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Rock mass assessment along the right bank of river Sutlej, Luhri, Himachal Pradesh, India

Rock mass assessment along the right bank of river Sutlej, Luhri, Himachal Pradesh, India Geomatics, Natural Hazards and Risk, 2015 Vol. 6, No. 3, 212–223, http://dx.doi.org/10.1080/19475705.2013.834486 Rock mass assessment along the right bank of river Sutlej, Luhri, Himachal Pradesh, India P.K. SINGH*, ASHUTOSH KAINTHOLA and T.N. SINGH Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai-400 076, India (Received 4 July 2013; accepted 10 August 2013) The study involves the characterization of rock mass along the right bank of river Sutlej, Luhri, Himachal Pradesh. This road connects to several important locations and therefore blockage due to slope failure may cause several problems. Lack of proper geotechnical/geological investigations has led to cutting of the natural hill slopes with improper design. The subsequent road cut slope has made this zone highly vulnerable and a threat to local commuters. The concerned area has varying lithology which are highly jointed and exposed all along the road cuts. The unrestrained slope in this zone is prone to recurrent failures due to high precipitation and seismicity, eventually causing loss of life and property. Therefore, the study helps in understanding the behaviour and mode of failure of the cut slope through geometrical relationship between structural discontinuities and surface topography. Several important parameters were determined to quantify the region based on available and widely used rockmass characterization techniques to develop a proper understanding. This will ultimately help in designing appropriate remedial measures to such vulnerable zones that will prevent further slope failure and resulting damage. 1. Introduction Slope failures are a general concern along the road cut hill slopes all around the world. The probability of failure increases exponentially in mountainous areas owing to new transportation corridor which are being constructed without proper under- standing of geological/geotechnical details of the area (Kainthola et al. 2012a, 2012b; Singh et al. 2013). The rock slopes are inherently dissected with several sets of discontinuities and inefficient design of the cut slope further exposes new rock surfa- ces which act as avenues for slope failures. Blasting and breakage induces multiple cracks and degrades their strength and stability. Improper excavation techniques employed for cutting these rocks for the construction and widening of roadways fur- ther deteriorates their strength. Hence, rock mass characterization is a vital tool for the construction and stability assessment of any civil engineering structure viz. dams, tunnels, road cut slopes, etc. The first step in the classification of rockmass is the kinematic analysis, which is a purely geometric means for determining the mode of failure in a jointed rock mass (Yoon et al. 2002). Planar and wedge failures are the most common type of slope fail- ure in highly jointed rock slopes. Therefore, a general understanding can be made *Corresponding author. Email: prakashks@iitb.ac.in 2013 Taylor & Francis Rock mass assessment along cut slopes 213 Figure 1. (a) Single plane sliding (b) Double plane sliding. J1 and J2 are the joint planes, arrow marks the direction of sliding, L1 is the true dip line of J1 and L12 is the line of intersec- tion of J1 and J2 (modified after Yoon et al. 2002). from the stereographic solution for kinematic analysis about the most critical joint sets responsible for the initiation of movement (figure 1). Although being a handy tool for quick stability analysis, it does not take into account rockmass properties, the in-situ existing conditions and discontinuity conditions, which is its major limitation. The aforementioned reason necessitates a method which can take into account all the important aspects used to characterize a rockmass. A number of methods have been developed henceforth, particularly for the classification of road cut slopes in highly jointed rock mass. Characterization deals with qualitative and quantitative assessment of rock masses for the purpose of designing proper support requirements and estimation of various rockmass parameters (Cai et al. 2004). Rock mass classification is an important tool for the appraisal of the behaviour of rock cut slopes based on the most significant inherent and structural parameters (Pantelidis 2009). Bieniawski (1973) designed a tool for classifying the rockmass based on the inherent properties, widely known as the rock mass rating (RMR). With the first use of these classification methods in cut slopes by Bieniawski (1979), several modifications have further been included. Their applications include various engineered structures like dam, tunnels, slopes and any underground excavations. Since it was specifically made for average to good strength rocks, it had some shortcomings. Rock quality designation (RQD), one of the six parameters used to estimate RMR, becomes meaningless for weaker rockmass which causes poor estimation of RMR particularly for RMR less than 25. RMR was improved by modified approaches such as slope mass rating (SMR) and geological strength index (GSI) which are more efficient and detailed for rockmass classification and better suited for weaker and jointed rockmass. Some of these classification tech- niques have also been used to estimate rockmass deformation modulus (Cai et al. 2004; Khabbazi et al. 2013). The present paper deals with the geo-mechanical characterization of hill cut slopes in the Luhri area of Himachal Pradesh, India. Previously, researchers have analysed the rock slope around this region along the left bank of river Sutlej based on soft computing and numerical methods (Sarkar & Singh 2008; Sarkar et al. 2008; Sarkar et al. 2009). Singh et al. (2010) have assessed the influence of static and seismic load- ing on a section of highly vulnerable cut slope in the outer part of the Kumaun Himalaya. Several studies related to slope stability have been conducted in Indian 214 P.K. Singh et al. Himalaya by different researchers and the general outcome of the research suggests that the concentration of recent landslides can be attributed to the neo-tectonic activ- ities in the vicinity of faults and boundary thrusts (Bartarya & Valdiya 1989; Valdiya 1995; Singh et al. 2010). Stability analysis requires detailed rockmass classification in this region, and therefore this study makes use of the various classification techniques available for the characterization of rockmass along the right bank of river Sutlej, Luhri, Himachal Pradesh. This will help in designing the road cut slopes and develop proper support requirements which can be later utilized as input parameters in numerical simulation for stability assessment. 