Research On Roughness And Peak Shear Strength For Rock Discontinuities Based On BAP

Rock mass is a kind of geological body that has certain structure. Inside the rock mass, discontinuities with direction, size and shape are developed, such as bedding planes, joints, faults, fractures, unconformable interfaces and so on. The existence of discontinuities makes rock mass different from any other materials, the physical and mechanical properties of rock mass are characterized with discontinuity, anisotropy, and non-homogeneity, also the integrity and overall strength of rock mass are reduced. The failure of rock mass usually does not occur through the part of intact material, but the weak structural plane within it. It can be said that the stability of rock mass is determined by the discontinuities, especially by the control ones. Therefore, research on the shear strength of the discontinuities is the key to evaluate the rock mass stability. One of the most important factors that have effect on the shear strength of the discontinuities is the roughness, and it has obvious connection with shear strength of the discontinuities. The rougher the discontinuity is, the greater the shear strength is. The study carried on peak shear strength estimation model based on the roughness has always been an important research topic. The reliability of estimation model much depends on the reasonable index used to represent the roughness of discontinuities. Unfortunately, the following problems exist in the current study of roughness of discontinuities,(1) The acquisition equipment is too backward to meet the quick evaluation of rock mass stability. To the rapid failure process of some rock mass, for example, high-speed long-distance landslide and some other important geological disaster emergency projects, time is valuable to save lives. This situation means the researchers have to react quickly to evaluate the stability of rock mass and predict the scope of the disaster and put forward rational treatment scheme. But due to time-consuming and low-accuracy of equipment, the advantages of the evaluation model cannot be shown.(2) The results of roughness evaluation depend much on the sampling direction, sampling size, and sampling interval. Lack of systematic study on the factors that has effects on roughness is also a research interest awaiting settlement.(3) The most of exiting evaluation index of roughness were proposed only based on the geometric information of the discontinuities, the mechanical properties was ignored. However, the roughness of discontinuities connects closely to the stress condition and stress direction.The objective of this paper is to carry systematic analysis of influence of sampling parameters, including sampling direction, sampling size and sampling intervals, on the roughness evaluation based on advanced sampling methods, to propose a more appropriate approach to describe the roughness of discontinuities, then to build a corresponding estimation model for peak shear strength of discontinuities base on the roughness index above mentioned.Taking Jiweishan landslide in Wulong, Chongqing as example, several research works is conducted as follows:(1)The rapid acquisition of the roughness data for natural rock discontinuities. LS05-1discontinuity is the limestone bedding plane that scattered in the accumulation area and the occurrence is not in-situ. ILRIS-36D3D laser scanner is applied to obtain the roughness data of the field size discontinuities, this kind of data is mainly used to evaluate the effect of sampling parameters on the roughness, to propose new representation method of roughness, to establish new estimation model of the peak shear strength based on BAP and so on. The preliminary point cloud date with sampling size of4.7×2.5m and sampling interval of0.8m has more than18,000,000points in total. On the other hand, all JW02-1BL, JW02-2BL, JW03-1BL, JW03-2BL, and JW03-3BR discontinuities belong to limestone joints collected on the sliding plane of landslide. The VZ-4003D laser scanner is employed for the rapid acquisition of topography information for small scale discontinuities. This kind of data is applied in the building of new estimation model for peak shear strength and corroboration of numerical simulation results. The sampling size and interval of original points cloud is0.181×0.362m and1mm respectively. There are about65,500points for each discontinuity. It is necessary and, also important, to conduct preprocessing for original point cloud data, including parsing, reduction, and denoising. Comprehensively combining the factors of computer calculation, accuracy and time together, the sampling size of LS05-1discontinuity is reduced to4.70×1.14m, the sampling interval is increased to0.01mm, after preprocessing near54,000points concerning to the roughness of discontinuity are left. In the meantime, as for JW02-1BL, JW02-2BL, JW03-1BL, JW03-2BL, and JW03-3BR discontinuities, their average sampling size are decreased to be0.06×0.06m, and the sampling interval keep constant with value of0.001m, the point cloud for each discontinuity has about3600points under this circumstance. Based on Matlab software, the code used to rebuild the topography of discontinuities surfer is completed.