Comparative study of the deformation modulus of rock mass

The deformation modulus of a rock mass (Em) is an important parameter in rock mechanics and engineering and its determination is a difficult task. Therefore, several equations were proposed using regression analysis of Em and rock mass class. Among them, widely used equations based on Q and RMR were selected for the calculation of Em values for different rock types which are encountered along the tunnel alignment of the Kulekhani III Hydroelectric Project, Nepal. From the results, it is found that some of the equations show Em value to be highly sensitive to the rock mass class while others show less sensitivity. Different values of Em were obtained from different equations for the same class and eight different rock types. Even a small change in RMR causes large variation in Em value. Maximum differences in obtained values are about 5–30 GPa for same rock class. In such condition, it becomes very difficult to choose deformability value to design engineering projects in rock mass. The graphs obtained from different equations show linear to fluctuating pattern depending upon the sensitivity of equation towards rock class.