Abstract: In triaxial loading and unloading tests of soft rock, the determination of compressive strength is a critical problem, which always tends to use empirical method，while less corresponding standards at present. Based on this, a set of triaxial loading and unloading tests of sandy mudstone were carried out and the experiment results show that: (1) Sandy mudstone samples always show obvious strain hardening characteristics in both triaxial loading and unloading tests when the confining pressure is more than 10 MPa. (2) The mechanical response differences of sandy mudstone under triaxial loading and unloading tests are obviously, the samples always have only one macro shear failure surface in triaxial unloading tests; while the samples have no obvious control fracture surface under the condition of triaxial loading, and the dilatancy effect is obvious. (3) The semi-log method was introduced into the data analysis of soft rock triaxial loading and unloading tests, which can determine the compressive strength more conveniently. (4) Based on the analysis of stress and strain increment in the process of triaxial loading and unloading tests, the deformation and failure process of soft rock can be divided into three stages: smooth stage, rule jump stage and sharp jump stage; the division has obvious physical significance, which can be used as an important reference for determining the compressive strength. Therefore, the two methods can be integrated used to determine the compressive strength of soft rock in triaxial loading and unloading tests. The methods for determining the compressive strength is very applicable, and can also provide reference for similar soft rock tests.

Key words: soft rock, compressive strength, failure characteristics, semi-log method