal Power Opencast Coal Mine is captive mine of Jindal’s 1000 MW (4 x 250 MW) thermal
power plant. The block is located between Longitudes - 83°29'40" to 83°32'32" (E) and Latitude
- 22°09'15" to 22°05'44" (N) falling in the topo sheet no. 64 N/12 (Survey of India).
Administratively, the block is under Gharghoda Tahsil of Raigarh District, Chhattisgarh. The
block is well connected by Road. It is about 60 km from Raigarh town, which is the district head
quarter and nearest railway station on Mumbai - Howrah Main Line.
4.2 Geology
In general area of the coal block - Jindal Power Open Cast Coal Mine is almost flat with small
undulations from surface the lithological section comprises about 3-4 m unconsolidated loose
soil/alluvium. Below the top soil there is weathered shale/sandstone up to 6–8 m depth. The
weathered shale/sandstone is competitively loose in nature and can be excavated without
blasting. Below weathered mental (which varies from 3 – 10 m), the rock is hard, compact and
massive in nature it can be excavated only after blasting.
In the sub-block IV/2 & IV/3 only lower groups of Gondwana seams have been deposited. The
general strike of the seams in NW-SE is almost uniform throughout the block. Two normal faults
of small magnitude have been deciphered based on the level difference of the floor of the seams,
though the presence of some minor faults of less than 5 m throw cannot be overruled.
The Mand Raigarh basin is a part of IB River - Mand - Korba master basin lying within the
Mahanadi graben. Sub block IV/2 & IV/3 of Gare-Pelma area is structurally undisturbed except
one small fault (throw 0-15 m) trending NE-SW with westerly throws. The strike of the bed is
NW-SE in general with dip varies from 2° to 6° southwesterly. In the sub block IV/2 & IV/3,
total 10 coal seams have been established. They are seam X to I in descending order. The
lithology of the seams and details of the seams are given in Table 4.1 and Table 4.2 respectively.
- 42 -
Table 4.1 Lithology of the seams
Coal Seam/Parting
Parting(m)
Banded fine grained sandstone
0.4
Carb shale
0.4
Sandstone
Grey shale
[OB]
4
Banded sandstone
1.5 - 2.5
Shale
1
Shaly coal
0.5
Banded sandstone
0.2
Shaly coal
0.2
Sandstone
0.2
Coal
0.2
Sandstone
2
[Seam IX A]
6
Coal
0.2
Shaly coal
0.5
[Seam IX ]
4.2
Fine grained banded sandstone
0.3
0.4
Carbonaceous shale
Fine sandstone
2.5
Grey shale
0.4
[ Parting ]
4-5
Seam VIII
4
Grey shale
2
Fine grained sandstone
4.5
[Parting]
6.5
Seam VII
5 - 5.5
- 43 -
Table 4.2 Details of the seams
Name of seam
Seam VII
Parting
Seam VIII
Parting
IX Seam
IX A Seam
Details
1. Spacing of joint: 90 cm.
2. Joint direction: 220- 310 0.
3. Dip of joint: 70 0.
1. Vertical joints: every 1m of dip 5-9 0.
2. Maximum joint spacing: 23cm.
1. Joint Spacing: 89 cm
2. Joint dip: 930
3. Strike: 135 0 SE
4. Dip of seam: 5-9 0
1. Dip amount: 50.
2. Joint direction: 210- 2600
3. Joint dip amount: 700
1. Strike: 1700
2. Joint Orientation: 850
3. Joint dip: 1620
1.
2.
3.
4.
5.
Joint Spacing: 4 m approx.
Bench slope angle: around 700.
7 joints/m
One dip side joint ( 4 joints /m )
One strike side joint ( 5 - 7 joints / m)
4.3 Data Collection
The objective of the investigation was to design stable slopes so that it facilitates safe operations.
The typical analysis ingredients are cohesion and angle of internal friction. These data represent the
engineering properties of the area under investigation.
