Stability Assessment of Rock Slope and Design of Rock Slope Reinforcement

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1 ISGTI April2018, IIT Delhi, India Stability Assessment of Rock Slope and Design of Rock Slope Reinforcement Kallam Naveen Reddy Adapa Murali Krishna Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati ABSTRACT: The stability analysis of rock slopes has been a challenging task because of the presence of discontinuities in various forms which result in different types of slope failures. Discontinuities are the weakest zones in the rock mass. Failure mechanism of a rock slope mainly depends on the characteristics of discontinuities. Discontinuities in the form of joints, bedding planes and faults create anisotropy in the rock mass. Stability assessment of rock slope is very much essential for suitable mitigation measurements. The stability of a rock slope is increased by using various stabilizing techniques. Rock bolting is one of the most common techniques used to stabilize the slope. In this paper, stability analysis of a rock slope located at Sairang station along the alignment of the Bairabi Sairang railway project is carried out. The study mainly focuses on the kinematic analysis of the rock slope to obtain the possible mode of failure using DIPS and the stability of the rock slope is analyzed using PHASE 2 which is a finite element method based shear strength reduction technique. Keywords:Rock Slope; Kinematic Analysis; Finite Element Method; Shear Strength Reduction; Rock bolting. 1. Introduction Failure mechanism of rock slope is very intricate phenomenon and mainly depends on the characteristics of discontinuities. Discontinuities in the form of joints, bedding planes and faults mainly decide the stability of rock slopes.discontinuities are the weakest parts in the rock mass and get affected when sheared under the load. Discontinuities create anisotropy in the rock mass and change the stress strain distribution within the rock mass. The mechanical behavior of rock masses is strongly affected by the properties and geometry of the discontinuities (Cai and Horri 1992).Rock slope fails in one or more combination of failure mechanisms, like circular failure, plane failure, wedge failure, toppling failure and buckling failure. Kinematic analysis is the most simplified failure analysis in terms of joint sets, bedding plane, cut slope and angle of internal friction but it is only suitable for preliminary design (Hoek and Bray, 1981; Coggan et al., 1998). Preliminary analysis of slope stability problem using kinematic approach is necessary to identify the orientation of critical discontinuity before proceeding into the strength based analysis. Static stability analysis is performed using Finite element method based onshear strength reduction (SSR) approach, which calculates the factor of safety based on strength reduction phenomenon. In order to introduce joints in the finite element model combined continuum interface element method is used in this study. In finite element combined continuum interface method, joint element is assumed as negligible thickness element in between intact rock element and deformation characteristics of interface elements follow Goodman joint element model (Goodman et al. 1968).Stability assessment of rock slope is very much essential for suitable mitigation measures. The stability of a vulnerable rock slope is increased by using various stabilizing techniques. Rock bolting is one of the most common techniques used to stabilize the slope (Tiwari and Latha 2016). The primary objective of this study is to investigate the mode of failure and critical discontinuity based on kinematic analysis and to determine the nature of rock slope failure based on the result obtained by performing stability analysis. 2. Background Kinematic analysis using friction method can be conducted in DIPS (Rocscience 2016a). Several researches have done to determine the mode of failure by kinematic analysis using DIPS (Singh et al., 2015, Ahmad et al., 2012, Sarkar et al., 2016, Umrao et al., 2011, Kumar and Sanoujam, 2007). Kinematic analysis is done to obtain a relationship and understanding between existing surface slope steepness and structural discontinuities. Kinematic analysis revealed that the landslides are caused by the wedge failure for the slope instability (Kumar and Sanoujam, 2007). Stability analysis using finite element method can be conducted in PHASE 2 (Rocscience 2016b).Several researchers conducted the stability analysis of a jointed rock slope using different techniques. Gupta et al., (2016) conducted the stability analysis of Surabhi landslide in the Uttaranchal located in Mussoorie using the FEM-SSR technique in PHASE 2. Pal Shilpa et al., (2012) conducted the dynamic analysis of Surabhi landslide in the Uttaranchal located in Mussoorie using the distinct element method (DEM) in UDEC. Kainthola et al., (2015) conducted the stability analysis of cut slopes along State Highway-72, using the finite element method in PHASE 2. Liu Yaqun et al., (2014) on the seismic stability analysis of a layered rock slope using pseudo static analysis in UDEC. Tiwari et al., (2014) performed the stability analysis of Himalayan rock slope using continuum interface approach by FEM-SSR technique in PHASE 2. Hatzor et al. (2004) carried out dynamic 2D stability analysis of upper terrace of King Herod s Palace in Masada, which is a highly discontinuous rock slope. Latha and Garaga (2010) performed the seismic slope stability analysis of a 350-m-high slope using the equivalent continuum approach in FLAC. Kanugo et al., 319

