Developing a Damage Model to Simulate Multiple-Step Loading Triaxial Compression Tests in Rocks

Abstract

Multiple-step loading triaxial compression test (ML-TCT) method is a useful tool to evaluate strength parameters of rock samples using a single specimen applying several loading/unloading. However, because of accumulated damages in the specimen with repeated cycles of axial loading/unloading, the shear strength is prone to be underestimated. A multiple-step loading damage (MLD) model was proposed to simulate ML-TCT results. Two series of ML-TCTs were carried out on a sedimentary soft rock of mudstone. The first series was to determine the geotechnical parameters to describe the MLD model, and the second series was to verify the model. The results demonstrated that the proposed MLD model was powerful to simulate ML-TCTs on the mudstone and modify the results of carried out tests to generate more reliable results. Moreover, a generalized MLD model was constructed. This model allows prediction of peak deviator stresses and the relevant excess pore water pressures in a ML-TCT for rocks having different strength which generally are affected by the previous loading history. The generalized MLD model indicates that the margin between shear strength parameters obtained by single-step loading triaxial compression tests and ML-TCTs, increases with an increase in the rock strength. Moreover, upper bound values for effective cohesion, c′, and lower bound values for, effective friction angle, ϕ′, was obtained in a ML-TCT with increasing effective confining pressure, σ′c. Whereas, upper bound values for ϕ′ and lower bound values for c′ predicted in a ML-TCT with decreasing σ′c. It was concluded that, ML-TCT increasing σ′c is preferable to ML-TCT decreasing σ′c.