The strength and deformational behaviour of rocks under loading conditions are important in underground excavations, mining and every civil engineering constructions as it determines the stability of such structures. This study aims to augment the existing know-how on such behaviour through analysis of point load tests conducted on anisotropic rocks of metamorphic nature. Three types of anisotropic rocks named augen gneiss, granitic gneiss, psammitic schist, and greenschist from different formations of the Lesser and Higher Himalaya of Central Nepal have been investigated. It is observed that the point load index strength of the anisotropic rocks is related to their grain size and the loading angle with respect to the planes of anisotropy. The anisotropic index shows pronounced effects on the surface energy distribution of the samples on the application of stress.
The strength of the rocks and their behaviour during excavations of varied nature is important for the stability of structures like tunnels, mines and other civil structures. It is therefore imperative to have a complete appraisal of the failure behaviour of rocks in advance, before commencing excavation of a structure in rock. Strength characteristics of rocks varies with their genetic nature and the variation in geo-mechanical properties is distinct in metamorphic rocks. This is primarily due to the strong anisotropy of such rocks. Accordingly, the strength and nature of the failure of such anisotropic rocks change with the orientation of stress. This has a strong bearing on the stability of the underground structures as their orientation with respect to the anisotropy defines the behaviour and stability of the rock mass. One of the methods to determine the geo-mechanical strength characteristics of rocks is the point load index strength test (PLI) that provides valuable information about the mechanical behaviour of rocks. Although indirect tensile testing of the rocks is the preferred test, the PLI is an easy method to arrive at such experimental values.
The anisotropy as proposed by [1] can be determined by testing rock samples in different directions. The anisotropic behaviour of the rocks in different index tests have been identified and studied by many researchers [1–4]. The failure behaviour of anisotropic rocks has been investigated by [1,5–13]. However, relating failure behaviours of anisotropic rocks with actual field conditions is a difficult process, if their directional strength properties are not considered. Therefore, for engineering projects, it is essential to estimate the strength, stress-strain, shear failure behaviour and failure mechanism of rocks accurately.
