Unloading Mechanics and Energy Characteristics of Sandstone under Different Intermediate Principal Stress Conditions

The TRW-3000 true triaxial rock testing machine was used to conduct loading and unloading tests of sandstone under different σ 2 , and the true triaxial lateral unloading mechanics and energy characteristics of sandstone under different σ 2 were studied. The experimental results show the following: (1) compared with the results of the loading test, the peak strength of the sandstone under the unloading σ 3 path is reduced, the unloading direction has obvious expansion and deformation, and the amount of expansion increases significantly with the increase of σ 2 ; sudden brittle failure occurs at the end of unloading. E gradually decreases with the increase of H, and it performs well to use the cubic polynomial to fit the curve of E-H. (2) The Mogi–Coulomb strength criterion can accurately describe the true triaxial strength characteristics of sandstone under loading and unloading conditions. Compared with the results of the loading test, the values of c and φ obtained based on this criterion under the unloading σ 3 path are reduced. (3) Under the condition of unloading σ 3 , U, U e , and U d , when the specimen is broken, are all linearly positively correlated with σ 2 . U d increases nonlinearly with the increase of H, and as σ 2 increases, the slope of the U d -H curve becomes larger, and the specimen consumes more energy under the same unloading amount. Most of the energy absorbed by the specimen under the unloading σ 3 path is converted into U e , but as σ 2 increases, U d   / U increases, and the energy consumed when the specimen is broken is greater.