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

Yingjie Zhang,Shuangfei Liu,Jiangteng Li,Gang Ma,Yunteng Wang

doi:10.1155/2021/5577321

Abstract

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.

Highlights

In recent years, a substantial number of laboratory tests were conducted to reveal the macromechanical and failure behaviors of rock masses under different loads, e g., uniaxial compression, conventional triaxial compression [8], and true triaxial compression [9, 10]
In order to investigate the influence of initiation and propagation of microcracks on the failure of rock materials, micromechanical methods [18,19,20] were employed to understand the failure of rock materials and establish the corresponding strength criteria and constitutive relations
Zhou et al [21] proposed a nonlinear three-dimensional strength criterion for rock-like materials based on the micromechanical methods, in which the effects of the intermediate principal stress on the failure of rock-like materials are taken into account

Summary

Materials and Methods

To explore the influence of σ2 on the true triaxial unloading mechanics and energy characteristics of sandstone, a true triaxial loading and unloading test was carried out. 30 s, when σ2 and σ3 remained unchanged, σ1 was loaded until specimens were destroyed. Part II: true triaxial lateral unloading: 30 s, when σ2 and σ3 remained unchanged, σ1 was loaded to a predetermined value (90% of the peak strength σ max 1 obtained by the true triaxial loading test under the same initial stress condition). Step 3: σ1 and σ2 were kept stable, and σ3 was unloaded until specimens were destroyed.

Results and Discussion of the Mechanical Characteristics

Results and Discussion of the Energy Characteristics

Conclusion