(2) Investment of the effect from sampling parameters on the roughness of discontinuities. Based on the pretreated point cloud data of the LS05-1discontinuity and supplemented date generated by Weierstrass-Mandelbrot fractal random functions, the influence of sampling direction, sample size and sampling interval on roughness is assessed in2D and3D level respectively. The analysis process obeys to the single variable principle with the other two factors remain fixed. For the anisotropy of roughness, from0°to330°, with a15°interval,24profiles are contracted from the geometry model of discontinuities, the fractal dimension D is computed to describe the roughness of each profiles, and for the3D joint surface, the improved stereographic projection pole diagram method is applied to calculate the quantity of normal vectors NV used to capture the anisotropy with respect to rock joint surface roughness; For the scale effect of roughness,9profiles with different length and9surface with different area was collected from pretreated point cloud data, the fractal dimension D for each profiles and surfaces is computed based on box counting method; For the interval effect of roughness, with a0.01m interval,100different intervals are selected from0.01m to1.00m, calculating the fractal dimension D for all cases. Practical solutions are put forward for each sampling parameters.(3) Based on Bright Area Percentage(BAP), a new representation method of rock joint roughness is proposed. Starting from the shear failure mechanism of rock discontinuities with no filling and low normal stress, the tiny plane facing the shear direction are taken as the object of study, a new index used to characterize the roughness is provided. The simulative light source program is written on the basis of Matlab, and definition and computing method for BAP are given according to the image segmentation technique, and value issue of the crucial parameters, gray threshold TL and light incidence angle β in the light source simulating process, is discussed to choose optimal one. Through describing the anisotropy, scale effect and the interval effect of the joint roughness, the applicability of the BAP is verified.(4) Establishment of new estimation model for the peak shear strength based on the BAP. Upon the platform of Matlab, the XYZ23DEC program is written to generate the three-dimensional complex discrete element model in short time. Through comparing with the results from direct shear testing, the physical and mechanical parameters for rocks and joints with regard to numerical simulation are adjusted and verified; on the basis of abovementioned parameters, the corroboration referring to reliability of the numerical model is carried out via describing the anisotropy, the scale effect and the sensibility on the normal stress of shear strength; by setting a reasonable sampling size and sampling interval, taking joint roughness index(BAP), anisotropy (shear direction a), rock strength(JCS), basic friction angle(φb) and normal stress (σ) into accounted,60direct shear simulation tests are conducted to provide the desired date for regression analysis. Finally, the relationship between BAP and the peak shear strength τp of discontinuities is established, and a estimation model (equation) is put forward to evaluate peak shear strength based on BAP.(5) Application of the new estimation model for the peak shearing strength to slope stability analysis. The new model is applied to the stability evaluation of Jiweishan landslide in Wulong, Chongqing. Combined with the new index BAP, normal stress σ, basic friction angle (φb and joint surface strength JCS, the range of shearing strength parameters such as internal friction φ and cohesion C for sliding plane are evaluated based on new estimation model of peak shear strength. By selecting the combination of the minimum, the maximum, and average as case study, failure evolutionary process of the sliding block is reappeared based on3DEC, and deformation characteristics of sliding block are also analyzed.Based on the abovementioned research content and techniques, the conclusions can be reached as followings:(1) With the3D laser scanning technology, the joint roughness data can be collected for a short time. The scanning time is directly related to the sampling interval and the sampling scope. The smaller the sampling interval is, the larger the sampling size is, and the greater scanning time will be. Additionally, the scanning time is supposed to be influenced by non-technical factors, for example improper operation, power supplement, operating environment, etc.; The original point cloud data need to be preprocessed, preprocessing of point cloud data is a try and error procedure, the improvement need to made on the basis of the feedback, and the time spent on the preprocessing depends greatly on the professional quality of the staff, and the more skilled the operator is, the shorter the work time will be cost.(2) Evaluation of the effect of sampling direction on the rock joint roughness shows:the roughness in different sampling directions is quite distinguished, and the most direct and effective way to solve the sampling direction error is to ensure that the sampling direction should be consistent with the interested direction in the data collecting process. The interested directions include sliding, shear, and seepage direction relating to rack mass. These interested directions should be identified in the geological investigating stage, for example, the surface scratches and fault steps is the clue to find the sliding and shear direction. Evaluation of the effect of sampling size on the rock joint roughness shows:sampling size has a significant impact on the roughness of rock joints, and the roughness will decrease with the increasing of sample size. When the size increases to the effective sampling size (3.5m for profile and3.5×0.84m for surface), the roughness is basically stable and no longer declines. The roughness size effect can be effectively eliminated by increasing the sampling size, but restricted by the limited range of measuring instruments, the result reveals that scale effect on roughness can be figured out by increasing the number of measurements, rather than enlarging the range of measurements aforesaid, and stronger practical meaning of previous approach is demonstrated at moment. Evaluation of the effect of sampling intervals on the rock joint roughness shows:discontinuities with different sampling intervals have distinguished roughness. Generally, with the sampling intervals decreased, more detailed information about the topography of joint surface is acquired, and the corresponding roughness increase. When the sampling interval decreases to be effective interval, the roughness of rock joint will trend to stable. On the other hand, when the sample interval is too large, a lot of geometric data of joint surface will be ignored, which makes the shape of rock discontinuity no longer have the fractal characteristics. The effective sampling interval has a positive correlation with the sampling size, and in order to both getting the real joint roughness and eliminating test period and cost, it is recommended that the ratio between sampling interval and sample size should be less than0.05. In this condition, the impact on the roughness from the sampling interval can be effectively reduced.