Jointed Rock Research Articles

Numerical study on compressive mechanical characteristics of filled jointed rock under confining pressure based on PFC

Confining pressure is an important factor affecting the strength and deformation characteristics of rock mass, it is of great significance to study the mechanical and deformation characteristics of jointed rock mass under confining pressure for the construction of deep underground engineering and the prevention of geological disasters. In order to study the mechanical and deformation characteristics of filled jointed rock under confining pressure, based on the laboratory experiment results of static uniaxial compression of filled jointed rock samples, the Particle Flow Code is used to conduct the numerical simulation. The strength characteristics, failure characteristics and micro-cracks development characteristics of filled jointed rock under different confining pressure levels, different joint inclination angles and different sample sizes are analyzed. The results show that the peak stress and peak strain increase with the increase of confining pressure level, and there is a strong linear relationship between peak stress and confining pressure level. The peak stress and initiation stress decrease first and then increase with the increase of joint inclination angle. With the increase of confining pressure level, the change law of initiation stress of filled jointed rock under different joint inclination angles is different. The confining pressure will prolong the development process of micro-cracks in filled jointed rock, which will make the distribution of micro-cracks more dispersed and the total number of micro-cracks increase. The failure mode changes from splitting failure to shear failure with the increase of confining pressure level. The change of joint inclination angle will seriously affect the failure mode and micro-cracks development characteristics of filled jointed rock.

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.

Drone Photogrammetry for Accurate and Efficient Rock Joint Roughness Assessment on Steep and Inaccessible Slopes

The roughness of rock joints exerts a substantial influence on the mechanical behavior of rock masses. In order to identify potential failure mechanisms and to design effective protection measures, the accurate measurement of joint roughness is essential. Traditional methods, such as contact profilometry, laser scanning, and close-range photogrammetry, encounter difficulties when assessing steep and inaccessible slopes, thus hindering the safety and precision of data collection. This study aims to assess the feasibility of utilizing drone photogrammetry to quantify the roughness of rock joints on steep and inaccessible slopes. Field experiments were conducted, and the results were compared to those of 3D laser scanning in order to validate the approach’s procedural details, applicability, and measurement accuracy. Under a 3 m image capture distance using drone photogrammetry, the root mean square error of the multiscale model-to-model cloud comparison (M3C2) distance and the average roughness measurement error were less than 0.5 mm and 10%, respectively. The results demonstrate the feasibility and potential of drone photogrammetry for joint roughness measurement challenges, providing a useful tool for practitioners and researchers pursuing innovative solutions for assessing rock joint roughness on precipitous and hazardous slopes.

Evolution Characteristics of Plastic Zone in Jointed Rock Mass of High-Temperature Hydraulic Tunnel

The evolution characteristics and the extent of the plastic zone in rock mass can reflect the failure characteristics and destruction degree of a hydraulic tunnel. In this research, we derived the equation of a plastic zone range based on a gateway-like structure high-temperature tunnel through a theoretical analysis. On-site monitoring and discrete element model simulation were combined to analyze the temperature field law and plastic zone evolution characteristics of the jointed rock mass at high temperatures. The results show that the joints affect the temperature field variation in rock mass, and the vertical and horizontal joint groups pose significantly greater influence than the inclined joint group on temperature field. The sensitivity of the joint internal friction angle and thermal expansion coefficient to the range of the plastic zone is relatively small. Under various joint spacings, the influence of horizontal and inclined joint groups on the plastic zone morphology decreases, while the vertical joint group exhibits an incremental influence on the plastic zone morphology. Similar to the influence of the temperature field, the vertical and horizontal joint groups have a significantly greater influence on the plastic zone range than the inclined joint group. Under various rock mass temperatures, the plastic zone in rock mass results in the occurrence of uneven expansions along the direction of the joint dip angle, which changes the potential failure direction of rock mass and increases the potential destruction degree of rock mass, whereas it exhibits a smaller uniform expansion perpendicular to the joint dip angle, and the boundary of the plastic zone coincides with the joint surface, relatively hindering the potential destruction degree of rock mass. The research results of this study have certain reference values for the stability control of jointed rock mass in high-temperature hydraulic tunnels.