4.4 Laboratory Test
4.4.1 Sample Preparation
Three rock samples are taken from undisturbed specimens by boring. After boring the samples
are cut into required dimension (Length/Diameter is greater than 2). The dimensions are given in
the Table 4.3.
- 44 -
Table 4.3 Dimensions of the tested samples
Sl. No.
Average Length(cm)
Average Diameter
(cm)
Length/Diameter
Ratio
Sample 1
11.5
5.38
2.1
Sample 2
11.49
5.43
2.1
Sample 3
11.38
5.55
2.1
4.4.2 Triaxial Testing Apparatus for Determination of Sample Properties
The equipment is designed for testing rock samples with a cell which is designed to withstand a
lateral pressure of 150 bar (150kgf/cm2) and can be used in AIM-050, Load Frame 500 kN
(50,000 kgf) capacity. Lateral pressure can be applied with the help of AIM – 246, Constant
Pressure System, 150 bar (150 kgf/cm2).
The equipment consists of a base which houses four valves these valves can be used for
measurement of pore pressure, top drainage, bottom drainage, and for entry /exit of cell water.
Base has a hole in the center for fixing the locating g pin and bottom pedestal of various sizes. It
also has ten threaded holes and two locating pins for aligning and clamping chamber with bolts
to base. Chamber has ten free holes and two lifting handles. Top cap is fixed with the chamber.
Top cap has an air plug and a pressure inlet plug. A grooved and lapped plunger which can be
lifted with the help of two pins provided on the top of the plunger.
Fig. 4.1 A typical triaxial test apparatus
- 45 -
Setting Up
1. First the cell is to be cleaned to be free from all foreign particles.
2. The chamber is removed by unscrewing ten Allen bolts with the help of Allen keys.
3. The base is cleaned and a thin layer of oil is applied on it.
4. The chamber is cleaned from inside and smeared with oil.
5. The locating pin is placed in the center hole. While the right size of pedestal is placed
with suitable combination on the locating pin.
6. The sample to be tested is placed on the pedestal. The same size loading pad is kept on
the top of the sample. The suitable copper tubes are connected with the bottom pedestal
and top loading pad. The chamber is placed in the locating pin and clamps it to the base
with the help of Allen bolts.
4.4.3 Test Procedure
1. The water is filled in the cell with the help of funnel and rubber tube through the valve
meant for this purpose.
2. The hose pipe from AIM – 246 is connected with constant pressure system to pressure
inlet plug and apply required lateral pressure around the sample.
3.
Designed level of cell pressure is built up using AIM - 246. The lateral pressure is to be
maintained constant while samples are subjected to different consolidation stress history
as well as during shear tests. The readings are recorded.
4. After the test is over, remove the loading pad, copper tube connections and pedestal. The
cell is cleaned and a thin layer of oil is put on the base and inside of chamber.
- 46 -
Table 4.4 Readings of proving and deviator and dial gauge
Sl.
Dial
Corrected
No.
