Experimental investigation of the mechanical properties of fine-grained sandstone in the triaxial cyclic loading test

Fine-grained sandstone is widely distributed in the Xiangjiaba Hydropower Station. To investigate the mechanical properties of the sandstone, a series of triaxial cyclic loading tests were conducted under the confining pressures of 5MPa, 15MPa, and 20MPa. According to the mechanical characteristics of the sandstone, a typical stress-strain curve can be divided into five stages: initial compression stage, linear elastic stage, strain hardening stage, strain softening stage and residual strength stage. Initial yield strength, peak strength, and residual strength linearly increase with the increasing of confining pressure, respectively. The initial yield stresses sigma y are approximately 69.0-74.0% of the peak strengths; while the residual strengths sigma s are 39.5-43.5% of the peak strengths under various confining pressures. Based on the experimental results, the strength and deformation parameters are obtained and analyzed. The strength parameters are calculated according to the Mohr-Coulomb criterion. The cohesion increases at first and then decreases with the increasing of internal variable kappa. The friction angle first gradually decreases, then it increases to the maximum and finally decreases to a stable value with the increasing of internal variable kappa. The maximal variations of friction angle and cohesion are approximately 29.1% and 62.5%, respectively. Variations in elastic modulus decrease, respectively, 10.3%, 6.1%, and 6.8% pre-peak strength under different confining pressures, and the variations in Poisson's ratio are, respectively, 71.1%, 36.2%, and 63.5%. The nonlinear mechanical behaviors of fine-grained sandstone are the result of the accumulation of irreversible deformation and damage evolution. Moreover, the hysteresis loop is obvious with the accumulation of irreversible deformation and damage evolution. Damage starts to evolve when the stress level is more than the damage threshold. The experimental investigation can enhance the knowledge of the nonlinear behaviors of rock material.