Shear Behavior Of Rock Joints Research Articles

Study on Mechanical Model for Postpeak Shear Behavior of Rock Joints Based on Degradation Characteristics of the 3D Morphology

The 3D morphology of the joint surface significantly influences the shear behavior of the jointrock. Constant normal load (CNL) direct shear tests with different shear displacement were conducted to understand the shear stress changing with joint roughness and damage degree during shear. The rough joint specimens were prepared using 3D scanning and printing techniques, and shear tests with different normal stresses and shear displacements were performed. Four different parameters and the damaged area quantitatively described by the image binarization and box dimension were calculated and compared to study the roughness evolution of joint surfaces. The experimental results demonstrated that the roughness parameter and shear stress decrease and approach constant values with increasing shear displacement. A JRC degradation model was presented based on regression analyses to evaluate the JRC values of rock joints under various displacements to replace it in the JRC–JCS model. Additionally, a new postshear behavior modeling was proposed for rock joints based on surface degradation characteristics under various initial joint roughness coefficients (JRC0) and normal stress. The stress–displacement curves resulting from the proposed modified model work well in predicting the postpeak stress–displacement curve, which can prove the effectiveness of the postpeak shear behavior modeling.

Can the splitting joint reproduce the characteristics of the natural joint in the lab? —A comparison study based on the roughness analysis and shear test

Splitting joints have been widely used to study the morphology characteristic and shear behavior of rock joints in the laboratory. There are some differences between the splitting joint and natural joint both in morphology characteristic and shear behavior. To quantitatively compare the differences, the present study used several splitting joints and natural joints to carry out the roughness analysis and direct shear test. Six profiles were extracted from each joint surface with a 30° interval to calculate the 2D roughness parameters (i.e., amplitude roughness parameter, average asperity inclination, root-mean-square of profile slope, and joint roughness coefficient). These parameters of the natural joints have a wide range and have obvious fluctuation. On the contrary, these parameters of the splitting red sandstone joints converge on a small range with tiny fluctuation, even calculated from different joint surfaces. The directional parameter (θ⁎max(C + 1)) was adopted to calculate the 3D roughness with a 5° interval. The directional parameter of the natural joints in range of 0° ∼ 360° forms a peanut, a prolate ellipse, or a gourd shape, presenting significant anisotropy. For the splitting joints, it forms a circle or a roughly circular shape, presenting insignificant anisotropy. The asperity heights of natural joints mainly present an irregular distribution, and they present an approximately normal distribution in few joints. For the splitting joints, the asperity heights generally obey a normal distribution. It indicates that there are huge differences between natural joint and splitting joint in morphology characteristic. To study the shear behavior, two types of joint were reconstructed for repeated test. The mechanical performance and failure characteristic of the splitting joint under different shear directions are similar. On the contrary, those of the natural joints under different shear directions are different, presenting obvious anisotropy. Based on the comparative analysis of the natural joints and the sandstone splitting joints in the present study, there is substantial difference between two types of joints both in morphology characteristic and shear behavior. The splitting joints cannot reproduce the characteristics of the natural joints well.

Shear Behavior and Asperity Damage of 3D Rough Joints under CNS Boundary Conditions Based on CZM Simulation

Although boundary conditions can significantly impact the shear behaviors and asperity damage evolution of jointed rocks, numerical studies on the damage of 3D rough rock joints under the constant normal stiffness (CNS) boundary condition have rarely been reported. In this work, the three-dimensional model of the irregular joint surface is established by using point cloud reconstruction technology. Based on the cohesive zone model (CZM), we simulate the shear behavior of three-dimensional rough rock joints under the CNS boundary condition, which is realized by using embedded spring elements implemented with a Python subroutine. We conducted laboratory direct shear tests under CNS boundary conditions. The agreement with the laboratory experimental results verifies the fidelity of the numerical method. Our results show that boundary conditions can significantly affect the shear behavior of rock joints, especially in the post-peak stage. Under the same initial normal stress, the peak shear stress and the number of microcracks in the asperities increase significantly with the increase of normal stiffness. The proportion of shear cracks positively correlates with the normal stiffness, indicating that the normal stiffness affects the joint failure mode. The damaged area and the volume of asperities increase with the increase of normal stiffness. Moreover, the distribution of shear-induced asperity loss becomes more nonuniform, and the loss of joint roughness increases rapidly and nonlinearly.

