Assessing rock mass properties for a hydropower project using deformability tests

Summary

Abstract

Rocks encountered at hydro power project sites in the Himalayas are generally jointed, anisotropic and show scatter in their engineering response. While, the intact rock strength can be obtained easily by testing specimens till failure in laboratory, assessment of the ultimate strength of rock mass by stressing it up to failure is extremely difficult, expensive, time consuming and many a times not feasible also. Alternative approaches are therefore preferred to assess the rock mass strength. This article advocates that rock mass strength can be obtained from the deformability of the mass which can be obtained by stressing the rock-mass to a very low level of stress. This hypothesis is based on outcome of large number of UCS tests performed on jointed block mass specimens. The hypothesis is utilised to assess the rock mass properties for a hydro power project located in higher Himalayas. The rocks at the site are intersected by a set of joints. Comprehensive laboratory tests were performed to get the intact rock properties and their probability distributions. A number of cyclic plate load tests were performed at the site to get the elastic modulus of the rock mass. These modulus values were than utilised to arrive at a set of UCS values of the rock mass. A non-linear strength criterion was then used to arrive at strength values under confinement. These results were used to simulate instabilities observed in a cut rock slope at the dam site. It is concluded that deformability of rock mass, which is relatively easy to obtain in the field, along with laboratory test results for intact rock, can be used successfully to arrive at strength of rock mass.