Behavior Of Discontinuities Research Articles

Influence of wetting and drying cycles on the shear behavior of discontinuities between two different rock types with various surface topographies

Influence of wetting and drying cycles on the shear behavior of discontinuities between two different rock types with various surface topographies

Effects of Undulation and Stress on the Shear Mechanical Behavior of Saw-Toothed Discontinuities Under Unloading Normal Stress

Effects of Undulation and Stress on the Shear Mechanical Behavior of Saw-Toothed Discontinuities Under Unloading Normal Stress

Extension of discontinuous deformation analysis method for coupled reactive transport problems

Fluid flow and the accompanying reactant transport in fractured rock masses are key concerns for understanding the performance and safety assessments of subsurface engineering problems, particularly in waste disposal and tunneling project. Nevertheless, predicting solute transport behavior remains a great challenge due to computational sophistication and numerical instability if numerous discontinuities and chemical reaction is taken into consideration. In this study, the partial differential equation describing the reactive transport of solute is solved and coupled with the Discontinuous Deformation Analysis (DDA) method. First, the extended DDA method is validated by simulating an indoor dissolution test on the single rock fracture. Then, it is applied to establish a discrete fracture network (DFN), and be further employed to investigate the reactive transport behavior and geochemical evolution in a case of acidic mine drainage in a fractured stratum. The study analyzes the effects of stress field, reaction speed, and diffusion coefficient on the reactive transport. Numerical results indicate that the conclusions reached are consistent with current findings, combined with its prominent features in simulating the mechanical behavior of discontinuities, demonstrating that the extended method is a promising tool for the coupled hydro-mechanical-chemo analysis.

A Simplified Approach to Estimate Anchoring Capacity of Blocky Rock Mass with Pressure Arch Theory

In this paper, a simplified method for predicting rock mass resistance against tensile forces from rock anchors (anchor) is presented. The interaction of anchors and rock mass was investigated using three-dimensional discontinuous numerical modelling. Several patterns of rock discontinuities were assumed in the numerical modelling while a single anchor is embedded in it. The numerical results show that the existence of a discontinuity set sub-parallel to the anchor significantly improves the rock mass resistance against the tensile force from the anchors. This phenomenon is due to the rock block interlocking at the sub-parallel discontinuity set. Rock block interlocking generates a zone of stress concentration inside the rock mass which has an arch shape (i.e., a pressure arch), resisting against the anchor's tensile force. The load-bearing capacity of the pressure arch plays a significant role in the resistance of the rock mass against the forces from the rock anchor. A voussoir beam analogy was utilised to study the load-bearing capacity of the pressure arch. A simplified analytical approach was developed to assess the load-bearing capacity of the voussoir beams. Then, it was used in combination with the weight of the mobilised rock mass by the anchor to assess the maximum anchoring resistance of the rock mass (anchoring capacity). The suggested method was calibrated by numerical modelling and relevant published pull-out test results. The technique developed in this paper shows the significance of rock block size, shear behaviour of rock discontinuities, Young's modulus of the rock mass, and uniaxial compressive and tensile strength of the intact rock in anchoring capacity of rock masses.

An adaptive finite element method for crack propagation based on a multifunctional super singular element

The traditional finite element method (FEM) often requires a large number of refined meshes to analyze the mechanical behavior of geometric discontinuities, its computational efficiency and convergence speed are affected. In this paper, an adaptive FEM for crack propagation based on a multifunctional super singular element (MSSE) at the crack tip is proposed for the fracture process investigation of two-dimensional (2D) materials. The adaptive FEM for crack propagation divides the crack tip neighborhood into the MSSE region, the protection element (PE) region and the background element (BE) region. The MSSE is established based on the numerical eigen-solution of the singular stress field near the crack tip, and can be used for anisotropic material and interfacial crack problems. The PE and BE regions are still discretized by the conventional finite element meshes. Since the node degrees of freedom between MSSE and conventional finite element are unified, they can be assembled directly without transition element. The MSSE can be used to solve the crack tip stress intensity factor (SIF) directly without refining the crack tip mesh. The adaptive meshing technique reduces the difficulty of remeshing during crack growth, thus facilitating the simulation of crack propagation processes. The adaptive crack propagation algorithm is used to analyze the crack propagation problems in isotropic materials, anisotropic materials and bi-materials. The calculated results demonstrate the effectiveness and universality of the crack tip MSSE. The method has potential for application in the simulation study of residual strength and fatigue fracture of complex mechanical structures.