Effect of roughness on the shear behavior of rock joints subjected to impact loading

The shear behavior is regarded as the dominant property of rock joints and is dramatically affected by the joint surface roughness. To date, the effect of surface roughness on the shear behavior of rock joints under static or cyclic loading conditions has been extensively studied, but such effect under impact loading conditions keeps unclear. To address this issue, a series of impact shear tests was performed using a novel-designed dynamic experimental system combined with the digital image correlation (DIC) technique. The dynamic shear strength, deformability and failure mode of the jointed specimens with various joint roughness coefficients (JRC) are comprehensively analyzed. Results show that the shear strength and shear displacement characteristics of the rock joint under the impact loading keep consistent with those under static loading conditions. However, the temporal variations of shear stress, slip displacement and normal displacement under the impact loading conditions show obviously different behaviors. An elastic rebound of the slip displacement occurs during the impact shearing and its value increases with increasing joint roughness. Two identifiable stages (i.e. compression and dilation) are observed in the normal displacement curves for the rougher rock joints, whereas the joints with small roughness only manifest normal compression displacement. Besides, as the roughness increases, the maximum compression tends to decrease, while the maximum dilation gradually increases. Moreover, the microstructural analysis based on scanning electron microscope (SEM) suggests that the roughness significantly affects the characteristics of the shear fractured zone enclosing the joint surface.

Effect of roughness and pore pressure on shear behavior of rock joint under true triaxial loading conditions

This paper presents the direct shear test to investigate the effect of the joint roughness and pore pressure on the shear behavior of rock joints. Recently, we have developed an experimental method for the direct shear test using true triaxial apparatus that can control three mutually perpendicular loads and pore pressure independently. The morphological properties of the joints were quantified using a non-contact 3D laser scanner to investigate the change in surface roughness due to shear and the effect of pore pressure. In addition, the time variation of shear load, normal displacement, and flow rate on the joint during shear sliding were measured, and the relationship with the change of the discontinuous surface after the experiment was discussed.

Geometrical aspects in the consideration of actual opposed surface along shearing direction in rock joints

Shear behaviour of rock joints is strongly influenced by the roughness of its surface as the different failure mechanisms will be conditioned by the shape and distribution of the asperities and undulations in it. Currently, this surface can be digitalized using scanning methods directly in the field or in representative laboratory samples. After that, the surface is digitally reproduced using triangulation methods. This allows to analyse it using stablished parameters that do not depend on the arbitrariness of the technician. Despite the fact that 2D roughness parameters are still of great importance, these techniques are nowadays provoking a tendency to implement 3D measuring methods that allow to treat all the surface simultaneously and thus gathering valuable information of the rock joint as a whole. The vast majority of the 3D most powerful methods consider the area of the surface that is opposed against the shearing direction in order to determine the roughness, anisotropy and peak shear strength of the rock joint. However, not always the whole surface of each triangle that faces against shearing direction should be considered. In this work, a new methodology to consider the actual area of each triangular element that is involved in the shearing process is mathematically derived. Therefore, improving the accuracy of current methods, as well as the truthfulness of the mechanical characterization of rock joints.

Method for visualizing the shear process of rock joints using 3D laser scanning and 3D printing techniques

This study presents a visualized approach for tracking joint surface morphology. Three-dimensional laser scanning (3DLS) and 3D printing (3DP) techniques are adopted to record progressive failure during rock joint shearing. The 3DP resin is used to create transparent specimens to reproduce the surface morphology of a natural joint precisely. The freezing method is employed to enhance the mechanical properties of the 3DP specimens to reproduce the properties of hard rock more accurately. A video camera containing a charge-coupled device (CCD) camera is utilized to record the evolution of damaged area of joint surface during the direct shear test. The optimal shooting distance and shooting angle are recommended to be 800 mm and 40°, respectively. The images captured by the CCD camera are corrected to quantitatively describe the damaged area on the joint surface. Verification indicates that this method can accurately describe the total sheared areas at different shear stages. These findings may contribute to elucidating the shear behavior of rock joints.

