Large scale plane strain experimental study on the mechanical properties of gravel soil under unloading and reloading condition

Regarding the repetitive loading and unloading issue in the plane strain engineering of water conservancy and transportation projects, the Xi'an University of Technology large-scale geotechnical triaxial apparatus modified with a pressure chamber of plane strain, was used to conduct consolidation and drainage plane strain unloading-reloading tests on two types of gravel 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 gravel soils under plane strain conditions. The results show the particle roundness, pore ratio, parent rock strength, and confining pressure of gravel soil can all affect the peak shear stress and shear dilation characteristics. Gravel soil with good particle roundness and small pore ratio has lower peak stress and is more likely to exhibit shear dilation characteristics. Low parent rock strength promotes strain softening and reduces peak stress, while increasing confining pressure can suppress shear dilation and increase peak stress; The volume 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 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 in 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 rebound gradient to initial tangent gradient decreases with increasing confining pressure.