A laboratory-testing-based study on mechanical properties and dilatancy characteristics of deeply buried mudstone under different stress loading rates

The damage and failure process of rock engineering is inseparable from rock dilatancy. Dilatancy is the phenomenon of rock volume increase after the differential stress exceeds the yield limit. X-ray diffraction, scanning electron microscope, and uniaxial compression tests under different stress loading rates (0.1, 0.5, 2.0, 5.0 kN/s) were carried out on mudstone specimens from drill holes in the mine site with a depth of nearly 1300 m. The results demonstrate that the mudstone is mainly composed of plagioclase. The distribution of various minerals in the rock is not uniform. The main component plagioclase is distributed in strips, filled with other minerals, the filling material is mainly chlorite. Under different stress loading rates, the axial strain curves are typical plastic-elastic-plastic curves, and each specimen has an obvious micro-crack compaction process. The lower the stress loading rate, the more pronounced the compaction process, that is, the smoother the curve. In the elastic stage, the slope of the curve increases as the stress loading rate increases, indicating that the rock’s elastic modulus rises. The one-way analysis of variance method was used to test the significance of the differences in each analysis item’s data. The results indicate that the stress loading rate (V_load) is related to the uniaxial compressive strength (σ_c), elastic modulus(E), maximum axial strain(ε_a,max), maximum radial strain(ε_d,max), the initial dilatancy strength(f^*), and the ratio of the initial dilatancy stress to the uniaxial compressive strength(f^*/σ_c) of mudstone. It does not correlate with the maximum volumetric strain value (ε_V,max), and the ratio of the initial dilatancy volumetric strain to the maximum volumetric strain (ε_V,d/ε_V,max). As the V_load increases, σ_c and E increase, when V_load increases 50 times from 0.1 kN/s to 5.0 kN/s, the measured σ_c increases from 26.8 MPa to 36.5 MPa, and E increases from 3.5 GPa to 5.9 GPa. The ε_a,max and ε_d,max decrease with increasing V_load while axial and radial strain rates increase. Hence, the likelihood of failure of mudstone rock under insufficient deformation increases with increasing V_load. The f^* and f^*/σ_c decrease as V_load increases, meaning that it is easier to enter the dilatancy stage with higher V_load. It is considered that the strength parameters, deformation and dilatancy parameters of mudstone are significantly dependent on V_load.