2. Study area 2.1. Geology The study area is a part of Lesser Himalaya falling between the Dhauladhar Range in the south and the Higher Himalayan range in the north (figure 2). The river Sutlej cuts the topography, exposing the basement rocks in the form of window. The domi- nant rocks in this region constitute of Shali and Rampur formations which is over- lain by Chail and Jutogh formations. The interface between these formations is marked by the presence of Chail and Jutogh thrusts. The Jutogh formation typically consists of mica schists and quartzties, whereas Chail formation comprises of mylon- ite gneiss, schists, phylltes with quartzite bands (Sharma 1977). The Shali formation comprises of Shali slate, Lower Shali limestone and Khaira quartzite. 2.2. Structural parameters The concerned area stretched over 9 km s has a varied lithology. Four different rock types are encountered downstream along the right bank of river Sutlej. These rocks are dissected with at least three major sets of discontinuities segmenting the rock into blocks. Four sections named, L-1, L-2, L-3 and L-4, have been chosen for the present study. While L-1 and L-2 are up north of Luhri town on the right bank, sections L-3 Figure 2. Google earth image showing different locations marked by different coloured marker points. Rock mass assessment along cut slopes 215 Figure 3. Field photographs for all the locations from the study area (L-1 to L-4). and L-4 are located downstream of it. The augen gneisses, quartzitic schist, carbona- ceous phyllite and dolomitic limestones are the dominant lithology at L-1, L-2, L-3 and L-4, respectively. The rocks are profusely jointed giving them a blocky appearance, block size vary- 3 3 ing between 0.1m to 2m . The rocks at locations L-2 and L-3 have comparatively smaller block sizes which is due to the presence of high percentage of mica content giving it a character of slaty cleavage (figure 3). Slope face is almost sub-vertical at all the locations. The attitudes of the discontinuities as measured during field inspec- tion are presented in table 1. 3. Kinematic analysis As the study deals with the rock mass characterization of road cut slope face, there- fore the identification of mode of failure is an important criterion to analyse the sta- bility of slopes. The inter-relationship between the orientations of the discontinuity and the slope face to determine the possible mode of failure is termed as kinematic analysis. Mode of failure in rock slopes are conveniently assessed by means of stereo- graphic projections of the discontinuity data and the slope geometry. The most Table 1. Structural data for all the locations used in rock mass classification and kinematic analysis. Location Lithology J1 (Foliation) J2 J3 Slope face L-1 Augen Gneiss 30/345 75/195 75/084 72/146 L-2 Quartzitic Schist 25/350 68/175 80/265 70/160 L-3 Carbonaceous Phyllite 26/026 70/205 75/129 72/143 L-4 Dolomitic Limestone 40/020 35/175 60/300 78/130 216 P.K. Singh et al. common types of slope failures in rockmass, governed by discontinuities, are planar, wedge and toppling failures. The objective of this analysis is to identify possible modes of failure along the road cut slopes, subsequently assessing the influence of most critical discontinuity affecting the stability of the slope face. Wedge failures can occur in two different ways depending upon the sliding planes of the wedge. Based on this, single plane sliding or planar failure will take place, if the sliding wedge is bounded by two joint planes and sliding occurs on only one of the planes. However, double plane sliding or wedge failure will occur, if sliding occurs in the ‘down dip direction’ of the line of intersection of both the joint planes (Hocking 1976; Yoon et al. 2002). The structural data for all the discontinuity planes of the four locations were plot- ted in the lower hemisphere of the equal area projection (figure 4). The first three locations show the characteristics of double plane sliding as the true dip of both the Figure 4. Kinematic analysis of the jointed rockmass for all the locations (shaded area shows possible failure envelope and red circle denotes angle of internal friction which is taken as 30 ). Rock mass assessment along cut slopes 217 joint planes (T and T ) falls outside the zone demarcated by true dip of slope face J2 J3 (T ) and the line of intersection of the two joint planes J2 and J3 (I ). L-4 does not sf J23 show significant signs of failure governed by discontinuities as the plunge of the intersection between either joints plot outside the failure envelope. The joint set, J2, proves to be the most critical in almost all the cases and therefore is likely to initiate slope failure. 