Evaluation of Open Pit Mine Slope Stability Analysis

590 International Journal of Earth Sciences and Engineering

ISSN 0974-5904, Vol. 04, No. 04, August 2011, pp. 590-600

#02040703 Copyright © 2011 CAFET-INNOVA TECHNICAL SOCIETY. All rights reserved.

Evaluation of Open Pit Mine Slope Stability Analysis

DHANANJAI VERMA1, RAHUL THAREJA2, ASHUTOSH KAINTHOLA1 and T. N. SINGH1 1Dept of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, India-400076

2Dept of Mining Engineering, Institute of Technology - Banaras Hindu University, Varanasi,

India-221005

Email: [email protected], [email protected], [email protected],

[email protected]

Abstract: The issues concerned with slope stability in the open cast mines have come to

forefront in the mining operations due to increasing pit depth. The cut slope stability has the

most prominent influence in the productivity and longevity of a mine, collapse of which can

lead enormous damages to man and machinery. It is always considered as economic burden

to mine production. A comprehensive study is necessitated to ensure stable slopes which

are aided by numerical, analytical, physical, kinematic and empirical analyses. In the

present study four cut slopes from a coal mine in Wardha Valley Coal field have been

analyzed using empirical and kinematic tools. The study has involved the classification and

prediction the probable failure mode of the slope mass using slope mass rating and

kinematic analysis. The analysis results have matched well with the field observations and

can help to protect the slope and ensure the safety for better productivity.

Keywords: Slope Stability, SMR, RQD, Kinematic Analysis, Wardha valley

Introduction:

Open cast mines call for the excavation of

the earth surface to reach the underlying

minerals of economic utility. The excavation

process requires cut slopes to be formed on

an earlier plain earth surface. Stability of the

cut slopes is crucial for the safe and

economical mining operations. The slope

stability is governed by the local geological

and geo-technical characteristics of the

slope forming mass and the prevailing

ground water conditions (Singh et al. 1998;

Singh et al. 1999). The design of the open

pit mine slopes is a deciding factor for

efficient exploitation of underground

minerals as well as for the safety of the

mine and the mineworkers which control the

economics of the operations. The ever

increasing pit depths and production

requirements from opencast mines subject

the design engineers and planners to work

under the constraints of two conflicting

requirements of stability and production.

Economics could be improved by steepening

the slope thereby reducing the amount of

waste excavation on the other side,

excessive steepening of slope could result in

failure leading to loss of life and damage to

property (Singh et al. 1989; Singh and

Singh, 1992). This scenario poses a big

question as to how to achieve an optimum

design – a compromise between a slope that

is flat enough to be safe and steep enough

to be economically acceptable. The

consequences of a slope failure could be

quite serious in terms of safety and

economics and are governed by the location

and extent of failure. Hence, the design of

the steepest slope with desired stability asks

for a detailed and reliable geotechnical

investigation. The factors, which mainly

influence the stability of a typical opencast

slope, are the shear strength parameters of

slope forming material, the presence and

characteristics of structural discontinuities in

the slope mass and the ground water

conditions (Singh and Monjezi, 2000; Singh

et al. 2008). There have been quite a

number of researchers who have proposed

the characterization of the rock mass

distinguishing them on the basis of strength

but there always persists a certain degree of

uncertainty while acquiring the field data for

designing a slope leading to erroneous rock

mass characterisation.

Rock Mass Classification Schemes:

Various researchers have proposed different

type of rock mass classification systems,

591 DHANANJAI VERMA, RAHUL THAREJA, ASHUTOSH KAINTHOLA

and T. N. SINGH

International Journal of Earth Sciences and Engineering


which find numerous applications in various

aspects of rock mechanics. Bieniawski

(1974) introduced the rock mass rating

(RMR). The RMR-system incorporates six

parameters, whose values are added to

obtain a total RMR rating to characterise a

rock mass. After 1974, the classification has

undergone several changes and it is

important to state which version of the

system is used Hoek–Brown (Hoek and

Brown, 1988). The rock mass strength

(RMS) classification (Stille et al. 1982) is a

modification of the RMR-system. The sum of

the parameters and the rating reduction

with respect to joints set is the RMS-value

for the rock mass. The Rock Quality

Designation index (RQD) was developed by

Deere (Deere et al. 1967) to provide a

quantitative estimate of rock mass quality

from drill core logs. This is also estimated

through indirectly on the number of

fractures and amount of softening or

alteration in the rock mass as observed in

the rock cores from a drill hole. The

geological strength index (GSI) was

introduced as a complement to their

generalised rock failure criterion (Hoek et al.

