| In the field of rock mechanics and engineering geology,the existence of rock joints often weakens the shear strength and stability of rock mass and has an important impact on engineering safety.Reasonable prediction on shear strength and deformation of rock joints is important for safety of rock structures,such as slope stability,tunnel excavation,durability of dam foundations,and design of rock-socketed piles.It is also helpful to a better understanding of the triggering mechanism of earthquakes that mainly occur between faults or plate boundaries due to the stick-slip frictional instability.In this paper,the shear test of rock joints and model study are taken as the research content,which involves the study of the roughness parameters of rock joints,the study of joint peak shear strength,the study of joint shear stiffness,and the study of joint shear deformation.The three-dimensional measurement test of granite joint surface was carried out;the direct shear tests under different stress conditions on the same morphology surface and different contact conditions were performed.Considering the relationship between the geometrical features of joint surface and the shear strength,a new three-dimensional roughness parameter system is proposed.Based on the new roughness parameter system,the sampling interval effect,size effect and anisotropy effect of joint surface roughness were analyzed.Based on the experimental results and the new roughness parameters,the peak shear strength model of the rock joints was established.On the basis of Hertz contact theory and GW model,a model considering the wear of asperities is proposed and the shear stiffness formula of the friction surface considering the change of the curvature radius of the asperity is obtained.Based on the experimental and theoretical analysis,the peak shear displacement model of rock joints was established.Based on the study of roughness characteristics of joint surface and the influence of roughness parameters on shear strength,this paper systematically carries out experimental research,model construction and theoretical analysis,aiming at establishing peak shear strength model,shear stiffness model and shear deformation model of rock joints.The main research contents and achievements are as follows:(1)New roughness parametersThe Brazilian splitting test was used to obtain the splitting joint surface,and the high precision point clouds were obtained by three-dimensional scanning technology.The joint surface is equivalent to a serious of rectangular asperities,and the effect of different geometric parameters on the shear strength is studied.The concept of the equivalent height difference is proposed based on the difference in the contribution of the two failure modes of the rectangular asperities to the shear strength,which can reflect that different geometric parameters of the asperities have different contribution to shear strength.On this basis,a roughness parameter AHD is proposed.Based on the fractal theory,the fractal dimension DAHD with vector character is proposed.Combining two roughness parameters AHD0 and DAHD,a roughness parameter system describing the shear strength of the rock joint is proposed.(2)Anisotropy,size and sampling interval effects of joint surface roughnessTo estimate the influence factors on the roughness of natural rock joints,based on the new roughness parameter AHD which can reflect that different geometric parameters of the asperities have different contribution to the shear strength,the variation rules of roughness under different shear directions,sizes and sampling intervals are analyzed by using the 3D laser scanning technology.In order to quantitatively characterize the roughness anisotropy of joint surface morphology,the anisotropy parameter AAHD is proposed based on the roughness parameter AHD in all shear directions,and it is more reasonable and comprehensive than the anisotropy parameter Ka proposed by Belem.The sampling interval effect of rough anisotropy and the size effect of rough anisotropy are further studied.The research shows that the joint surface roughness depends on shear directions,sampling intervals and the research sizes;the roughness size effect depends on the shear direction,but tends to be stable with the increase of the research size;the roughness anisotropy also depends on the sampling intervals and size.(3)Shear test of rock jointsWith the help of 3D printing technology and 3D laser scanning technology,cement mortar joint samples with certain surface morphology are prepared.Shear test of 20 sets of matching joint samples were performed under a constant normal load and a uniform shear displacement.The shear displacement-stress curves are obtained.The influence factors of peak shear strength,peak displacement and shear stiffness of test joints are studied.It is found that the wear distribution is approximately the same as that of the blue area in the equivalent height difference map,and the larger the blue area is,the more serious the joint wears.These coincidences show that the equivalent height difference distribution plays an important role in joint shear resistance,and the roughness parameter based on which is reasonable.Direct shear tests of 8 sets of cavity-containing joint samples with the normal stress of 2 MPa were performed.The results show that the shear stress-displacement curve of the cavity-containing joint samples is similar to that of the matching joints,and the shear strength of the cavity-containing joints decreases with the increase of the cavity percentage.(4)The new peak shear strength modelBased on the shear test results and theoretical analysis of the rock joints,the influencing factors of the peak shear strength are discussed,and the mechanism of these factors affecting the peak shear strength are analyzed.It is verified that the new proposed roughness parameters have an obvious influence on the peak shear strength.A function describing the variation of peak dilatation angle with normal stress is proposed.Combining the new roughness parameters with peak dilatation angle,a peak shear strength model is proposed.The peak shear strength model of cavity-containing joints is obtained by considering the reduction of the strength is due to the existence of cavity ratio on the basis of peak shear strength of matching rock joints.The obtained peak shear strength model for cavity-containing joints only reflects the empirical relationship between the cavity percentage and peak shear strength,it does not reveal how the cavity affects the peak shear strength of the rock joints.In this paper,it is considered that the cavity percentage affects the characteristics of the surface roughness,namely,the equivalent roughness of the generalized contact joint surface.And then the shear strength of cavity-containing joints can be estimated with the new proposed peak shear strength model using the equivalent roughness.(5)The shear deformation of rock jointsOn the basis of the Hertz contact theory and GW model,the shear stiffness formula of the friction surface is derived when the asperity curvature is constant.However,rock joints are prone to wear when shearing,and the curvature radius of the asperities will increase as the degree of wear increases.The critical normal force for a single asperity is obtained,which shows that the normal force corresponds to the curvature radius of asperity.A model considering the wear of asperities is proposed.In this model,with the increase of the normal force,the higher-order asperity gradually breaks.Damaged rock masses are sheared and pushed to both sides of the asperity and the impact of the damaged rock masses on the remaining asperity is ignored.The remaining asperity is smoother than the previous one,and the curvature radius becomes larger.Furthermore,the shear stiffness formula of the friction surface considering the change of curvature radius of the asperity is obtained.The calculation results of the model can well reflect the test results:for the given joint surface,with the increase of normal force,the joint surface gradually becomes smooth;for different joint surfaces,with the increase of roughness,the joint surface is easier to be smoothed.Taking into account the main factors of normal stress and joint roughness,an empirical formula of peak displacement suitable for natural rock joints is proposed based on experimental results and regression analysis.The estimation of the peak shear displacement model corresponds well with the test results. |
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