SLOPE STABILITY INVESTIGATIONS OF ROAD CUT SLOPES: NH-58, RISHIKESH TO DEVPRAYAG

Abstract

The Himalayan orogenic belt is well known for its active neotectonism, frequent seismicity, and multifarious geological and geotechnical environment. This mountain belt is also characterized by multiple phases of deformation and metamorphism that formed rugged topography with extensively elevated mountains and deep ravines. Substantial exogenic and endogenic factors are contributing towards widespread slope destabilization. From a couple of decades, the acceleration in rising demands for urbanization and constrain for mega civil engineering projects in the eco-sensitive terrains like the Himalayas are prime factors that are deteriorating the geoenvironmental fragility of the region. The acceleration in such pessimistic factors had amplified rampant landslides in the region. The swift progression and intervention of anthropogenic factors are being questioned since long but effective endeavor is rarely being undertaken to reduce them. Consequently, every year numerous death tolls and massive loss infrastructure are being proclaimed from distinct sectors of the region particularly along transportation corridors. Rampant slope failures are deliberately endangering the plight of the Himalayan ecosystem. Due to the lack of railways and airways network, roadways have exceptional significance in hassle-free transportation and communication in the Himalayan terrain. Transportation corridors within perilous and precarious Himalayan region experience incessant landslides, particularly along the sections that are manifested by geological discontinuities. Due to enormous deformation and contraction, the rock mass at certain sections along the highway is extensively fractured and sheared. One such route is national highway-58 which connects Indo-Gangetic plains to mountainous region leading to Badrinath in India. NH-58 has remarkable importance due to the extensive inflow of tourists from different parts of the world and pilgrimage activities. Chronic and recurrent landslides cause frequent blockage along the highway. During a traverse along NH-58, extensively high sub-vertical to vertical cut sections can be witnessed with rare preventive measures. Moreover, civilizations along seasonal valleys, debris dumps along hairpin bends are some common adverse practices along the highway. Demarcation of landslide-prone sections, quantification of the probability of slope failures and correspondingly implementation of restitutive and remedial measures are some major pre-requisite to attaining safe and economically functional design along the highway. The present investigation encompasses the geotechnical appraisal along the strategic tactical transportation route. The investigation incorporates diverse issues pertaining to a variety of landslides along NH-58 from Rishikesh to Devprayag, Uttrakhand. Road cut slope stability appraisal is being conducted in prolonged stages including preliminary literature survey, demarcation of hazard-prone zones along the highway, extensive field survey to obtain data pertaining to slope stability, laboratory experiments to assess geomechanical properties. ii The results were synthesized by integrating field and laboratory data. During the preliminary stage of the project, hazard-prone sections were demarcated. By considering complex terrain conditions, vulnerable road cut slopes were identified and targeted for much detailed inspection was conducted via distinct proxies. The failure pattern was assessed during the initial field surveys and different approaches were selected to deal with rock and debris failures. In rock slopes, Rock mass classification technique viz. Rock Mass Rating (RMR), Slope Mass Rating (SMR), Continuous Slope Mass Rating (CSMR), Geological Strength Index (GSI) were applied to assess the stability levels of twenty vulnerable road cut slopes. Furthermore, the kinematic analysis technique was also carried out to identify the potential for different modes of structurally controlled failures. Moreover, much advanced and computer-aided numerical modeling technique was also performed by using plain strain simulator ‘Phase 2D’. Shear strength reduction technique was conducted by finite element modeling. The non-linear Generalized Hoek-Brown (GHB) criterion was adopted. Critical SRF (equivalent to factor of safety) and shear strain contours were determined for each cut slope. According to FoS values, the cut slopes have been categorized into three classes. For FoS less than 1 as unstable, for 1 to 1.3 as marginally stable and for FoS greater than 1.3 as stable. As per the outcomes, five slopes (S6, S7, S18, S19, and S20) are unstable; four slopes (S2, S9, S13, and S17) are marginally stable while S1, S3, S4, S5, S8, S10, S11, S12, S14, S15 and S16 are stable. The above categorization is based on overall FoS which considers the mass failure only. However, there are certain slopes in the investigated section that are having reasonably fair FoS but having varying potential for occasional block failures. Such failures can be evaluated by kinematic analysis and accordingly remedies may be undertaken. Furthermore, the linear Mohr-Coulomb (MC) criterion was also adopted for comparative analysis. The critical obtained by employing GHB and MC was compared. For lower FoS, both GHB and MC are giving similar outcomes but for higher FoS values, MC underestimates the increase of stability and disparities among outcomes. As Himalayan rock mass comprises of jointed rock mass, the non-linear GHB criterion is much applicable as compared to linear MC criterion. In debris slopes, circular and talus failures were evidenced during initial field surveys and keeping such aspects in mind numerical models were prepared by incorporating bedrock. Geological and geotechnical data pertaining to slope stability was collected during extensive field inspection and representative samples of debris were collected. Geomechanical properties were determined by rigorous laboratory experiments as per the standard procedures. The deterministic assessment of eight vulnerable road cut debris slopes is being conducted by using iii different limit equilibrium methods and finite element modeling. In all debris slopes, MC criterion was adopted while for debris slopes having shallow underlying bedrock GHB (for rock) and MC (for debris) criteria were applied simultaneously. FoS and shear strain contours were determined for each slope to assess determine the overall stability grade and pattern of failure. Similar to rock slopes, the categorization was being done on the basis of critical SRF values that determine the overall stability of slopes. From eight investigated road cut debris slopes, six slopes (L1, L3, L4, L5, L6, and L7) are unstable; slope L8 is marginally stable whereas slope L2 is stable. Furthermore, multi-parameter sensitivity analysis was also conducted in the limit equilibrium technique to evaluate the influence of geomechanical parameters instability of slopes. According to the outcomes obtained by various methods, an attempt has been made to suggest general guidelines for prevention and stabilization of critical slopes along the highway and it has been also recommended to conduct similar geotechnical investigations in hazard-prone Himalayan and similar terrains.