2 Stability Assessment of Rock Slope and Design of Rock Slope Reinforcement (2013) performed the stability analysis of rock slopes using finite element method in PHASE 2. In this paper, stability analysis of a rock slope located at Sairang station along the alignment of the Bairabi Sairang railway project is carried out. The study mainly focuses on the kinematic analysis of the rock slope to obtain the possible mode of failure using DIPS and the stability of the rock slope is analyzed using PHASE 2 which is a finite element method based shear strength reduction technique.the observations further utilized to obtain the details of rock bolt installation i.e. bolt diameter, bolt length, bolt orientation, tensile strength and spacing of the bolts. 3. Description of the site The site is located at Sairang station between chainage of 49.2 km to 51.2 km along the alignment of the Bairabi- Sairang railway project. The rock slope is about 18 m high with an intermediate bench of 3m width. The area has been subjected to controlled blasting to pave way for the railway track alignment. The application of blasting has induced severe instability at the location which is mainly due to the presence of bedding planes dipping out of the face. The mineralogy of the rock slope is mainly composed of thinly bedded fully persistent fine-grained shale (Mithresh, 2017). The rock slope has undergone serious fragmentation due to the blasting process resulting in formation of rock debris at various sections of the slope. Joint orientation was measured using Brunton compass to determine the dip and dip direction of the joint planes. For the present slope, in addition to the bedding plane, two other prominent joint sets were observed.the details about the orientation of the slope and the discontinuities are given in Table-1. The joint planes are impersistent withpersistence ranging from 2 m to about 8 m (Mithresh, 2017). Table-1 Slope and joint orientation details Discontinuities Dip ( ) Dip Direction ( ) Bedding plane (B.P) Joint 1 (J 1 ) Joint 2 (J 2 ) Slope Evaluation of Rock Strength Parameters The strength parameters are evaluated by conducting the simple point load test on rock lumps of varying thickness which then utilises various empirical relations to arrive at a detailed geotechnical data for stability analyses of the rock slope.the strength parameters are adopted from the laboratory test results presented in Mithresh and Krishna (2017).The value of point load index is found with respect to standard size of 50 mm using the equation, I L (50) P (1) 0.75 ( A) 50 The conversion factor of point load value to UCS value is taken as 14 which is as preferred for soft rocks (Agustawijaya 2007). The UCS value thus obtained confirms shale to be classifiedunder soft rocks. Intact rock parameter m i and modulus ratio MR for shale rocks are considered to be 6 and200 respectively (Hoek 2000). The value of elastic modulus is found out using the equation, E MR * (2) i ci Geological Strength Index (GSI) and Rock Mass Rating (RMR) are other important parameters which require to be evaluated for a complete understanding of the characteristics of the rockmass in the area.the damage factor was taken as D = 0.7 due to good blasting carried out at Sairang site location.properties of rock mass and joints are shown in Table-2. Table-2 Propertiesof Rock Mass and Joints Parameter Point Load Strength, I L(50) (MPa) Uni-axial compressive strength, σ ci (MPa) Value Elastic Modulus, E i (GPa) 2 Unit Weight (KN/m 3 ) Geological Strength Index (GSI) Rock Mass Parameter Joint compressive strength, JCS (MPa) Joint roughness coefficient, JRC 27 m b s a Joint cohesion, kpa 20 Joint internal friction angle (º) 30 Normal stiffness, (MPa/m) Shear stiffness, (MPa/m) 7 7 J1 665 J2 222 J3 222 J J J Kinematic Analysis of Rock Slope The most causative factors of slope failure are multiple joint sets, weathering of the rock mass and high intensity of rainfall. The analysis has been carried out using friction cone method in DIPS (Rocscience, 2016a). A friction circle of 30 o equal to joint friction angle was plotted on the stereo-net to obtain the mode of failure for the rock slopes which are depicted in the Fig

