Study On Mechanical Properties And Permeability Mechanism Of Sand-Like Mudstone Assemblages Under Seepage Stress Coupling

In recent years,the stability and safety issues of deep geological exploitation and underground engineering have been a research topic actively promoted in China Composite rock is a common and special type of rock mass in deep underground engineering,but it is quite different from single rocks.Composite rock masses are not only influenced by factors such as layer thickness ratio,layer planes,and structural plane dip angles,but also by geophysical fields such as stress field and seepage field Therefore,research on the mechanical properties and permeation mechanism of composite rock masses is essential.The main research methods and results are as follows:(1)To accurately represent the composition of sand-mudstone blend rocks,we conducted similar model tests using ordinary Portland cement,quartz sand,and silica powder.We prepared two types of rocks with varying mechanical properties(similar to sandstone and mudstone to determine the physical and mechanical characteristics of each type.Using these results as a reference,we carried out multiple matching experiments to identify the most suitable ratios.After careful analysis,we identified two mix ratios that best resemble sandstone-like and mudstone-like rocks.(2)Five groups of composite rocks with different layer thickness ratios(0,0.25,0.5,0.75,1)were made using the two optimal mixing ratios mentioned above.In order to study the damage mechanical characteristics of the composite rock composed of sandstone and mudstone,conventional uniaxial and triaxial compression tests were carried out.In the uniaxial test,acoustic emission technology was combined to synchronously record the acoustic emission data and the whole process of crack initiation and propagation in rock samples.The strength and deformation characteristics of the composite rock under conventional uniaxial and triaxial tests were analyzed.The results show that the compressive strength of the rock sample first decreases and then increases.When the layer thickness ratio is 0.25,the uniaxial compressive strength of the rock sample is the lowest.When the layer thickness ratio is 1,the compressive strength of the rock sample is the highest.The elastic modulus follows the same trend as compressive strength.(3)Performed permeability experiments on combination rocks of sandy rock and mudstone with different thickness ratios,different confining pressures(10MPa,15 MPa,20MPa,25 MPa,30MPa),and different pore pressures(2MPa,3MPa,4MPa,6MPa).The results show that when the confining pressure and pore pressure are consistent,the permeability is negatively correlated with the thickness ratio,and the proportion of sandy rock is the dominant factor affecting the permeability of the combination rocks.When the thickness ratio and pore pressure are consistent,the permeability of the combination rocks decreases with the increase of confining pressure.(4)Based on the principle of effective stress and the law of mass conservation,a coupling equation for seepage and stress in a sand-mudstone composite rock under elastic conditions was established by introducing the elastic modulus of the composite rock.This equation fully characterizes the influence of the layer thickness ratio on the stress and seepage fields of the composite rock.During the loading process,damage occurs in the rock.The influence of the damage on the stress and seepage fields of the composite rock is characterized by the elastic modulus after damage and the modified permeability coefficient.A coupled seepage-stress model for sand-mudstone composite rock considering different layer thickness ratios and damage is proposed.(5)The established coupled equations of seepage and stress for the composite rock of sand-mudstone were input to the coupling software COMSOL Multiphysics for numerical simulation to verify the reliability of the experimental results.By analyzing the influence law of the ratio of layer thickness on gas pressure and permeability,it was revealed that the ratio of layer thickness is an important factor affecting the mechanical properties and permeation mechanism of the composite rock of sand-mudstone.The experimental results were consistent with the simulation results,providing theoretical support for deep stratum development and safe construction of underground engineering.