Applicability of Critical State Concept in Determination of Triaxial Strength of Non-Persistent Jointed Rock

Abstract

Rocks encountered in civil and mining engineering structures are generally intersected by discontinuities. The triaxial strength behaviour of a jointed rock intersected by a discontinuity has been a topic of keen interest for the engineering fraternity. It is well established that the strength behaviour of such rocks is highly non-linear. In the past, Barton’s critical state concept has been used to correctly define the curvature of the non-linear failure envelopes of intact and jointed rocks. The aim of the present study has been to evaluate the applicability of the critical state concept for the rock, which is intersected by non-persistent discontinuity having weak foreign material. Triaxial strength tests were conducted on specimens of rock like model material. The specimens were intersected by a non-persistent discontinuity with various combinations of orientation, degree of persistence and number of joint segments. The range of confining pressure was varied from zero (uniaxial loading condition) to reasonably high (critical state) values. The study indicates that the rock intersected by discontinuity comprising of joint segments (having weak foreign material) and intact rock bridges, do follow the critical state concept. However, the triaxial strength behaviour is substantially affected by the presence of non-persistent joints. It is shown that the critical state concept based Modified Mohr–Coulomb (MMC) criterion can be used with confidence to model the triaxial strength of rocks. Correlations have been suggested to assess the criterion parameters based on attributes of the discontinuity and intact rock.