Joint Asperity Research Articles

Investigation of local mechanical behavior during the shear process of rough rock joints using DEM

Rough joints are prevalent in natural rock masses, and their shear mechanical properties are crucial for ensuring the safety and stability of these masses. Existing studies often neglect the contribution of various joint asperities to shear stress resistance. This study addresses this gap by eluci-dating the local shear mechanisms of different asperities in rough joints throughout the shear process. This was achieved through a combination of quantitative and qualitative analyses using the Discrete Element Method (DEM). Initially, Barton's ten standard roughness joint profiles were dig-itized, and rough joints were modeled using the Modified Smooth Joint Model (MSJM). Direct shear tests on joint specimens under constant nor-mal stress were performed using servo-controlled methods. Subsequently, the shear stress behavior was analyzed in relation to the test results. The failure modes of the joint specimens were examined in detail using crack tracking and force chain analysis. The study also explored the relation-ship between the number of cracks and shear stress. Additionally, variations in average stress across ten different segments of joints during shear were monitored using the measuring circle function. The shear resistance contributions of local joint segments were quantitatively assessed using three stress indices: maximum stress, upper limit of maximum stress, and the distribution range of maximum stress. The Joint Roughness Coeffi-cient (JRC) of joints was found to correlate well with these indices. Overall, the progressive failure of local joint segments was analyzed both qual-itatively and quantitatively. The study confirmed that the local shear mechanisms of different joint segments during the shear process are distinct.

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.

Peak shear strength model considering the dilation and shear failure effect of actual contact joint asperities

Peak shear strength model considering the dilation and shear failure effect of actual contact joint asperities

Experimental study on the normal deformation of joint under dynamic compressions

Understanding the effect of loading rate on the normal deformation characteristics of joints has been an important topic for rock dynamics and rock engineering activities. In this paper, a series of static and quasi-static cyclic compression tests are performed on intact and jointed specimens, to reveal the mechanism of the dynamic normal behaviour of joints. The rate and peak value of the compression loading in each cycle are carefully determined to relate the dynamic normal characteristics of joint to the damage of joint asperities. The effect of loading rate on the normal stress-closure curve in different cycles of loading is analyzed. The results indicate that the loading rate has little or no effect on the normal deformation of joint under compression when the joint behaves in an elastic manner and no damage is caused to joint asperities. And the rate-dependent normal behaviour of joint, on the other hand, is found to be due to the failure of joint asperities, arising from the rate-dependent strength of the material.

Simulation investigation of mechanical and failure characteristics of jointed rock with different shapes of joint asperities under compression loading

Simulation investigation of mechanical and failure characteristics of jointed rock with different shapes of joint asperities under compression loading

Investigation of the shear failure of rock joints using the four-dimensional lattice spring model

This research adopts the four-dimensional lattice spring model (4D-LSM) to investigate the shear strength and failure mechanism of irregular rock joints and the influence of rock heterogeneity. The 4D-LSM has been used for a wide range of rock engineering applications, however its ability for simulating direct shear tests has not been investigated in detail. The material strength parameters were calibrated using uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) tests on granite specimens while the joint parameters were calibrated from direct shear tests on saw-cut UCS specimens and rock joints containing a single triangular asperity. From UCS tests, a calibration method is proposed to adjust the excessive machine deformation due to the high brittleness of granite, which is further applied to the direct shear test results. It was found that the strength of rock joints is strongly influenced by the intact rock properties. To investigate the influence of joint geometry and rock heterogeneity, three joint asperity angles and five heterogeneous models were simulated using the calibrated 4D-LSM model. Simulation results were able to capture the influence of different joint asperity angles on the shear strength and failure mechanism as reported in previous experimental and numerical studies. With larger asperity angles, higher shear strengths were obtained. Meanwhile, rock heterogeneity represented by particles with lower strength will reduce the shear strength and alter the failure mechanism, especially at a high porosity. This strength reduction was observed to be greater for steep-angle asperities and becomes less significant for flatter asperities. Similar conclusions were also obtained based on the simulations of natural joint profiles. From these findings, it is suggested that the characterisation of rock heterogeneity is crucial to estimate the strength of rock joints. In addition to the roughness parameters , heterogeneity parameters such as porosity are fundamental for rock engineering applications. • The ability of the (4D-LSM) for simulating the direct shear of irregular rock joints is verified against experimental results. • A calibration is developed for 4D-LSM to model rock joints by integrating uniaxial compressive and direct shear tests. • The influence of rock joint surface on its shear strength is investigated numerically by using the calibrated 4D-LSM. • The influence of different rock heterogeneities on the shear strength of the joints is numerically studied.