Gauge
Area
Reading
σ3 = 100 kPa
σ3 = 200 kPa
σ3 = 300 kPa
Proving
Deviator
Deviator
Proving
Deviator
Deviator
Proving
Deviator
Deviator
Reading
Reading
stress
Reading
Reading
stress
Reading
Reading
stress
(kPa)
(kPa)
(kPa)
1
0
12.19
0
0
0
0
0
0
0
0
0
2
50
12.25
16
54.4
44.40
16
54.4
44.4036
21
71.4
58.279
3
100
12.31
34
115.6
93.88
21
71.4
57.9869
33
112.2
91.122
4
150
12.38
39
132.6
107.14
33
112.2
90.662
49
166.6
134.62
5
200
12.44
45
153
123.0
62
210.8
169.47
59
200.6
161.269
6
250
12.50
56
190.4
152.28
87
295.8
236.591
86
292.4
233.87
7
300
12.57
68
231.2
183.97
109
370.6
294.899
112
380.8
303.015
8
350
12.63
99
336.6
266.46
121
411.4
325.678
149
506.6
401.04
9
400
12.70
114
387.6
305.24
143
486.2
382.897
176
598.4
471.258
10
450
12.76
132
448.8
351.60
157
533.8
418.194
205
697
546.05
11
500
12.83
139
472.6
368.31
179
608.6
474.299
244
829.6
646.529
12
550
12.90
146
496.4
384.82
196
666.4
516.61
265
901
698.478
13
600
12.97
152
516.8
398.51
219
744.6
574.179
283
962.2
741.975
14
650
13.04
163
554.2
425.08
241
819.4
628.498
301
1023.4
784.970
15
700
13.11
192
652.8
498.03
258
877.2
669.234
319
1084.6
827.463
16
750
13.18
211
717.4
544.37
268
911.2
691.436
346
1176.4
892.674
17
800
13.25
234
795.6
600.45
276
938.4
708.226
379
1288.6
972.528
18
850
13.32
252
856.8
643.12
298
1013.2
760.523
389
1322.6
992.763
19
900
13.40
261
887.4
662.45
314
1067.6
796.978
406
1380.4
1030.48
20
950
13.47
265
901
668.91
332
1128.8
838.034
411
1397.4
1037.44
- 47 -
21
1000
13.54
267
907.8
670.23
341
1159.4
855.997
417
1417.8
1046.77
22
1050
13.62
267
907.8
666.51
345
1173
861.226
421
1431.4
1050.94
23
1100
13.70
-
-
-
345
1173
856.415
423
1438.2
1050.03
24
1150
13.77
-
-
-
-
-
-
424
1441.6
1046.60
25
1200
13.85
-
-
-
-
-
-
424
1441.6
1040.69
4.4.4 Plotting of Mohr’s Circle
With σ3 = 100 kPa, 200 kPa and 300 kPa respectively and the total stress σ1 = 670 kPa, 861 kPa
and 1050 kPa the respective Mohr’s circles are drawn. Mohr’s circle showed cohesion and angle
of internal friction as 180 kPa, and 26 degrees, respectively.
Fig. 4.2 Mohr’s circle for determination of cohesion and angle of internal friction
4.5 Parametric studies
Parametric studies were conducted through numerical models (FLAC/Slope) to study the effect
of cohesion (140-220 kPa) and friction angle (20°-30° at the interval of 2°). Pit slope angle was
varied from 35° to 55° at an interval of 5°.
- 48 -
Fig. 4.3 Projected pit slope
Table 4.5 Safety factors for various slope angles (Depth= 116m)
Sl. No.
Slope angle(°)
Cohesion(kPa)
Friction angle(°)
Factor of
Safety
1
35
180
26
1.47
2
40
180
26
1.32
3
45
180
26
1.2
4
50
180
26
1.09
5
55
180
26
1.0
Fig. 4.4 Some models developed by FLAC/Slope with varying cohesion and friction angle
- 49 -
a) Depth= 116m, C=180 kPa, Slope angle= 35°, Friction angle = 26° (FOS = 1.47)
b) Depth= 116m, C=180 kPa, Slope angle= 45°, Friction angle = 26° (FOS = 1.2)
c) Depth= 116m, C=180 kPa, Slope angle= 55°, Friction angle = 26° (FOS = 1.0)
d) Depth= 116m, C=140 kPa, Slope angle= 45°, Friction angle = 30° (FOS = 1.21)
- 50 -
e) Depth= 116m, C=160 kPa, Slope angle= 45°, Friction angle = 26° (FOS = 1.08)
f) Depth= 116m, C=200 kPa, Slope angle= 45°, Friction angle = 22° (FOS = 1.08)
g) Depth= 116m, C=220 kPa, Slope angle= 45°, Friction angle = 20° (FOS = 1.12)
- 51 -
Table 4.6 Safety factors for various C and Ø values (Depth= 116m)
Sl. No.
