Study On Evolution Law And Mechanism Of Seepage Characteristics Of Single Fractured Sandstone Under Water-rock Interaction

In this paper,the sandstone in the water-level fluctuation zone of the bank slope of Wu dongde Hydropower Station is taken as the research object.By combining the laboratory test and numerical simulation methods,the seepage process of fractured rock mass in the reservoir bank slope during the process of reservoir water level rise and fall is simulated.The influence mechanism of water-rock interaction on seepage characteristics of fractured rock mass is discussed.The main research contents and results are as follows:(1)Carry out the dry-wet circulating water-rock interaction test and analyze the changes of ion concentration of soaking solution and the micro-structure and morphology characteristics of fractured surface in different test periods,so as to reveal the change mechanism of seepage characteristics of fractured rock mass.The results show that with the increase of water-rock interaction period,the ion concentration in the immersion solution,the microstructure and morphological characteristics of the fracture surface change with the rule of "first fast and then slow".The main reason is that the physical and chemical reactions in the water-rock interaction soften and dissolve the rock minerals,erode the constituent minerals and crystal structure of the rock,and form a porous honeycomb structure and a new flow channel.Additionally,water-rock interaction also destroyed the coordination of fracture surface morphology,resulting in a complex and diverse morphology with increased numbers and heights of convex and concave fracture surfaces,as well as increased fluctuation,discreteness,and overall roughness of morphology.(2)The seepage flow rate in different test periods was obtained through the single fracture seepage test and its change rule was analyzed.The results show that when the seepage water pressure is constant,the seepage flow,the equivalent hydraulic gap width and the permeability coefficient decrease exponentially with the increase of confining pressure.When the confining pressure is constant,the seepage flow increases linearly with the increase of the osmotic water pressure,but the equivalent hydraulic gap width and permeability coefficient generally fluctuate around a constant value.Under different confining pressures,the seepage characteristics vary slightly with the water-rock interaction period,but the maximum reduction is in the first period.When the seepage flow rate is small,the seepage flow rate fluctuates with the increase of the roughness coefficient JRC,which is not a monotonic trend.Under the action of low confining pressure,the roughness of fracture surface JRC has a great influence on seepage flow,but with the increase of confining pressure,the roughness coefficient JRC obviously reduces the resistance to water flow in fractures.(3)The process of single fracture seepage under water rock interaction is the transition process of the fracture surface from the nail bed model to the cave protrusion model.After rock sample splitting,many mineral particles with different sizes are produced on the new surface to support each other,and there is a certain gap width.After the water rock interaction,the mineral particles on the surface of the sample soften and dissolve,and the microstructure and morphology characteristics of the crack surface change.Under confining pressure,the deformation of the crack surface increases and the strength decreases.The diameter of the cave between fracture surfaces decreases and the contact area of the bulge increases,and the seepage path extends and the resistance of water flow increases,which eventually leads to a significant decrease in the seepage characteristics in the first period.(4)Based on Monte Carlo method,the fracture network model is established and the size effect and anisotropic characteristics of fractured rock mass are analyzed.The results shows that the permeability coefficient of fractured rock mass has obvious size effect and anisotropic characteristics.After water-rock interaction,the unit size of permeability coefficient of fractured rock mass increases,the radius of permeability ellipse increases,and the main permeability coefficient,permeability tensor and main permeability direction decrease.