Evaluation of progressive damage of discontinuity asperities due to shearing by means photogrammetric survey

It is well known that the mechanical behaviour of rock discontinuities strongly influences the stability of slopes and fractured rock walls. With this end, particular attention must be paid to the analysis of the roughness of natural discontinuities, which represents a peculiar geometric feature strongly influencing their shear strength. The paper describes an experimental procedure carried out at laboratory scale on natural rock discontinuities to measure the shear strength and the roughness of their surfaces to analyse the progressive damage of the asperities during shearing process. The direct shear tests along discontinuities were coupled to photogrammetric surveys of the surfaces carried out before the tests (natural surfaces), after the first cycle and at the end of the last cycle. This allowed the reconstruction of the digital surface models of the intact and degraded surfaces. Through analytical procedures, the data obtained were processed to obtain geometric descriptors and adequately estimate the Joint Roughness Coefficient (JRC), analysing several profiles extracted along the direction in which the mechanical tests were conducted. The comparison between the experimental results and the roughness surface direct measure showed that discontinuities, even at the small scale, have an inhomogeneous roughness and that discontinuity degree of damage is a progressive process influenced by the state of confinement applied during the tests.

Failure strength and fracture characteristics of rock with discontinuity under indirect tension

Large-scale discontinuities can significantly affect the mechanical properties of rock masses. However, the tensile behavior of rock discontinuities is often less investigated. To study the statistical characteristics of failure strength and fracture characteristics of rock discontinuities, Brazilian disc tests were conducted on limestone specimens with a single natural discontinuity at different load-discontinuity angles ( β ). In this study, β = 0 ° and β = 90 ° correspond to the discontinuity parallel and perpendicular to loading direction, respectively. The results show that Brazilian failure strength (BFS) can reasonably represent the tensile strength of rock with discontinuities, by comparing the BFS and tensile stress in the disc center at peak force. The two-parameter Weibull distribution can capture the statistical BFS characteristics of rock discontinuities parallel to loading direction ( β = 0 ° ) and at different load-discontinuity angles ( β ≠ 0 ° ). All specimens with discontinuity at different load-discontinuity angles show more plastic deformational behaviour than intact rock specimen. With increasing β , the mean BFS of limestone with discontinuity increases before reaching a plateau at β = 45 ° . The single plane of weakness theory best explains the BFS of fractured limestone with β . Only a specific segment of pre-existing rock discontinuity could affect the fracture process. When β = 0 ° , interfacial cracks and alternative cracks formed. When β ≠ 0 ° , mixed failure mode with shear and tensile failure occurred, particularly when β = 30 ° and β = 60 ° . The findings can contribute to better understanding the failure and fracture characteristics of rock with discontinuities, particularly the interaction of pre-existing discontinuities with stress-induced fracturing.

Structural control within flawed rock specimens under external loading as visualized through repeating nucleation on multiple sites by acoustic emission (AE)

SUMMARYThe behaviour of discontinuities in rock mass have been experimentally investigated extensively for decades by a type of laboratory analogue of flawed rock specimens, taking the artificially prepared flaw(s) as the analogue of rock mass discontinuity(ies). The role of macroscale flaw(s) as controlling structure has been generally neglected. Here, we conduct detailed characterizations of the acoustic emission (AE) from the rock fracture process on the pre-flawed specimens under loading. Though the same phenomena from the literature can be reproduced within our tests, detailed spatial-temporal investigation on the AE events at the nucleation point suggests a different physical interpretation. Artificial flaw(s) play a non-negligible role as a controlling structure for the following rock fracture process including a series of nucleation points alteration via stress transfer. The interlocking parallelogram (IP) control and single-large-structural (SLS) control are two fundamental mechanisms initiated by the artificial flaws. Each of these two mechanisms are observed under a wide range of flaw settings and can potentially coexist within the across length scales. The scale-invariance feature is observed to be fundamentally different within the scale of specimens under SLS and/or IP control, suggesting the incompleteness at the specimen-scale for IP control. Therefore, no ‘fracture coalescence’ zone can be separately investigated without considering its external rock fracture evolution and confirming scale-invariance. Therefore, structural controlling mechanisms must be considered due to the pattern and AE characterization results.

