Scale Effect on Strength and Failure Modes of Open Pit Cut Slope of Wardha Valley Coalfield in India

Dhananjai Verma1, Deepankar Choudhury, M.ASCE2, P. G. Ranjith3 and T. N. Singh4

1Research Scholar, IITB- Monash Research Academy, Mumbai - 400076, India; email: geodhananjai@gmail.com

2Associate Professor, Department of Civil Engineering, Indian Institute of Technology Bombay, Mumbai - 400076, India; email: dc@civil.iitb.ac.in

3Associate Professor, Department of Civil Engineering, Monash University, Melbourne, Victoria 3145, Australia; email: ranjith.pg@monash.edu

4Professor, Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai - 400076, India; email: tnsingh@iitb.ac.in

ABSTRACT Stability of slopes is one of the basic problems in open cast mines. Economical and safe open pit slopes involve maximization of the angle of inclination of the slope with proper stability. Studies on stability require a complete knowledge of the local geology. Stability of rock slopes depends on the scale of the slope, type of rock, orientation and density of the discontinuities etc. With an increase in size of rock mass, the chances of presence of joints and fractures increases and hence the strength of the rock material reduces due to the inclusion of more discontinuities in the rock mass. In the present study, a mine slope at Wardha Valley Coalfield in India has been numerically analyzed to take into account the effect of number of joint sets on the stability. The results show a reduction in factor of safety with an increase in joint sets.

Keywords: Rock mass, Wardha Valley, Slope stability, FOS, Scale effect.

INTRODUCTION Surface mining activity in India is increasing at a rapid rate to bridge the gap between demand and supply of coal to the energy sector. Due to the deepening of mines, most surface mines suffer from problems related to slope stability. The presence of water reduces the frictional strength of the slope material, and the geo-mechanical properties are reduced further due to the presence of pore water pressure. The flow of water may augment the seepage, leading to the formation of tension cracks across the slope. Tension cracks are also possible due to shocks and vibration caused by poor blasting (Singh and Singh, 1992; Singh et al. 1994; Khandelwal and Singh, 2009).

Slope stability analysis requires more in-depth understanding of the mechanics involved. Several researchers like Choudhury and Subba Rao (2006), Choudhury et al. (2007), Choudhury and Modi (2008) and many others had studied the slope stability problems

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but mostly for soil slopes. The design of the slopes of open cast mines is becoming more and more significant due to the continual increase in the working depth of the mines. A minor change in the slope geometry may affect economy as well as mine stability. Strength properties of the rock mass depend on the material and the frequency of discontinuities present in the rock mass. This is known as the scale effect on the strength of the rock mass. The main factors which affect slopes are the stress conditions acting on the particular area, the presence of geological structures on the scale effect, the strength of the rock mass with different frequencies of discontinuities and the slope geometry.

The scale effect in rock mechanics can be said to be the summation of the mechanical properties of rock on the size of tested samples and the discontinuities encountered in them. Extrapolating small-scale laboratory tests to full-scale structures incorporates the effect of structure size on strength. The sliding surface in a slope consists of a single continuous plane over the full area of the surface, or a complex surface made up of both discontinuities and fractures through intact rock. According to Brunetto (2004), even small changes in shear strength can result in significant changes in the safe height or angle of a slope.

With development of modern method of excavation and increase in excavation depth, it has been pragmatic that some discontinuities are developed parallel to the excavation periphery in hard rock masses (Germanovich and Dyskin, 2000). The fractures parallel to the excavation periphery in deep underground openings have been studied by engineers and researchers. For example, Diederichs (2007) paid much attention to the tensile failure of hard rock. This researcher carried out a comparison of two curves showing the long-term strength of laboratory samples and the in situ strength of hard rock. It was concluded that in situ strength of hard rock is less than the long-term strength of laboratory samples when the confining stress is relatively low compared with Uniaxial Compressive Strength (UCS), and spalling failure may occur under this confining stress condition. To understand the failure process in hard rock, Eberhardt (1998) and Eberhardt et al. (1999) had conducted a number of experiments to identify and characterize the brittle fracture process by uniaxial compression testing of pink Lac du Bonnet granite in Canada. The shape of the specimens was cylindrical and the height-to-diameter ratio was approximately 2.25. The compression failure of concrete columns and the size effect on compression strength of columns have been carefully studied (Bazant and Xiang 1997; Bazant et al. 1999).

The Geological Strength Index (GSI) was introduced as a complement to the generalized rock failure criterion. The GSI-system was used to overcome the deficiencies in Rock Mass Rating (RMR) for very poor quality rock masses. The Rock Quality Designation (RQD) index was developed by Deere et al. (1967) to provide a quantitative estimate of rock mass quality from drill core logs. RQD is defined as the percentage of intact core pieces longer than 100 mm (4 inches) in the total length of core. The core should be at least NX size (54 mm in diameter). Palmström (1982) suggested that, when no core is available but discontinuity traces are visible in surface exposures, the RQD in that case

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may be estimated from the number of discontinuities per unit volume. The suggested relationship for clay-free rock masses is given as:

RQD = 115 - 3.3 Jv (1)

where Jv is the sum of the number of joints per unit length for all joint (discontinuity) sets known as the volumetric joint count.