Impact of loading rate on the mechanical behavior of jointed rock

The impact of loading rate is a vital issue in the study of the time-dependent behavior of rock masses. A sample containing a single inclined joint is represented by the particle flow code and used as an analog to examine the rate-dependent behavior of jointed rock. A series of numerical triaxial compression tests are completed at various loading rates on specimens containing this single joint inclined at 30°, 45°, and 60° with respect to the orientation of the confining stress of 5 MPa. Observations are recovered for the evolution of stress–strain, deformation and energy release together with resulting failure mode. Rate sensitivities of four parameters defining a smooth joint model (normal stiffness, shear stiffness, stiffness ratio and friction coefficient) are used to represent observed response. We find that competition between mechanical damping and inertial force results in the strain rate effect and that the peak strength of the rock specimens increases with increasing loading rate. The step-wise form of both the stress–strain and kinetic energy can be used as indicators of the onset of dynamic failure. These numerical experiments are consistent with observations from laboratory experiments on identical samples where rate effects in the jointed rock are accentuated over those apparent in intact rocks. Change in the loading rate has a significant effect on the failure mode for specific joint angles relative to the confining stress. The joint friction coefficient is the main rate dependent factor controlling behavior and is an important factor in defining the significance of loading rate effects.

Shear behaviours and roughness degeneration based on a quantified rock joint surface description

The asperity wear of rock joints significantly affects their shear behaviour. This study discusses the wear damage of the asperities on the joint surface, highlighting the roughness degradation characteristics during the shear process. The direct shear experiment of artificial specimens containing rock joints was conducted under different normal stresses based on three-dimensional scanning technology. These experimental results showed the contribution of joint wear to roughness degeneration, such as the height, zone, and volume of asperity degeneration. The wear coefficient of the rock joint was obtained based on the volume wear of asperities in the laboratory experiment. The functional relationship between the friction coefficient and wear coefficient is subsequently determined. To quantitatively analyse the wear damage of a joint surface, a calculation method for determining the wear depth of the rock joint after shearing was proposed based on wear theory. The relationship between the ultimate dilation and wear depth was analysed. A coefficient m, which can describe the damage degree of the joint surface, and a prediction method of joint surface roughness after shearing are established. Good agreement between analytical predictions and measured values demonstrates the capability of the developed model. Lastly, the sensitivity factors on the wear depth are explored.

Investigation of long-wavelength elastic wave propagation through wet bentonite-filled rock joints

The saturation of the compacted bentonite buffer in the deep geological repository can cause bentonite swelling, intrusion into rock fractures, and erosion. Inevitably, erosion and subsequent bentonite mass loss due to groundwater inflow can aggravate the overall integrity of the engineered barrier system. Therefore, the coupled hydro-mechanical interaction between the buffer and rock during groundwater inflow and bentonite intrusion should be evaluated to guarantee the long-term safety of deep geological disposal. This study investigated the effect of bentonite erosion and intrusion on the elastic wave propagation characteristics in jointed rocks using a quasi-static resonant column test. Jointed rock specimens with different joint conditions (i.e. joint surface saturation and bentonite filling) were prepared using granite rock discs sampled from the Korea Underground Research Tunnel (KURT) and Gyeongju bentonite. The long-wavelength longitudinal and shear wave velocities were measured under different normal stress levels. A Hertzian-type power model was used to fit the wave velocities, and the relationship between the two fitted parameters provided the trend of joint conditions. Numerical simulations using three-dimensional distinct element code (3DEC) were conducted to better understand how the long-wavelength wave propagates through wet bentonite-filled rock joints.