Small-scale size effects on the shearing behavior and sliding stiffness reduction-displacement curves of rock-plastic gouge simulant under low normal stresses

Assessment of the influence of laboratory specimen size (and scale effects) is critical to be obtained in the study of geological structures. The present work examines the influence of the small-scale size of specimens (in the range of a few tens of millimeters) on the behavior of analogue discontinuities with highly plastic clay filling material using a custom-built apparatus. The results showed that the displacement required to reach the peak friction increased with the specimen size, while the shearing stiffness showed an opposite trend. A slight increasing tendency of peak friction with increasing specimen size was also observed, but it needs further testing to be assertive. The weakening behavior in the gouge layer increased at larger normal stresses leading to significant post-peak softening, which was intensified as the gouge layer size increased. Application of pre-consolidation to the gouge layer intensified the weakening behavior. The degree of non-linearity in the stress-displacement curves and rigidity of the specimens were also quantified, and a hyperbolic-type model was implemented to measure the shear stress mobilization rate based on the stress-displacement curves. The data were further integrated with that from previously published works on experiments at smaller and larger scales to enhance the findings from this work on scale effects on the mechanical behavior of geological structures.

The evolution of dynamic energy during drop hammer testing of Brazilian disk with non-persistent joints: An extensive experimental investigation

Rock mass is well known as a discontinuous, heterogeneous, and anisotropic material. The behavior and strength of rock mass is heavily controlled by the condition and orientation of discontinuities (faults, joints, bedding planes) and discontinuity sets. Under dynamic loading conditions, rock bridges along non-persistent discontinuity planes may crack, and a fully persistent discontinuity may form, potentially affecting the stability of a rock structure. The study of the dynamic behavior of rock discontinuities has critical implications for civil engineering, the mining industry, and any other areas where rock mass is utilized as a structural foundation in areas prone to dynamic loading conditions, such as those formed during earthquake events. In this paper, cement-mortar-based Brazilian disks containing open, non-persistent joints were constructed and subjected to impact loading to investigate their impact energy behavior. The effect of some parameters, such as joint continuity factor (the relationship between joint length and rock bridge length), bridge angle, joint spacing, joint orientation, and impact angle were investigated to estimate the required Dynamic Energy for Crack Initiation (DECI), Dynamic Energy for Crack Coalescence (DECC) and failure pattern of specimens. The results of the experiments revealed an increasingly continuous joint reduces the DECI and DECC, while larger joint spacings past the middle value of those experimented increase the DECI and DECC. The bridge angle and loading direction do not affect DECI, but by increasing bridge angle DECC decreases, and it increases by increasing loading direction angle. Finally, an optimization analysis was conducted which showed that joint spacing and joint continuity factors significantly affects DECI, and joint continuity factor and loading direction have significant effect on DECC.

Influence of Initial Stress and Deformation States on the Shear Creep Behavior of Rock Discontinuities with Different Joint Roughness Coefficients

Influence of Initial Stress and Deformation States on the Shear Creep Behavior of Rock Discontinuities with Different Joint Roughness Coefficients

A New Double Shearing Testing Equipment for the Shear Behaviour of Rocks and Discontinuities under Static and Shock Loads

The dynamic mechanical behaviour and characteristics of rocks have been investigated using different techniques to evaluate the engineering structures under dynamic loads. The authors devised a new experimental apparatus to investigate the behaviour of rocks under shock waves. A new double shearing jig shown in figure 1 was developed and it was used to study the shearing of rocks and discontinuities under static and impact loading conditions using the device. Double shearing tests involve various rocks and discontinuities. The authors describe the experimental results under shock load and static loading and the results are compared with each other and their implications are discussed in rock mechanics and rock engineering. Although the system is quite simple, it is shown that it is very practical to study the shear behaviour of both intact rocks and discontinuities.