(3) In the shear failure process of the discontinuities with no filling and low normal stress, the tiny planes facing the direction is likely to primarily occur shear failure, these planes play a great role in shear resistance. Therefore the new joint roughness representative method should be focused on the these kind of tiny planes; the selection of gray threshold TL is the key, which should be combined with the failure mechanics mechanism of the discontinuities, to describe the roughness appropriately, and it should follow the specific conditions principle to select; The simulated light incidence angle β has a significant impact on the calculated result of BAP, and the BAP will increase with the increasing of incidence angle β, the best recommended angle β is located in the range of from35°to70°; In specific cases, the selection of TL and β should mutually coordinate and then make a reasonable combination; Based on the BAP method, the anisotropy, size effect and interval effect of rock discontinuities can be well reflected, this method has a unique advantage since both of geometric and mechanical information are considered in the roughness evaluation.(4) The numerical simulation results which is combined with direct shear tests in laboratory show:the numerical experiments can reflect the anisotropy, the size effect and sensitivity to normal stress of the shear strength very well; the value of the gray threshold TL which is used in the image segmentation, is affected by the normal stress σ. Taking the LS05-1rock joint as a studying case, with the light incidence angle β=35°, a variety of reasonable gray threshold TL corresponded with the normal stress σ are found as follows,When the normal stress σ=0.5MPa, the gray threshold TL=160;When the normal stress σ=1.OMPa, the gray threshold TL=155;When the normal stress σ=1.5MPa, the gray threshold TL=150;When the normal stress σ=2.OMPa, the gray threshold TL=145;When the normal stress σ=2.5MPa, the gray threshold TL=140.Based on large quantities of numerical simulation data and multivariate statistical analysis, the new estimation model of the rock joint peak shear strength is built,(5) The recommended values of9pairs of cohesion C and friction angle φ are estimated on the basis of27cases, and compared with the data form published literature, the values have the similar statistical regularity with other scholars. The minimum combination of cohesion C and friction angle φ is φ=3.977°and C=0.443MPa, and the maximum combination of C and φ is φ=38.080°, C=1.917MPa, and average combination of cohesion C and friction angle φ is φ=17.162°, C=1.094MPa. Minimum one is relatively in line with the actual situation. According to case with minimum combination, the displacement variation law of the slide mass failure in the early stage is simulated, and the result shows that the sliding block has different deformation law at different part. The leading and trailing edges of the sliding block have large displacement, the leading edge moves along the strike of T2fracture, and the trailing edge slides along the crossing direction of T2fracture and sliding surface, the failure firstly occurs at leading edge of the sliding block due to the existence of the free space. At the same location of the sliding block, the displacement magnitude of the upper one is larger than the lower one, and the upper block will happen failure firstly. The displacement magnitude in the outer side (east side) is also bigger than the inner side, and in the trailing edge the difference is more manifest. As the movement is not synchronized for the outer and inner block, rotational motion is taken place during the failure process. Compared with field geological investigation, the rationality of the numerical simulation result is verified and it is proved that the new estimation model for peak shear strength has made a successful application on the stability analysis for slope.The main innovation points of this paper are:(1) The new representation method named BAP for discontinuities roughness is proposed. This method is a3D evaluation method, the anisotropy, size effect and interval effect of the roughness can be considered using this method, and what is more, the shear failure mechanism of the rock mass can be captured with it. Based on point cloud data and code made in Matlab software, BAP method make a great improvement on roughness evaluation accuracy and period(2) Based on BAP, the new estimation model for the peak shear strength is established. This mode is put forward on the basis of reasonable sample size and sampling interval, taking the normal stress σ, the basic friction angle φb, the surface strength parameter JCS, the roughness BAP and shear direction into account. The structure of model is relatively simple but with great mechanical meaning. The statistical parameters involved in the model are matched out on the basis of plenty of numerical experiments, which are relatively reliable. Seeing from the engineering application, the model also has a good application, and can succeed in estimating rational shear failure mechanical parameters of discontinuities.