A Comprehensive Method for Similarity Evaluation in Discrete Fracture Network Modeling of Jointed Rock Masses

A Comprehensive Method for Similarity Evaluation in Discrete Fracture Network Modeling of Jointed Rock Masses

Three-Dimensional Discrete Element Analysis of Jointed Rock Slope Stability Based on the Universal Elliptical Disc Model

Three-Dimensional Discrete Element Analysis of Jointed Rock Slope Stability Based on the Universal Elliptical Disc Model

Failure characteristics of rocks with non-persistent joints under local load.

Jointed rocks under local load are ubiquitous in civil engineering. The instability and failure of jointed rocks are fatal to engineering safety. This paper numerically investigated the effects of loading area and joint angle on the strength dividing points, energy evolution, and crack distribution characteristics of non-persistent jointed rocks. The results demonstrated that the closer the absolute value of joint angle to 45° and the smaller the loading area, the lower the strength dividing points of rocks. The curves of rock joint angle versus total energy at peak and of elastic energy versus amplitude of post-peak abrupt energy change render a W-shape distribution. Meanwhile, compared with joint angle, loading area has more influence on rock energy input. The larger the loading area, the higher the crack fractal dimension, the crack entropy, and the penetration rate. Tensile cracks outnumber shear cracks when jointed rocks are damaged, and shear cracks increases significantly at the post-peak stage.

Numerical simulation study on shear resistance of anchorage joints considering tensile–shear fracture criterion of 2G-NPR bolt

2G-NPR bolt (the 2nd generation Negative Poisson’s Ratio bolt) is a new type of bolt with high strength, high toughness and no yield platform. It has significant effects on improving the shear strength of jointed rock mass and controlling the stability of surrounding rock. To achieve an accurate simulation of bolted joint shear tests, we have studied a numerical simulation method that takes into account the 2G-NPR bolt's tensile–shear fracture criterion. Firstly, the indoor experimental study on the tensile–shear mechanical properties of 2G-NPR bolt is carried out to explore its mechanical properties under different tensile–shear angles, and the fracture criterion of 2G-NPR bolt considering the tensile–shear angle is established. Then, a three-dimensional numerical simulation method considering the tensile–shear mechanical constitutive and fracture criterion of 2G-NPR bolt, the elastoplastic mechanical behavior of surrounding rock and the damage and deterioration of grouting body is proposed. The feasibility and accuracy of the method are verified by comparing with the indoor shear test results of 2G-NPR bolt anchorage joints. Finally, based on the numerical simulation results, the deformation and stress of the bolt, the distribution of the plastic zone of the rock mass, the stress distribution and the damage of the grouting body are analyzed in detail. The research results can provide a good reference value for the practical engineering application and shear mechanical performance analysis of 2G-NPR bolt.

The Mechanism of Rockbolts in Jointed Rock under Uniaxial Tension

In this research, the rockbolt mechanism of a jointed rock mass under uniaxial tension was systematically revealed with a laboratory test and a numerical simulation. It was found that the rockbolt rock mass experienced five stages under uniaxial tension, the densification stage, elastic stage, plastic deformation stage, progressive debonding stage, and complete debonding stage. The stress-strain curve and ultimate tensile strength of a rockbolt rock mass were analyzed by taking the rockbolt spacing and rockbolt angle as variables. It was found that the improvement effect of the reduction of the rockbolt spacing on the ultimate tensile strength was limited. When the rockbolt spacing was reduced to a certain limit, the stress concentration area between adjacent rockbolts was connected and destroyed, resulting in the increase of the rockbolt rock strength becoming smaller, and even having a downward trend. The increase of the rockbolt angle led to the change of the stress mode and failure mode of the whole structure, and the ultimate tensile strength first increased and then decreased. The optimal rockbolt angle was between 60° and 70°. It is worth noting that there was an obvious mechanical occlusion between the thread on the rockbolt surface and the rock mass, resulting in the multi-stage step-down characteristic of the stress-strain curve in the complete debonding stage. The results of this study can provide a reference for the design and construction of similar projects.

