| In the construction of deep engineering,the stress environment of rocks is complex,and the authenticity of laboratory simulation is highly challenged.Electrical resistivity is one of the most important properties of rocks,and the resistivity can effectively reflect internal fissure changes during rock failure under stress,thus achieving rock monitoring and disaster warning.The frictional effect at the end of the rock is a key focus of rock mechanics research,and an increasing number of scholars consider the influence of end friction in rock loading tests,analyzing the changes in rock samples under different end conditions.However,the special electric field environment in loaded rock resistivity testing restricts most reduce-friction methods,so the influence of end frictional effects is often ignored in experiments.To address this issue,this thesis conducts relevant research on the development of reduce-friction pressure heads for multiple-field coupled rock resistivity testing.Firstly,the reduce-friction pressure head was developed through material selection and preliminary testing.Based on the reduce-friction principles of rolling friction and dispersed sliding blocks,the reduce-friction pressure head was designed and the reduce-friction effect of dispersed sliding blocks was verified through numerical simulation.The advantages and disadvantages of different materials were analyzed,and the advantages of various materials were fully utilized to complete the machining of reduce-friction pressure heads.The reduce-friction pressure head was subjected to comparison tests with conventional reduce-friction methods,and the preliminary test results showed that the reduce-friction pressure head had a lower coefficient of friction and lower peak strength of the sample than the copper sheet.However,polytetrafluoroethylene has the lowest coefficient of friction,but cannot be used in resistivity simulation tests due to its own insulation.The reduce-friction pressure head should meet the requirements of multiple-field rock resistivity testing,so its performance was tested in single-axis and conventional triaxial resistivity tests.The reduce-friction pressure head was compared with other electrically conductive and reduce-friction layers,and the test results showed that the reduce-friction pressure head could achieve good reduce-friction effects in resistivity testing,with the strain on the upper,middle,and lower parts of the rock sample having small differences and the overall deformation being relatively uniform.In the single-axis compression test,under different reduce-friction conditions,the marble produced three failure modes after failure: X-shaped conjugate shear failure,shear failure,and tensile splitting failure,indicating that there is a limit to the friction coefficient,which changes the failure mode of the rock sample.In the conventional triaxial compression test,the sample failure mode was shear failure,with slight differences in the sample failure angle under different end conditions and confining pressure conditions.The resistivity showed a long-term decreasing and a rapid increasing trend before fragmentation under all reduce-friction methods.Under poor reduce-friction effects,the stress-strain curve of the rock sample dropped at the moment of failure,and the resistivity simultaneously increased suddenly.Under good reducefriction effects,the resistivity of the rock sample similarly increased significantly during the failure stage,but the turning point of the resistivity surge was earlier than that of the stress-strain turning point.Considering the end friction can clearly show the premonition of failure of the rock sample when it is about to fragment,the consideration of the end frictional effect in resistivity testing has a more beneficial effect on the accuracy of rock sample fragmentation and warning.Discrete element simulation was used to simulate the crack evolution and resistivity of different end friction under different end friction conditions,and to determine the friction coefficient of the loading plate and the failure morphology of the rock sample under good reducefriction effects.There were significant differences in the internal fissure evolution of the sample from loading to failure under different end friction coefficients.The low friction coefficient was not related to good reduce-friction effects,and the reduce-friction effect still needed to be judged based on the low dispersion of the overall stress-strain of the rock sample.Solving the end reducefriction problem requires finding a friction coefficient that matches the sample to achieve the best results.In addition,the similar principle of using hydro-electric anomalies simulation was used in the simulation to obtain the relationship between the rock sample resistivity and the end friction.The end friction mainly affects the resistivity during the rupture stage.Considering the effect of end friction on the study of resistivity,it has a better effect on early warning.There are signs of increasing resistivity before the sample is damaged.Considering the effect of end friction on the study of rock sample crack development and resistivity in the laboratory loading process is of great significance for early warning. |
