Shear Strength Of Rock Joints Research Articles

New 2D roughness parameters with geometric and physical meanings for rock joints and their correlation with joint roughness coefficient

Determining the joint roughness accurately will better serve the peak shear strength estimation models of rock joints used for stability assessment of rock masses. Considering the defects of the existing quantitative characterization parameters for two-dimensional (2D) joint roughness, especially the lack of explicit geometric and physical meaning, we proposed two new 2D roughness parameters, θ2D and h2D. The former, θ2D, represents the average inclination angle of all potential contact asperities over the entire joint profile, while the latter, h2D, characterizes their average undulation height. Both parameters are closely related to the shear strength of rock joints. Then, the roughness parameters θ2D and h2D of 102 rock joint profiles digitized at 0.5 mm sampling interval were calculated, and a new nonlinear regression equation for the determination of the 2D joint roughness coefficient (JRC) was established by combining the calculated results of the two roughness parameters. It was verified that the proposed equation could give accurate JRC estimation values of the 10 standard profiles of rock joints. Through the comparative analysis of the experimental data collected from earlier studies for the peak shear strength of 73 rock joint samples and corresponding estimated values, the equation was further verified to be applicable and accurate for estimating the JRC values of rock joints. Furthermore, we discussed the effects of shear direction and sampling interval on roughness and further provided another equation that could be applied to estimate the JRC values of joint profiles at the sampling interval of 1.0 mm.

A method to predict the peak shear strength of rock joints based on machine learning

A method to predict the peak shear strength of rock joints based on machine learning

Determination of the minimum number of specimens required for laboratory testing of the shear strength of rock joints

The shear strength of rock joints is critical in determining rock mass stability. However, the reliability of shear strength determined by testing only a few rock joint specimens is questionable due to the inherent heterogeneity of rock joints. This study aimed to investigate shear strength heterogeneity and determine the required minimum number of specimens (RMN) for obtaining reliable values. Specimens were extracted from various positions within a natural rock joint, and their roughness and shear strength variations were examined. A significant correlation between shear strength heterogeneity and joint roughness was found. Monte Carlo simulation was used to simulate engineering practice sampling procedures. The effectiveness of specimen numbers under different normal stresses was evaluated using relative error and corresponding probability. Results showed that the RMN depended on acceptable relative error, specified probability, and normal stress. For acceptable relative errors of 5% to 10% and corresponding probabilities of 85–95%, the RMN exhibits a decrease with an increase in normal stress, while it demonstrates an increase when considering acceptable relative error and probability requirements. A smaller RMN was needed under high normal stress for the same rock joint, demonstrating a decrease in shear strength heterogeneity with increasing normal stress. However, commonly used specimen numbers like 3 or 5 did not always align with the recommended RMN. The results also emphasize the significance of considering rock material properties when determining the RMN. The proposed method provides a quantitative and reliable tool for determining the RMN, considering the heterogeneity of rock joints.

The Influence of Different Shear Directions on the Shear Resistance Characteristics of Rock Joints

The joint roughness coefficient JRC of rock joints is an important parameter for measuring the geometric morphology of rock joints. However, the parameter obtained from traditional calculation methods has certain limitations in reflecting the differences in shear strength of rock joints in different shear directions with the same orientation. Firstly, native joint surface test blocks were cast using three-dimensional reverse reconstruction technology. Subsequently, direct shear tests were conducted under different normal stress and shear direction conditions, followed by numerical simulations using the finite difference software Flac3d. The JRC coefficient calculation method proposed by Yuan was modified and extended by considering the percentage of climbing sections and the effective contact area during the ramping process. This study indicates that the numerical simulation results are in good agreement with the shear test results. The introduction of the shear climbing rate SCR and the curvature coefficient of profile CCP is used to reflect the geometric differences of the joint surfaces in different shear directions. These two parameters are nonlinearly fitted with the experimental results, leading to a calculation formula that characterizes the shear strength characteristics of the joint surfaces in different shear directions. This novel formula is an extension of the JRC–JCS model.