4. Rock mass classification 4.1. Rock mass rating Demarcation of zones based on characteristics of rock mass is done by incorpo- rating all those parameters which influence the rock mass strength. Therefore, RMR proposed by Bieniawski (1979) was initially adopted for classifying basic the rock mass in the study area. This method is slightly different from the usual RMR, as it considers only first five parameters viz. strength of intact rock, RQD, spacing of discontinuities, condition of discontinuities and groundwater conditions. Based on estimated RMR values, any engineering structure can be categorized into five classes (table 2).RMR is an integral parameter for the basic estimation of SMR. 4.2. Slope mass rating Romana (1985) attempted to extend the rock mass classification system for slope assessment. The five parameters employed in the RMR system can be applied for basic different engineering conditions (slope stability problems) by adjusting their values (table 3). In comparison to RMR , the detailed quantitative definition of the cor- basic rection factors is one of the most important advantages of SMR classification (Tomas et al. 2007, 2012). Several researchers have also used continuous SMR (CSMR) approach, a modification of SMR by Romana (1985) for slope (Tomas et al. 2007; Umrao et al. 2011). SMR is a commonly used geo-mechanical classifica- tion for the characterization of rock slopes and can be numerically presented as follows: RMR ¼ RMR þðF  F  F Þþ F : ð1Þ Slope Basic 1 2 3 4 4.3. Geological strength index RMR system largely depends on RQD, which is not defined clearly for weak rock masses or RMR less than 25 (Hoek & Brown 1997). Therefore, to account for Table 2. Categorization of rock class based on the values of RMR. RMR 100–81 80–61 60–41 40–21 <20 Rock class I II II IV V 218 P.K. Singh et al. Table 3. Values of adjustment factors for different joint orientations (modified after Romana 1985). Very Very Cause of slope failure favourable Favourable Fair Unfavourable unfavourable P a  a j s T a  a  180 >30 30–20 20–10 10–5 <5 j s W a  a i s P/W/T F 0.15 0.40 0.70 0.85 1.0 P b W b <20 20–30 30–35 35–40 >45 P/W 0.15 0.40 0.70 0.85 1.0 TF 1.0 1.0 1.0 1.0 1.0 P b  b j s W b  b >10 10–0 0 0–(10) <10 i s T b þ b <110 100–120 >120 – – j s P/W/T F 0 6 25 50 60 P: planar failure; T: toppling failure; W: wedge failure; a : slope strike; a : joint strike; a : plunge direction s j i of line of intersection; b : slope dip; b : joint dip; b : plunge of line of intersection. s j i this inconsistency, GSI system was developed and modified over the years for the classification of jointed rockmass and several other important rockmass properties. GSI assesses several important rockmass properties required as an input in numer- ical analysis of the engineered structures such as road cut slopes, design of tunnels, etc. Hoek and Brown failure criterion is one of the most widely used failure crite- ria for jointed rockmass which depends on GSI. This classification is based on visual inspection of the lithology, structure and condition of discontinuity surfaces on the exposed outcrops in tunnels, road cuts or any surface/underground excava- tions (Marinos et al. 2005). In spite of this, Marinos et al. (2005) cautioned the use of GSI in rockmasses with ‘clearly defined structural continuity and excavated faces in rocks with the spacing of discontinuities similar in magnitude to the dimensions of slope under consideration’. GSI has been correlated with the RMR which can directly be used to calculate GSI, if the value of RMR is Basic known, GSI ¼ RMR  5: ð2Þ Basic Sonmez and Ulusay (1999) have further modified GSI for their applicability in rock slopes by introducing parameters such as surface condition and structure rating (figure 5). Structure rating was defined based on volumetric joint count, whereas surface condition ratings were estimated from parameters such as rough- ness, filling and weathering. The addition of the above-mentioned rating parameters (structure and surface condition ratings) particularly emphasize on the characteris- tics of structural discontinuities, and thus are extremely useful in classifying highly jointed rockmass. Therefore, the study makes use of this modified version of GSI to classify the rockmass and to estimate several important parameters which can be later used in the numerical simulation for stability analysis, excavation of tunnels or any civil structures. Rock mass assessment along cut slopes 219 Figure 5. The modified GSI classification system for slope stability analysis (after Sonmez & Ulusay 1999). 5. Results and discussion The rockmass classification in the study area required a detailed field investigation, which led to acquisition large structural data and various rockmass parameters. These data helped in studying kinematic analysis, RMR , SMR and finally GSI. basic 220 P.K. Singh et al. The results of kinematic analysis helped in identifying the most critical joint set and the type of failure associated with it at a particular site. Out of all four locations, three locations (L-1, L-2 and L-3) are predicted to undergo wedge failure by typical double plane sliding and occasional small planar failure. The fourth location (L-4) is stable due to structural restrains, but the possibility of small block detachment can- not be overlooked especially in the upper portion of slope. RMR was calculated from several rockmass parameters based on visual Basic inspection and laboratory experimentation (table 4). The result shows that the first two locations (L-1 and L-2) fall in ‘good’ category, while the third falls in ‘poor’ and fourth falls in ‘fair’ category. Based on these values, several other important parame- ters can be deduced like deformation modulus as well as design of excavation meth- ods and proper support system requirements. SMR, which is more robust for the classification of cut slopes, has also been esti- mated for the slope stability analysis in this region. This method is preferred over others because it emphasizes on the relation between structural discontinuities and surface topography. The analysis of cut slopes by SMR is likely to give more detailed idea about the stability of the region. The adjustment factors for all the locations have been carefully estimated from kinematic analysis (table 5). Accordingly, L-1 falls in good category, while the L-2 and L-4 are in normal category with L-3 being in the bad category. According to Romana (1985), the L-1 is stable