Rock variability and establishing confining pressure levels for triaxial tests on rocks

Abstract

In teaching rock mechanics, often the theoretical part of the lectures are demonstrated by laboratory sessions, which are helpful in supplementing the teaching of rock mechanics principles to the students. In these laboratory sessions, an unconfined compressive strength (UCS) test can be followed by a conventional triaxial compressive strength (TCS) test on similar rock specimens. The former test is the one used most in the characterization of rock strength and in many rock classification systems; while the latter test has been widely used for the determination of design parameters such as C and f because of the equipment simplicity and convenient specimen preparation and testing procedures. However, the experience of the authors in conducting such sessions has shown that problems are encountered while performing the conventional triaxial compressive strength tests and these can be misleading to the students. The conventional TCS test consists of applying a hydraulic pressure to the external surface of a cylindrical rock specimen and at the same time axially loading the specimen in compression with increasing increments until failure of the rock. The procedure for measuring the rock strengths has been standardized by the American Society for Testing and Materials (ASTM) [1,2] and the International Society for Rock Mechanics (ISRM) [3,4]. Neither in the ASTM nor in the ISRM