Scale Effects on Shear Strength of Rough Rock Joints Caused by Normal Stress Conditions

Scale effects on the mechanical behavior of rock joints have been extensively studied in rocks and rock-like materials. However, limited attention has been paid to understanding scale effects on the shear strength of rock joints in relation to normal stress σn applied to rock samples under direct shear tests. In this research, a two-dimensional particle flow code (PFC2D) is adopted to build a synthetic sandstone rock model with a standard joint roughness coefficient (JRC) profile. The manufactured rock model, which is adjusted by the experiment data and tested by the empirical Barton’s shear strength criterion, is then used to research scale effects on the shear strength of rock joints caused by normal stresses. It is found that the failure type can be affected by JRC and σn. Therefore, a scale effect index (SEI) that is equal to JRC plus two times σn (MPa) is proposed to identify the types of shear failure. Overall, shearing off asperities is the main failure mechanism for rock samples with SEI > 14, which leads to negative scale effects. It is also found that the degree of scale effects on the shear strength of rock joints is more obvious at low normal stress conditions, where σn < 2 MPa.

A consecutive joint shear strength model considering the 3D roughness of real contact joint surface

A consecutive joint shear strength model for soft rock joints is proposed in this paper, which takes into account the degradation law of the actual contact three-dimensional (3D) roughness. The essence of the degradation of the maximum possible dilation angle is the degradation of the 3D average equivalent dip angle of the actual contact joint asperities. Firstly, models for calculating the maximum possible dilation angle at the initial and residual shear stress stages are proposed by analyzing the 3D joint morphology characteristics of the corresponding shear stages. Secondly, the variation law of the maximum possible dilation angle is quantified by studying the degradation law of the joint micro convex body. Based on the variation law of the maximum possible dilation angle, the maximum possible shear strength model is proposed. Furthermore, a method to calculate the shear stiffness degradation in the plastic stage is proposed. According to the maximum possible shear strength of rock joints, the shear stress-shear displacement prediction model of rock joints is obtained. The new model reveals that there is a close relationship between joint shear strength and actual contact joint roughness, and the degradation of shear strength after the peak is due to the degradation of actual contact joint roughness.

Integration of rock joint roughness into the Mohr-Coulomb shear behaviour model – application to dam safety analysis

Based on a previous experimental study that we conducted on granite joint replicas including thirty direct shear tests, this article proposes to integrate the joint roughness in the Mohr-Coulomb shear behaviour model (MC). The model integrates joint roughness into constitutive stress-displacement relationships describing mechanical behaviour of rock joint. The shear strength of rock joint is assessed by the MC shear criterion that takes into account the apparent cohesion. This MC model integrating rock joint roughness component is validated against other experimental results from the literature. It is able to accurately predict the peak shear strength of an unbounded rock joint with an average relative error of 7.9%. It is finally used to assess the role of joint roughness on the shear behaviour of a gravity dam foundation.

Experimental study on shear performance of saw-tooth rock joint with weak interlayer under different moisture contents and filling degrees

Rock joints are important factors affecting natural disasters such as landslides, and weak interlayer is the decisive factor for rock joint stability. Therefore, it is necessary to carry out shear test research on rock joints with weak interlayer. In this paper, the red sandstone was used as rock joint material, and consistent saw-tooth rock joints were produced by the 3D carving technology. According to the on-site composition analysis, the weak interlayer used in the test was obtained through remodeling. Weak interlayer with different moisture contents and filling degrees were produced by the Constant Dry Mass Method (CDMM). Shear strength test of rock joint with weak interlayer of different filling degree and moisture content under multi-level normal stress was carried out by the self-developed direct shear apparatus. The test results show that peak shear strength of rock joint with weak interlayer increases firstly and then decreases with the raise of moisture content. Peak shear strength of rock joint with weak interlayer decreases firstly and then tends to a stable value with the raise of filling degree. As the filling degree of weak interlayer changes from small (less than 1.0) to large (more than 1.0), dilatancy mode of saw-tooth rock joint changes from dilatation to contraction during shearing. Peak shear strength of rock joint with weak interlayer increases with the raise of normal stress.