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 roughness parameter considering joint material properties and peak shear strength model for rock joints

This study aims at proposing a reasonable roughness parameter that can reflect the peak shear strength (PSS) of rock joints. Firstly, the contribution of the asperities with different apparent dip angles to shear strength is studied. Then the shear strength of the entire joint asperities is derived. The results showed that the PSS of the entire joint asperities is proportional to a key parameter θτ, which is related to the geometric character of the joint surface and the joint material properties. The parameter θτ is taken as the new roughness parameter, and it is reasonable to associate the PSS with the geometric characteristics of the joint surface. Based on the new roughness parameter and shear test results of 20 sets of joint specimens, a new PSS model for rock joints is proposed. The new model is validated with the artificial joints in this paper and real rock joints in published studies. Results showed that it is suitable for different types of rock joints except for gneiss joints. The new model has the form of the Mohr-Coulomb model, which can directly reflect the relationship between the 3D roughness parameters and the peak dilation angle.

Three-Dimensional Combined Finite-Discrete Element Modeling of Shear Fracture Process in Direct Shearing of Rough Concrete–Rock Joints

A three-dimensional combined finite-discrete element element method (FDEM), parallelized by a general-purpose graphic-processing-unit (GPGPU), was applied to identify the fracture process of rough concrete–rock joints under direct shearing. The development process of shear resistance under the complex interaction between the rough concrete–rock joint surfaces, i.e., asperity dilatation, sliding, and degradation, was numerically simulated in terms of various asperity roughness under constant normal confinement. It was found that joint roughness significantly affects the development of overall joint shear resistance. The main mechanism for the joint shear resistance was identified as asperity sliding in the case of smoother joint roughness and asperity degradation in the case of rougher joint asperity. Moreover, it was established that the bulk internal friction angle increased with asperity angle increments in the Mohr–Coulomb criterion, and these results follow Patton’s theoretical model. Finally, the friction coefficient in FDEM appears to be an important parameter for simulating the direct shear test because the friction coefficient affects the bulk shear strength as well as the bulk internal friction angle. In addition, the friction coefficient of the rock–concrete joints contributes to the variation of the internal friction angle at the smooth joint than the rough joint.

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.

Effect of contact surface area on frictional behaviour of dry and saturated rock joints

Frictional behaviour of joints/faults is of great importance in many geo-engineering applications across different scales. It is known that the joint surface roughness is one of the critical parameters controlling the frictional behaviour of rock joints. While nominal normal and shear stresses acting across the joint asperities are calculated based on overall shearing area, the actual normal and shear stresses can be much greater due to smaller actual contact area. The actual contact area on the other hand is known to be related to the joint surface roughness. In order to investigate the effect of contact area on the shear behaviour of rock joints, an extensive set of direct shear experiments at different normal stresses on dry and saturated shale, limestone and sandstone tensile joints were undertaken. To compare the frictional properties of natural and artificial samples, synthetic samples were also fabricated, and their frictional properties were evaluated. Results of this study revealed that the predominant parameter controlling the frictional strength is the actual contact area. While the common models in predicting the shear strength of rock joints were unable to fit the experimental data based on nominal contact area, the results based on actual contact area fit the measured values accurately. In addition, it was observed that when the actual contact area is considered, Mohr-Coulomb criterion is sufficient to fit the experimental data irrespective of the rock type, joint surface roughness or whether dry or saturated. This highlights that the Mohr-Coulomb criterion can appropriately predict the frictional strength if the actual contact area can be estimated. More importantly, the results showed that the surface roughness cannot dictate the shear behaviour of rock joints without considering the actual contact area.