1
2
3
4
Cohesion (kPa)
Friction Angle (˚)
Factor of Safety
20
0.91
22
0.92
24
0.97
26
1.03
28
1.09
30
1.21
20
0.92
22
0.97
24
1.03
26
1.08
28
1.14
30
1.2
20
1.03
22
1.08
24
1.13
26
1.19
28
1.25
30
1.31
20
1.12
22
1.13
24
1.19
26
1.25
28
1.31
30
1.44
140
160
200
220
- 52 -
1.8
Friction angle vs Factor of safety
Factor of safety
1.6
Cohesion (kPa)
140
160
200
220
1.4
1.2
1.0
0.8
18
20
22
24
26
28
30
32
34
36
38
40
Friction angle(in degrees)
Fig. 4.5 Variation of factor of safety with friction angle for different cohesion
4.6 Result and Discussion
1. Based on Table 4.5 it is concluded that as the pit slope angle increases, the stability of the
slopes decreases. The slope angle of 45° is having a factor of safety of 1.2 which is quite safe
and matches with theory. Lower the pit slope angle, higher is the stripping (mining of waste
rock), which will in turn have direct consequences on the economy of the mining operation.
2. Based on Table 4.6 it is concluded that as the cohesion and angle of internal friction
increases, the factor of safety increases. As the cohesion increases, the binding property enhances
which makes the slopes stable. High water content can weaken cohesion because abundant water
both lubricates and adds weight to a mass. Moreover alternating expansion by wetting and
contraction by drying of water reduces strength of cohesion.
3. While running the numerical model FLAC/Slope it was observed that factor of safety changes
with change in the resolution of the numerical mesh (coarse, medium and fine). Incase of coarse
mesh the factor of safety is quite approximate, while in fine mesh the factor of safety converges
to the nearest possible value making it more accurate. However, calculation in coarse mesh is
faster than in fine mesh. So depending upon the requirement and time availability of modeler, the
mesh has to be selected.
- 53 -
CHAPTER: 05
CONCLUSION
- 54 -
CHAPTER: 05
CONCLUSION
5.1 Conclusion
Opencast mining is a very cost-effective mining method allowing a high grade of mechanization
and large production volumes. Mining depths in open pits have increased steadily during the last
decade which has the increased risk of large scale stability problems. It is necessary to assess the
different types of slope failure and take cost effective suitable measures to prevent, eliminate and
minimize risk.
The different types of the slope stability analysis techniques and software are available for slope
design. Numerical modelling is a very versatile tool and enables us to simulate failure behavior and
deforming materials. FLAC/Slope is user friendly software which is operated entirely from
FLAC’s graphical interface (the GIIC) and provides for rapid creation of models for soil/rock
slopes and solution of their stability condition. Moreover it has advantages over a limit
equilibrium solution like any failure mode develops naturally; there is no need to specify a range
of trial surfaces in advance and multiple failure surfaces (or complex internal yielding) evolve
naturally, if the conditions give rise to them. In this project, an attempt has been made to get
acquaintance with the powerful features of FLAC/Slope in analysis and design of stable slopes in
opencast mines. Data was also collected from Jindal Opencast Mine with 116m ultimate pit
depth at Raigarh in Chhattisgarh State to assess the effects of cohesion and angle of internal
friction on design of stable slope using FLAC/Slope.
The parametric study which was carried by varying the cohesion, angle of internal friction and
ultimate slope angle showed that with increase in ultimate slope angle, the factor of safety
decreases. Moreover cohesion and angle of internal friction are quite important factors affecting
slope stability. With increase in both the parameters the stability increases. Conduct of slope
stability assessment in Indian mines is mostly based on empirical and observational approach;
hence effort is made by statutory bodies to have more application of analytical numerical
modelling in this field to make slope assessment and design scientific. This will ensure that
- 55 -
suitable corrective actions can be taken in a timely manner to minimize the slope failures and the
associated risks.