System reliability analysis of joint rock slopes with the stepped sliding surface considering hydraulic and seismic actions

System reliability analysis of joint rock slopes with the stepped sliding surface considering hydraulic and seismic actions

Experimental and Numerical Study of Joint Persistence Effect on the Non-persistent Jointed Rock Mass’ Tension Failure Behavior

Experimental and Numerical Study of Joint Persistence Effect on the Non-persistent Jointed Rock Mass’ Tension Failure Behavior

Planar Slope Failure in Heavily Jointed Rock: Tension Cracks and Nonlinear Strength

This work proposes the calculation of the factor of safety for a strongly jointed rock mass in the case of plane failure with a tensile crack whose exact position or depth is not known but is expected to exist. This calculation is performed by applying the non-linear failure criteria of the Focus Procedure of Úcar and Hoek–Brown’s and implementing the necessary formulae in a spreadsheet. The aim is to provide a simple, cost-effective, and easy-to-use procedure that is useful in the early stages of a project or as a starting point for more detailed investigations. Besides slope geometry and strength parameters, the required parameters are the RMR of the rock mass or its mi, depending on the criterion used. The proposed procedure allows for the estimation of the factor of safety, the position and depth of the tensile crack, and the inclination of the failure plane in the most unfavorable case, with reasonable accuracy, using an iterative process with the conventional tools available in common spreadsheet programs. An example is provided in which an accuracy of 86–96% for the factor of safety is obtained.

A New 3D Statistical Parameter for Determining Roughness of Joint Surfaces Considering Shear Direction and Asperity Features

The engineering rock mass is a multi-fracture body crisscrossed by geologic structure planes, and its safety is affected by the rock joints' mechanical properties and morphological features. The joint roughness coefficient (JRC) is a vital characterization method to describe joint surface roughness. Its precise quantization is a crucial factor in predicting shear strength. However, JRC in recent research did not reflect the directional effect of the joint morphology. A new 3D roughness parameter WAD (weighted apparent dip) was proposed to estimate JRC based on the apparent dip angle of the contact unit. The proposed parameter comprehensively reflects the contribution of shear direction and asperity features to the roughness. Based on the ten standard roughness profiles, we investigated the directionality of the WAD and studied the influence of the sampling interval (Sai) on WAD and their correlation with JRC. And the practicability of WAD in evaluating JRC was well verified by the comparison between the indoor test and calculated results with different roughness and shear directions, and the results showed that WAD could accurately estimate the shear strength of the joint rock.

Dynamic splitting response and mechanical effect of joint angle in sandstone containing various weak-filling joints

Weak-filling joints substantially degrade the integrity and load-bearing capacity of rock masses, thus increasing the complexity and unpredictability of their mechanical response behavior. One of the main mechanical properties of jointed rocks is their splitting performance, which heavily influences their failure mode. To study the effects of joint angles on splitting mechanical properties, we conducted dynamic splitting tests on sandstone samples with seven different inclination angles of weak-filling joints. These tests were conducted using the Brazilian disc method and a modified split Hopkinson bar apparatus. The splitting mechanical response, energy rate characteristics, joint angle damage factors, failure modes, micromorphology characteristics, and the mechanical effects of joint angles were systematically analyzed. The findings suggested that varying joint angles led to different initial damage levels in the rock mass, which can be described by a power exponential function. The observed relationship between peak load and joint angle was corroborated by integrating a damage parameter into the theoretical derivation of the mechanical effect of the joint angle. Weak-filling joints reduced the brittle failure characteristics and peak loads of the samples but enhanced their ability to deform. Furthermore, the energy reflection rate exhibited an inverse relationship with the joint angle, which is contrary to the trends observed in energy transmission and absorption rates. Additionally, both macroscopic and microscopic morphology analyses confirmed that the joint angle primarily determined the fracture trajectory of the samples, causing a gradual shift in the failure modes of jointed samples from shear to tensile failure.

