Natural Rock Joints Research Articles

Investigating asperity damage of natural rock joints in polycrystalline rocks under confining pressure using grain-based model

A cohesive grain-based model (GBM) was employed to investigate the asperity damage response of jointed Aue granite under confined compression. The cohesive GBM was able to characterise both inter- and intra-grain contacts in distinct element method (DEM). We calibrated the model against the laboratory data, including confined and unconfined compression tests as well as Brazilian tensile test of Aue granite. We generated synthetic Aue granite specimens, including three different rock joint profiles with various joint roughness coefficients (JRC) from smooth to very rough (i.e. 4.6, 10.2, and 17.5). We conducted confined compression tests on the synthetic specimens under 2, 5, 10, and 40 MPa of confining pressures. The numerical results revealed that at high confining pressure (i.e. 40 MPa), the rock joint profile had a negligible influence of the damage response of the specimen, and only contributed to the reduction of strength. For the other numerical experiments, the intensity of asperity damage caused by grain crushing was more pronounced when the confining pressure was high. We concluded that the cohesive GBM framework has the potential to be used as a virtual laboratory for investigating the shear behaviour of jointed granitic rocks, which is challenging to be studied in the laboratory.

Effects of tensile stresses on wave propagation across stylolitic rock joints

External stresses, i.e., compressive, shear and tensile stresses, strongly affect seismic wave propagation across rock joints. The present study experimentally investigates the effects of tensile stresses on wave behaviors across single natural rock joints by simultaneously conducting ultrasonic measurements and direct tensile tests. Laboratory results show that increasing tensile stress causes decreases in transmission coefficient and spectral amplitudes for both ultrasonic P- and S-waves. Additionally, for the weaker joint under tension, wave velocity is evidently reduced by the increasing tensile stress. The comparison of measured spectra with theoretically predicted spectra indicates that ultrasonic wave properties of single rock joints under tension could be well described by the displacement discontinuity model (DDM). It is also found that values of joint static and dynamic specific stiffness decrease with increasing tensile stress.

Prediction of Peak Shear Strength of Natural, Unfilled Rock Joints Accounting for Matedness Based on Measured Aperture

The mechanical behaviour of natural, unfilled rock joints is influenced by the interaction between surface roughness and matedness of the contact surfaces. In the field, natural rock joints normally exhibit a mismatch between the contact surfaces, mainly due to different geological processes such as weathering or deformations. Various attempts have been made to estimate how matedness of rock joints influences their peak shear strength. However, the proposed methodologies imply certain difficulties since they are intended to estimate the matedness of rock joints based mainly on visual inspection, and by relating an initial shear displacement to the length of the analysed sample or by relating the opening of saw-tooth and two-dimensional joint profiles with the degree of interlocking. Therefore, a tested peak shear strength criterion for natural, unfilled rock joints that realistically accounts for the influence of matedness on their peak shear strength is still lacking. This paper presents a methodology where objective measurements of the average aperture of natural, unfilled rock joints are used to estimate their matedness as a step in the prediction of the peak shear strength. This measured average aperture is based on high-resolution optical scanning of the surface roughness. The proposed relationship between measured average aperture and matedness of natural rock joints has been included in a further developed peak shear strength criterion. The verification against ten natural rock joint samples of coarse-grained granite showed that the revised criterion can predict the peak shear strength considering rock joint matedness.