Development of neuro-fuzzy models for predicting shear behavior of rock joints

The purpose of this article is to present predictive models of dilation and shear stress of rock discontinuities by applying the neuro-fuzzy technique, which uses a) the high capacity of artificial neural networks (ANN) to understand and to model complex multivariate phenomena, and b) the concepts of fuzzy sets theory to consider the variability of the input parameters in the proposed models’ responses. To develop the proposed models, experimental results were obtained from large-scale direct shear tests performed on different types of rock discontinuities and boundary conditions. The input variables of the proposed neuro-fuzzy models are the normal boundary stiffness, the ratio of fill thickness to asperity height, the initial normal stress, the joint roughness coefficient, the uniaxial compressive strength of the intact rock, the basic friction angle of the intact rock, the friction angle of the infill, and the shear displacement. The proposed models for dilation and shear stress provided results that fitted satisfactorily the experimental data, and the analyses of their performances indicated that they can represent the influence of the input variables on the shear behavior parameters of the rock discontinuities. The results from the neuro-fuzzy systems developed are also closer to the experimental data than those estimated by using traditional analytical methodologies existing in Rock Mechanics. This occurs because once considering the uncertainty of the input data, a more representative shear behavior prediction can be made by the neuro-fuzzy models.

A comparative evaluation of shear behaviour of rock joints under static and dynamic loading

A comparative evaluation of shear behaviour of rock joints under static and dynamic loading

Investigation on the Shear Behavior of Rough Rock Joints Prepared by Three-Dimensional Engraving Technique

Abstract Laboratory shear test is an important and widely used method to investigate the shear behavior of rock joints, and the preparation of joint specimens is a key factor that influences test results. In order to study the shear mechanism of rough rock joints with different morphologies in the laboratory, an innovative three-dimensional (3D) engraving technique was employed to produce specimens with rough joints. Intact sandstone and cement mortar blocks were used as model materials. The engraving parameters were determined to ensure a satisfactory engraving precision and to save as much time as possible for an engraving process. Three Barton profiles with joint roughness coefficient (JRC) values of 2.8, 10.8, and 18.7 were selected as the morphology prototypes for automatically engraving the model material blocks to obtain rough joint specimens. Shear tests were then conducted under four normal stress levels in the laboratory. In addition, numerical simulations on joints with the same physico-mechanical properties, surface morphology, and boundary conditions as the joint tested in the laboratory were performed. The failure characteristics, shear behavior curves, and shear strength obtained from numerical and experimental studies were analyzed considering the effect of joint materials, joint roughness, and applied normal stress. The shear mechanism of rough joints was investigated, and the analysis indicated that the results of shear tests by simulation and experiment were similar. The results revealed that it is feasible to make rough joints by engraving either real rocks or commonly used model materials (e.g., cement mortar).

Modeling the shearing behavior of discontinuous rock mass incorporating dilation of joint aperture

The shearing characteristics of rock joints govern the mechanical performance of discontinuous rock masses. The present study investigated the normal compression and shear behavior of rock joints through cyclic compression tests and direct shear tests. The irreversible relationship between the normal stress and the normal closure was confirmed by cyclic compression tests on rock joints. An elastoplastic model was proposed incorporating the aperture variation. A modified version of the critical state framework was developed for modeling the shearing and dilation behavior. Specifically, the initial state of the rock joints was determined according to the stress history. The proposed model for the mechanical behavior of rock joints was validated by predicting the experimental results. A parameter analysis was also performed to highlight the difference in the shear behavior of the rock joints due to the difference in the initial apertures.

Experimental investigation of the effect of thermal treatment on shear characteristics of healed rock joints

Understanding the temperature-dependent shear behavior of rock joints is significant for the design, construction and maintenance of deep rock engineering. However, the shear characteristics of healed rock joints under high temperatures are still poorly understood. In this paper, to capture the thermal effect on the shear behavior of healed rock joints, a series of direct shear tests are performed with the aid of X-ray diffraction and scanning electron microscope techniques. The testing results demonstrate that the shear resistance of the healed joints is slightly lower than that of the intact rock, but much greater than that of clean joints. Compared with intact rocks, the ductile failure characteristics of the healed joints after high temperature thermal treatment are more significant. The peak shear strength, shear stiffness and dilation angle of healed joints are greatly influenced by thermal treatment temperature, while the ultimate shear strength is almost independent on with temperatures. The influence of thermal treatment on shear resistance is attributed to the physical and chemical changes. The thermal decomposition of the filling minerals at around 400 °C is believed to be the main cause of the change in thermal-induced shear behavior of the healed joints. The findings of this work are useful to understand the mechanical behavior of jointed rock mass under high temperatures.

Investigating the Effects of Cement and Polymer Grouting on the Shear Behavior of Rock Joints.

