Significance and response of rock mass during excavation of open slopes in mountains for construction of major civil structures - a case study on the behaviour of excavated slopes viz a viz treatment provided for slope stability

The great Himalayas are still actively orogenic and are constituted by a complex sequence of igneous, sedimentary and metamorphic rocks dominated by shear zones, thrust zones, joints, active faults and igneous intrusions. The presence of such discordant features causes the rock mass to behave in an anisotropic manner. Studies are continuing to understand rock mass behaviour and to fix influencing parameters for design of open slopes. In Himalayan geology the long term stability of an unsupported slope, the design of support system or preventive/corrective measures to be adopted for an existing slope is still a difficult task. During excavation & support of large unsupported slopes, typical problems are encountered, the rock strata changes very frequently and mass behaviour becomes highly unpredictable. It becomes a difficult task to decide the adoption of supports based on changing rock mass. The works may remain disrupted many days in many projects due to failure of the slopes and pending decisions for providing heavy supports or flexible supports. At many places the failure of slopes have adversely affected the construction program for example Subansiri Project Power House Back slope, Parbati Stage –II Power House back slope, Teesta Low Dam Project Slope failures, Surge Shaft wall failure in Rangit Project & Teesta Stage-V Project and slope failure on the road leading to Duhangan weir in AD Project. Various cases of failures have been highlighted and discussed in brief to indicate the extent of damage it causes in a project of high importance.