Abstract
Conventional triaxial loading and unloading tests were carried out on sandstone samples in the Zigong area, of Sichuan Province, China. The changes in the elastic modulus of the unloading curves under different confining pressures were calculated, and the evolution law of the nonlinear properties of rock was analyzed. The results show that the rock is subjected to nonlinear damage during initial compaction, the elastic phase, destruction, and postpeak unloading. Moreover, the nonlinear behaviors of rock are restrained by the confining pressures. On this basis, a nonlinear stress-strain relationship affected by the average stress is proposed to describe nonlinear behaviors in the initial compaction stage. According to the test data, the evolution laws of various energies inside the rock during loading and unloading cycles are obtained. The results show that the external work is transformed into elastic energy and damage dissipated energy. Based on the energy analysis, the energy balance equation is established according to the law of energy conservation. By deriving the energy balance equation, the damage evolution equation of sandstone under triaxial loading is solved to establish a continuous constitutive model. The calculation results of the model are compared with the test results from two aspects of loading and postpeak unloading. The comparison results show that the proposed model, which reflects the whole stress-strain process and nonlinear properties of rock, could also describe the stress-strain relationship at the postpeak unloading stage to some extent.
Highlights
In the process of underground mining, the surrounding rock of the working space is always in a complex stress state
The nonlinear characteristics of the initial compaction stage are not considered in the previous model, so the mechanical behaviors of rock in the compaction stage cannot be well described by Zhu et al The previous model established by Cao could represent nonlinear characteristics, but the residual strength is not considered in the model
Compared with the test results, the following conclusions can be obtained: (1) An elastic modulus model with the average stress as the influencing factor is established to describe the nonlinear behaviors during loading
Summary
In the process of underground mining, the surrounding rock of the working space is always in a complex stress state. Xie et al [41, 42] explored the energy mechanism in the process of rock deformation and discussed the relationship between energy release and rock failure On this basis, energy dissipation and damage variables were calculated, and a damage evolution equation was established. Combined with the Weibull distribution model, a statistical damage model considering energy evolution under triaxial compression was proposed by Liu. Based on thermodynamics theory, Qin et al [44, 45] interpreted the process of rock damage and failure by an energy evolution mechanism. Liu et al [46] carried out a uniaxial cyclic loading test and proposed the concept of the compaction coefficient to modify the damage equation They established an energy dissipation model of rock, and the model results can reflect the loading and unloading stressstrain curves before the peak stress. We explored the simulation of postpeak unloading curves by the proposed model
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