model which will restrict displacement in x and y direction at the base and roller boundary condition is assigned to the side of the slope which will allow movement in the vertical y direction. Fig.

The values of trend and plunge at the point of intersection of two discontinuity planes are 229 o and 30 o.in kinematic analysis, only the effect of joint orientation is studied.

2) of the rock slope has been developed in two-dimensional elasto-plastic finite element analysis program, PHASE 2 (Rocscience, 2016b).

3 model which will restrict displacement in x and y direction at the base and roller boundary condition is assigned to the side of the slope which will allow movement in the vertical y direction. Fig. 1 Kinematic analysis for wedge failure From the Fig.1, it is clearly observed that the rock slope section has potential for occurrence of wedge failure. The values of trend and plunge at the point of intersection of two discontinuity planes are 229 o and 30 o.in kinematic analysis, only the effect of joint orientation is studied. Influence of joint spacing, persistence, strength characteristics of rock, self-weight of slope and external forces are not considered. 6. Static Stability Analysis of Rock Slope Numerical model (Fig. 2) of the rock slope has been developed in two-dimensional elasto-plastic finite element analysis program, PHASE 2 (Rocscience, 2016b). Joints are introduced in between intact rock as negligible thickness interface elements formulated by Goodman et al. 1968, which connects two intact rock elements. Shear and normal stiffness of the interface element governs the displacement of the jointed system. Joints are introduced as elastic-perfectly plastic elements where Mohr- Coulomb strength criteria have been considered to represent joint strength parameters.generalized Hoek Brown failure criteria are used for intact rock. The ratio of the horizontal stress to vertical stress in the rock mass is considered to be 0.5 (Eberhardt et al., 2004). Six nodded triangular meshing element of gradation factor 0.1 and uniform size 1m is used to generate the finite element mesh.optimum size of mesh in this study is obtained using mesh convergence study. In the process of mesh convergence analysis, the size of mesh is changed till the mesh size does not change the solution results. Fixed boundary condition is assumed at the base of the Fig. 2 Numerical model of rock slope Static stability analysis was conducted using the shear strength reduction technique proposed by Matsui and Sam (1992). An iterative search algorithm was used in shear strength reduction technique to obtain the critical strength reduction factor (SRF). In this method, the parameters of joints and intact rock are reduced by sequential factor and the stress-deformation behavior of the same is obtained in each step until the slope failed. The reduction factor at the point of failure is the critical strength reduction factor, which is similar to factor of safety of the slope. If the initial material strength parameters are c and ɸ, then the reduced strength parameters, c f and ɸ f can be obtained using the following equations. c f f c SRF 1 tan tan ( ) SRF (3) (4) Where, SRF is the strength reduction factor. Fig. 3Critical SRF and Displacement response of rock slope Figure 3 represents the critical strength reductionfactor (SRF) and displacement response of the rock slope. The critical strength reduction factor (SRF) for the slope is 1.22 (less than recommended value 1.5) and the displacement of the rock slope observed after the analysis was 40.8 mm, hence the slope is to be stabilized to reduce the displacement and to increase the stability. 321

Stability Assessment of Rock Slope and Design of Rock Slope Reinforcement 7. Design of Reinforcements In the present study, end anchorage bolts are used to reinforce the slope.

When the bolts are installed at an angle flatter than the normal to the joint (Fig. 4a), FOS has been found to be maximum for this orientation.