Modelling Shear Behaviour of Joint Based on Joint Surface Degradation During Shearing

An empirical model of the shear behaviour of rock joints is suggested based on a recently proposed joint surface degradation investigation method. Plaster joint replicas were tested with various surface roughnesses, normal stresses, and shear displacements. Joint surfaces with identical joint surface roughness were tested under the same normal stress and the tests were stopped at different shear displacements to investigate the joint surface degradation during shearing. Moreover, the joint surfaces before and after the shear tests were digitized and the digital surfaces were aligned with the help of reference points attached to the sides of the specimens. As a result, the joint surface deformation and void space during shearing could be determined. An empirical model was built based on analysis of the failure mode and failure process of the joint asperities under shearing; the model also takes into account that the basic friction angle varies with normal stress acting on the contact area. The concept of the equivalent effective asperity dipping angle, de, which can be regarded as the representative angle along which the two surfaces slide, was proposed and employed in the derivation of the joint shear and dilation model. Additional shear tests were conducted to verify the proposed model by comparing it with the modified Barton model.

Non-linear Shear Strength Criterion for a Rock Joint with Consideration of Friction Variation

Under shearing, joint asperities get sheared off and damaged. Moreover, shearing and sliding and the interaction between normal and shear stresses occur simultaneously. The nonlinearity of the shear strength envelope is closely related to the joint material, normal stress level, and morphological characteristics. This study analyzed the correlation of the friction angle with the normal stress level to overcome overestimation or underestimation at relatively low or high normal stress levels in the JRC–JCS empirical model. A hyperbolic function was adopted to describe the degradation of friction for different normal stress levels, and the modified non-linear shear criterion $$\tau = \sigma_{\text{n}} \tan (\phi_{\text{r}} + \Delta \phi /(1 + a\sigma_{\text{n}} /{\text{JCS}}))$$ was proposed. Furthermore, the proposed model avoids direct connection to the surface morphology, which is convenient for practical use. Statistical analysis of the experimental data and comparison with the JRC–JCS model verified the validity of the proposed model and present an excellent method for characterizing the non-linearity of the failure envelope for the rock joint.

Effect of rock joint roughness on its cyclic shear behavior

Rock joints are often subjected to dynamic loads induced by earthquake and blasting during mining and rock cutting. Hence, cyclic shear load can be induced along the joints and it is important to evaluate the shear behavior of rock joint under this condition. In the present study, synthetic rock joints were prepared with plaster of Paris (PoP). Regular joints were simulated by keeping regular asperity with asperity angles of 15°–15° and 30°–30°, and irregular rock joints which are closer to natural joints were replicated by keeping the asperity angles of 15°–30° and 15°–45°. The sample size and amplitude of roughness were kept the same for both regular and irregular joints which were 298 mm × 298 mm × 125 mm and 5 mm, respectively. Shear test was performed on these joints using a large-scale direct shear testing machine by keeping the frequency and amplitude of shear load under constant cyclic condition with different normal stress values. As expected, the shear strength of rock joints increased with the increases in the asperity angle and normal load during the first cycle of shearing or static load. With the increase of the number of shear cycles, the shear strength decreased for all the asperity angles but the rate of reduction was more in case of high asperity angles. Test results indicated that shear strength of irregular joints was higher than that of regular joints at different cycles of shearing at low normal stress. Shearing and degradation of joint asperities on regular joints were the same between loading and unloading, but different for irregular joints. Shear strength and joint degradation were more significant on the slope of asperity with higher angles on the irregular joint until two angles of asperities became equal during the cycle of shearing and it started behaving like regular joints for subsequent cycles.

Rock Joint Asperities and Mechanical Strength of Concrete

Mechanical interactions between concrete foundations of large civil engineering structures (tunnels, bridges or dams) and the asperity surfaces of rock masses represent a useful topic for investigation. It is obvious that such large objects exert huge pressures on bedrocks and this might result in surprising variations of mechanical properties of the materials used in foundations. The present contribution evaluates possible changes of the compressive strength of concrete caused by the invasive acting of asperity-like needles penetrating into the volume of this material. The experimental arrangement simulates mechanical interactions between sharp asperities of bedrocks and the cement-based materials placed in the foundations of large civil engineering structures.