Order of Intermittent Rock Fractured Surfaces

According to chaos theory, some underlying patterns can disclose the order of disordered systems. Here, it has been discussed that intermittency of rough rock fractured surfaces is an orderable disorder at intermediate length scales. However, this kind of disorder is more complicated than simple fractal or even multi-scaling behaviours. It is planned to deal with some multifractal spectra that systematically change as a function of the analysed domain. Accordingly, some parameters are introduced that can perfectly take into account such systematic behaviour and quantify the intermittency of the studied surfaces. This framework can be used to quantify and model the roughness of fractured surfaces as a prerequisite factor for the analysis of fluid flow in rock media as well as the shear strength of rock joints. Ultimately, the presented framework can be used for analysing the intermittency of time series and developing new models for predicting, for instance, seismic or flood events in a short time with higher accuracy.

Influence of Water Content on the Shear Strength of Rock Joints with Clay-Rich Fillings

Influence of Water Content on the Shear Strength of Rock Joints with Clay-Rich Fillings

3D morphology reconstruction of rock joints from 2D profile measurement by a profilograph

Rock mass stability is closely related to the shear strength of rock joints. Measuring the joint surface morphology is of vital importance for evaluating the shear strength of rock joints. Although non-contact measurement methods such as laser scanning are advantageous in high precision, the characteristics of high purchase cost or rental fees and being susceptible to the operating environment have weakened their practicability for in-situ batch measurement of the three-dimensional (3D) morphology of joints. To facilitate the possibility of utilizing cost-effective contact-based devices to replace the non-contact ones, we proposed a methodology to reconstruct the 3D morphology of rock joints using two-dimensional (2D) joint profiles measured by contact measurement methods (e.g., a profilograph). First, through a comprehensive comparison of commonly used spatial interpolation methods, the V4 method from the Griddata function is determined as the optimal one for the proposed methodology. Then, based on the optimal spatial interpolation method, the appropriate profile interval is determined through a referential band derived in this study. Finally, the performance of the proposed methodology was validated from experimental and theoretical perspectives. Results show that the relative errors of the test cases are within the engineering permissible error range, and more than 92% of the test cases are associated with an error less than 5%. In addition, the influences of the joint mechanical parameters, joint roughness level, and joint size on the appropriate profile interval are investigated. This methodology offers a cost-effective alternative for in-situ batch measurement of joint 3D morphology in the field survey of rock joints.

Representative sampling method for laboratory testing on shear strength of rock joints

The direct shear test is commonly used to determine the shear strength of rock joints. The measured shear strength, however, varies greatly with specimen positions due to the heterogeneity of rock joints. As a result, selecting specimens that represent the overall properties of rock joints is usually difficult. This paper aims to investigate the heterogeneity of rock joints and propose a new sampling method for selecting representative specimens. The roughness and shear strength variations of specimens taken from different positions of a natural rock joint were analyzed, and it was discovered that the heterogeneity of rock joint roughness is responsible for the heterogeneity of shear strength. The limitations of the traditional sampling method based on visual judgment were extensively investigated, revealing that shear strength parameters acquired by the traditional method contain large coefficient of variation (COV) values. To acquire trustworthy shear strength parameters, we proposed a representative sampling method based on the maximum likelihood estimation of the overall properties of rock joints. The number of determined representative specimens increases with the increase of normal stress. Representative specimens determined under low normal stresses can likewise exhibit the overall properties of the rock joint when subjected to high normal stresses. The Mohr-Coulomb and a nonlinear criterion were used to validate the derived representative specimens, demonstrating that the proposed method can produce reliable shear strength parameters and shear strength envelopes. Particularly, the determined representative specimens could derive shear strength parameters with relative errors less than 10% and COV values less than 0.1. The proposed method provides a quantitative and reliable tool for determining representative specimens to obtain reliable shear strength of rock joints.

