Experimental investigation and failure mechanism analysis for dacite under true triaxial unloading conditions

Strong unloading damaged zones at both slopes of the dam of a hydropower station in Southwest China have been attributed to the stress release. To study the deformation and failure mechanism of dacite (dam site material) under unloading conditions, three groups of true triaxial unloading tests were conducted on the rock sample with an initial minimum principal stress sigma(0)(3) = 5 MPa, and initial intermediate principal stresses sigma(0)(2) = 10, 15, and 20 MPa. The experimental results indicate that the strain in the direction of sigma(3) is the most sensitive to the unloading of sigma(3), whereas the three principal strains are further suppressed with increasing sigma(2). Ductile deformation is a failure precursor of dacite under unloading conditions. Based on the analysis of the failure mode and acoustic emission monitoring data, it has been concluded that the unloading failure of dacite is caused by the initiation, propagation, and coalescence of microcracks, which form localized tension macrocracks. The observed anisotropic damage evolution is found to be determined by stiffness degradation and quantified the degree of fracture during the final unloading stage. The results indicate that damage is anisotropic under the true triaxial unloading conditions, and that the damage omega(3) is the most sensitive to the unloading stress sigma(3). Therefore, the deformation or damage in the direction of sigma(3) can be used as a guide in the design of engineering excavation plans.