Experimental Study On Permeability Evolution Of Fractured Rock Under Complex Stress Conditions

As a common complex medium in underground engineering construction,the permeability characteristics of fractured rock are greatly affected by stress.Therefore,the experimental study on the permeability evolution of fractured rock under complex stress conditions plays an important role in engineering design,construction and safe operation.In this paper,experimental research,theoretical analysis and numerical simulation methods are adopted to conduct research.Based on the two experimental schemes of regular fracture and random distribution fracture,the experimental study of red sandstone and mudstone is carried out under complex stress conditions.Firstly,the conventional triaxial mechanical test and the intact rock permeability test were carried out for the red sandstone,and the relevant physical and mechanical parameters and the permeability evolution of the intact rock were obtained.Secondly,the permeability test was carried out for the regular fractured rock,and the permeability evolution under different stress conditions were obtained.Finally,the permeability test was carried out on random fractured rock,and the evolution law of permeability under complex stress conditions was mastered.In this paper,the permeability evolution law of different fractured rocks is analyzed.The permeability evolution mechanical model of fracture rock considering the number of cracks is constructed.The numerical simulation considering permeability evolution is carried out,and the correctness and effectiveness of permeability evolution mechanical model is verified.The permeability evolution law of fractured rock under complex stress environment is systematically grasped;relevant results can provide guidance and reference for solving and preventing water seepage problems in engineering and correctly evaluating the safety and stability of actual engineering.The main research contents and conclusions are as follows:(1)The basic mechanical parameters of rock are obtained by conventional triaxial compression test.The stress-strain curve exhibits distinct brittle characteristics,including the compaction phase,the linear elastic phase,the yield phase,and the failure phase.The results of intact rock permeability test show that the permeability-fluid pressure curve has obvious phase characteristics.The permeability of rock sample varies from 10^-16m²?10^-17m²,which is no more than one order of magnitude.The permeability decreases as the hydrostatic pressure increases.During the axial compression cycle loading and unloading test,the permeability was irreversible.The influence of axial pressure on permeability is obviously less than that of fluid pressure and hydrostatic pressure.(2)Permeability tests under different stress conditions were carried out on the regular fractured rocks and random distribution fractured rocks.According to the test results,the experimental results show that there is a linear relationship between seepage velocity and hydraulic gradient,so the seepage is laminar and conforms to linear Darcy's law.The results of permeability test show that the fluid pressure-permeability curve accords with the exponential function,and the hydrostatic pressure-permeability curve accords with the negative exponential function.Through the axial compression cycle loading and unloading test,it is found that the permeability is irreversibly reduced during the loading and unloading process;in the variable confining pressure and axial compression tests,there is a negative exponential function relationship between the permeability and the axial pressure,and there is also a relationship between the relational parameters and the confining pressure.(3)The evolution of permeability of fractured rock is mainly caused by external stress and fluid pressure.Based on the pore-fracture dual medium seepage theory and experimental results,it is defined that the permeability of fractured rock is composed of fracture permeability and matrix permeability.The interaction between fracture system and matrix system during deformation is analyzed,and the permeability evolution mechanical model considering the fracture form,fluid pressure,confining pressure and axial pressure is established,and the model parameters are determined according to the test results.(4)Based on the spatial and distribution characteristics of the fracture,the Monte-Carlo simulation method is used to generate the numerical model of the fractured rock.The permeability evolution mechanics model was embedded in FLAC3D through the Fish programming language.The simulation study of the permeability evolution of the fractured rock under the experimental conditions was carried out,and the validity of the model is verified.The permeability evolution of fractured rock under complex stress conditions is systematically grasped.