Z shape joints under uniaxial compression

In this paper, the effects of angle of Z shape non-persistent joint on the compressive behaviour of joint's bridge area under uniaxial compression load have been investigated. Furtheremore experimental test and Particle flow code in two dimension (PFC2D) have been used. concrete samples with dimension of 50 mmx50 mmx100 mm were prepared. Tensile strength of model material was 1 MPa. Within the specimen, three Z shape non-persistent notches were provided. The notch length was 1.5 cm. when two upper and lower notch have horizontal direction, the middle joint angle were 45o, 90o and 135o. When dips of two upper and lower notch was 90o, the middle joint angle were 45o, 90o and 135o degree. When dips of two upper and lower notch was 135 degree, the middle joint angle were 45o, 90o and 135o. Totally, 9 different configuration systems were prepared for Z shape non-persistent joints. Also, 37 models with different Z shape non-persistent notch angle were prepared in numerical model. The axial load with rate of 0.005 mm/s was applied to the model. Results indicated that the failure process was mostly governed by the Z shape non-persistent joint angle. The compressive strengths of the specimens were related to the fracture pattern and failure mechanism of the discontinuities. In addition it was shown that the compresssive behaviour of discontinuities is depend on the number of the induced tensile cracks which are increased by increasing the middle joint angle. The middle joint has significant effect on the failure pattern. In fixed upper and lower joints angles, when the middle joint angle is 90o the failure stress has maximum value. On the other hand, it has minimum value when the middle joint angle was 135o. In fixed middle joint angle, the samples have minimum value when the upper and lower joints angles are 135o. At the end, the failure pattern and strength results of the experiential tests and the numerical simulations were similar.

Relationship between point load index and mode II fracture toughness of granite

Experimental and numerical methods were used to investigate the relationship between point load index and mode II fracture toughness of granite. A punch-through shear test was used to measure the mode II fracure toughness of granite. Point load test was performed to measured the point load index of jointed granite. Three granite samples with dimension of 20 mmx150 mmx40 mm consisting parallel non-persisent joint were prepared in the laboratory for punch test. Also six recangular specimen with echelon joint was prepared for point load test. Cuncurrent with experimental tests, numerical simulations have been done for punch test by PFC2D and poin load test by PFC3D. Numerical model for punch test has dimension of 100 mmx120 mm. similar to those for joints configuration systems in the experimental test, three models with different rock bridge lengths were prepared. Also, numerical model for point load test has dimension of 100 mmx100 mmx40 mm. six models consisting non-persistent joint were prepared. The punch testing results showed that the failure process was mostly governed by the rock bridge lengh. The shear strengths of the specimens were related to the fracture pattern and failure mechanism of the discontinuities. It was shown that the shear behaviour of discontinuities is related to the number of the induced tensile cracks which are increased by increasing the rock bridge length. The strength of samples decreases by increasing the joint length. The point load testing results showed that the tensile cracks initiate beneath the loading cone and propagates through the intact rock till coalescence with notch tips. The value of point load index has close relationship with mode II fracture toughness obtained by punch test. The failure pattern and failure load are similar in both methods i.e., the experimental testing and the numerical simulation methods.

Crack growth properties in granite specimens consisting non-persistent joint under punch shear through test

Experimental and numerical methods were used to investigate the effect of echelon notchs on the shear behaviour of joint’s bridge area in granite. A punch-through shear test was used to model the granite cracks under shear loading. Granite samples with dimension of 20 mm×150 mm×40 mm were prepared in the laboratory. Within the specimen model and near the edges, four edge notches were provided. Nine different configuration systems were prepared for notches. In these configurations, the length of each notch was taken as 3 cm, 4cm and 5 cm. Assuming a plane strain condition, special rectangular models were prepared with dimensions of 100 mm×100 mm using particle flow code in two dimensions. similar to those for joints configuration systems in the experimental tests i.e. 9 models with different rock bridge lengths and rock bridge joint angle were prepared. The axial load was applied to the punch through the central portion of the model. This testing showed that the failure process was mostly governed by the rock bridge length and rock bridge angle. The shear strengths of the specimens were related to the fracture pattern and failure mechanism of the discontinuities. It was shown that the shear behaviour of discontinuities is related to the number of the induced tensile cracks which are increased by increasing the rock bridge angle. The strength of samples decreases by increasing the joint length. The failure pattern and failure strength are similar in both methods i.e. the experimental testing and the numerical simulation methods.