This study carried out a comparison between cement grouting and chemical grouting, using epoxy and polyurethane, with respect to their effects on the shear behavior of joints. Joint replicas, with three different grades of surface roughness, were molded and grouted by means of cement and epoxy grouts of various mixtures. To investigate their shear behavior, samples were subjected to direct shear tests under constant normal load (CNL) condition. According to the results obtained, grouting improves the overall shear strength of the rock joints. All the grouted samples yielded higher maximum and residual shear strength in comparison with the non-grouted joint. Grouting resulted in an improvement in the cohesion of all the samples. However, a fall in friction angle by 5.26° in the sample with JRC of nine was observed, yet it was reduced by 2.36° and 3.26° for joints with JRC of 14 and 19, respectively. Cement grouts were found to have a more brittle behavior, whereas the chemical grouts were more ductile. Higher amounts of cement used in the grout mixture do not provide as much cohesion and only increase the brittleness of the grout. As a result of being more brittle, cement grout breaks into small pieces and joint planes are in better contact during shearing; consequently, there would be less of a fall in friction angle as opposed to epoxy grout whose ductile characteristic prevents grout chipping; therefore, joint planes are not in contact and a greater fall in the friction angle occurs. There was no noticeable change in the cohesion of the larger grouted joints. However, the friction angle of both natural and grouted joints increased in the larger joint. This can be related to the distribution of random peaks and valleys on the joint surface, which increases with the joint size.

Numerical investigations of joints with different surface roughness under shear and normal loads

A large number of discontinuities and weak planes exist in the natural system of rock masses. The presence of these discontinuities and weak planes reduces the strength of the rock masses. Thus, a thorough understanding of the shear behavior of jointed rocks is very important for many engineering projects. In this study, multistage normal loading (30, 60, 90, 180, and 360 kN) direct shear tests were conducted on a smooth planar joint using a large shear box device. Four numerical models with different joint asperities were built in parallel using Fast Lagrangian Analysis of Continua in three dimensions (FLAC3D), and the shear behavior was numerically analyzed. According to the experimental and numerical simulation results, joint roughness and normal force are the two most important factors that influence the shear behavior. The peak shear force increased as the normal load and joint asperity increased, but the normal displacement increased as the joint asperity increased and the normal load decreased. For the plane joint surface or joint surface with small asperity, the vertical displacement on the applied shear force side tended to go down, whereas it tended to go up on the opposite side. For the joint surface with large asperity, the vertical displacement on both sides went up. However, the value in the applied shear force side for the joint surface with large asperity was lower than that for the joint with smaller asperity. During shearing, the inclination of the upper box of the specimen is an important characteristic that causes the nonuniform distribution of normal stress, shear stress, and contact area of the interface. Furthermore, we developed a method for calculating the aperture size during shearing and discovered that the aperture size increased as the joint asperity increased. This research provides insight into the shear behavior of rock joints with various asperities.

A new peak shear strength criterion of three-dimensional rock joints considering the scale effects

The scale effect is one of the important factors affecting the shear behavior of rock joints. However, the nature of the scale dependency of rock joints is still unknown. In this study, to evaluate the effect of scale on the mechanical behavior of the joint, we selected three natural surface joints with different geometries. The sizes of these surfaces were greater than 2500 cm2. The replica joints were prepared from these three surfaces with dimensions of 50 × 50 mm2, 100 × 100 mm2, and 200 × 200 mm2. Three types of materials were used with uniaxial strengths of 12.5, 26, and 35.2 MPa. In total, 90 direct shear tests were carried out on joint samples. Finally, using the results of direct shear tests on joint specimens, a criterion was proposed for estimating the shear strength of natural joints which considered the scale effect. Compared to the previous similar criteria, in the present criterion, to quantify the roughness of the surfaces, the joint surfaces were considered three-dimensional and several roughness parameters (instead of one parameter) were used to capture both first- and second-order roughness of surfaces. The scale effect was also considered the changes in the joint dimensions (in some previous similar criteria, changes in scanning resolution of the surfaces have been considered the scale effect). Furthermore, since the surfaces of artificial joints were copied from the natural joint surfaces (instead of tensile joint surfaces), it can be expected the results of the present criterion would have a better estimate of the shear strength of natural joints.