while the L-2 and L-4 are partially stable and L-3 being unstable. The results obtained from this study seem to be very reasonable as there was a slope failure post-monsoon near to the third location in the same rock type (figure 6). Like other classifications, once SMR has been estimated, several support measures and strengthening techniques are available for proper slope design to mitigate the risk of slope failures. For the purpose of rock slope stability assessment, modified GSI was also used. This method considers several additional important rockmass parameters, including Table 4. The values of different parameters used for the estimation of rock mass rating for the studied location. Location Discontinuity Condition of Groundwater UCS RQD spacing discontinuity surface condition RMR Basic L-1 12 17 15 14 12 70 L-2 12 15 15 13 12 67 L-3 7 8 8 11 4 38 L-4 7 13 8 14 7 49 Table 5. The values of correction factors obtained for the estimation of slope mass rating of the studied area. Location F1 F2 F3 F4 Adjustment factor SMR L-1 0.85 1 60 5.6 64.4 L-2 0.15 1 50 0 7.5 59.5 L-3 0.15 1 50 0 7.5 30.5 L-4 0.15 0.70 60 0 6.3 42.7 Rock mass assessment along cut slopes 221 Table 6. GSI values obtained from surface condition rating and structure rating for the studied area. Location Roughness rating Weathering rating Infilling rating Jv Structure rating GSI L-1 5 6 4 2.4 77 66 L-2 1 6 2 8 77 62 L-3 3 1 0 37 25 29 L-4 5 3 2 18.9 36 40 Figure 6. Slope failure in carbonaceous phyllite at L-3 during monsoon about 3 km s down- stream of Luhri town (arrow shows direction of sliding and curve marks the extent of failure). Figure 7. Histogram plot showing correlation of computed and empirically estimated GSI for the studied area. 222 P.K. Singh et al. in-situ geological condition. The values of structure and surface condition rating have been obtained by detailed field investigation (table 6). The computed GSI values for the first two locations correlate well with empirically obtained GSI through RMR (figure 7). The variation in L-3 and L-4 may be either due to reduction in basic groundwater rating or application of more detailed analysis from the modified GSI for weaker rockmass. Hoek and Brown rockmass parameters (m and s) and rock mass deformation modulus can be assessed empirically, once GSI has been estimated. This will be a valuable input for numerical simulation of the cut slopes for stability analysis. Conclusion For the present study, four locations (L-1 to L-4) along the right bank of Sutlej river, Himachal Pradesh, India, were chosen and investigated on the basis of differ- ent geotechnical and stability attributes. Kinematic analysis, RMR , SMR and basic GSI were determined for all these sections. Based on the kinematic analysis, it can be concluded that the local joint set ‘J2’ is the most critical and majorly responsible for slope movements. Wedge failure by double plane sliding is predicted to be the most common type of failure in all the sections except the L-4. RMR has also basic been calculated as an essential parameter required for the estimation of SMR. SMR values show that the first two locations (L-1 and L-2) may not need any major support but the L-3 and L-4 will certainly require instant support measures to prevent the risk of collapse. The use of modified GSI helps in more detailed and efficient classification of the rock masses in this region. The GSI values can further help in estimating Hoek and Brown rockmass parameters (m and s) which can be included in numerical simulation for the stability analysis of highly jointed rockmass. Acknowledgements The support provided by General Manager, Luhri Hydro Electric Project and the team of M/S Sutluj Jal Vidyut Nigam (SJVNL) during field investigation is highly appreciated. The authors would also like to thank the reviewers, whose critical comments helped to produce the paper in its current form. Funding The first author would like to thank the University Grants Commission (UGC), India, for the funding. References Bartarya SK, Valdiya KS. 1989. Landslides and erosion in the catchments of the Gaula river, Kumaun Lesser Himalaya, India. Mt Res Develop. 9:405–419. Bieniawski ZT. 1973. Engineering classification of jointed rock masses. Trans S Afr Inst Civil Engineers. 15:335–344. Bieniawski ZT. 1979. The geomechanics classificationinrockengineering applications.In: Proceedings of the 4th International Congress on Rock Mechanics; Montreux; p. 41–48. Rock mass assessment along cut slopes 223 Cai M, Kaisera PK, Unob H, Tasakab Y, Minami M. 2004. Estimation of rock mass deforma- tion modulus and strength of jointed hard rock masses using the GSI system. Int J Rock Mech Mining Sci. 41:3–19 Hocking G. 1976. A method for distinguishing between single and double plane sliding of tetrahedral wedges. Int J Rock Mech Mining Sci. 13:225–226, Geomechanics Abstract. Hoek E, Brown ET. 1997. Practical estimates of rock mass strength. Int J Rock Mech Mining Sci. 34 (8):1165–1186. Kainthola A, Singh PK, Wasnik AB, Sazid M, Singh TN. 2012a. Finite element analysis of road cut slopes using Hoek and Brown failure criterion. Int J Earth Sci Eng. 5 (5):1100–1109. Kainthola A, Singh PK, Wasnik AB, Sazid M, Singh TN. 2012b, Distinct element modelling of Mahabaleshwar road cut hill slope. Int J Geomaterials. 2:105–113. Khabbazi A, Ghafoori M, Lashkaripour GR, Cheshomi A. 2013. Estimation of the rock mass deformation modulus using a rock classification system. Geomech Geoeng: Int J. 8 (1):46–52. Marinos V, Marinos P, Hoek E. 2005. The geological strength index: applications and limita- tions. Bull Eng Geol Environ. 64:55–65. Pantelidis L. 2009. Rock slope stability assessment through rock mass classification systems. Int J Rock Mech Mining Sci. 46:315–325. Romana M. 1985. New adjustment ratings for application of Bieniawski classification to slopes. In: Proceedings of International Symposium Role of Rock Mechanics; ISRM; Zacatecas, Mexico; p. 49–53. Sarkar K, Sazid M, Khandelwal M, Singh TN. 2009. Stability analysis of soil slope in Luhri area, Himachal Pradesh. Mining Eng J. 10(6):21–27. Sarkar K, Singh TN. 2008. Slope failure analysis in road cut slope by numerical method. In: ISRM International Symposium; Tehran; p. 635–642. Sarkar K, Gulati A, Singh TN. 2008. Landslide susceptibility analysis using artificial neural networks and GIS in Luhri area, Himachal Pradesh. J Indian Landslides. 