1995) which are in turn, used when

estimating the parameters “s”, “a” and “mb”

in the Hoek–Brown criterion, using empirical

equations. The GSI-system was introduced

to overcome the deficiencies in RMR for very

poor quality rock masses. The original GSI-

table has been subject to several minor

revisions, as well as additions to classify,

particularly weak and soil-like rock masses

(Hoek et al. 2002). The aim of present study

is to assess and evaluate the condition of

slope of Wardha Valley Coalfield (WVC). The

mine is infested with problems related to

slope stability owing to the low strength

slope forming material, its heterogeneity,

anisotropy and the discontinuity guided

failures. The problem of stability is more

aggravated due to presence of Wardha

River/ Ground water and incessant burning

of the coal in some pits. The Slope stability

problem of the cut slopes in the area calls

for a detailed geotechnical investigation for

the scientific and systematic mitigation.

There are number of approaches to assess

the behaviour of slope using different

modelling methods like limit equilibrium,

analytical and kinematic tools, physical and

numerical models as well as intelligent

models. The economic and safe design can

be achieved by a systematic approach like

Slope Mass Rating (SMR) (Romana, 1985).

It is used to assess the health of the slope

and is one of the most accepted, versatile

and widely used tool. This tool provides

quick assessment about the behaviour of

slope at a given site. The aim of the present

paper is identifying potentially hazardous

rock cut slopes using the slope mass rating

(SMR) approach in open cast coal mine of

Wardha Valley Coal Field (WVC) Nagpur,

India.

Slope Mass Rating:

Slope Mass Rating is a modified RMR system

for slope, developed by Romana (1985).

SMR is a useful rating tool for evaluation of

slope instability risk based on slope face

relation with geological discontinuities.

SMR = RMR - (F1.F2.F3) + F4

The adjustment rating of joints is the

product of three factors:

F1 depends upon the parallelism between

joints and slope face strike. It ranges from

1.0 to 0.15. The values are empirically

established by the formula:

F1 = (1 - Sin A)2

Where A =Angle between the strike slope

face and joints.

F2 refers to the joint dip angle in the planar

mode of failure. Its values range from 1.00

to 0.15. The empirically established formula

is F2 = tan2 Bj

Where Bj = joint dip angle. F3 indicates to

the relationship between slope face and joint

dip. In planar mode of failure, F3 refers to

the probability of joints daylighting on the

slope face. Condition is favourable when

slope face and joints are parallel and

unfavourable when slope dips 10 degree

more than joints. F4 - Adjustment factor for

method of excavation has been fixed

empirically and are follows Natural slope =

+15, Presplitting = +10, Smooth blasting =

+8, Normal blasting = 0, Deficient blasting

= -8 and Mechanical excavation = 0.

According to the SMR values, Romana

(1985) defined five stability classes. They

are described in Table 1.

592 Evaluation of Open Pit Mine Slope Stability Analysis



Table 1: Stability Classes as per SMR Values (Romana, 1985)

Class V IV III II I

SMR

Values 0-20 21-40 41-60 61-80 81-100

Rock

mass

description

Very bad Bad Normal Good Very good

Stability Completely

unstable Unstable Partial stable Stable

Completely

stable

Failures

Big planar

or soil like

or circular

failure

Planar or big

wedges

Planar along

Some joints and

Many wedge

failure

Some

block

failure

No failure

Probability

of Failure 0.9 0.6 0.4 0.2 0

Study Area:

Wardha Valley Coalfield (WVC) is a NW–SE

elongated structural basin with its coal

bearing seams spreading over an area of

800 sq km along a length of 116 km

situated towards the south of the city of

Nagpur in the central region of India (Figure

1).The slope forming materials in these

mines mainly consists of variable soil, shales

and sandstones as overburden followed by a

composite of coal seam of 15–21 m

thickness. The Wardha River forms a major

drainage system of the area and flows from

NW to SE along the central part of WVC. The

study mainly focused on the Ghugus open

cast coal mine. The Ghugus open cast mine

lies exactly East of the Wardha River The

depth of open cast mine at present is around

95-100m. Some coal seams are submerged

in water throughout the year, particularly the

lower benches. WCL has taken on strip

mining method for Ghugus and the other

mines. At present, they are operating on the

11th cut of mine and going down further

along the dip of the seam. Four locations

have been selected for the present study for

the assessment of the slope stability.

Methodology:

A geological and geotechnical study was

record out to observe the geological data

viz. discontinuities present in the rock mass,

bedding planes, slope geometry and the

hydro-geological conditions. Rock samples

were collected from four different slope

locations in the study area to determine the

various geo-mechanical properties in

laboratory.

Figure 1: Study Area and Sample Collecting

Location of different Coalmines (Jhanwar &.

