Research On Thermo-Hydro-Mechanical Coupling Of Brittle Rock Based On Microcrack Model

With the development of infrastructure and deep resources mining in our country,the problem of thermo-hydro-mechanical coupling is becoming an important research topic in engineering practice.Due to the complexity of thermo-hydro-mechanical coupling,the present research is still unable to make a reasonable prediction of thermo-hydro-mechanical coupling of brittle rock.Based on the summary of existing research results,with the microcrack model and the maximum circumferential stress theory,the microcrack coalescence model was eatablished under the thermo-hydro-mechanical coupling state.Different from the traditional Phenomenological method,on the basis of the microcrack propagation and coalescence mechanism model,the thermo-hydro-mechanical coupling constitutive model of brittle rock was deduced directly from the point of microcrack.In order to further verify the accuracy of the theoretical model,through the laboratory experiment about thermo-mechanical coupling and thermo-hydro-mechanical coupling,the influence of high temperature heat treatment,pore water pressure and confining pressure on the rock mechanical properties and failure modes was fully studied.At the same time,the polarizing microscope test was used to directly observe and analyse the initial distribution of brittle rock microcrack.Finally,with the concrete stress and strain data,the constitutive model of microcrack is verified.The research results show that:(1)Through the research on the microcrack propagation slip model,a reasonable microcrack propagation model is proposed which emphasized the characteristics of crack propagation direction parallel to the direction of maximum principal stress.Moreover,the theoretical formula of the crack growth length and effective confining pressure is derived.Through the theoretical analysis,it is found that the increase of the effective confining pressure can obviously inhibit the propagation of microcrack.(2)Considering the process of the brittle rock's thermo-hydro-mechanical coupling,the increase of confining pressure causes the failure mode of rock to be the transition from splitting failure to shear failure.It is necessary to take into account both the closure effect of rock microcracks and the effect of uncoordinated deformation at high temperature.On the whole,when the heat treatment temperature is high,the failure mode of rock tends to shear failure.However,the increase of pore water pressure will slow down the process,and with the increase of temperature,the pore water pressure will be more obvious.(3)Introduce the concept of natural strain,a constitutive model of microcrack compaction section is deduced.Furthermore,the constitutive model of the linear elastic stage and the constitutive model of the micro crack compaction section are unified.Based on the simplification of the steady growth stage of microcrack,the stress-strain constitutive model of circumferential strain is established.On the basis of the model of the microcrack coalescence zone,the stress-strain constitutive model is derived.(4)With the increase of the heat treatment temperature,the change of the microcrack property is affected by the two aspects of incompatible deformation and thermal stress closure.When the heat treatment temperature is relatively low,the mechanical properties of rock may be slightly improved.With the further increase of heat treatment temperature,the effect of incompatible deformation is gradually dominant,and the initiation of microcracks is isotropic.With the increase of heat treatment temperature,mechanical properties of rock first increased slightly.With the heat treatment temperature continues to rise,the mechanical properties of granite began to appear obvious deterioration.At the same time,confining pressure has the characteristics of inhibiting the deterioration of rock,and the increase of pore water pressure has the trend of accelerated deterioration.(5)The increase of heat treatment temperature and confining pressure can increase the ductility of the rock,while the increase of pore water pressure is one of the most important factors to increase the brittleness of rock.But for the plastic strain,the magnitude of the value is small,which is easy to be affected by the error of the equipment.(6)The genetic algorithm was applied to verify the rock constitutive model of thermo-hydro-mechanical coupling.From the simulation results,it is obvious that the constitutive model of brittle rock can be used to express the whole process of stress and strain,and the precision is high.(7)The applicability of the crack model to the interpretation of the thermo-hydro-mechanical coupling problem was verified by a numerical example.For the anti wing crack problem,it is mainly caused by stress concentration.When the effective confining pressure increases,the anti wing crack gradually plays an important role in rock failure.The decrease of the bearing capacity of rock mass is mainly related to the shear slip of micro cracks.The dilatancy stress of rock can be considered as the long-term strength of rock.Under certain loading conditions,the stress and strain curves show some fluctuation,and the results are consistent with the results of stress-strain analysis of unstable crack growth stages.