Effect of Confining Pressure on Post-Peak Behaviour of Igneous Rock

Many hydroelectric, metro rail and nuclear repository projects are under construction in the country. The structures for these projects, which include dams, tunnels, powerhouse caverns, are mostly constructed on/in rocks. The characterization of the behaviour of rock forms essential part of the analysis and design of these structures. Many of the rocks around tunnels and underground caverns exhibit strain softening behaviour under loading. Strain softening is defined as the progressive loss of strength when material is compressed beyond peak. The present paper deals with the testing of an igneous rock, i.e. basalt from hydroelectric project located in Madhya Pradesh in India. The testing has been conducted using closed-loop servo-controlled testing machine under strain-controlled loading. The specimens were prepared as per ISRM standards. The diameter of the specimen was 54 mm, and the length of the specimen was 108 mm. The index properties of the rock were determined as per ISRM standards. Triaxial tests were conducted under various confining pressures at a constant axial strain rate of 9.259 × 10⁻⁶ s. The strains were measured with the help of strain gauges and specially designed extensometers. The stress–strain–volume change relationships of the tests are presented and discussed in this paper. The behaviour of basalt is also predicted using an elastoplastic constitutive model, i.e. Mohr-Coulomb strain softening model using FLAC. The material parameters for the model are determined from the experimental results. The stress–strain–volume change response of the basalt is then predicted using the material parameters and is compared with the observed results. The predictions using the constitutive modes are found satisfactory and comparable with the experimentally observed results.