Severity Prediction of Stress-Induced Instabilities During Subsurface Excavation in the Himalaya: Case Studies

Time-dependent deformation in rock under high in-situ stress conditions in the Himalaya poses a number of unique challenges in underground excavation leading to time and cost overrun. Objective of this paper is severity prediction of potential stress-induced instabilities based on the back analysis of already-encountered rock bursting and squeezing incidences in two deep-seated tunnels located in two different geological formations in the Himalaya. Geomechanical properties of the encountered rock and different in-situ stress factors like σθ (tangential stress), σV (vertical stress), σH (horizontal Stress), σcm (compressive strength of rock mass) and SRF (stress reduction factor) have been determined from the already-encountered zone for severity analysis. Based on the analysis, three prediction theories were adopted for identifying potential zones of stress-induced instabilities during tunnel excavation. First prediction approach was attempted utilizing storage energy potential of the rock based on its stress–strain characteristics under uniaxial compression testing in the laboratory. The second approach involves utilization of different in-situ stresses to determine modified overload factor (OFM) proposed by Deere et al. (1969). The third approach is to determine in-situ vertical stress to intact rock strength ratio and incorporate in empirical stability classification proposed by Hoek and Brown in Underground excavations in rock, Institution of Mining and Metallurgy, London, 1980). These stress factors are then incorporated in empirical rock mass classification RMR to propose modified RMR for true assessment of rock mass classification in rock bursting and squeezing ground conditions. In squeezing ground, support pressure has been determined from Goel (1994) approach. Finally, based on the above analysis, a site-specific severity prediction model has been proposed which can be used as an impetus for future study. This attempt may assist in framing a comprehensive geotechnical baseline report by predicting potential geotechnical hazards under stress-induced conditions often encountered during tunneling in the Himalaya and adopting suitable risk mitigation measures.