Cracking behaviors and chaotic characteristics of sandstone with unfilled and filled dentate flaw

Natural rock joints usually contain filling materials such as sand, clay and broken rock fragments as weak bonds, which heavily affect the mechanical responses and cracking behaviors of rock mass in engineering practice. This study aims to investigate the effects of filling materials on mechanical properties and fracture behaviors of sandstone specimens containing a single dentate flaw under uniaxial compression, and to quantitatively character the damage degrees of flawed rocks. The cracking process of tested specimens is monitored and recorded by acoustic emission (AE) and photographic capturing technologies. The effects of filling on mechanical properties of flawed rocks are first studied. The relation among the AE parameters, the axial stress-time curve and the cracking process is constructed. A qualitative comparative analysis regarding the cracking behaviors between the filled and unfilled specimens is performed. The experimental results show that the filling affects the location and type of crack initiation and propagation behaviors. The chaos characteristics of flawed rocks are quantitatively explored by the entropy-value theory based on AE time-series. The dynamic entropy-value evolution characteristics can be employed to evaluate the stability of flawed rock masses, and it can be considered as an early warning of ultimate failure of flawed rocks. This study provides a better understanding of mechanical properties, fracture behaviors and damage assessment of rock mass containing unfilled and filled dentate flaw.

Shear-related roughness classification and strength model of natural rock joint based on fuzzy comprehensive evaluation

This paper investigated shear-related roughness classification based on Fuzzy comprehensive evaluation, and established a new strength model of natural rock joint. The joint profiles were extracted from 3D data to evaluate roughness, and a series of direct shear tests were carried out on three kinds of natural rock joints. The height statistical parameters Sm, Sq, Ss and Sk, and the textural statistical parameters Si, Sc, and Z2 were measured by Talysurf morphology instrument. Considering the existence of strong positive relations between Sm and Sq, and Z2 and Si, the four morphological parameters of Si, Sq, Sc, and Sk are considered as the main influencing factors related with rock joint shear behaviors. A new fracture roughness coefficient FRC is proposed based on fuzzy comprehensive evaluation (FCE), which considers that the influence of multi morphology parameters on the roughness of rock joint surface, and generally, the FRC is higher than the JRC. Compared with the previously published shear strength models, the proposed FRC-JCS shear strength model, which can more comprehensively reflect the influence of joint surface morphology parameters to shear strength, are in better agreement with the experimental data.

Characterization of rock joint surface anisotropy considering the contribution ratios of undulations in different directions

Anisotropy in rock joint is strongly dependent on undulating surface morphology. Recent research of the morphology showed the parameter can express the different types of anisotropic characteristics of the joint surface separately. This report aims to analyze the common characteristic of the anisotropic distribution and exhibit the anisotropic variation trend. The joint morphology function consists of two morphology functions of regular plane in orthogonal directions, and the anisotropic variation determined by the contribution ratios of the two morphology. The roughness weight ratio in orthogonal direction of joint surface is used as an index to describe the anisotropic variation behavior, which proposes the anisotropic variation coefficient (AVC). On this basis, it is divided into 5 levels from strong anisotropic to isotropic. According to the assumption of anisotropic arc distribution, the anisotropic analytic function is derived and the agreement between the deduced curves and measured data therefore suggests the possibility of defining the morphology anisotropy through the index AVC. Finally, we verify the characteristic of three natural rock joints, and prove the proposed function can reflect the anisotropic distribution trend. The new index can be used to describe the anisotropic variation behaviour of rock joint surfaces.

An Experimental and Numerical Study on the Influence of Filling Materials on Double-Crack Propagation

Filling materials such as clay or sand widely exist in natural rock joints and work as weak bonds between the joint surfaces. The fillings affect rock deformation and failure behavior, and show different influences in terms of single crack or multiple cracks. While most of the literature have focused on unfilled cracks in brittle materials, this study aims to investigate various filling materials on the crack behavior, e.g., initiation, secondary cracks and peak strength. In this paper, the crack propagation in rock-like specimens with double-filled and unfilled cracks are investigated experimentally and numerically. Uniaxial compression tests were conducted and the experimental observations indicate that the peak stress and first crack initiation stress of the specimens vary with different geometries and different filling materials, while the crack initiation location and the pattern of crack coalescence show similar behavior between filled and unfilled cracks. In parallel to the experimental tests, numerical simulations were carried out using a modified phase field model (PFM) to complement the experiments and provide a new perspective. The PFM is found to produce consistent stress–strain curve, strength, and crack patterns with those observed in the experimental tests for both unfilled and filled cracks.