1(1):11–20. Sharma VP. 1977. Geology of the Kulu-Rampur belt, Himachal Pradesh. Memoirs Geological Surv India. 106:235–407. Singh TN, Verma AK, Sarkar K. 2010. Static and dynamic analysis of a landslide. Geomatics, Nat Hazards Risk. 1(4):323–338. Singh PK, Wasnik AB, Kainthola, A, Sazid, M, Singh TN. 2013. The stability of road cut cliff face along SH-121: a case Study. Nat Hazards. 68(2):497–507. Sonmez H, Ulusay R. 1999. Modifications to the geological strength index (GSI) and their applicability to stability of slopes. Int J Rock Mech Mining Sci. 36:743–760. Tomas R, Cuenca A, Cano M, Garcıa-Barba J. 2012. A graphical approach for slope mass rating (SMR). Eng Geol. 124:67–76 Tomas R, Delgadob J, Sero JB. 2007. Modification of slope mass rating (SMR) by continuous functions. Int J Rock Mech Mining Sci. 44:1062–1069. Umrao RK, Singh R, Ahmad M, Singh TN. 2011. Stability analysis of cut slopes using contin- uous slope mass rating and kinematic analysis in Rudraprayag district, Uttarakhand. Geomaterials. 1:79–87 Valdiya KS. 1995. Accelerated erosion and landslides prone zones in the Himalayan region. Gyanodaya Prakashan, Nainital. 17:39–53. Yoon WS, Jeong UJ, Kim JH. 2002. Kinematic analysis for sliding failure of multi-faced rock slopes. Eng Geol. 67:51–61. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png "Geomatics, Natural Hazards and Risk" Taylor & Francis

Rock mass assessment along the right bank of river Sutlej, Luhri, Himachal Pradesh, India

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Geomatics, Natural Hazards and Risk, 2015 Vol. 6, No. 3, 212–223, http://dx.doi.org/10.1080/19475705.2013.834486 Rock mass assessment along the right bank of river Sutlej, Luhri, Himachal Pradesh, India P.K. SINGH*, ASHUTOSH KAINTHOLA and T.N. SINGH Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai-400 076, India (Received 4 July 2013; accepted 10 August 2013) The study involves the characterization of rock mass along the right bank of river Sutlej, Luhri, Himachal Pradesh. This road connects to several important locations and therefore blockage due to slope failure may cause several problems. Lack of proper geotechnical/geological investigations has led to cutting of the natural hill slopes with improper design. The subsequent road cut slope has made this zone highly vulnerable and a threat to local commuters. The concerned area has varying lithology which are highly jointed and exposed all along the road cuts. The unrestrained slope in this zone is prone to recurrent failures due to high precipitation and seismicity, eventually causing loss of life and property. Therefore, the study helps in understanding the behaviour and mode of failure of the cut slope through geometrical relationship between structural discontinuities and surface topography. Several important parameters were determined to quantify the region based on available and widely used rockmass characterization techniques to develop a proper understanding. This will ultimately help in designing appropriate remedial measures to such vulnerable zones that will prevent further slope failure and resulting damage. 1. Introduction Slope failures are a general concern along the road cut hill slopes all around the world. The probability of failure increases exponentially in mountainous areas owing to new transportation corridor which are being constructed without proper under- standing of geological/geotechnical details of the area (Kainthola et al. 2012a, 2012b; Singh et al. 2013). The rock slopes are inherently dissected with several sets of discontinuities and inefficient design of the cut slope further exposes new rock surfa- ces which act as avenues for slope failures. Blasting and breakage induces multiple cracks and degrades their strength and stability. Improper excavation techniques employed for cutting these rocks for the construction and widening of roadways fur- ther deteriorates their strength. Hence, rock mass characterization is a vital tool for the construction and stability assessment of any civil engineering structure viz. dams, tunnels, road cut slopes, etc. The first step in the classification of rockmass is the kinematic analysis, which is a purely geometric means for determining the mode of failure in a jointed rock mass (Yoon et al. 2002). Planar and wedge failures are the most common type of slope fail- ure in highly jointed rock slopes. Therefore, a general understanding can be made *Corresponding author. Email: prakashks@iitb.ac.in 2013 Taylor & Francis Rock mass assessment along cut slopes 213 Figure 1. (a) Single plane sliding (b) Double plane sliding. J1 and J2 are the joint planes, arrow marks the direction of sliding, L1 is the true dip line of J1 and L12 is the line of intersec- tion of J1 and J2 (modified after Yoon et al. 2002). from the stereographic solution for kinematic analysis about the most critical joint sets responsible for the initiation of movement (figure 1). Although being a handy tool for quick stability analysis, it does not take into account rockmass properties, the in-situ existing conditions and discontinuity conditions, which is its major limitation. The aforementioned reason necessitates a method which can take into account all the important aspects used to characterize a rockmass. A number of methods have been developed henceforth, particularly for the classification of road cut slopes in highly jointed rock mass. Characterization deals with qualitative and quantitative assessment of rock masses for the purpose of designing proper support requirements and estimation of various rockmass parameters (Cai et al. 2004). Rock mass classification is an important tool