Cyclic triaxial test investigation on tuffs with different water content at Badantoru Hydropower Station in Indonesia

The weakening effect of water in rocks has been extensively investigated over the past few decades. The degradation of the strength and stiffness of rocks can vary greatly with rock type. To investigate the waterweakening effect on the mechanical properties and failure modes of a Hydropower station's bedrock, static and cyclic triaxial compressive tests were carried out on dry, natural, and saturated tuffs (from the Badantoru Hydropower Station in Indonesia) under different confining pressures. Analysis of the failure modes, stress-strain curves, and brittleness index revealed that all of the samples exhibited typical brittle fracture behavior, i.e., a stress drop, volumetric dilation, and no compaction bands. The decreases in the peak stress, residual stress, and elastic modulus with increasing water content verify the water-weakening effect. The correlations between the water content and the confining pressure and other characteristic parameters calculated using the Pearson correlation coefficient indicate that the peak stress and elastic modulus are negatively correlated with the water content, and the peak stress, peak strain, and elastic modulus are positively correlated with the confining pressure. Compared with the Mohr-Coulomb criterion, the Hoek-Brown criterion can better describe the failure behavior of tuffs. Two damage variables, DE and D epsilon, were defined based on the hysteretic energy and residual strain. Both can characterize the damage evolution and failure mechanism, and DE can describe the stages of the damage evolution in more detail.