Effects Of Joint Roughness Research Articles

The temperature-dependent shear strength of ice-filled joints in rock mass considering the effect of joint roughness, opening and shear rates

Abstract. Global warming causes many rockfall activities in alpine mountains, especially when ice-filled joints in the rock mass become thawed. The warming and thawing of frozen soils and intact rocks have been widely studied in the past several decades; however, the variation of shear strengths of ice-filled joints is not fully understood. In this study, a series of compression–shear experiments were conducted to investigate the shear strength of ice-filled rock joints by considering the effects of joint roughness, temperature, opening, shear rates and normal stress. The joint roughness can improve the shear strength of ice-filled joints. However, the contribution of joint roughness is controlled by some noticeable bulges instead of the joint roughness coefficient (JRC) index. The peak shear strength of the ice-filled joint linearly increases with the increasing aggregation of rupture ice area before these noticeable bulges. As the joint opening increases, the effect of joint roughness decreases, and the shear strength of ice-filled joints tends to be equal to the shear strength of pure ice. In addition, the shear strength quickly reduces with increasing temperature from −15 to −0.5 ∘C. The shear failure mode changes from shear cracking of joint ice to the shear debonding of the ice–rock interface above −1 ∘C. Increasing shear rate decreases the shear strength of ice-filled joints because the joint ice displays the brittle failure phenomenon at a high shear rate. The peak shear strength of the ice-filled joint linearly increases with the increase in normal stress. Moreover, it is also proved that the Mohr–Coulomb criterion can be used to characterize the shear strength of ice-filled joints under different normal stresses. This research can provide a better understanding of the warming degradation mechanism of ice-filled joints by considering the abovementioned important influencing factors.

Effects of Joint Roughness, Shear Rate, and Normal Stress on Shear Behavior and Acoustic Emission Characteristics in Two Parallel Coplanar Intermittently Jointed Rock: An Experimental Study

Effects of Joint Roughness, Shear Rate, and Normal Stress on Shear Behavior and Acoustic Emission Characteristics in Two Parallel Coplanar Intermittently Jointed Rock: An Experimental Study

Effect of Joint Roughness and Infill Thickness on Shear Characteristics of Rock Mass

The joint planes in fractured rock mass will reduce the tensile strength and shear strength of rock mass, which has a decisive influence on the deformation and failure mode of engineering rock mass. Furthermore, infill also exists between the joint surfaces, which will also affect the shear characteristics of the joint. A method of importing standard joint profile into PFC2D was proposed, and a series of numerical simulation tests were carried out to study the effect of joint roughness and infill thickness on the shear characteristics of joints. The numerical results revealed that rock bolts profoundly improved the shear strength of the infilled rock joints, enhanced the toughness of the joint surface, increased the number of micro-cracks, and made the dilatation more obvious. The shear stress and the normal displacement of unbolted or bolted infilled rock joints increased with increasing the joint roughness and decreasing the infill thickness. The maximum horizontal compression stress in the middle of the bolt gradually increased with the increase of joint roughness coefficient. Different roughness has different effects on the number of micro-cracks in the sample. The number of total cracks and tensile cracks of the bolted and unbolted specimens increased with the increase of joint roughness coefficient, while the shear cracks remained almost the same. Through the study of the coupling effect of joint roughness and infill thickness on peak shear stress, results can be obtained as follows. The unbolted samples are highly sensitive to JRC changes. The greater the infill thickness, the greater the sensitivity of unbolted samples to JRC changes. The reinforcement effect of the bolt will strengthen the meshing strength between the joint surface and the filling material; that is, the meshing strength is positively correlated with joint roughness and negatively correlated with filling thickness.

The Effect of Joint Roughness on Shear Behavior of 3D-Printed Samples Containing a Non-Persistent Joint

The goal of this paper is to study the applicability of 3D printing technology to assess the effect of joint roughness on the shear strength of weakness planes with non-persistent discontinuities. Three disc-shaped profiles were generated to make joints with low, intermediate, and high levels of roughness. Powder-based 3D printing technology was applied to provide two types of samples: Type-A samples (joint samples) and Type-B samples (samples with a non-persistent joint). Type-A samples were printed to assess the shear behavior of 3D-printed joints, and Type-B samples were printed to investigate the joint roughness and rock bridge cohesion contributions to the shear strength of partially discontinuous planes. For comparison purposes, several plaster samples containing a non-persistent joint were cast as well. Three series of direct shear experiments were performed on Type-A, Type-B, and plaster samples under constant normal load conditions. The effects of two parameters, namely normal stress and joint roughness, on the shear behavior of the 3D-printed specimens were separately investigated, and the interaction between them was analyzed. The evaluation of the experimental results indicates the existence of two-way interaction between the joint roughness and the applied normal stress of Type-B samples. The experimental results obtained from plaster samples were compared with those obtained from Type-B samples. The comparison reveals that 3D-printed samples properly reflect the effects of joint roughness and normal stress on the shear strength of partially discontinuous planes, although their prepeak and post-peak behaviors are different from those of plaster specimens.

