Research On The Peak Shear Strength Model Of Rock Joint Based On Fine Carving And Its Engineering Application

In the practice of water conservancy and hydropower engineering,it is often necessary to evaluate and deal with the potential hazards brought by the rock mass to the project,and the premise is to find out the spatial distribution and mechanical properties of the internal joint of the engineering rock mass.The shear strength of the joint affects the mechanical properties of the joint,which are an indispensable part of the research work.At the same time,the statistical characteristics of the shear strength parameters of the joint are uncertain due to the limited sample size,which further affects the stability evaluation of the engineering rock mass and the final result of the prevention and control engineering support design.Therefore,in addition to quantitatively assessing the roughness and peak shear strength of the joint to obtain effective shear strength parameters,the research of engineering rock mass joint also needs to measure the impact of the statistical uncertainty in shear strength paramters of joints on the mechanical response of the prevention and control engineering under limited samples and determine the optimal design method to improve the robustness for the prevention and treatment engineering resisting on variation in the statistical characteristics of the shear strength parameters of joints,which can provide the corresponding scientific basis and technical guarantee for the safety evaluation and support design of the prevention and control project.Therefore,the research on the peak shear strength model of rock mass joint and robust design for prevention and control engineering has great theoretical significance and application value.Based on the Key Project of National Natural Science Foundation and the National Natural Science Fund Project,the thesis carried out the research on carving technology of joints,the quantitative method for determining the joint roughness,the joint peak shear strength model and robust optimization design of slope considering the statistical uncertainty of shear strength parameters in joint,etc.Firstly,in view of the situation where repeated failure tests cannot be performed on the joint,according to the high-speed CNC carving related theory and technology,combined with the physical and mechanical characteristics of rock joints,the process flow of joint carving and the determination method of key design parameters are proposed to provide the basis and support for shear and seepage test of the joints.Secondly,analyzing the drawbacks of classical joint roughness index of root mean square of inclination angle,and improving the index algorithm based on local fitting method in point cloud technology.At the same time,considering the amplitude height of rock joints,a new roughness index was put forward,and the 112 profiles in literature was used to establish a new quantitative model between the joint roughness coefficient(JRC)and the new roughness index.In order to improve the applicability of the quantization model in practice,the model was further extended to three dimensions,and the peak shear strength model of joint was proposed based on JRC-JCS.In addition,based on the shear data of high-strength hard rock joint prepared by carving,the model of peak shear strength is modified.In view of the statistical uncertainty of random variables,a robustness index was proposed which can reflect the influence of the statistical variability of shear strength parameters on the the prevention and control engineering support design,and based on this,the robustness optimization design process was simplified.Taking the spillway high slope of Rumei Hydropower station as an engineering example,the size effect and anisotropy of the large size joint on the study area are studied.The roughness of the joint and peak shear strength parameters are evaluated by using the new roughness index and related quantitative model,and the stability of the excavated high slope is evaluated.Finally,considering the statistical variability of shear strength parameters,taking the sensitivity of variability and cost of anchoring slope as the optimization objectives,and the failure probability as the safety constraint,the robustness design results of anchoring slope are given.The main research contents,conclusions and main innovations are as follows:(1)The process flow of joint carving and the method to determine the key design parameters of tool path planning are put forward.According to the related theories and technology in the field of high speed carving,a complete set of carving production process planning are summarized,including the rock blanks selection and processing,the carving stage division,the choice of process benchmark,the type of cutting tool,the way of cutting path,path planning and process etc.combining with the characteristics and difficulties of joint carving;Based on the differential geometry theory,the key design parameters of the tool path were studied,the selection criteria of the cutter radius was determined,the determination method of the critical projection interval was given,the influence of design point interval on the key design parameters was studied,and the suggested parameters of the cutter radius and projection interval were given.Considering both accuracy and efficiency,it is suggested that the design point interval should be 0.4?1.0mm.(2)A quantitative evaluation parameter of the joint roughness which can differentially express the roughness characteristics of joints is proposed.First of all,starting from the definition of Z2,redrawing the integral interval,the height deritative replaced by micro line segment slope in the original discretization formula is improved to the one expressed as a point cloud tangent vector,thus the new algorithm of root mean square and the corresponding indicator–Z_n is proposed,while taking into account the anisotropy chracteristics,Z_n is to be further revised to Z'_n.The degree of fitting with JRC from Barton's ten standard profiles reflects the superiority of Z_n and Z'_n,in order to coordinate with the dimension of JRC,the dimensionless correction is carried out and ?_n⁺ is born.Secondly,in the shear process,the height characteristics of the convex body towards the shear direction also have significant contribution to the roughness characteristics.The point cloud height difference parameters H is modified,and only the point cloud height difference involved in the shear is taken into account,and an anisotropic root mean square roughness index H⁽¹⁰⁾is proposed.Finally,a new roughness index HD is established by combining the inclination angle index ?_n⁺ and amplitude height index H⁽¹⁰⁾.The new roughness index HD can characterize the morphological characteristics of roughness from the aspects of the amplitude height,inclination angle and shear direction of joint.