Co-seismic landslide hazard assessment of Uttarakhand state (India) based on the modified Newmark model

Co-seismic landslides are the most dangerous geological hazards in seismically active mountainous regions. These landslides cause damage to roads, drainage pipelines, buildings, agriculture, and loss of human lives. It is crucial but difficult to precisely map the hazards resulting from earthquake-induced landslides. Newmark's approach is widely used to determine the permanent displacement of a probable sliding mass and simulate the mechanism of seismically-induced slope failure. The present study assesses co-seismic landslide hazards for Uttarakhand state (India), which falls under the highest seismically active zones as per the seismic code of India. It proposes an improved Newmark’s approach aby incorporating rock joint shear strength parameters and the size effect of possible sliding surfaces. Static factor of safety and critical acceleration maps were prepared for the study area by combining various geological, geotechnical, and topographical parameters. The ground motion parameters were determined in terms of surface-level Arias Intensity. Newmark permanent displacement map was prepared by taking critical acceleration and Arias Intensity as input parameters. Finally, the modified Newmark’s model results were compared with the conventional Newmark’s model using the area under curve (AUC) analysis. This assessment approach can be used effectively to forecast the area affected by co-seismic landslide hazards and prepare guidelines for major infrastructure projects and post-disaster damage studies.

Using PFC2D to simulate the shear behaviour of joints in hard crystalline rock

Significant uncertainties remain regarding the assessment of the peak shear strength of rock joints. In recent years, Particle Flow Code (PFC) has been used to simulate shear tests of rock joints. Although previous studies showed PFC’s capability to simulate rock joint shear behaviour, it is uncertain how different parameters in PFC should be combined to realistically capture roughness and strength of asperities in contact of actual rock joints. Under low normal stresses, the shear behaviour of well-mated hard crystalline joints is governed by the interaction between asperities of some tenths of a millimetre. This paper investigates the capability of PFC2D to realistically simulate the peak shear strength of hard crystalline rock joints under different constant normal stress magnitudes. The simulated two-dimensional profiles were selected from the digitised joint surface obtained with optical scanning measurements. To realistically capture surface roughness and asperity strength in PFC2D, different values of joint segment length, particle resolution per segment, and bond strength between particles were studied and calibrated while taking into account the laboratory observations. The results of the numerical simulations in the PFC2D environment show that the simulated peak shear strength using the profile containing the steepest asperity is in good agreement with that measured in the laboratory. The joint profile needs to be represented by both a magnitude of segment length that captures the grain size, and at least two particles per segment. The bond strength calibration needs to account for both asperity size and the number of particles in contact during shearing.

Exploring Effect of Microproperties on Shear Strength of Rock Joints through Physical and Numerical Modeling

Exploring Effect of Microproperties on Shear Strength of Rock Joints through Physical and Numerical Modeling

Peak Shear Strength of Natural, Unfilled Rock Joints in the Field Based on Data from Drill Cores – A Conceptual Study Based on Large Laboratory Shear Tests

Significant uncertainties remain regarding the field assessment of the peak shear strength of rock joints. These uncertainties mainly originate from the lack of a verified methodology that would permit prediction of rock joints’ peak shear strength accounting for their surface area, while using information available from smaller samples. This paper investigates a methodology that uses objective observations of the 3D roughness and joint aperture from drill cores to predict the peak shear strength of large natural, unfilled rock joints in the field. The presented methodology has been tested in the laboratory on two natural, unfilled rock joint samples of granite. The joint surface area of the tested samples was of approximately 500 × 300 mm. In this study, the drill cores utilised to predict the peak shear strength of the rock joint samples are simulated based on a subdivision of their digitised surfaces obtained through high-resolution laser scanning. The peak shear strength of the tested samples based on the digitised surfaces of the simulated drill cores is predicted by applying a peak shear strength criterion that accounts for 3D roughness, matedness, and specimen size. The results of the performed analysis and laboratory experiments show that data from the simulated drill cores contain the necessary information to predict the peak shear strength of the tested rock joint samples. The main benefit of this approach is that it may enable the prediction of the peak shear strength in the field under conditions of difficult access.