Estimation of the shear strength of fractured Gosford sandstone based on fractal theory and numerical modelling

The effects of joint roughness on the peak shear strength of fractured rock mass has been extensively investigated. Among these investigations, Barton's JRC-JCS model has been well recognized for its capability to characterize surface geometry. However, shear strength estimation is still a major challenge because of the difficulties involved in accurately calculating the joint roughness coefficient (JRC). Thus, alternative methods such as statistic methods based on fractal theory have been proposed to evaluate the impact of surface morphology on rock joints. Extensive evidences show that, the application of the fractal dimension alone cannot produce a satisfactory estimation of either the JRC or the peak shear strength. Therefore, this paper aims to investigate the peak shear strength of rock joints by adopting the fractal theory and via a numerical simulation. Gosford sandstone taken from Sydney Basin is adopted as the rock materials for both numerical validation and shear strength investigation. Along with numerical simulations on shear strength, a linear function has been proposed to describe the correlations among the root-mean-square of the first derivative of fracture profile (Z2), fractal dimension and the standard deviation. The peak shear strength is then accurately interpreted as a function of joint roughness indices: fractal dimension (D), standard deviation (SD) and Z2. Predictions by the proposed equation are compared with both the numerical simulation results and that reproduced by Barton's equation. Good agreement has been achieved even when the joint roughness coefficients (JRC) is varied which proves that the proposed equation is highly accurate and can be reused in other related applications.

Impacts of stress and roughness on steady-state creep of synthetic rock joints under shear

This study presents a systematic experimental investigation on the effects of joint roughness on the steady stage creep of rock joints. Multi-stage creep tests were conducted on synthetic rock joints with different roughness characterised by joint roughness coefficient (JRC, 0–2, 6–8 and 18–20) under different normal stress levels (10–30% of the uniaxial compressive strength, σc). Each creep test includes five loading steps at shear stress of 50–90% of the peak shear strength (τp). This study focuses on the steady-state creep rate of rock joints, which is characteristic of the time-dependent deformation behaviour, and stability of rock mass. The experimental results show that steady-state creep rate is highly affected by the magnitude of JRC and normal stress level. The creep rate decreases as the JRC increases and the relationship exhibits an exponential trend. The normal stress level affects heavily the creep rate at low values. The influence diminishes when normalised normal stress exceeds 0·2σc because the influence of joint roughness becomes less important and the creep rate is close to that of intact rock. The experimental results reported herein provide valuable data regarding the time-dependent deformation behaviour of rock joints, which have been rarely reported. This will be of significance to the improvements in theoretical models and engineering design.

Investigation on the shear properties of discontinuities at the interface between different rock types in the Badong formation, China

Recent research has paid little attention to the difference in shear strength between discontinuities with different joint wall material (DDJM) and discontinuities with identical joint wall material (DIJM) and what are the controlling factors of shear properties of DDJM. Bedding planes between argillaceous limestone and mudstone in the Badong formation in the Three Gorges area of China were investigated using physical model tests. The effect of joint wall material combination on the shear properties of DDJM was first revealed by conducting tests on a series of artificial joint specimens that were made of different joint wall materials and with varying joint surface topographies under different normal stresses. The results indicate that: the peak shear strength of DDJM is not equal to the lower peak shear strength of DIJM for two joint wall materials of the DDJM, even so it is closer to the lower peak shear strength than to the higher one; effect of joint roughness and normal stress on peak shear strength of DDJM is similar with DIJM; effect of joint wall material combination on peak shear strength of DDJM is greater with larger roughness and higher normal stresses. Based on the data acquired from the physical model tests, a new empirical equation to estimate the peak shear strength of DDJM was developed. The capability of the equation was validated by comparing estimates with data from direct shear tests on natural DDJM samples gained from the Badong formation. Finally, the proposed empirical equation was applied to the stability analysis of a rock slope in the Badong formation.

The Effects of Joint Roughness on Stress Wave Propagation Based on Projection Cover Method

To study the influence of smooth and rough joint on stress wave propagation in rock mass and the mechanical properties of jointed rock, experimental research was carried out using Split Hopkinson Pressure Bar. The influence of fractal dimension of rock joints on stress wave propagation was studied. The results showed that the transmission coefficients and equivalent stiffness of the smooth joints rock specimen reduce very little compared to the intact rock specimen, which indicates that the smooth joints have very little influence on stress wave propagation in the experiment. The transmission coefficient and equivalent stiffness of joint rock specimen were influenced by the difference of joint forms. The bigger of the joint roughness (fractal dimension) are, the smaller of the transmission coefficient and stiffness of the rock joint specimen.