5.2 Scope for Future work
For the parametric studies, only cohesion and friction angle have been considered. However this
study can be extended to individual bench angles where all the benches may not be of same
height. The conditions assumed during this analysis are such that there is no effect of water table
and geological disturbances. Along with cohesion and friction angle other parameters like effect
of geological disturbances, water table and blasting can be carried out. For slope stability
analysis other numerical models such as UDEC and Galena can also be used in order to compare
the sensitivity and utility of the different software.
- 56 -
REFERENCES
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Engineers, A.I.M.E, pp. 83-94.
2. Bieniawski, Z.T. (1984), “Input Parameters in Mining”, Rock Mechanics Design in
Mining and Tunneling, A.A. Balkema, Netherland, Edition-8, pp.55-92.
3. Call, R. D. & Savely, J. P. (1990), “Open Pit Rock Mechanics”, In Surface Mining, 2nd
Edition (ed. B. A. Kennedy), Society for Mining, Metallurgy and Exploration, Inc., pp. 860882.
4. Call, R. D., Nicholas, D.E. & Savely, J.P. (1976), “Aitik Slope Stability Study”, Pincock,
Allen & Holt, Inc. Report to Boliden Aktiebolag, Gallivare, Sweden.
5. Coates, D. F. (1977), “Pit Slope Manual”, CANMET (Canada Centre for Mineral and
Energy Technology), CANMET REPORT , pp 126p
6. Corbyn, J.A. (1978). “Stress Distribution in Laminar Rock during Sliding Failure”, Int. J.
Rock Mechanics, Vol. 15, pp.113-119.
7. Farmer, I. (1983), “Engineering Behavior of Rocks” Chapman & Hall, U.S.A., pp.145167
8. Goodman, R.E. (1975), “Introduction to Rock Mechanics”, John Wiley & sons, U.S.A.,
pp.187-194
9. Hoek, E. (1970), “Estimating the Stability of Excavated Slopes in Opencast Mines”, Trans.
Instn. Min. Metall. (Sect. A: Min. industry), 79, pp. A109-A132.
10. Hoek, E. (197la), “Influence of Rock Structure in the Stability of Rock Slopes”, In Stability
in Open Pit Mining, Proc. 1st International Conference on Stability in Open Pit Mining
- 57 -
(Vancouver, November 23-25, 1970), New York: Society of Mining Engineers, A.I.M.E, pp.
49-63.
11. Hoek, E. & Bray, J.W. (1980), “Rock Slope Engineering”, Institute of Mining &
Metallurgy, London, pp.45-67.
12. Itasca. (2001), “FLAC Version 5.0. Manual”, Minneapolis: ICG.
13. Nordlund, E. & Radberg, G. (1995). “Bergmekanik. Kurskompendium”, Tekniska
Hogskolan iLulea, pp. 191.
14. Sage, R., Toews, N. Yu, Y. & Coates, D.F., (1977) “Pit Slope Manual Supplement 5-2 —
Rotational Shear Sliding: Analyses and Computer Programs”, CANMET (Canada Centre for
Mineral and Energy Technology), CANMET REPORT 77-17, 92 p.
15. Winkelmann, R. (1984) “Operating Layout & Phase Plan”, Open pit Mine Planning and
Design, Chapter-3A, pp.207-217
16. Zhang, Y., Bandopadhyay, S., Liao, G. (1989), “An Analysis of Progressive Slope
Failures in Brittle Rocks”, Int. J. of Surface Mining, Vol. 3, pp.221-227.
17. http://www.rocscience.com/products/slide/Speight.pdf
18. http://geoinfo.usc.edu/bardet/reports/Journal_papers/5simplex.pdf
19. http://www.infomine.com/publications/docs/Brawner1997.pdf
- 58 -
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