Experimental And Modeling Study On Shear Flow Features Of Structure Plane

Fractured rock masses which with complicated mechanical and hydraulic properties,is commonly encountered during the construction of rock engineering activities such as dam foundation,nuclear waste disposal,ore minerals,oil and natural gas production,deep tunnels,etc.Fractured rock masses are comprised of rock matrix and numerous structural planes.In general,structural planes dominate the stability and seepage behavior of these rock engineering activities.Therefore,understanding the mechanical properties and fluid flow behavior of a single rough-walled fracture is fundamental and of great importance to ensure performance and safety of these engineering projects.In this study,the unfilled roughness structure plane was studied by combining the methods of laboratory tests,theory analysis and numerical modeling.In order to study the shear flow features of rough-walled structure palnes systematically,the follow works have been carried out in this research.Firstly,joint roughness has been studied using three dimensional morphology scanning systems.Secondly,direct shear tests have been carried out under different normal stresses.Thirdly,Shear flow tests have been carried out on six rock fractures.Fourthly,the nonlinear flow model for calculating the nonlinear flow behavior in rock fracture during shear has been derived.At last,numerical simulation fluid flow through rock fracture has been conducted on five fractures with different dislocation.The main research contents and results are as follows:(1)The roughness of five types of rock fracture has been measured using three dimensional morphology scanning systems.The anisotropic characteristics of joint roughness have been studied using three dimensional morphology parameter ?max*(C +1)and Joint Roughness Coefficient(JRC).The results show that the three dimensional morphology parameter ?max*/(C+1)can well capture the anisotropic characteristics of rock joint roughness.However,Joint Roughness Coefficient is insufficient to explain the anisotropic characteristics of forward and backward directions.The polar diagram of three dimensional morphology parameter is an ellipsoid.The degree of anisotropic has been analyzed on split joints and natural joints.It is show that the anisotropic phenomenon is more evident for natural rock joints than split joints.There is no direct relationship between degree of anisotropic and joint roughness coefficients.A power law relationship has been established between JRC and three dimensional morphology parameter ?max*(C+1)by regression analyzing on the experimental data.(2)Direct shear tests have been conducted on 20 group split joints with normal stress ranged between 0.325 MPa and 8.0 MPa.The relation between three dimensional morphology and peak shear strength of rock joints has been studied.The results show that shear resistance of rock joint is provided by the override and shear failure of asperities.It was found that the peak shear strength not only related to applied normal stress but also 3D morphology characteristics.A new peak shear strength criterion has been proposed based on quantified three dimensional joint roughness parameters.A comparison between the new model,the Grasselli's model,and the Xia's model is conducted from the perspective of both the rationality of the formula and the prediction accuracy.It seems that the proposed new model has some advantages in predicting the peak shear strength of rock joints.(3)A simple and effective sealing technique for coupled shear flow system is reported.Shear flow tests have been conducted on six group granitic rock fractures.The shear flow tests results show that the correlation between pressure gradient and volume flowrate is demonstrated to be nonlinear.The Darcy law is likely to overestimate the conductivity of rock fracture.The linear coefficient a and the nonlinear coefficient b in Forchheimer's law is quite sensitive to the shear deformation,especially,at the stage of 0-4mm shear displacement.As the shear displacement is increased from 0 to 10mm,the coefficients a and b experience about 1-2 and 1-3 orders of magnitude reduction,respectively.The critical Reynolds number varies as a function of the shear deformation.With the increase of shear displacement,the critical Reynolds number presents an increasing trend.A possible explanation of this result is that the influence of the roughness on flow behavior decreasing with the increase of shear displacement.For all the cases in this study,the critical Reynolds number is ranged between 1.5 and 13.0.(4)A Forchheimer-based flow model for describing fluid flow through rock fracture during shear has been established.The linear and nonlinear coefficients in Forchheimer equation were formulated using mechanical aperture and fracture roughness coefficients.The dilation curve was divided into three phases and analytical expressions for modeling the dilation curve were established by incorporating the joint roughness coefficient(JRC)and mobilized roughness coefficient(JRCmob).Shear flow tests were conducted on marble and granite fractures with normal stress ranged between 0.5 and 3.0MPa.The obtained experimental data are used to verify the proposed model.The results show that the proposed model can well predict the flow in rock fracture.(5)The five geometry models of fracture flow with different dislocation have been build.Numerical simulation fluid flow through rock fracture has been conducted on five fractures.The results show that with the increasing of shear displacement,the aperture distribution becomes more and more heterogeneous and the flow paths become more and more tortuous.The flow results clearly show channeling flow along the preferential paths.Loss of inertial force is the essential cause of non-linearity flow in rock fractures.It is found that the inertial force losses may induced by the following two reasons.As the aperture distribution is homogeneous,the high velocity may induce nonlinear flow in rock fracture.As the velocity is low,the tortuosity of flow paths may induce nonlinear flow in rock fracture.