A new multiple-step loading triaxial test method for brittle rocks

Abstract

To evaluate mechanical properties of rocks by triaxial compression (TC) testing, several identical samples are required. Multiple-step loading (ML) TC testing permits determination of a failure envelope for different confinement stress levels using a single sample. The test consists of a series of consolidation and shearing steps, and in each step the axial loading is stopped or reversed before a complete failure. In the method suggested by the International Society of Rock Mechanics (ISRM), axial load should be increased until the axial stress-strain curve shows a horizontal tangent. However, in most brittle rocks, at peak stress failure occurs almost instantaneously and without any precursory signs. In this situation, attempting the ML test is very difficult. A new ML loading test method is proposed for the determination of the triaxial compression strength envelope in brittle rocks. A series of uniaxial compressive strength (UCS), single-step loading (SL) and ML TC tests were conducted on pyritic schist rock samples. It was observed that, during axial loading, the point at which the value of secant Young's modulus is maximum ( E sec =max), is the most suitable criterion for estimating the imminent rock failure. Using the proposed method, the peak and residual strengths at different confining pressures, asobtained by a set of SL TC tests, can be determined rather accurately from the results of a ML test using a single specimen. Finally, it was concluded that, in brittle rocks, ML test increasing σ 3 produces more reliable results as compare to ML test decreasing σ 3.