Thote, 2011)

The aim of the study is to characterize the

rock mass forming the slope. The rock

samples collected from the field include

different types of Sandstones, Coal, Shaley

Coal and Shale. Uniaxial Compressive

Strength (UCS) of the samples was

determined by loading the NX sized core

samples using Universal Testing Machine

(UTM) (ISRM, 1978, 1981).


and T. N. SINGH



Table 2: Uniaxial Compressive Strength of

the Rock Samples

Sl.

No.

Rock

Type

UCS

(MPa) GSI

1 Sandstone 20-Oct 35-50

2 Shale 15-Jun 25-35

3 Shaley

Coal 13-Aug 35-40

4 Coal 9.5-15 30-45

GSI values were also tabulated from the

field with help of GSI chart given by Hoek

and Brown (1998) and adjusted from Hoek

1994). The resultant value of UCS and GSI

are given in Table 2. The Rock Quality

Designation (RQD) was obtained from the

volumetric joint count (JV) (Palmstrom,

1982).

Four locations were selected for recording of

field data which is described below:

Location 1:

Location 1 has beds of Shaley coal and shale,

which have three set of joints named as J1, J2 and J3 (Figure 2). Bedding plane and the slope

are dipping in the same direction i.e. west,

but inclination of slope is considerably more.

The discontinuity data has been tabulated

with their adjustment factors for different

joint conditions in Table 3(a,b).

Figure 2: Field View of Location 1 with Marked Joint Set.

Table 3a: Orientation of Discontinuities and Slopes (Location 1)

Joint Strike Dip angle Dip direction

J1 N 330° 80° ENE

J2 N 175° 55° W

J3 N 250° 55° SE

Slope N 180° 55° W

Bedding plane N 20° 8° W

Table 3b: Adjustment Factor for different Discontinuities (Location 1)

Conditions F1 F2 F3 F1* F2* F3

J1 and slope 0.15 1.0 50 7.5

J2 and slope 1.0 1.0 25 25

J3 and slope 0.65 1.0 25 16.25

*Normal Blasting and Mechanical Excavation is the Case so F4 = 0.




Location 2:

Location 2 has layers of shale and

Sandstone, with two prominent joint set

marked as J1 and J2 (Figure 3). Bedding plane

and the slope orientation are similar to

Location 1. The rock mass is highly

weathered. The discontinuity data has been

tabulated with their adjustment factors for

different joint conditions in Table 4(a, b).



J1 N 180° 30° W

J2 N 185° 60° W

Slope N 180° 45° W




J1 and slope 1.0 0.57 25 14.25

J2 and slope 1.0 1.0 60 60.0


Figure 3: Field View of Location 2 with Marked Joint Set and Highly Weathered Rock Mass.

Location 3:

The slope in location 3 is composed of shale

and Sandstone, which have three sets of

joints marked as J1, J2 and J3 (Figure 4). This

slope is steeply inclined as compared to

Location 1 and 2 and J2 and J3 are forming a

wedge. The discontinuity data has been




and T. N. SINGH



Figure 4: Field View of Location 3 with Broken Rock Mass.



J1 N 10° 81° W

J2 N 100° 60° SSW

J3 N 50° 70° NW

Slope N 180° 60° W




J1 and slope 0.15 1.0 50 7.5

J2 and slope 0.70 1.0 6 4.2

J3 and slope 0.85 1.0 25 21.25


Location 4:

Shaley coal, Shale and Sandstone form the

slope in this location which is traversed by

two sets of joint marked as J1, J2 and J3 (Figure 5). Bedding plane and the slope are

dipping in the same direction, but the slope in

this location is steeper as compared to

location 3. The strata are highly weathered

which indicates continuous spalling due to

the presence of vertical joints which have

less spacing as compared to the inclined

joints. The discontinuity data has been






Figure 5: Field View of Location 4 with Vertical Joints.



J1 N 45° 75° SE

J2 N 10° 90° -

Slope N 180° 70° W




J1 and slope 0.75 1.0 25 18.75

J2 and slope 0.55 1.0 50 27.5


Analysis of Slope:

Rock mass characterization is another tool

to classify the slope particularly in mines.

