Abstract:

To provide reference for a highway tunnel construction, unloading confining pressure tests are carried out on standard siltstone samples with constant axial deformation by using MTS815.04 electro-hydraulic servo-controlled rigid testing system. Results show that there are three stages during the whole test process initial stage, crack growth stage, and failure stage. In the failure stage, the major cracks propagate along the direction of the maximum principal stress, and eventually a tension failure plane which is approximately parallel to the maximum principal stress is formed. The variation curve of axial pressure vs. confining pressure σ₃ during the unloading process is analyzed. It can be found that the rock is more vulnerable to unloading destruction compared with loading destruction, because when the confining pressure is reduced to a certain degree, the cracks spread rapidly due to the tensile stress, and the friction between the expanded cracks disappears at the same time. The mechanical parameters of the siltstone samples under unloading are obtained, among which the elastic modulus and Poisson’s ratio μ show a non-linear change with the volumetric strain ε_v.

Key words: siltstone , unloading failure , strength characteristics , parameter variation