4 Stability Assessment of Rock Slope and Design of Rock Slope Reinforcement 7. Design of Reinforcements In the present study, end anchorage bolts are used to reinforce the slope.an end-anchored bolt is represented as a 1-D deformable element whose interactionwith the finite element meshes is through the endpoints only (Rocscience 2016b). When the bolts are installed at an angle flatter than the normal to the joint (Fig. 4a), FOS has been found to be maximum for this orientation. The normal component of mobilized tension (Tn) neutralizes dilation and the shear component of mobilized tension (Ts) acts inreverse direction of the driving force. Combined effect of these two mechanisms increase the FOS of the rock slope if the bolts are installed at an angle flatter than the normal to the joint. (a) Fig. 5 Numerical model of reinforced rock slope Based on the trial and error results, the optimum bolt parameters for the current study are as follows: Bolt Orientation = Normal to the slope (45 o ) Bolt diameter = 20 mm Bolt Length = 4 m Bolt Spacing = 0.75 m Bolt Tensile Capacity = 0.25 MN. The Factor of Safety obtained after stabilization of rock slope is 1.62 which is more than the required value 1.5 and the displacementof the rock slope observed after the analysis was 29.7 mm and the displacement response of the rock slope after stabilization is as shown in Fig 6. (b) Fig.4 Bolts installed at an angle (a) flatter than normal to the joint (b) steeper than normal to the joint If the bolts are installed steeper than the normal to the joint (Fig. 4b), the shear component of the bolt tension (Ts) acts down the joint plane which increases the total driving force resulting in the reduction of factor of safety of the rock slope. For the current analysis, bolts are installed normal to the slope face (flatter than normal to the joint, J 2 ). The bolt orientation, bolt length and bolt spacing are evaluated based on the trial and error method. Different combinations of bolt parameters such as bolt diameter, bolt length, bolt spacing and bolt tensile capacity are used to obtain the effective parameters of the bolt. Figure 5 represents the numerical model of the reinforced rock slope. Fig. 6 Critical SRF and Displacement response of reinforced rock slope From the analysis, the factor of safety for the unreinforced case under static condition was 1.22, which was increasedto 1.62 after reinforcing the slope.provision of reinforcement increased the factor of safety becauseit modifies the normal and shearing forces acting along asliding plane. Similarly,the maximum displacement observed for the unreinforced case under static condition was 40.8 mm, which was decreasedto 29.7 mm after reinforcing the slope. 8. Summary and Conclusions Stability analysis of a rock slope located at Sairang station along the alignment of the Bairabi Sairang railway project is presented in this paper. Kinematic analysis of the rock slope is carried out to obtain the possible mode of failure using DIPS and the stability of the rock slope is analyzed using PHASE 2 which is a finite element method based shear strength reduction technique. Kinematic analysis concludes that the rock slope had the potential for occurrence of wedgefailure. The slope is 322

