Research On The Mechanism Of Rock Energy Evolution Under Graded Unloading Condition

The rapid development of China’s infrastructure engineering construction has resulted in a large number of rocky high and steep slopes,underground tunnel projects,and water conservancy projects all inevitably encountering the problem of surrounding rock-graded unloading.The deformation and failure of deep rock mass under mining disturbance are macroscopically characterized by high energy input and high strain rate,and the process of rock failure is accompanied by energy conversion and release,with obvious failure behavior and fracture law and energy characteristics.Studying the conversion,storage,and release characteristics of energy in this process is the key to ensuring the safe and efficient exploitation of deep resources.We systematically carry out dynamic experimental research on different lithologies,different confining pressures,and different stress paths,and study the differences in mechanical properties of rocks under static and cyclic loading conditions based on obtaining the characteristics of physical properties of rocks at different burial depths.The apparent and detailed characteristics of rock failure under different unloading conditions are obtained,and the nonlinear mechanisms such as energy conversion and release are discussed,to provide a reliable theoretical basis for the mining of deep resources and scientific evaluation of their long-term stability.Based on this paper,the cyclic loading and unloading mode and the step unloading mode of confining pressure are compared with the conventional triaxial unloading mode,and the analysis from the perspective of the stress-strain relationship and strength criterion alone has certain limitations.This paper analyzes the influence of stress paths on rock unloading from the energy point of view.Based on the study of the energy evolution mechanism of rocks under graded unloading conditions,the relevant research is carried out in four aspects,namely,the rock energy evolution law under the confining pressure cyclic loading and unloading conditions,the rock energy characteristics under the step unloading conditions,the rock energy nonlinear evolution model and the analysis of the rock fine view failure mechanism.(1)Uniaxial compression tests,conventional triaxial loading tests,and six sets of cyclic loading and unloading tests under different stress paths were carried out for different rocks.Based on the rock energy calculation method,the evolution curves of energy parameters(input energy,elastic energy,and dissipation energy)of the whole process of rock deformation and damage were systematically obtained,and the evolution of distortion energy and bulk energy in conventional triaxial loading tests were also analyzed.At the same time,the laws of mechanical parameter characteristics,crack volume strain,crack axial deviatoric strain,crack radial deviatoric strain,migration of hysteresis loop,and irrecoverable strain are elaborated and discussed,and it is found that the energy storage and dissipation characteristics of rocks with different lithologies and different initial confining pressures show different change patterns.(2)Four series of uniaxial and different perimeter pressure step unloading tests and two sets of conventional triaxial unloading tests were carried out for different rocks.The energy calculation method of rocks was elaborated,the energy evolution law,energy characteristics,and strain energy growth rate were analyzed,and the energy characteristics between different lithologies were compared and analyzed.The chaos theory was applied to the step unloading test of the confining pressure.It is shown that the radial energy iterative growth factor was more sensitive than the axial energy iterative factor under the confining pressure step unloading condition.The last step growth rate of conventional triaxial strain energy under the same unloading condition was higher than the growth rate of step unloading strain energy,in which the axial strain energy was higher than 29.18%(average value)and the radial strain energy growth rate was higher than15.02%(average value).It indicates that the step unloading makes the energy transformation more adequate,and the elastic strain energy released during rock damage is smaller than that of conventional triaxial unloading,while the sudden release of a large amount of elastic energy will induce.The step unloading path provides a safer unloading mode for the high-stress and high-confining pressure unloading process.(3)The evolution model of strain was applied to the cyclic loading and unloading test,and its universality was verified.The rocking energy evolution model was proposed and applied to fitting energy and cycle numbers.The generalizability law was verified by fitting and analyzing 125 sets of cyclic loading and unloading data under different stress path conditions,initial confining pressures,and lithological conditions.The results show that: 37.6% and 45.6% of the axial(radial)strain energy data were fitted above 0.95;90.4% and 93.6% were fitted above 0.80,respectively.Since energy is the result of stress-strain coupling,the fitting results were slightly lower compared with strain,but still achieved better results.The evolution model provides strong data support for the exploration of energy mechanisms,fatigue damage evolution analysis,and fatigue life prediction.(4)The comparative analysis of T2 spectra and imaging of rocks before and after damage under different stress paths and the number of cycles using NMR,the pore migration pattern can identify the development pattern of internal pores,damage degree,size,and location of damage surface of rock samples before and after damage from a microscopic perspective,which provides strong support for the analysis of macroscopic damage patterns and energy mechanisms.The SEM analysis of the microscopic morphology of the fracture under different cyclic loading and unloading conditions,different step unloading conditions,different unloading amplitudes,different lithologies,and different stress paths under low and high confining pressure conditions were also analyzed.There are some differences in the forms of through-crystal fracture and along-crystal fracture at the corresponding fractures for tensional damage,shear damage,or mixed tensional-shear damage of rocks under different conditions,and the morphological analysis of SEM can provide a microscopic reference for the mechanism of rock energy evolution and fracture mode.