for the appraisal of the behaviour of rock cut slopes based on the most significant inherent and structural parameters (Pantelidis 2009). Bieniawski (1973) designed a tool for classifying the rockmass based on the inherent properties, widely known as the rock mass rating (RMR). With the first use of these classification methods in cut slopes by Bieniawski (1979), several modifications have further been included. Their applications include various engineered structures like dam, tunnels, slopes and any underground excavations. Since it was specifically made for average to good strength rocks, it had some shortcomings. Rock quality designation (RQD), one of the six parameters used to estimate RMR, becomes meaningless for weaker rockmass which causes poor estimation of RMR particularly for RMR less than 25. RMR was improved by modified approaches such as slope mass rating (SMR) and geological strength index (GSI) which are more efficient and detailed for rockmass classification and better suited for weaker and jointed rockmass. Some of these classification tech- niques have also been used to estimate rockmass deformation modulus (Cai et al. 2004; Khabbazi et al. 2013). The present paper deals with the geo-mechanical characterization of hill cut slopes in the Luhri area of Himachal Pradesh, India. Previously, researchers have analysed the rock slope around this region along the left bank of river Sutlej based on soft computing and numerical methods (Sarkar & Singh 2008; Sarkar et al. 2008; Sarkar et al. 2009). Singh et al. (2010) have assessed the influence of static and seismic load- ing on a section of highly vulnerable cut slope in the outer part of the Kumaun Himalaya. Several studies related to slope stability have been conducted in Indian 214 P.K. Singh et al. Himalaya by different researchers and the general outcome of the research suggests that the concentration of recent landslides can be attributed to the neo-tectonic activ- ities in the vicinity of faults and boundary thrusts (Bartarya & Valdiya 1989; Valdiya 1995; Singh et al. 2010). Stability analysis requires detailed rockmass classification in this region, and therefore this study makes use of the various classification techniques available for the characterization of rockmass along the right bank of river Sutlej, Luhri, Himachal Pradesh. This will help in designing the road cut slopes and develop proper support requirements which can be later utilized as input parameters in numerical simulation for stability assessment. 2. Study area 2.1. Geology The study area is a part of Lesser Himalaya falling between the Dhauladhar Range in the south and the Higher Himalayan range in the north (figure 2). The river Sutlej cuts the topography, exposing the basement rocks in the form of window. The domi- nant rocks in this region constitute of Shali and Rampur formations which is over- lain by Chail and Jutogh formations. The interface between these formations is marked by the presence of Chail and Jutogh thrusts. The Jutogh formation typically consists of mica schists and quartzties, whereas Chail formation comprises of mylon- ite gneiss, schists, phylltes with quartzite bands (Sharma 1977). The Shali formation comprises of Shali slate, Lower Shali limestone and Khaira quartzite. 2.2. Structural parameters The concerned area stretched over 9 km s has a varied lithology. Four different rock types are encountered downstream along the right bank of river Sutlej. These rocks are dissected with at least three major sets of discontinuities segmenting the rock into blocks. Four sections named, L-1, L-2, L-3 and L-4, have been chosen for the present study. While L-1 and L-2 are up north of Luhri town on the right bank, sections L-3 Figure 2. Google earth image showing different locations marked by different coloured marker points. Rock mass assessment along cut slopes 215 Figure 3. Field photographs for all the locations from the study area (L-1 to L-4). and L-4 are located downstream of it. The augen gneisses, quartzitic schist, carbona- ceous phyllite and dolomitic limestones are the dominant lithology at L-1, L-2, L-3 and L-4, respectively. The rocks are profusely jointed giving them a blocky appearance, block size vary- 3 3 ing between 0.1m to 2m . The rocks at locations L-2 and L-3 have comparatively smaller block sizes which is due to the presence of high percentage of mica content giving it a character of slaty cleavage (figure 3). Slope face is almost sub-vertical at all the locations. The attitudes of the discontinuities as measured during field inspec- tion are presented in table 1. 3. Kinematic analysis As the study deals with the rock mass characterization of road cut slope face, there- fore the identification of mode of failure is an important criterion to analyse the sta- bility of slopes. The inter-relationship between the orientations of the discontinuity and the slope face to determine the possible mode of failure is termed as kinematic analysis. Mode of failure in rock slopes are conveniently assessed by means of stereo- graphic projections of the discontinuity data and the slope geometry. The most Table 1. Structural data for all the locations used in rock mass classification and kinematic analysis. Location Lithology J1 (Foliation) J2 J3 Slope face L-1 Augen Gneiss 30/345 75/195 75/084 72/146 L-2 Quartzitic Schist 25/350 68/175 80/265 70/160 L-3 Carbonaceous Phyllite 26/026 70/205 75/129 72/143 L-4 Dolomitic Limestone 40/020 35/175 60/300 78/130 216 P.K. Singh et al. common types of slope failures in rockmass, governed by discontinuities, are planar, wedge and toppling failures. The objective of this analysis is to identify possible modes of failure along the road cut slopes, subsequently assessing the influence of most critical discontinuity affecting the stability of the slope face. Wedge failures can occur in two different ways depending upon the sliding planes of the wedge. Based on this, single plane sliding or planar failure will take place, if the sliding wedge is bounded by two