Experimental Study of Mechanical Behavior of Gneiss Considering the Orientation of Schistosity under True Triaxial Compression

Abstract

Rocks with a layered structure (bedding or foliation) usually exhibit different levels of anisotropy in terms of deformation, strength, and failure mode under multiaxial stress conditions. This anisotropy is influenced by the spatial relationship between the internal layered structure and the principal stresses; however, relatively few experimental studies have been conducted due to various limitations, resulting in insufficient knowledge of the failure mechanism of layered rocks. A series of true triaxial compression tests for a gneiss were carried out considering the orientation of schistosity. The results show that the orientation of schistosity has a significant impact on the strength and failure of this gneiss. More specifically, under the same intermediate principal stress σ2, the strength of specimens increases as the angle (ω) between the strike of schistosity and the direction of σ2 increases. The greater the angle ω, the more sensitive the strength varies with σ2. When σ2 is low, the schistosity plane acts as a weak plane to control the failure of the specimen, and the main failure plane is parallel to the schistosity. As σ2 increases, the weakening effect of schistosity decreases, and the influence of σ2 on the failure is enhanced. In this case, the failure of cutting through the rock matrix more commonly occurs, and the failure plane dips toward σ3 and strikes parallel to σ2.