Scale Dependence of Waviness and Unevenness of Natural Rock Joints through Fractal Analysis

The scale dependence of surface roughness is critical in characterising the hydromechanical properties of field-scale rock joints but is still not well understood, particularly when different orders of roughness are considered. We experimentally reveal the scale dependence of two-order roughness, i.e., waviness and unevenness through fractal parameters using the triangular prism surface area method (TPM). The surfaces of three natural joints of granite with the same dimension of 1000 mm×1000 mm are digitised using a 3D laser scanner at three different measurement resolutions. Waviness and unevenness are quantitatively separated by considering the area variation of joint surface as grid size changes. The corresponding fractal dimensions of waviness and unevenness in sampling window sizes ranging from 100 mm×100 mm to 1000 mm×1000 mm at an interval of 100 mm×100 mm are determined. We find that both the fractal dimensions of waviness and unevenness vary as the window size increases. No obvious stationarity threshold has been found for the three rock joint samples, indicating the surface roughness of natural rock joints should be quantified at the scale of the rock mass in the field.

A New Representative Sampling Method for Series Size Rock Joint Surfaces

The greatest variability in both shear strength and roughness exists for joint samples with smaller size, which underscores the necessity of performing representative sampling. This study aims to provide a representative sampling method for series size joint surfaces. The progressive coverage statistical method is introduced to provide the sufficient sample capacity for series sampling sizes by setting different propulsion spaces. The statistical law of the joint surface morphology at different sampling sizes is measured by the 3D roughness parameter with {{theta }}_{max }^{ast }/({C}+1). Through an application in nine natural large-scale rock joints, nine consecutive sampling sizes from 100 mm × 100 mm to 900 mm × 900 mm are selected and 121 samples are successfully acquired from each sampling size. According to the frequency distribution of roughness statistics, a new sampling method combining the layering principle and K-medoids clustering algorithm is proposed to screen representative joint samples for each sampling size. The sampling results that meet the test accuracy requirements suggest the possibility of realizing an intelligent sampling method. In addition, the representative of the interlayer cluster center is validated. Finally, the comparison results with the traditional stratified sampling method prove that the proposed method has better stability.

Research on the anisotropy, size effect, and sampling interval effect of joint surface roughness

In order to explore the factors impacting on the roughness of natural rock joints, a new roughness parameter AHD (the average equivalent height difference) is proposed based on study of joint surface by using the 3D laser scanning technology. The new parameter AHD cannot only reflect the roughness anisotropy of joint surface, but the sampling interval effect and the size effect on roughness. To quantitatively characterize the roughness anisotropy of joint surface, a new anisotropy parameter AAHD is established based on the new roughness parameter AHD. The new parameter AAHD is more comprehensive and reasonable than the anisotropy parameter Ka obtained by Belem because of its consideration on surface morphology in all shear directions, while Ka only considers the largest and the smallest values of roughness parameter. Based on the new parameter AAHD, the sampling interval effect and the size effect on the roughness anisotropy are further studied. The obtained results show that the roughness of joint surface is determined by shear directions, sampling intervals, and research sizes (which refers to window sampling size). It is found that the size effect on roughness depends on shear directions and stabilizes when research size increases to a certain value, while the roughness anisotropy is determined by sampling intervals and research size as well.