A new shear strength model incorporating influence of infill materials for rock joints

Overall mechanical behaviour of jointed rock masses is predominantly controlled by the shear strength of rock joints which contain infill materials. Barton’s empirical model has been shown to be one of the most successful model in predicting joint shear strength (Barton and Choubey in Rock Mech 10:1–54, 1977). However, the Barton model does not express the effect of infill material explicitly, but only incorporates influence of infill into the factors of joint-wall compressive strength (JCS) and residual friction angle. Although there are a large number of studies carried out on the effects of infill materials on the shear strength of rock joints, none of them incorporated infill influence directly into the Barton’s formulation. This paper investigates the effect of infill thickness on JCS and the overall joint shear strength. The interface between wall rock and infill material thicknesses ranging from 1 to 5 mm is simulated by using smooth-joint model. A natural joint is digitally scanned and the roughness geometry is used in all PFC3D models. The results showed that JCS does not show a significant variation at high infill thickness up to a threshold limit of 2 mm. However, JCS decreases drastically at low infill thickness. As expected the shear strength of filled joints reduces from fresh joints to the infill materials, as the infill thickness increases. The reduction is inversely related to the infill thickness. The reduction factor k has a negative exponential relation with the normal stress, indicating that the effect of infill material is stronger at lower normal stress while effect of joint roughness is stronger at higher normal stress.

Microscale modeling of fluid flow‐geomechanics‐seismicity: Relationship between permeability and seismic source response in deformed rock joints

AbstractStudying rock joint deformation including both slippage and opening mechanisms provides an opportunity to investigate the connection between the permeability and seismic source mechanisms. A microscale fluid flow‐geomechanics‐seismicity model was built to evaluate the transport response and failure mechanism of microcracks developed along a joint in Berea sandstone samples during deformation. The modeling method considers comprehensive grain‐cement interactions. Fluid flow behavior is obtained through a realistic network model of the pore space in the compacted assembly. The geometric description of the complex pore structure is characterized to predict permeability of the rock sample as a function of rock deformation by using a dynamic pore network model. As a result of microcracks development, forces and displacements in grains involved in bond breakage are measured to determine seismic moment tensor. Shear and nonshear displacements are applied to the joint samples to investigate their effects on permeability evolution and failure mechanism of microcracks during joint deformation. In addition, the effect of joint roughness is analyzed by performing numerical compression tests. We also investigate how confining pressure affects volumetric deformation leading to opening or closure of developed microcracks and permeability changes of samples with joints.

Modeling the anisotropic behavior of jointed rock mass using a modified smooth-joint model

This study proposes a modified smooth-joint model using the distinct element method (DEM) to simulate the anisotropic behavior of jointed rock mass. Instead of one fixed value for the friction angle, the proposed model adopts a relation of the roughness angle and the normal stress on the joint face to consider the effect of joint roughness and orientation on the shear strength. To simulate the behavior of jointed rock mass, two contact models: the parallel-bond model and the modified smooth-joint model, were used. The first model was used to simulate the cement between the particles, and the second model was applied to simulate the sliding effect on the joint face. The proposed model was validated with the experimental results of the artificial rock mass. The simulated results show that the failure modes of the rock mass, including the split mode, mixed mode, and sliding mode, could be fulfilled. The stress–strain curves in the various joint orientations were well simulated. Furthermore, compared with the existing joint models, the proposed model has better agreement with the experimental results, especially in the mixed and sliding modes.

A Constitutive Model for Shear Behavior of Rock Joints Based on Three-Dimensional Quantification of Joint Roughness

A new constitutive model to describe the shear behavior of rock joints under constant normal stiffness (CNS) and constant normal load (CNL) conditions is proposed. The model was developed using an empirical approach based on the results of a total of 362 direct shear tests on tensile fractured rock joints and replicas of tensile joints and on a new quantitative roughness parameter. This parameter, the active roughness coefficient C r, is derived from the features of the effective roughness mobilized at the contact areas during shearing. The model involves a shear strength criterion and the relations between stresses and displacements in the normal and shear directions, where the effects of the boundary conditions and joint properties are considered by the shape indices C d and C f. The model can be used to predict the shear behavior under CNS as well as CNL conditions. The shear behavior obtained from the experimental results is generally in good agreement with that estimated by the proposed model, and the effects of joint roughness, initial normal stress, and normal stiffness are reasonably reflected in the model.