Shear behaviour of rock joints under cyclic loading

For the design of civil and mining engineering geo-structures, it is often necessary to investigate the behaviour and the role of discontinuities on the rock mass performance. In many cases, failure is governed by the shear behaviour of discontinuities in excavations. In this context, evaluation of the effects of small repetitive earthquakes on the shear strength parameters of rock joints especially in tunnels and dam foundations is also important. This paper presents the results of a systematic study carried out on the cyclic shear behaviour of artificial rock joints under constant normal stiffness (CNS) conditions, as many of the previous studies have been conducted under constant normal load (CNL) conditions. To understand the basic mechanisms involved, idealized joint samples were subjected to cyclic loading using a large scale direct shear apparatus for different stress amplitudes. The mechanisms behind the shearing under cyclic loading at low and high level of normal stresses have been investigated and compared with CNL conditions experiments. The results indicate that asperity sliding and shearing are the two main mechanisms governing the mechanical behaviour of joints under cyclic loading. Moreover, the joint cyclic dilation is overestimated under the CNL conditions.