Investigation of the effects of nonstationary features on rock joint roughness using the laser scanning technique

Accurately estimating rock joint roughness is crucial for understanding the shear mechanism and permeability behavior of a rock mass. Several influencing factors, including anisotropy, measurement noise, and the scale effect and sampling interval, have been considered. However, little attention is paid to the influences of nonstationary features on the roughness assessment. In this study, a portable laser scanner was employed to collect high-density 3D point clouds of ten natural rock joint specimens. Based on two parameters, namely, the bright area percentage (BAP) and θ*max/(C + 1), where θ*max is the maximum apparent dip angle and C is a dimensionless fitting parameter, the rock joint roughness was determined before and after removing nonstationary features, and a comparison showed that nonstationary features have a considerable influence on the roughness. Subsequently, an approach was proposed to remove nonstationary features through the conversion of spatial coordinates, and an application to a roughness evaluation illustrated that similar trends are observed between the BAP and θ*max/(C + 1) with respect to the point clouds of ten rock joints whose nonstationary features were removed. These findings reveal that nonstationary features should be removed to improve the accuracy and comparability of the roughness assessments.

A numerical study to investigate the influence of surface roughness and boundary condition on the shear behaviour of rock joints

The shear behaviours of rock joints with and without rock bolt support are numerically studied using the discrete element code PFC2D. A cohesive contact model was employed to reproduce the damage response of the synthetic intact rock (i.e. asperity degradation). We used the smooth-joint model to simulate the micro-scale roughness of the joint surface. We calibrated and validated the proposed numerical framework against the experimental results. A series of numerical constant normal stiffness (CNS) direct shear tests were conducted on idealised and natural rock joints to investigate the influence of the boundary condition on the shear behaviour of rock joints. In particular, the importance of CNS condition, surface roughness, asperity angle, and the initial normal stress were studied. Additionally, the shear and damage mechanism of bolted rock joints under constant normal load (CNL) and CNS condition was numerically investigated. The results presented show that the shear resistance of the joint increases under both CNL and CNS conditions, but at a high degree of roughness, no significant enhancement was observed on the value of peak shear stress.

Class Ratio Transform with an Application to Describing the Roughness Anisotropy of Natural Rock Joints

In this study, we explore the potential of class ratio transform with an application to describing the roughness anisotropy of natural rock joints. Roughness smooth coefficient, used for suitably smoothing the roughness parameter values to realize an anisotropic model, is proposed to represent the apparent anisotropy of surface roughness. The geometric irregularities of roughness parameters in polar plots allow transforming to a regular roughness asperity pattern, which can be readily approximated by the ellipse function. The joint roughness coefficients in different orientations of natural rock joints were measured and revealed to be identical after applying the smoothing process using the class ratio transform method. The results show that the roughness smooth coefficient increases with sample size but decreases as azimuthal interval narrows. This method demonstrates the ability in describing the roughness anisotropy and inferring the roughness parameters Z2, Rp, and .

New Method for Characterizing the Shear Damage of Natural Rock Joint Based on 3D Engraving and 3D Scanning

AbstractA lack of joint specimens with the same natural surface morphology has limited detailed experimental study of the shearing performances of joints. In this technical note, we propose a way t...

A New Criterion for Peak Shear Strength of Rock Joints with a 3D Roughness Parameter

With the help of 3D printing and 3D laser scanning techniques, cement mortar joint samples with a certain surface morphology were prepared. Shear tests of 20 sets of matching joint samples and 8 sets of joint samples with different percentages of cavity area were performed under constant normal load and uniform shear displacement. The results show that the distribution characteristics of the equivalent height difference based on the new roughness description were in accordance with the distribution of the wearing area after shearing, and the effect of cavities on the peak shear strength is essentially due to the influence of the cavities on the roughness of the joint surface. The relationships between the peak shear strength and the two roughness parameters were discussed, and a new criterion for predicting the peak shear strength of rock joints was proposed. It was noted that the roughness parameter system adopted in this paper, which can describe the peak shear strength, was reasonable. The roughness anisotropy of the five joint surfaces was discussed, and a corresponding quantification parameter AAHD accounting for the roughness anisotropy was proposed. The roughness of the joint surface has a positive size effect, a negative size effect and no size effect in a certain direction. However, no matter the direction, the roughness parameters will gradually stabilize as the research scale increases. A similar relationship between the roughness anisotropy parameters and research scales was also observed. To check the applicability of the proposed criterion for estimating the peak shear strength of natural rock joints, 12 sets of rock joints with the same surface morphology were produced based on a numerically controlled engraving technique. Under the same loading conditions as those in the shear tests of the cement mortar replication joints, the peak shear strengths of the rock joints were tested, and the results indicated that the new criterion was also applicable to predict the peak shear strength of natural rock joints.