5 found to be unstable as the critical strength reduction factor (SRF) is less than recommended value.hence the slope is to be stabilized to reduce the displacement and to increase the stability. Reinforcement in the form of rock bolts was recommended for stabilizing the slope sections and the reinforcement layout was arrived at based on trial analyses.provision of reinforcement increased the factor of safetyor strength reduction factor of the slope section. References Agustawijaya, D. S. (2007). The uniaxial compressive strength of soft rock. Civil Engineering Dimension, 9(1), pp-9. Cai, M., & Horii, H. (1992). A constitutive model of highly jointed rock masses. Mechanics of Materials 13: Coggan, J. S., Stead, D., & Eyre, J. M. (1998). Evaluation of techniques for quarry slope stability assessment. Transactions of the Institution of Mining and Metallurgy. Section B. Applied Earth Science, 107. Eberhardt, E., Stead, D., & Coggan, J. S. (2004). Numerical analysis of initiation and progressive failure in natural rock slopes the 1991 Randa rockslide. International Journal of Rock Mechanics and Mining Sciences, 41(1), Ghosh, S., Kumar, A., & Bora, A. (2014). Analyzing the stability of a failing rock slope for suggesting suitable mitigation measure: a case study from the Theng rockslide, Sikkim Himalayas, India. Bulletin of Engineering Geology and the Environment, 73(4), Gupta, V., Bhasin, R. K., Kaynia, A. M., Kumar, V., Saini, A. S., Tandon, R. S., & Pabst, T. (2016). Finite element analysis of failed slope by shear strength reduction technique: a case study for Surabhi Resort Landslide, Mussoorie Township, Garhwal Himalaya. Geomatics, Natural Hazards and Risk, 7(5), Hatzor, Y. H., Arzi, A. A., Zaslavsky, Y., & Shapira, A. (2004). Dynamic stability analysis of jointed rock slopes using the DDA method: King Herod's Palace, Masada, Israel. International Journal of Rock Mechanics and Mining Sciences, 41(5), Hoek, E., & Bray, J. D. (1981). Rock slope engineering. CRC Press. Kanungo, D. P., Pain, A., Sharma, S. (2013) Finite element modeling approach to assess the stability of debris and rock slopes: a case study from the Indian Himalayas. Journal of the International Society for the Prevention and Mitigation of Natural Hazards 69:1-24 Kumar, A., & Sanoujam, M. (2007). Landslide studies along the national highway (NH 39) in Manipur. Natural hazards, 40(3), Latha, G. M., & Garaga, A. (2010). Seismic stability analysis of a Himalayan rock slope. Rock Mechanics and Rock Engineering, 43(6), Liu, Y., Li, H., Xiao, K., Li, J., Xia, X., & Liu, B. (2014). Seismic stability analysis of a layered rock slope. Computers and Geotechnics, 55, Matsui, T., & San, K. C. (1992). Finite element slope stability analysis by shear strength reduction technique. Soils and foundations, 32(1), Mithresh, K. P. (2017) Characterization and Stability Analyses of Rock Slopes (Post Graduation Thesis, Indian Institute of Technology, Guwahati). Mithresh, K. P. and Krishna, A.M. (2017) Stability of Rock Slopes in Soft and Stratified Rock Mass Geotechnics for Natural and Engineered Sustainable Technologies: Indian Geotechnical Conference (GeoNEst: IGC-2017), Guwahati, India, pp Pal, S., Kaynia, A. M., Bhasin, R. K., & Paul, D. K. (2012). Earthquake stability analysis of rock slopes: a case study. Rock Mechanics and Rock Engineering, 45(2), Rocscience Inc. (2016a). DIPS v7.0. Graphical and statistical analysis of orientation data. Toronto, Ontario, Canada. Rocscience Inc. (2016b). PHASE 2 Version Finite Element Analysis for Excavations and Slopes. Toronto, Ontario, Canada. Rocscience Inc. (2016c). ROCDATA Version Rock, Soil and Discontinuity Strength Analysis. Toronto, Ontario, Canada. Sarkar, K., Singh, A. K., Niyogi, A., Behera, P. K., Verma, A. K., & Singh, T. N. (2016). The assessment of slope stability along NH-22 in Rampur-Jhakri Area, Himachal Pradesh. Journal of the Geological Society of India, 88(3), Tiwari, G., & Latha, G. M. (2016). Design of Rock Slope Reinforcement: An Himalayan Case Study. Rock Mechanics and Rock Engineering, 49(6), Tiwari, G., Gali, M. L., & Rao, V. R. (2014). Finite Element Study of a Rock Slope Using Continuum- Interface Approach. Umrao, R. K., Singh, R., Ahmad, M., & Singh, T. N. (2011). Stability analysis of cut slopes using continuous slope mass rating and kinematic analysis in Rudraprayag district, Uttarakhand. Geomaterials, 1(03),

Stability Assessment of a Heavily Jointed Rock Slope using Limit Equilibrium and Finite Element Methods

Indian Geotechnical Conference 2017 GeoNEst 14-16 December 2017, IIT Guwahati, India Stability Assessment of a Heavily Jointed Rock Slope using Limit Equilibrium and Finite Element Methods Aswathi CK Amalesh