Estimation of rock mass deformation modulus using variations in transmissivity and RQD with depth

Discontinuity normal stiffness and deformation modulus of large scale rock masses are very difficult to determine. A method for estimation of discontinuity normal stiffness based on the decrease in transmissivity with depth has been proposed by the authors in a former paper. In the current study, the method is further developed by accounting for the changes in both discontinuity aperture and frequency with depth, which are key factors that cause the transmissivity to decrease with depth. The discontinuity frequency can be estimated from RQD measurements, which are readily available in most geotechnical investigations. The transmissivity data from packer tests are usually available in geotechnical investigations for hydropower plants. For a rock mass in a dam site mainly controlled by lithostatic stress, based on transmissivity and RQD data at different depths, the change in discontinuity aperture with depth can be linked to the change in aperture with stress, which defines the normal stiffness of discontinuities. In the case study, the discontinuity normal stiffness is successfully estimated by using transmissivity and RQD data, and the result shows that the normal stiffness increases with stress (depth) and the rate of normal stiffness versus stress (depth) decreases with stress (depth), which is consistent with experimental studies. The estimated normal stiffness has been utilized to calculate the rock mass deformation modulus using an equivalent model. The result of deformation modulus by the proposed method is close to that obtained by using in situ measurements, as well as by using empirical models relating RQD to deformation modulus.

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