RQD is a directionally dependent parameter and its value may change significantly, depending on the borehole orientation. The use of the volumetric joint can be quite useful in reducing this directional dependence.

FIELD INVESTIGATIONS The Wardha Valley coalfields cover an area of about 1440 km2 between latitudes ~19°28'N and 20°27’N and longitudes ~78°50'E and 79°45'E and are located in the Chandrapur and Yeotmal districts of Maharashtra state, India. They extend over a length of 150 km from NW to SE in an almost straight line. For the present study, the Ghugus open-cast coal mine, which has coal seams of lower Gondwana formation, was selected.

Slope stability assumes a special significance in the Wardha Valley coalfield (WVC) where open-cast mines are becoming deeper due to constraints of land availability, and the deposits are affected by geological discontinuities in the form of joints, fractures and faults. The stability of open pit slopes in this coal field is greatly influenced by these structural features and the constrained benching patterns. From the field, rock blocks were collected for the determination of geo-mechanical properties. The numbers of discontinuities were observed and surface conditions for GSI values of different locations are tabulated in Table 1. The number of discontinuities was also counted in one cubic meter of the cut surface of the mine for determination of RQD values, which are tabulated in Table 2. A general field photograph of the study area is given in Figure 1.

Table 1: GSI values of different locations in the Wardha Valley Coalfield

Sr. No

GSI

No. of discontinuities, Location 1

Surface Condition

No. of discontinuities, Location 2

Surface Condition

1 80-70 2 Good 2 Good 2 70-60 3 Fair 3 Fair 3 60-50 5 Good 5 Poor 4 50-40 8 Poor 8 Poor 5 40-30 4 Fair 4 Poor

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Table 2: RQD values at different locations of the Wardha Valley Coalfield Sr. No.

RQD (%)

No. of discontinuities, Location-1

Jv (Location-1)

No. discontinuities, Location-2

Jv (Location-2)

1 75-90 2 5 3 72 50-75 4 13 7 173 25-50 8 22 10 264 <25 9 31 11 32

Figure1. A general field view of the study area at Wardha Valley Coalfield.

LABORATORY EXPERIMENTS The geo-mechanical properties of rock samples were measured from cores (NX size) as well as rectangular blocks of different sizes. UCS was measured from rectangular blocks of different sizes (L/W ratio 2 to 2.5) with help of Universal Testing Machine (UTM). The angle of internal friction and cohesion were calculated with the help of triaxial cell and Mohr diagram. For the triaxial test, NX size (54mm dia) was used. One single value of cohesion and angle of internal friction, three samples were tested plated on Mohr diagram and finally got the values. Assume a number of test samples were cut from a block of rock containing a smooth, planar discontinuity. Furthermore, the discontinuity contains a cemented infilling material such that a tensile force would have to be applied to the two halves of the sample in order to separate them. Each sample is subjected to a force at right angles to the discontinuity surface (normal stress, σ), and a force is applied in the direction parallel to the discontinuity (shear stress, τ) while the shear displacement (δs) is measured. The tested properties are given in Tables 3a and 3b. EVALUATION OF TESTED DATA SET In order to find the scale effect of the strength of geo-material, UCS values were obtained for different sizes of rock samples. It is observed that with an increase in the size of

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sample/area, UCS decreases (Figure 2). This is because of the presence of discontinuities in the samples, since discontinuities affect the strength of the material. It also depends on the nature of the discontinuity.

Table3a: Geo-mechanical properties of sample with different sizes.

Sr. No Cross sectional area (cm sq.)

UCS (MPa), Location-1

UCS (MPa), Location-2

1 25 15 13 2 64 10 11 3 81 8 8 4 100 7 6 5 121 5 4

Table3b: Geo-mechanical properties of core samples.

Sr. No. Unit weight (MN/m3)

Friction angle (degree)

Cohesion (MPa)

1 0.022 25 0.58 2 0.025 23 0.60 3 0.021 21 0.55 4 0.022 27 0.49 5 0.023 26 0.63

Average value 0.0226 24.4 0.57

Location-1

0

20

40

60

80

100

120

140

4 6 8 10 12 14 16

UCS (MPa)

Cro

ss-s

ectio

nal a

rea

(cm

sq.)

Figure 2. Relationship of sample size and UCS, a) location-1, b) location-2

The RQD value has been observed to be inversly proportional to the number of discontinuties present in the cut surface (Fig. 3a). The numbers of discontinuties were counted on the face of cut surfaces and RQD values were calculated by the empirical equation given by Palmström (1982). UCS values also decrease as the numbers of discontinuities increase at that particular site (Fig. 3b).

