Determination of rockmass characteristics is very important in planning, design and construction of any engineering structures on the rock or within the rockmass. Field investigation includes determination of modulus of deformation, modulus of elasticity, shear strength parameters and stress measurements and others on rock mass. Of many rockmass parameters, deformability characteristics are of much more importance to determine different modulus values where the response of rock mass to the applied load is desired. This is an important and useful input parameter for the stability analysis of dams, slopes and underground structures of hydroelectric projects, nuclear power projects and other similar structures.

This paper is an attempt to discern the influence of sample size variation directly related to the geological structural features, the testing methodology on the deformability characteristics of the rock mass. In the paper rock mass deformability characteristics determined by in-situ investigations at Kotlibhel H.E. Project, Stage-IA across river Bhagirathi, located near Devprayag in districts of Pauri and Tehri Garhwal of Uttrakhand State, in North India are presented. A comparative study of the results of laboratory and in situ investigation (Plate load tests and Goodman Jack tests) in the drifts on both right and left banks are discussed considering that the parent rocks at both the banks were weak and weathered phyllites.

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