(3)A quantitative model of roughness index between HD and JRC was established based on 112 joint data in literature.The adjust multiple correlation coefficient of the model is as high as 0.89,indicating that there is a high correlation between roughness index HD and JRC,which can be used to calculate the JRC of the joint.Compared with reverse JRC,the average relative error of JRC based on HD is 11.70%,while that based on Z2 calculation is 28.82%,and the JRC calculated by Z2 is generally smaller than true ones.This indicates that the contribution of rinclination angle,amplitude height and shear direction cannot be ignored when evaluating JRC.(4)Based on roughness index HD,the peak shear strength model of rock joint was proposed.Since HD is a roughness index for two-dimensional profiles,it is impossible to fully evaluate the contribution of the three-dimensional effect of the topography to the peak shear strength of the rock joints.Therefore,the local fitting method based on principal component analysis is proposed to realize the conversion of two-dimensional roughness index HD to three-dimensional one,and the calculation method of three-dimensional roughness index HD3D and three-dimensional JRC3D is obtained.Furthermore,based on the JRC-JCS model,a quantitative evaluation model for the peak shear strength of rock joints is proposed.Through comparison with the point cloud morphology data and direct shear test data of the sandstone joints in the literature,it is found that the relative error between the estimated value and the experimental value is evenly distributed on both sides of the zero value,the absolute relative error range is 1.09%?19.21%,and the average relative error is 6.54%.This shows that the strength model proposed in this thesis is effective in estimating the peak shear strength of sandstone joints.Considering the initial boundary conditions of normal stress of the dilatancy angle,a modified peak shear strength model based on JRC and JCS parameters was further proposed based on the direct shear test data of carved high-strength rigid gabbro based on the proposed carving technology.Compared with the laboratory direct shear test data,it is found that the peak shear strength of gabbro and sandstone is evaluated well with an absolute relative error range of 0.08%-36.37%and an average relative error of 9.83%.They are better than 1.09%-38.21%and 17%based on JRC-JCS model.(5)A measure index called sensitivity index of variability is proposed,which can consider the sensitivity of statistical uncertainty including shear strength parameter of joint to slope engineering failure probability.On the basis of the principle of reliability analysis,the standard deviation function(traditional robustness index)of the failure probability with respect to the standard deviation of the random variable variability is obtained according to the Taylor expansion,and further combined with the essence of the robust optimization design,a sensitive coefficient which can reflect the system failure probability with the statistical variability of shear strength parameters is proposed,and with the aid of the Euclidean norm.a comprehensive sensitivity index that uniformly reflects the system's comprehensive sensitivity to different rock and soil parameters—sensitivity index of variability is proposed.It is also used as a robustness index to further simplify the optimization design process of reliability-based robustness.With the help of the classic rock wedge slope excavation case,a robust optimization design framework based on sensitivity index of variability was presented considering the statistical uncertainty of the occurrence and of shear strength parameters of rock joint.(6)The stability analysis of the spillway high slope at the middle dam site of Rumei Hydropower station was completed.Firstly,according to the engineering geological profile of the spillway high slope and the field data,the overall failure mode of the high slope before and after excavation is analyzed.The results show that the slope does not have the sliding condition along the bottom slip surface composed of hard joint and soft joint before excavation,so the slope is in a stable state before excavation.After excavation,on the one hand,due to stress relaxation and unloading,and on the other hand,because the bottom slip surface penetrates to the slope surface at the front edge of the slope,the overall plane slip is very likely to occur.Secondly,the morphology of large-size joints collected on site is analyzed,and the roughness index HD3D proposed in this thesis is used to quantitatively evaluate the joint roughness coefficient JRC,and then the peak shear strength under different normal stresses is estimated according to the proposed peak shear strength model.According to the Mohr-Coulomb criterion,the shear strength parameters,cohesion and internal friction angle of the hard joints were obtained,which are relatively consistent with the field direct shear test data,which shows the effectiveness of the peak shear strength model proposed in this thesis;Finally,based on the obtained parameters,the stability of the high spillway slope was evaluated,and it was found that the safety factors of the slope after excavation under natural,heavy rain and seismic conditions were 1.11,0.92 and 0.94.It indicates that timely follow-up support measures are needed during slope excavation.(7)According to the robust design method,the anchorage support scheme of spillway high slope after excavation is given.Aiming at the design of anchoring support for the excavation slope,the sensitivity of failure probability of the anchoring slope to the statistical uncertainty of the shear strength parameters of hard and weak joints—sensitivity index of variability—is taken as a robustness index.At the same time,the robust and cost are taken as the two objective functions,the failure probability of the anchored slope is set as the safety constraint,and the easy-to-control indicators such as the diameter of the pre-stressed cable,the length of the anchoring end,the anchoring inclination,the vertical and horizontal spacing of the pre-stressed cables,etc.are used as the design parameters,multi-objective optimization designs for the robustness of anchored slopes under three different working conditions were carried out,and NSGA-II was used to identify the corresponding Pareto front and the target performance(robustness and economy)of the non-dominated solution,then the corresponding knee points were solved by the normal boundary intersection method.After comparison and analysis,the one obtained under seismic condition is finally used as the optimal design solution for the spillway anchored slope.The corresponding design parameters are 100tons of pre-stressed cables,the length of anchorage end is 8m,and the vertical and horizontal row spacing is 3.5m×3m,and the anchor cable is arranged downwardly by10°.In addition,compared with the initial deisn proposal,the robust optimal scheme of the anchorage support of the high slope at the outlet of the No.1 Tailwater tunnel in Wudongde in compared with the initial design one were analyzed.Under natural conditions,the cost of the single width section was 1.53×10⁴ yuan less than that of the initial design,but rainstorm rain conditions,it was only 4000 yuan higher than that of the initial design.The safety and robustness of the robust design scheme are optimal than that of the initial design in both natural and rainstorm conditions,indicating that the robust optimization design has great advantages.