Modeling induced acoustic velocity anisotropy during triaxial tests of rocks

A model based on theory proposed by Hudson is developed to simulate the changes of acoustic velocities of rocks during triaxial induced compaction. The effect of triaxial loading on the acoustic velocity is simulated by considering two sets of microcracks, one with normals parallel to the principal stress, and the other with normals randomly distributed in the plane perpendicular to the principal stress. The total effect is the sum of those. The model is used to match the experimental data and to obtain microcrack parameters which are used to estimate the crack density and aspect ratio combined with the stress-strain data. The results show that when the axial load is low (less than 50% of the ultimate strength), cracks normal to the loading direction are closed with increasing load, while cracks parallel to the loading direction remain unaffected. At higher loads, cracks parallel to the loading direction are opened, but cracks normal to the loading direction are kept closed. Theoretical analyses show that the dilatancy is caused by the opening of microcracks.