Investigation of Rock Joint and Fracture Influence on Delayed Blasting Performance

Geological structures such as joints and faults in rock mass have a significant influence on open-pit mining. Hence, it is critical to develop an understanding of dynamic joint behavior under blasting loading. This, in turn, can provide both theoretical and practical guidance to improve blasting rock fragmentation and associated bucket excavating efficiency. In this paper, delayed blasting on a highwall bench at an open-pit mine is used as an example; a nonlinear joint blasting model is also constructed. By simplifying the blasting wave propagation velocity and combining the relevant stress and displacement theories of type I and II cracks, equipotential diagrams of the stress and displacement field with the vibration velocity of the particle are obtained. Additionally, ANSYS is used to analyze the distribution of the stress field. This is able to be visualized by the degree of color change post-processing. It is concluded that, with the attenuation of the detonation wave energy, the stress exhibited a decreasing trend in this process. According to the distribution of the peak effective stress, it is found that the peak value first increases to 10–12 MPa and then shows a downward trend.

Experimental investigation and strength model of rough ice-filled joints under tensile and shear loading

Warming softening of ice-filled joints has caused many rockfall disasters in cold regions. A series tensile and shear strength tests were conducted to investigate the thawing softening mechanism of ice-filled joints by considering the effect of the joint opening, temperature and roughness. The joint opening has little influence on the tensile strength of ice-filled joints, but the temperature is a remarkable influencing factor. When the temperature increases from −15 °C to −0.5 °C, the average tensile strength of ice-filled joint reduces from 0.64 MPa to 0.22 MPa, which displays a constant logarithmic decay function against the freezing temperature and it is similar to the change rule of pure ice. The shear strength of the ice-filled joint decreases with increasing the infill thickness, and tends to be the shear strength of pure ice beyond the critical infilling thickness. In addition, all the tensile failures are inside the joint ice, however, the shear failure is mainly caused by the debonding of ice-rock interface when the temperature increases to −0.5 °C. It implies that the tensile bonding strength of ice-rock interface also is larger than that of the solid ice itself even at a high freezing temperature. Based on the experimental results, a new shear strength model of the ice-filled joint is proposed for Barton's joint profiles by considering the effect of temperature and infill thickness. It has been validated that this model is accurate to predict the shear strength of ice-filled joint, which may be used to estimate the stability of freezing joint rock slope in cold regions.

Shear behavior and off-fault damage of saw-cut smooth and tension-induced rough joints in granite

The damage of rock joints or fractures upon shear includes the surface damage occurring at the contact asperities and the damage beneath the shear surface within the host rock. The latter is commonly known as off-fault damage and has been much less investigated than the surface damage. The main contribution of this study is to compare the results of direct shear tests conducted on saw-cut planar joints and tension-induced rough granite joints under normal stresses ranging from 1 MPa to 50 MPa. The shear-induced off-fault damages are quantified and compared with the optical microscope observation. Our results clearly show that the planar joints slip stably under all the normal stresses except under 50 MPa, where some local fractures and regular stick-slip occur towards the end of the test. Both post-peak stress drop and stick-slip occur for all the rough joints. The residual shear strength envelopes for the rough joints and the peak shear strength envelope for the planar joints almost overlap. The root mean square (RMS) of asperity height for the rough joints decreases while it increases for the planar joint after shear, and a larger normal stress usually leads to a more significant decrease or increase in RMS. Besides, the extent of off-fault damage (or damage zone) increases with normal stress for both planar and rough joints, and it is restricted to a very thin layer with limited micro-cracks beneath the planar joint surface. In comparison, the thickness of the damage zone for the rough joints is about an order of magnitude larger than that of the planar joints, and the coalesced micro-cracks are generally inclined to the shear direction with acute angles. The findings obtained in this study contribute to a better understanding on the frictional behavior and damage characteristics of rock joints or fractures with different roughness.