Effect of Cyclic Loading on the Shear Behaviours of Both Unfilled and Infilled Rough Rock Joints Under Constant Normal Stiffness Conditions

The present study experimentally investigated variations in the mechanical behaviours of natural rough rock joints during shearing under cyclic loading and constant normal stiffness conditions, using a servo-controlled shear testing apparatus. The influences of initial normal stress (σn0), normal stiffness (kn) and shear velocity (v) on the shear behaviours are estimated and analysed. The results show that the shear stress (τ), normal stress (σn) and normal displacement (δv) for both unfilled and infilled rock joints decrease with the increase in the number of cycles (N), especially in the N range of 1–2. This is because some asperities on the joint surface are sheared during the first shear process, and the subsequent shear tests for N > 2 were subjected to the frictional process. The σn0 and kn both contribute significantly to the variations in the shear behaviour of rock joints. For unfilled rock joints, increasing σn0 from 2 to 4 MPa increases the shear stress and normal stress by 128.5% and 106.5%, respectively, when shear displacement (δh) = 2 mm and N = 1. Increasing kn from 3 to 5 GPa/m enhances the shear stress and normal stress by 19.4% and 10.4%, respectively, when δh = 2 mm and N = 1. For infilled rock joints, the shear stress and normal stress increase with increasing σn0 when N 0.78. Using the proposed models, the fillings decrease the τ and σn by approximately 24.96–65.52% and 9.38–57.95%, respectively, while increasing the normal displacement (δv) by 0.5 mm on average during the entire shear process.

Neutrosophic Function with NNs for Analyzing and Expressing Anisotropy Characteristic and Scale Effect of Joint Surface Roughness

The shear behavior of rock mass significantly depends upon the surface roughness of rock joints which is generally characterized by the anisotropy characteristic and the scale effect. The large-scale natural rock joint surfaces, at Qingshi Town, southeast of Changshan County, Zhejiang Province, China, were used as a case study to analyze the roughness characteristics. A statistical assessment of joint roughness coefficient (JRC) indicated the roughness anisotropy of different sized rock joints. The lower limit (JRCmean-σ) was regarded as the determinate information, and the difference between lower and upper limits represented indeterminate information. The neutrosophic number (NN) was calculated to express the various JRC values. The parametric equations for JRC anisotropic ellipse were presented based on the JRC statistical assessment of joint profiles of various orientations. The JRC values of different sized joint samples were then quantitatively described by the neutrosophic function. Finally, a neutrosophic parameter ψ for evaluating the scale effect on the surface roughness anisotropy was introduced using the ratio of maximum directional roughness to minimum directional roughness. The case study indicates that the proposed method has the superiority in moving forward from subjective assessment to quantitative and objective analysis on anisotropy characteristic and scale effect of joint surface roughness.

Failure behavior and crack evolution mechanism of a non-persistent jointed rock mass containing a circular hole