Determination of Joint Surface Roughness Based on 3D Statistical Morphology Characteristic

Roughness significantly affects the shear behavior of rock joints, which are widely encountered in geotechnical engineering. Since the existing calculation methods on the joint roughness coefficient (JRC) fail to obtain a sufficiently accurate value of JRC, a new determination method was proposed in this study, where the 3D laser scanning technique and self‐compiled Python code, as well as the statistical parameter methods, were applied. Then, the shear strength of jointed rock was evaluated via Barton′s model, and therefore, a comprehensive comparison between the calculating results and experimental results was executed. Ultimately, the influencing factors of roughness profile extraction on the accuracy of JRC value, such as the measuring point interval, profile number, and measuring direction, were investigated. The results show that (1) equipped with the 3D laser scanning technique, the roughness profiles can be accurately extracted via the self‐compiled Python code, (2) an excellent consistency of shear strength could be observed between the calculating value and experimental results, verifying the validity and accuracy of the proposed method, and (3) a smaller measuring point interval can produce a more accurate digital profile and more accurate JRC value. To a certain extent, the more the sample numbers of profiles, the smaller the value of JRC.

Modeling shear behavior of rock joints: A focus on interaction of influencing parameters

One of the principal issues in the design of surface and underground structures on/in rock is the shear behavior of rock discontinuities that irreparable damages are caused when neglecting it. Several parameters are involved in the shear behavior of rock discontinuities which can interact with each other. The purpose of this study is to model the shear behavior of rock joints with regard to the individual impact of the influencing parameters and their interaction. To do so, a number of direct shear tests was performed where the values of independent parameters including the compressive strength (σc), normal stress (σn), modulus of elasticity (E), joint roughness coefficient (JRC) and basic friction angle (ϕb) were varied. Then, the response surface methodology (RSM) was used to incorporate the individual influence and interaction of the parameters into constitutive relationships for the peak shear strength, peak shear displacement, and dilation angle. For the shear strength, the most significant interaction was found to be between E and ϕb, for the peak shear displacement between JRC and E, and for the dilation angle between σc and E. The results show that the proposed model performs better in estimating the shear behavior characteristics than the model obtained from the nonlinear regression and the Barton-Bandis model. Therefore, the application of RSM is a winning approach for modeling the rock shear behavior which is capable of modeling the most complex interactions and provides a reliable performance.

Shear Strength of Rock Joints and Its Estimation

This study examined the shear behavior of rock joints and proposed an analytical model for estimating the shear strength at rock joints. Numerous experimental tests indicated that the shear behavior at rock joint depends on many factors including the distribution of initial joint asperity, asperity angle and strength, applied normal stress, and progressive degradation of asperity. Nevertheless, most pre-existing strength models do not consider these features on shearing at rock joints enough. In this study an improved shear strength model, which can consider the complex joint shearing features observed from many experimental tests, was developed and proposed. The improved model considers the joint features more reliably and overcomes the limitations of the pre-existing strength models. The proposed model was compared with some experimental test results along with some pre-existing strength models. The comparison clearly indicated that the improved model can estimate the shear strength at rock joints more reliably than the pre-existing models. From the study results it is expected that a better estimate of joint shear strength could be obtained in the future for various rock, joint, stress, and shearing conditions.

Analytical Modeling of the Shear Behavior of Rock Joints with Two-Order Asperity Dilation and Degradation

Abstract Stability evaluation of rock structures requires a reliable model to assess the shear behavior of rock joints. This study reports an analytical model to predict the complete shear process ...

Experimental Study of Shear Behavior of Rock Joints under Two Types of Boundary Conditions: Constant Normal Load and Constant Normal Stiffness

Shear destruction of rock mass along weakened planes is one of the main reasons of the failure of career boards, underground workings and rock slopes. The paper deals with laboratory testing of rock joints under direct shear loading. Special test and measurement complex was created to study deformation and strength characteristics of rock joints. Test specimens were made from single slab of sandstone of homogeneous structure. Shear tests have been conducted under constant normal load (CNL) and constant normal stiffness (CNS) boundary conditions at three values of normal compressive stress: 0.5 MPa, 1.2 MPa and 1.8 MPa. The analysis of test results allowed to reveal patterns of changes of deformation-strength properties of rock joints depending on the level of normal load, boundary conditions and roughness values. The ultimate shear stress is always higher under CNS boundary condition than CNL for the same values of normal compressive stress. Shear stress practically retains constant value after reaching the ultimate stress under CNL boundary conditions and it decreases under CNS conditions. The values of ultimate shear stresses, shear modulus and the length of linear section on “shear stress–shear displacement” curve increases with increasing surface roughness at the equal levels of normal compressive stress.