Anisotropic Mechanical Properties of Sandstone Under Unloading Confining Pressure at High Temperatures

Abstract

High geothermal temperatures appear to be unfavorable for tunnel constructions in layered sandstone rocks with large overburden. The main objective of our research was to investigate the anisotropic mechanical characteristics of layered sandstones at various levels of geothermal temperature, to find answers to the following research questions: which temperature level is critical and which angle of orientation of the bedding is most unfavorable under unloading stress conditions in tunnel excavation. We carried out conventional triaxial unloading tests at five different temperatures from 20 to 150 $${^{\circ }}\hbox {C}$$ . The obtained results show there is a critical temperature value between 120 and 150 $${^{\circ }}\hbox {C}$$ , at which the strength properties of sandstone strongly decrease and the Poisson’s ratio strongly increases, as compared to those at lower temperatures. From 20 to 150 $${^{\circ }}\hbox {C}$$ , the sandstone failure mechanism changes from shear failure to tension failure. The elastic modulus of the sandstone decreases, the Poisson’s ratio slightly increases, and the peak strength decreases first and then increases, with increasing bedding angles. The bedding angle of 30 $${^{\circ }}$$ is the most unfavorable angle for the mechanical characteristics of sandstone. The ratios of $$\sigma _{ci}/\sigma _{f }$$ and $$\sigma _{cd}/\sigma _{f }$$ are not affected by the bedding angle, and these ratios are almost stable when the temperature increases from 20 to 120 $${^{\circ }}\hbox {C}$$ . However, they increase at 150 $${^{\circ }}\hbox {C}$$ . The tested layered sandstone is a low anisotropy rock, but from 20 to 150 $${^{\circ }}\hbox {C}$$ the anisotropy slightly increases.