Any theory or model studies cannot provide the degree of reliability required for a good comprehension of deformations and in turn the stability of structures in rock. This is due to the influence of discontinuities viz. bedding, fractures, joints, faults, folds in the rock mass. Understanding of these can be enhanced by making in-situ measurements, if the test area is representative of the rock in the full scale structure. Among the rock mass parameters, it is important to know the deformability characteristics for assessing expected displacements during excavation, to check stability and also to design proper rock support measurements. Deformability characteristics can be measured by different methods, normally Plate Jacking test, Plate Load test and Goodman Jack test are in practice. Each method has its own merits and demerits; choice depends on the requirement of design. In the present paper, findings of 17 Nos. of in-situ tests along with estimated values of modulus from different classification schemes of RMR and RQD are discussed. It is observed that the modulus of deformation estimated from RMR is higher than that determined from in-situ plate load test whereas the modulus of deformation estimated from RQD is lower in comparison with the in-situ results obtained from Goodman Jack test.
Deformability of rock mass is characterized by a modulus describing the relationship between the applied load and the resulting deformation. For the determination of deformability characteristics both laboratory and in-situ methods are available, of which laboratory testing will be carried out on intact rock and in-situ testing on rock mass, which contains weaknesses besides the intact rock material. Hence, the modulus values obtained from laboratory testing will be in the order of five to twenty times higher than in-situ values . All in-situ deformability tests are expensive and difficult to conduct. Initial preparation at the test site is particularly time consuming. The interpretation of measured in-situ data is another difficult aspect, as in rock mass not only the geological, lithological, physical, mechanical and environmental factors, the structural features namely bedding planes, shear planes, joints, fractures, fault and the gauge material present will decide the response of the rock mass . There are several methods available for measuring the deformability characteristics of rock masses; includes the Plate load test, Goodman jack test, Plate jacking test, Flat jack test, Cable jacking test, Radial jack test and rock dilatometer test. Other than this, several authors have proposed empirical relationship for estimating the value of an isotropic rock mass deformation modulus on the basis of classification scheme such as Rock Mass Rating (RMR), Tunnelling Quatility Index (Q), Geological Strength Index (GSI) and Rock Quality Designation (RQD) .
The Kotlibhel H E Project, Stage II is located near Kaudiyala Village in districts of Pauri and Tehri Garhwal of the state of Uttrakhand. It envisages the construction of 58.6m high concrete gravity dam across river Ganga about 29 km downstream of Devpryag to generate hydropower.