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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.

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