Coupling Characteristic Of Radial Flow And Shear Within A Single Quasi-sandstone Fracture

The seepage and shear coupling characteristics of rock mass fractures are issues of great concern to geological engineering such as water conservancy,mining and petroleum engineering,nuclear waste storage engineering,etc.,and are also research hotspots in the fields of rock hydraulics and hydrological engineering geology.Single fractures are basic units of the rock mass fracture network,which determines basic permeability characteristics of groundwater in the rock stratum,and radial seepage is one of the main forms of groundwater seepage,so it is of great significance to study characteristics of the radial seepage and shear coupling of single fracture.In this paper,single quasi-sandstone fractures are taken as the research object,and the method of combining laboratory test,theoretical analysis and numerical simulation is used to study the shear characteristics of regular tooth structural plane,the radial seepage and shear characteristics of single fracture,the nonlinear seepage law of non matched fracture and the radial seepage model of rotating dislocation fracture.The main research contents and achievements are as follows:(1)Quasi-sandstone samples with high strength gypsum as similar materials were prepared.Based on the Three Similarity Theorems and taking high-strength gypsum as the research object,quasi-sandstone materials meeting various physical and mechanical parameters of sandstone were prepared,and a radial seepage and shear coupling test was carried out.The results show that the quasi-sandstone structural plane prepared in this paper meets the requirements of radial seepage and shear coupling test.(2)The peak shear strength model in the form of dilatancy angle product and the dilatancy constitutive model considering piecewise function under normal pressure were established.The shear tests of smooth structural plane,rough matched structural plane and rough non matched structural structural plane were carried out under 5 normal pressures.The mechanical responses of various structural planes under different normal pressures in the shear process were analyzed.The results show that the shear capacity of quasi-sandstone structural planes increases with normal pressure,and the rough quasi-sandstone structural planes form weak interlayers after shear failure.The dilatancy curve of matched structural plane is divided into three sections,i.e.initial compression section,macroscopic dilatancy section and post-peak compression section.(3)The influence law of different normal pressures on grain size of the fracture fillings in the shear process was revealed,and a seepage model considering grain size of the fillings and normal pressure was proposed.Radial seepage and shear tests with different normal pressures,different shear velocities and different combinations of fracture surfaces were carried out against quasi-sandstone fractures,and the influence of different conditions on failure state of fracture surfaces and changes to gap width were analyzed.The results show that gap width and flow rate decrease with the increase of normal pressure and increase with the generation of fillings,while the grain size of fillings decreases with the increase of normal pressure.(4)The influence mechanism of the cavity shape of the inlet on nonlinear characteristics of the radial seepage was expounded.A radial seepage and shear coupling test was carried out on one set of matched fracture surfaces and two sets of non matched fracture surfaces,and a numerical simulation of two sets of non matched fracture surfaces was carried out with the COMSOL Multiphysics software.The influence of relative contact rate and shear displacement on fracture seepage quantity was analyzed,and the nonlinear seepage law of non matched fracture was discussed.The results show that the opening distribution inside the non-meshed fracture cavity is the main reason for the flow rate,and the shape of the inlet section has a great influence on flow pattern of the radial seepage.In addition,the flow area is the main factor that affects the nonlinear coefficient b of the Forchheimer equation for radial seepage when the fracture opening and the gap width distribution are unchanged.(5)A radial seepage model considering the shape of fracture cavity with rotational dislocation was established.Radial seepage tests under fixed normal pressure were carried out on 8 groups of fractures under rotating dislocation conditions.Through analysis of the seepage results,it is shown that the influence of the shape of fracture cavity with rotational dislocation on the test results can not be ignored,and the flow pattern shows obvious anisotropy.On this basis,the tortuosity of structural plane was redefined.According to model of the equivalent trench flow,the expression of correction coefficient ? was derived,and the modified cubic law of radial seepage was obtained.The comparative analysis shows that the calculations of cubic law of the radial seepage established in this paper are closer to the measured values.