joint planes and sliding occurs on only one of the planes. However, double plane sliding or wedge failure will occur, if sliding occurs in the ‘down dip direction’ of the line of intersection of both the joint planes (Hocking 1976; Yoon et al. 2002). The structural data for all the discontinuity planes of the four locations were plot- ted in the lower hemisphere of the equal area projection (figure 4). The first three locations show the characteristics of double plane sliding as the true dip of both the Figure 4. Kinematic analysis of the jointed rockmass for all the locations (shaded area shows possible failure envelope and red circle denotes angle of internal friction which is taken as 30 ). Rock mass assessment along cut slopes 217 joint planes (T and T ) falls outside the zone demarcated by true dip of slope face J2 J3 (T ) and the line of intersection of the two joint planes J2 and J3 (I ). L-4 does not sf J23 show significant signs of failure governed by discontinuities as the plunge of the intersection between either joints plot outside the failure envelope. The joint set, J2, proves to be the most critical in almost all the cases and therefore is likely to initiate slope failure. 4. Rock mass classification 4.1. Rock mass rating Demarcation of zones based on characteristics of rock mass is done by incorpo- rating all those parameters which influence the rock mass strength. Therefore, RMR proposed by Bieniawski (1979) was initially adopted for classifying basic the rock mass in the study area. This method is slightly different from the usual RMR, as it considers only first five parameters viz. strength of intact rock, RQD, spacing of discontinuities, condition of discontinuities and groundwater conditions. Based on estimated RMR values, any engineering structure can be categorized into five classes (table 2).RMR is an integral parameter for the basic estimation of SMR. 4.2. Slope mass rating Romana (1985) attempted to extend the rock mass classification system for slope assessment. The five parameters employed in the RMR system can be applied for basic different engineering conditions (slope stability problems) by adjusting their values (table 3). In comparison to RMR , the detailed quantitative definition of the cor- basic rection factors is one of the most important advantages of SMR classification (Tomas et al. 2007, 2012). Several researchers have also used continuous SMR (CSMR) approach, a modification of SMR by Romana (1985) for slope (Tomas et al. 2007; Umrao et al. 2011). SMR is a commonly used geo-mechanical classifica- tion for the characterization of rock slopes and can be numerically presented as follows: RMR ¼ RMR þðF  F  F Þþ F : ð1Þ Slope Basic 1 2 3 4 4.3. Geological strength index RMR system largely depends on RQD, which is not defined clearly for weak rock masses or RMR less than 25 (Hoek & Brown 1997). Therefore, to account for Table 2. Categorization of rock class based on the values of RMR. RMR 100–81 80–61 60–41 40–21 <20 Rock class I II II IV V 218 P.K. Singh et al. Table 3. Values of adjustment factors for different joint orientations (modified after Romana 1985). Very Very Cause of slope failure favourable Favourable Fair Unfavourable unfavourable P a  a j s T a  a  180 >30 30–20 20–10 10–5 <5 j s W a  a i s P/W/T F 0.15 0.40 0.70 0.85 1.0 P b W b <20 20–30 30–35 35–40 >45 P/W 0.15 0.40 0.70 0.85 1.0 TF 1.0 1.0 1.0 1.0 1.0 P b  b j s W b  b >10 10–0 0 0–(10) <10 i s T b þ b <110 100–120 >120 – – j s P/W/T F 0 6 25 50 60 P: planar failure; T: toppling failure; W: wedge failure; a : slope strike; a : joint strike; a : plunge direction s j i of line of intersection; b : slope dip; b : joint dip; b : plunge of line of intersection. s j i this inconsistency, GSI system was developed and modified over the years for the classification of jointed rockmass and several other important rockmass properties. GSI assesses several important rockmass properties required as an input in numer- ical analysis of the engineered structures such as road cut slopes, design of tunnels, etc. Hoek and Brown failure criterion is one of the most widely used failure crite- ria for jointed rockmass which depends on GSI. This classification is based on visual inspection of the lithology, structure and condition of discontinuity surfaces on the exposed outcrops in tunnels, road cuts or any surface/underground excava- tions (Marinos et al. 2005). In spite of this, Marinos et al. (2005) cautioned the use of GSI in rockmasses with ‘clearly defined structural continuity and excavated faces in rocks with the spacing of discontinuities similar in magnitude to the dimensions of slope under consideration’. GSI has been correlated with the RMR which can directly be used to calculate GSI, if the value of RMR is Basic known, GSI ¼ RMR  5: ð2Þ Basic Sonmez and Ulusay (1999) have further modified GSI for their applicability in rock slopes by introducing parameters such as surface condition and structure rating (figure 5). Structure rating was defined based on volumetric joint count, whereas surface condition ratings were estimated from parameters such as rough- ness, filling and weathering. The addition of the above-mentioned rating parameters (structure and surface condition ratings) particularly emphasize on the characteris- tics of structural discontinuities, and thus are extremely useful in classifying highly jointed rockmass. Therefore, the study makes use of this modified version of GSI to classify the rockmass and to estimate several important parameters which can be later used in the numerical simulation for stability analysis, excavation of tunnels or any civil structures. Rock mass assessment along cut slopes 219 Figure 5. The modified GSI classification system for slope stability analysis (after Sonmez & Ulusay 1999). 