Prediction of Unconfined Compressive Strength for Jointed Rocks Using Point Load Index Based on Joint Asperity Angle

This research paper is aimed to briefly highlight the correlation between unconfined compressive strength and point load index for jointed rocks based on joint asperity & orientation. In this observe, specimens were tested to obtain their unconfined compression strength and point load index for a different joint condition. The different joint conditions considered for this study were clean joint and joint filled condition. For both clean joint and joint filled specimens were prepared by various asperity angles of 30°, 45°, 60° and 90° with different orientation angles such as 0°, 30°, 45°, 60°, 90°. Plaster of Paris was used as model material to simulate weak rock mass in the field. By testing intact model specimens for unconfined compressive strength leads to revealing of optimum moisture content for further testing. The curing period for the model specimens is 3 days at room temperature. To simulate jointed rocks, various moulds of different orientation of joint with respect to major principal stress are prepared separately. The inner diameter of the mould is 50 mm and height is 100 mm. After casting, a rough joint was created by cutting the prepared sample using the cutter. The specimens are tested for both clean joint and joint filled condition to determine the favorable joint orientation and asperity angle. After curing, the specimens are tested for unconfined compressive strength and Point load index. The new multi-linear correlation for determining unconfined compressive strength with the help of point load index is developed and cross checked with equations formed for actual rock. On comparing both results it is found that the new equation can suitable for assessing the unconfined compressive strength of limestone and serpentinite rocks through point load index value.

Direct Measurement of the Contact Normal Stress Distribution between Two Smooth Joint Surfaces

The shear strength of rock joints is highly depended upon the failure mode of joint asperity. At lower normal stress slide-up of one asperity up over another mode, however at high normal stress the joint asperities are sheared off at the base. This research uses pressure measurement film to directly measure the contact normal stress between smooth joint surfaces. It demonstrates that the density of color impression is capable of capturing the normal stress distribution behavior. The contact normal stress distribution during shearing is changed. After shearing, the contact stress becomes large. This increase in contact normal stress is to fracture the joint wall material.

EVALUATION OF SHEAR STRENGTH OF MODEL ROCK JOINTS BY EXPERIMENTAL STUDY

In this paper, variation of the shear strength of artificial rock joints under constant normal loading condition is studied. Idealised joint surfaces were prepared using a developed molding method with special mortar and shear tests were performed on these samples under CNL conditions. Different levels of normal load and shear displacement were applied on the samples to study joint behaviour before and during considerable relative shear displacement. Nine types of saw-tooth joints have been selected for simplicity of modelling to quantify the effect of CNL conditions on joint shear behaviour. It was found that the shear strength of joints is related to rate of shear displacement, joint roughness (varying joint asperity angles) and applied normal stress condition. Finally, based on the experimental results and observations made of sheared joint samples, a new peak shear strength envelope is proposed to model sawtooth type joints tested under CNL conditions.

Effect of asperity order on the shear response of three-dimensional joints by focusing on damage area

The shear resistance of rock joints has always been of paramount importance in stability analysis of surface and underground rock structures. Many parameters influence the shear strength of rock joints among which surface asperity is highly crucial that has encouraged many researchers to work on. Among the pioneers working on the subject, Patton [1] was the first researcher who systematically studied the effect of joint asperity using simple physical models with two-dimensional toothshaped asperity. Patton proposed the following bilinear shear strength criterion:

Underestimation of roughness in rough rock joints

AbstractNumerous studies have been made to improve Barton's shear strength model for the quantification of rock joints. However, in these previous studies, the roughness and shear strength of the rock joint have been underestimated especially for relatively high undulated profiles (joint roughness coefficient (JRC) >14). The main factors of roughness underestimation in rough rock joints are investigated for the proper quantification of rock joint roughness. The aliasing effect and the roughness characteristics are analyzed by using artificial joint profiles and natural rock joint profiles. A 3D camera scanner is adopted to verify the main source of underestimation when using conventional measurement methods. Shear strength tests are carried out by using two types of shear apparatus to study the roughness mobilization characteristics, which may also affect the roughness underestimation. The results of joint roughness assessment, such as aliasing and undulation of waviness, show that the roughness can be underestimated in relatively rough joint profiles (JRC>14). At least two components of roughness parameters are needed to properly represent the joint roughness, for example, the amplitude and the inclination angle of joint asperity. Roughness mobilization is affected by both the normal stress and the asperity scale. Copyright © 2008 John Wiley & Sons, Ltd.