SMR is the most common classification

scheme, which is frequently used by

different researchers for the stability

analyses of cut slopes in various mines

(Jhanwar et al. 2010; Pradhan et al. 2011;

Singh et al. 2011). Slope Mass Rating is

main tool to understand the rock mass

behaviour of slopes in surface mines, which

always poses serious problems due to

increase in depth and slope angle. Due to

the presence of various geological

complexities, the problem of stability is

more aggravated. Singh et al (2011) have

corroborated the results of numerical

simulations with the SMR value and matched

the results well with the field condition. In

the present study, all the locations have

different joint density with the bedding

planes having a dip slope. RMR weightage

and subsequently its SMR value is given in

Table 7. SMR ratings for Locations 1 and 3

fall under the SMR class III, which is

described as 'normal' rock mass and is

partially stable with a 0.4, probability of

failure. Figures 2 & 4 clearly indicate that

the possible failure mode is planer and

wedge type. Locations 2 and 4 are falling

under the SMR class IV, which is considered

to have 'bad' rock. The stability of this slope

face is observed as unstable and chances of

failure are planer and toppling with a 0.6,

probability of failure.


and T. N. SINGH



Table 7: RMR, RQD and SMR Values of different Locations

Parameters Location 1 Location 2 Location 3 Location 4

UCS in MPa

(Rating)

10-15

2

13-16

2

14-19

2

12-16

2

RQD from JV

(Rating)

30-45

8

25-40

8

40-55

13

30-50

8

Spacing of

discontinuities

in cm

(Rating)

40-90

15

40-100

15

20-40

10

15-40

10

Conditions of

discontinuities

(Rating)

Rough surfaces

slightly

weathered

25

Slightly rough

surface highly

weathered

20

Rough surface

slightly

weathered

25

Slightly rough

surface highly

weathered

20

Ground Water

condition

(Rating)

Dry

15

Dry

15

Dry

15

Damp

10

RMR basic 65 60 65 50

F1 0.60 1.0 0.57 0.65

F2 1.0 0.78 1.0 1.0

F3 33.33 42.5 27.0 37.5

F4 0 0 0 0

F1* F2* F3 19.99 33.15 15.39 24.37

SMR Value 45.01 26.85 49.61 25.63

SMR Class III IV III IV

Rock Mass

Description Normal Bad Normal Bad

Stability Partially stable Unstable Partially

stable Unstable

Failure

Planar along

some joint and

many wedges

Planar or big

wedges

Planar along

some joint

and many

wedges

Planar or big

wedges

Probability of

Failure 0.4 0.6 0.4 0.6

Kinematic Analysis of Slopes:

Kinematic slope stability analysis was

carried out using the Stereonet plots. It is

an easy tool to analyse the planar and

wedge failures in the rock slopes. The

structural data is geometrically plotted in an

equal area net to establish the mode and

probability of failure. At location 1,

intersected by three joint set present, shows

the possibility of wedge failures as indicated

by the plot (Figure 6 a). Joint set, J3, which

are parallel to the slope and forms a critical

failure plane. The results are supported by

the field observation of that particular

location (Figure 2). At location 2, there are

two joint set, which is running parallel to the

slope face. The J2 has a steeper dip as

compared to J1 which results in day lighting

on the slope face (Figure 3). There is chance

of planer failure due to the presence of J2

(Figure 6 b).




Figure 6: Equal Area Stereonet Plot of all Location A) Location 1, B) Location 2.

At location 3, there are three set of joint

present, which are forming a wedge

between the slope face and joint sets J2 & J3

(Figure 7a) . As J1 is steeply inclined to the

slope, there is a likelihood of a toppling

failure (Figure 4). At location 4, there are

two set of joints present. J1 is steeply

inclined with the slope face (Figure 5) and

the probable failure mode would be toppling

as similar to location 3, whereas J2 is

represented as vertical joint (Figure 7b).

The slope face in Location 4 is slightly

steeper than the all above locations. Here,

the joint have almost a similar strike, with

variation in dip amount.

Figure 7: Equal Area Stereonet Plot of all Location A) Location 3, B) Location 4.

Conclusion:

In the present study four vulnerable

locations were examined using a slope mass

rating classification scheme which were

further investigated through kinematic

analysis. The maximum three joint sets

were observed at a particular location. The

slopes in the studied locations have same

strike direction with slight variation in their

inclination towards west. The bedding has a


and T. N. SINGH



dip slope towards west gently inclined at an

angle between 6° to 8°. The SMR study of

different locations indicates that the rock

mass are partially stable to unstable with

probability of failure 0.4 to 0.6 (SMR class

III to IV), which is further confirmed by

kinematic analysis. The failure is of wedge,

toppling and planer mode which is matching

with field investigations and observations.

For long term stability and its sustainability,

the slope requires immediate attention to

prevent and mitigate chances of failure in

order to enhance the productivity of the

mine. The similar approach should be

adapted to other virgin areas prior to the

beginning of the excavation to understand

the mode and mechanism of probable

failures.

Acknowledgment:

The authors would like to thank the

management of WCL, India, for their

cooperation and support during the field

work. The views expressed in the article are

those of the author and are not necessarily

any organization or institution.

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Evaluation of Open Pit Mine Slope Stability Analysis

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