Introduction

Knowledge of the mechanical properties of rocks at high pressures and temperatures is fundamental not only for material science but also for earth science, such as for solving the mechanism of earthquakes and tectonic processes. The mechanical properties of rocks under pressure are usually studied in triaxial testing. When cylindrical samples are subjected axially to simple compression in the presence of superimposed hydrostatic pressure, the latter is called the confining pressure, commonly designated by σ 3 (= σ 2 ), and the axial compressive stress surpassing the confining pressure is called the differential stress, commonly designated by σ 1 — σ 3 .