In natural rock masses, joints or flaws usually occur in sets that are more or less parallel and regularly spaced. Many problems occurred in tunnel engineering mainly result from the failure of joints or faults because they are weaker compared with the rock matrix. Therefore, it is important to study the interaction of joints with the tunnel, as well as the interactions between themselves. In this research, rock-like specimens of non-persistent jointed rock masses containing a circular hole are prepared and tested under uniaxial compression. The test results show that the peak strength and elastic modulus of the non-persistent jointed rock specimens display a “U” type variation with respect to the joint inclination. The multiple joints can be simplified and regarded as a combination of three types of two-joints relative location, and the crack coalescence types and failure modes are in accordance with the classic crack coalescence types of the two joints. The crack coalescence between the joints and a circular hole can be categorized into three modes with respect to the joint inclination. The Discrete Element Method (DEM) model of the jointed rock specimen that contains a circular hole is established in particle flow code 2D (PFC2D). The simulation results show a good agreement with the experiment results. Furthermore, the displacement fields at the crack zones reveal that the coalescence of joints is mainly caused by tensile cracks and that shear slipping cracks easily occur at the sidewalls of the circular hole. By comparing the parallel-bond force fields in jointed models containing a circular hole with no a circular hole, it is shown that the circular hole can change the stress state at the top and bottom. The joints at a joint inclination of 90° have the smallest effect on the mechanical behavior of the rock specimen.

Fracture analysis of sandstone with a single filled flaw under uniaxial compression

Natural rock joints with infilling materials such as clay, broken rock fragments, or even cement mortar infusion material are common in the field of rock engineering. The fracture behavior of a rock with a filled flaw is related to both the flaw inclination and mechanical properties of the infilling materials. In this study, uniaxial compression experiments were performed for sandstone specimens with different flaw inclinations varying from 0° to 90°, and four kinds of infilling conditions, including no filler, gypsum filler, cement filler, and resin filler were considered. The damage process of specimens was revealed and traced by acoustic emission (AE) and digital image correlation (DIC) to comprehensively understand the cracking evolution. Macroscopic cracking information was recorded with a high-resolution camera, and crack patterns were identified and classified. The influence of infilling material and flaw inclination on the sandstone cracking process was studied. The tensile stress was found to be suppressed due to the existence of infilling materials, and different infilling materials have different effects on the crack initiation stress. As a result, specimens with a filled flaw have larger crack initiation stress levels and smaller crack inclination angles than specimens with an unfilled flaw. The effects of infilling materials and flaw inclination angles on crack initiation are analyzed with a theoretical model based on the Coulomb friction law. The compressibility effect of infilling materials on the cracking behavior is also investigated through displacement and strain fields.

Experimental Study on 3D Roughness and Shear Failure Mechanism of Rock Mass Discontinuity

A set of systematic experimental methods, including 3D accuracy scanning and identification of discontinuous surface topography, physical model construction, and laboratory direct shear experiment under different directions and normal stresses, was proposed to research the influence of discontinuity roughness on strength and deformation of discontinuity. During physical model construction of discontinuity, three types of discontinuity and rough natural rock joint surface models were constructed and moulded. Meanwhile, many influence factors of discontinuity surface topography, such as asperity inclination angle (AIA), asperity height (AH), normal stress (NS), and shear direction (SD), were considered during the direct shear experiment. On the basis of the experimental results, it can be found that there were two types of failure modes under different loading conditions, which were named “failure by shearing through the asperities” and “failure by sliding over the asperities”. The obvious stress concentration phenomenon, climbing, and cutting effects appeared in the process of the direct shear experiment. In addition, the accurate identification of surface topography of natural rough rock joint surface was carried out using three-dimensional sensing system (3DSS) and self-programming software before and after the experiment. The subsamples with the same surface topography as the original samples were moulded using a self-developed instrument. Then, the mechanical behavior of the original samples and subsamples for the natural rough rock joint surface under different shear directions and normal stresses was studied. The results show that the shear displacement under different shear directions and normal stresses is very large before it reaches the failure state. And the residual strength of the original samples is higher than that of the subsamples. In addition, failure modes of the subsamples are main failure by shearing through the asperities due to the significant difference between peak shear strength and residual strength. The failure modes for parts of the original samples are failure by sliding over the asperities. The change ratio of area for the discontinuity after the experiment depends on surface topography, strength of heave on the surface of discontinuity, and particle size of minerals on the surface of discontinuity.