Location-2

020406080

100120140160

3 8 13 18UCS (MPa)

Cro

ss se

ctio

nal a

rea

(cm

sq.)

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The GSI values of that site were also calculated using the chart provided by Hoek and Brown (1998) and adjusted by Hoek (1994). The GSI values of geo-material vary according to the number of discontinuities encountered and depending on the surface condition of the particular site (Fig. 4). As the number of discontinuities on the surface and the rating of the surface condition decreased, GSI values also decreased.

It is generally accepted that there is a significant reduction in strength with an increase in sample size, and this general relation quantifying the phenomena is the current benchmark. Hoek and Karzulovi (2000) suggested that this reduction in strength is due to the increased probability that failure of the rock mass will occur as the sample size increases. This will reach an asymptotic minimum level at a set sample size that will depend on the type and condition of the rock. RQD and UCS are directly affected by the presence of fractures in the rock mass. As number of fracture increases in the rock mass, RQD and UCS both decrease (Fig. 3).

02468

1012

75-90 50-75 25-50 <25

RQD (%)

Num

ber

of d

iscon

tinut

ies No. of discontinuties, Location-1

No. discontinuties, Location-2

Figure 3. Relationship between (A) RQD and discontinuties. B) RQD and UCS.

0123456789

80-70 70-60 60-50 50-40 40-30 30-20

GSI

Num

ber o

f disc

ontin

utie No. of discontinuties, Location 1

No. of discontinuties, Location 2

Figure 4. Relationship between GSI and discontinuties

NUMERICAL SIMULATION There are number of techniques by which one can analyze the slope prior to excavation and post excavation. There are many different ways to compute the factor of safety

0

2

4

6

8

10

12

14

16

75-90 50-75 25-50 < 25

RQD (%)

UC

S (M

Pa)

UC S (Mpa), L ocation-1

UC S (Mpa), L ocation-2

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(FOS) of open cut slopes including limit equilibrium method, finite element method, finite difference method, discrete element method etc. In recent past, with increase in computational capacity, the finite element method has been widely used for initial stage of slope stability analysis. For certain types of complex geotechnical problems, finite element method offers realistic results over linear equilibrium methods (Griffiths and Lane, 1999; Kainthola et al. 2011).

Numerical simulations have been used with different numbers of joint sets and heights of slopes. The scale effect of the shear strength of a fractured rock mass surface includes the effect of specimen size and the geometry configurations of the joint surface which finally affects the factor of safety (FOS). It has been found in some studies that the shear strength of joint is inversely proportional to specimen size (Giani et al. 1992), while in other cases, strength increases with specimen size (Giani et al. 1992; Brown et al. 1977).

In the present study, rock slopes of different heights with different numbers of joint sets at a constant inclination of 60° have been simulated using PHASE2 (Version 7.0, Finite Stress Element Analysis). The first slope analyzed is of 28m height with one set of discontinuities. The factor of safety in this case was calculated to be 1.48 (Fig. 5). In the second analysis, the height of the open cut slope was taken to be 29 m with two cross sets of discontinuities. The FOS value reduced to 1.23 with the introduction of the joint sets (Fig. 6). In the third case, a 30 m slope with three sets of discontinuities was analyzed. The value of the factor of safety further reduced to 0.89 (Fig. 7). Overall FOS decreases as number of fracture / discontinuity in the rock mass increases. FOS is also depending on the height of the slope as it is indicating the number of fractures in the rock mass. The increase in discontinuities decreases the rock mass strength which destabilizes the slope. The increase in height also adds to the vertical load on the slope leading to reduction in the factor of safety (Figures 8a and 8b). CONCLUSIONS The aim of the present study was to discover the change in strength parameters of geo-materials with increase in scale effect. The UCS, RQD and GSI values show reductions with an increase in joint density in the rock mass. Similarly, the FOS values also decrease with an increase in the number of discontinuities in the slope. Another observation is that the FOS also decreases with an increase in the height of the open cut slope. The increased probability of failure can be attributed to the inclusion of larger numbers of discontinuities within the rock mass due the increase in the height of the slope. This also suggests that geo-mechanical properties should be used with proper care for simulating the actual conditions in the analysis. ACKNOWLEDGEMENTS Authors thank the management of WCL, India, for the cooperation and support during the field work. Also the funding received from BHP Billiton, Australia through the IITB-Monash Research Academy to carry out the research work is acknowledged.

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Figure5. A 28m cut slope with one set of discontinuity with FOS = 1.48

Figure6. A 29m cut slope with two sets of discontinuity with FOS = 1.23

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Figure7. A 30m cut slope with three sets of discontinuity with FOS = 0.89

Figure 8. A) Relationship between FOS and number of discontinuties. B). Plot between

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