5. Results and discussion The rockmass classification in the study area required a detailed field investigation, which led to acquisition large structural data and various rockmass parameters. These data helped in studying kinematic analysis, RMR , SMR and finally GSI. basic 220 P.K. Singh et al. The results of kinematic analysis helped in identifying the most critical joint set and the type of failure associated with it at a particular site. Out of all four locations, three locations (L-1, L-2 and L-3) are predicted to undergo wedge failure by typical double plane sliding and occasional small planar failure. The fourth location (L-4) is stable due to structural restrains, but the possibility of small block detachment can- not be overlooked especially in the upper portion of slope. RMR was calculated from several rockmass parameters based on visual Basic inspection and laboratory experimentation (table 4). The result shows that the first two locations (L-1 and L-2) fall in ‘good’ category, while the third falls in ‘poor’ and fourth falls in ‘fair’ category. Based on these values, several other important parame- ters can be deduced like deformation modulus as well as design of excavation meth- ods and proper support system requirements. SMR, which is more robust for the classification of cut slopes, has also been esti- mated for the slope stability analysis in this region. This method is preferred over others because it emphasizes on the relation between structural discontinuities and surface topography. The analysis of cut slopes by SMR is likely to give more detailed idea about the stability of the region. The adjustment factors for all the locations have been carefully estimated from kinematic analysis (table 5). Accordingly, L-1 falls in good category, while the L-2 and L-4 are in normal category with L-3 being in the bad category. According to Romana (1985), the L-1 is stable while the L-2 and L-4 are partially stable and L-3 being unstable. The results obtained from this study seem to be very reasonable as there was a slope failure post-monsoon near to the third location in the same rock type (figure 6). Like other classifications, once SMR has been estimated, several support measures and strengthening techniques are available for proper slope design to mitigate the risk of slope failures. For the purpose of rock slope stability assessment, modified GSI was also used. This method considers several additional important rockmass parameters, including Table 4. The values of different parameters used for the estimation of rock mass rating for the studied location. Location Discontinuity Condition of Groundwater UCS RQD spacing discontinuity surface condition RMR Basic L-1 12 17 15 14 12 70 L-2 12 15 15 13 12 67 L-3 7 8 8 11 4 38 L-4 7 13 8 14 7 49 Table 5. The values of correction factors obtained for the estimation of slope mass rating of the studied area. Location F1 F2 F3 F4 Adjustment factor SMR L-1 0.85 1 60 5.6 64.4 L-2 0.15 1 50 0 7.5 59.5 L-3 0.15 1 50 0 7.5 30.5 L-4 0.15 0.70 60 0 6.3 42.7 Rock mass assessment along cut slopes 221 Table 6. GSI values obtained from surface condition rating and structure rating for the studied area. Location Roughness rating Weathering rating Infilling rating Jv Structure rating GSI L-1 5 6 4 2.4 77 66 L-2 1 6 2 8 77 62 L-3 3 1 0 37 25 29 L-4 5 3 2 18.9 36 40 Figure 6. Slope failure in carbonaceous phyllite at L-3 during monsoon about 3 km s down- stream of Luhri town (arrow shows direction of sliding and curve marks the extent of failure). Figure 7. Histogram plot showing correlation of computed and empirically estimated GSI for the studied area. 222 P.K. Singh et al. in-situ geological condition. The values of structure and surface condition rating have been obtained by detailed field investigation (table 6). The computed GSI values for the first two locations correlate well with empirically obtained GSI through RMR (figure 7). The variation in L-3 and L-4 may be either due to reduction in basic groundwater rating or application of more detailed analysis from the modified GSI for weaker rockmass. Hoek and Brown rockmass parameters (m and s) and rock mass deformation modulus can be assessed empirically, once GSI has been estimated. This will be a valuable input for numerical simulation of the cut slopes for stability analysis. Conclusion For the present study, four locations (L-1 to L-4) along the right bank of Sutlej river, Himachal Pradesh, India, were chosen and investigated on the basis of differ- ent geotechnical and stability attributes. Kinematic analysis, RMR , SMR and basic GSI were determined for all these sections. Based on the kinematic analysis, it can be concluded that the local joint set ‘J2’ is the most critical and majorly responsible for slope movements. Wedge failure by double plane sliding is predicted to be the most common type of failure in all the sections except the L-4. RMR has also basic been calculated as an essential parameter required for the estimation of SMR. SMR values show that the first two locations (L-1 and L-2) may not need any major support but the L-3 and L-4 will certainly require instant support measures to prevent the risk of collapse. The use of modified GSI helps in more detailed and efficient classification of the rock masses in this region. The GSI values can further help in estimating Hoek and Brown rockmass parameters (m and s) which can be included in numerical simulation for the stability analysis of highly jointed rockmass. Acknowledgements The support provided by General Manager, Luhri Hydro Electric Project and the team of M/S Sutluj Jal Vidyut Nigam (SJVNL) during field investigation is highly appreciated. The authors would also like to thank the reviewers, whose critical comments helped to produce the paper in its current form. Funding The first author would like to thank the University Grants Commission (UGC), India, for the funding. References Bartarya SK, Valdiya KS. 1989. Landslides and erosion in the catchments of the Gaula river, Kumaun Lesser Himalaya, India. Mt Res Develop. 9:405–419. Bieniawski ZT. 1973. Engineering classification of jointed rock masses. Trans S Afr Inst Civil Engineers. 15:335–344. Bieniawski ZT. 1979. 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