Empirical methods to estimate the strength of jointed rock masses

Abstract The ultimate strength and deformation of jointed rock mass are important parameters that designers look for in selecting sites for foundations of civil and mining engineering structures in rocks. In the field, it is extremely difficult to stress a rock mass to its ultimate failure stress. Consequently, the field tests are performed up to a certain predefined stress level. It is shown in this study that the results of these tests can be used to arrive at the ultimate strength of the rock mass. A large number of uniaxial compressive strength (UCS) tests were conducted on the specimens of jointed block mass having various combinations of orientations and different levels of interlocking of joints. Four dominating modes of failure were observed. The strength and the tangent (elastic) modulus values of the mass for a specific failure mode, when plotted on the Deere–Miller classification chart, are found to follow an empirical straight line, indicating strong correlation between them. Using the gradient of this empirical line, correlations have thus been suggested to assess the ultimate strength of the jointed rock mass. The findings of the study have been verified by applying it to estimate the ultimate rock mass strength of nine rock types from few dam sites in the lower Himalayas. The ultimate strength obtained by the present methodology is compared with that obtained through the Q classification system. It is concluded that reasonably good estimates on field strength of jointed rocks are possible by using the correlations suggested in this study.