Large-scale plane strain tests on mechanical properties of gravelly soils under unloading and reloading conditions

Aiming at the repetitive loading and unloading issue in the plane strain of water conservancy and transportation projects, the large-scale geotechnical triaxial apparatus of Xi'an University of Technology modified with a pressure chamber of plane strain, was used to conduct consolidation and drainage plane strain unloading-reloading tests on two types of gravelly soils under different confining pressures. The mechanical properties investigated include the stress-strain relationship and the unloading rebound strain and unloading-reloading rebound gradient of gravelly soils under plane strain conditions. The results show the particle roundness, pore ratio, parent rock strength and confining pressure of the gravelly soils can all affect the peak shear stress and shear dilation characteristics. The gravelly soils with good particle roundness and small pore ratio have lower peak stress and are more likely to exhibit shear dilation characteristics. Low parent rock strength promotes strain softening and reduces the peak stress, while the increasing confining pressure can suppress shear dilation and increase the peak stress. The volumetric strain and shear strain unloading rebound amount are related to the stress level and confining pressure. The rebound modulus first increases and then decreases with the increase of the stress level, and when the density is high, the rebound modulus is higher; The unloading rebound volumetric strain and the generalized shear strain are influenced by the stress level during unloading and the consolidation confining pressure, while the unloading-reloading rebound gradient is related to the proportion of elastic strain during the loading process. During the unloading-reloading process, the rotation of the principal stress axis is observed, and the stress in the non-strain direction is not always the intermediate principal stress. The rebound gradient is significantly greater than the initial tangent gradient, and the ratio of the rebound gradient to the initial tangent gradient decreases with the increasing confining pressure.