Shear behaviour of infilled rock joints under different boundary conditions

The use of commercial software for the analysis and design of rock slopes, underground structures, mining projects, foundations of infrastructure projects, piles and so on is becoming more commonplace. However, such software sometimes has limitations and cannot be used directly for determining the strength and deformation behaviour of rock joints when they are subjected to external loads due to construction. Correct predictions of the strength and deformation behaviour of rock joints are important for safe, economical and sustainable design. The strength and deformation behaviour of unfilled and infilled rock joints were numerically studied using the commercial software Universal Distinct Element Code (Udec) under different boundary conditions (constant normal stiffness (CNS) and constant normal load (CNL)). The numerical results were compared with experimental results of physical model tests. The strength predicted by the numerical model was in close agreement with the experimental results at low initial normal stress and for CNL boundary conditions, as asperity degradation during the shearing process was almost same throughout the test. However, the numerical model did not correctly predict the shear strength of rock joints under CNS boundary conditions and at high normal stress under both CNL and CNS conditions because Udec does not consider the effect of asperity degradation during shearing. The Udec code was thus modified for applicability under both CNL and CNS boundary conditions and at high initial normal stress at CNL boundary conditions. Comparison of the predicted and experimental results indicated most predictions to lie in the 95% confidence band.

Influence of material bridges on shear strength of unfilled intermittent rock joints

According to the well-known Jaeger's theory, the minimum possible rock mass shear strength as a discontinuum actually corresponds to the shear strength of rock joints. Since failures in rock masses due to loads caused by civil structures or civil works occur mainly by exceeding their shear strength, the shear strength of rock joints has huge practical significance. Therefore, in this paper it was decided to analyse a factor which can have a very important influence on the shear strength of unfilled rock joints. Namely, the influence of the presence of material bridges and initial joints between them, their number, size, mutual distance and orientation relative to the shearing direction were analyzed using results of laboratory or numerical CNS direct shear tests which were carried out by different researchers around the world. Except for increasing its peak and residual value, the professional public is currently unaware of the impact of this factor on the shear strength of intermittent rock joints. Based on the carried out analysis, appropriate conclusions have been made.

Intelligently Predict the Rock Joint Shear Strength Using the Support Vector Regression and Firefly Algorithm

AbstractTo propose an effective and reasonable excavation plan for rock joints to control the overall stability of the surrounding rock mass and predict and prevent engineering disasters, this study is aimed at predicting the rock joint shear strength using the combined algorithm by the support vector regression (SVR) and firefly algorithm (FA). The dataset of rock joint shear strength collected was employed as the output of the prediction, using the joint roughness coefficient (JRC), uniaxial compressive strength (σc), normal stress (σn), and basic friction angle (φb) as the input for the machine learning. Based on the database of rock joint shear strength, the training subset and test subset for machine learning processes are developed to realize the prediction and evaluation processes. The results showed that the FA algorithm can adjust the hyperparameters effectively and accurately, obtaining the optimized SVR model to complete the prediction of rock joint shear strength. For the testing results, the developed model was able to obtain values of 0.9825 and 0.2334 for the coefficient of determination and root-mean-square error, showing the good applicability of the SVR-FA model to establish the nonlinear relationship between the input variables and the rock joint shear strength. Results of the importance scores showed that σn is the most important factor that affects the rock joint shear strength while σc has the least significant effect. As a factor influencing the shear stiffness from the perspective of physical appearance, the change of the JRC value has a significant impact on the rock joint shear strength.

The importance of accounting for matedness when predicting the peak shear strength of rock joints

The contribution from both surface roughness and matedness in the peak shear strength of rock joints is not yet well understood. To be able to account for the influence of matedness on the peak shear strength of rock joints, both surface roughness and aperture need to be considered. Technical developments over the past few decades have shown that both surface roughness and aperture can be accurately measured using optical scanning. This technique has been utilized to account for surface roughness parameters in various shear strength criteria that assume a perfect match between joint surfaces. This paper investigates and compares the capabilities of two shear strength criteria to predict the peak shear strength of rock joints with different matedness. The analysis performed shows that both approaches have their strengths and limitations. For instance, accounting for the matedness of unmated rock joints based on their surface aperture gives better predictions of the peak shear strength. On the other hand, accounting for shearing failure mode becomes relevant at high normal loads. A possible way forward to reduce the limitations of these criteria could be to combine their strengths.