Evolution of contact area and aperture during the shearing process of natural rock fractures

The role of surface geometry and its evolution during shearing process on the hydromechanical behaviour of rock discontinuities has long remained an open question for geologist. During the shearing process, only a fraction of discontinuity is in contact and the actual stresses on these contacts are considerably greater than nominal stresses. However, measuring the contact area during shearing process has been a significant experimental challenge due to difficulties in detection of the surface evolution while the shearing process takes place. In this study, the evolution of contact area and fracture aperture during the shearing process were measured using a novel experimental set-up coupled with X-ray micro computed tomography technique. In these experiments, the contact area and aperture of natural fractures were measured during the shearing process in slightly smooth (shale) and rough surfaces (sandstone) specimens. It was shown (within resolution limit) that for the shale specimen, the contact area decreases from 24% to 11%. Contrarily, the contact area increased from 5% to 13% for the sandstone specimen. The volumetric aperture of both specimens, on the other hand, increased during shearing process indicating that the evolution of contact area and aperture is a complex process. This process is likely to be controlled by initial mismatching of the surfaces, dilation, damage of the asperities, and gouge production. The obtained results of this study have significant implication on prediction of shearing behaviour of faults and fractures through measuring the actual stresses acting on the fracture surfaces.

Physical test and PFC2D simulation of the failure mechanism of echelon joint under uniaxial compression

Experimental and discrete element methods were used to investigate the effects of echelon non-persistent joint on the failure behaviour of joint's bridge area under uniaxial compressive test. Concrete samples with dimension of 150 mmx100 mmx50 mm were prepared. Uniaxial compressive strength and tensile strength of concrete were 14 MPa and 1MPa, respectivly. Within the specimen, three echelon non-persistent notches were provided. These joints were distributed on the three diagonal plane. the angle of diagonal plane related to horizontal axis were 15o, 30o and 45o. The angle of joints related to diagonal plane were 30o, 45o, 60o. Totally, 9 different configuration systems were prepared for non-persistent joint. In these configurations, the length of joints were taken as 2 cm. Similar to those for joints configuration systems in the experimental tests, 9 models with different echelon non-persistent joint were prepared in numerical model. The axial load was applied to the model by rate of 0.05 mm/min. the results show that the failure process was mostly governed by both of the non-persistent joint angle and diagonal plane angle. The compressive strengths of the specimens were related to the fracture pattern and failure mechanism of the discontinuities. It was shown that the shear behaviour of discontinuities is related to the number of the induced tensile cracks which are increased by increasing the joint angle. The strength of samples increase by increasing both of the joint angle and diagonal plane angle. The failure pattern and failure strength are similar in both methods i.e. the experimental testing and the numerical simulation methods.

Experimental Study on Shear Behavior and Acoustic Emission Characteristics of Nonpersistent Joints

The shear behavior of rock discontinuities controls the stability of rock masses to a great extent. In this paper, laboratory shear tests were performed on rock-like materials with different cracks to study the effect of nonpersistent joints on the shear behavior of rock masses. The results show that the variation trends of the shear stress-displacement curves of specimens with different cracks are generally similar and have the same stage characteristics. When the crack length is relatively short, the elastic stage is prolonged, the peak shear strength decreases, and the shear displacement corresponding to the peak shear strength and the residual shear strength increases with the increase of the crack length. When the crack length is relatively long, the elastic stage is shortened, the peak shear strength decreases, and the shear displacement corresponding to the peak shear strength increases with the increase of the crack length. The peak shear stress gradually decreases with the increase of the crack length. The shear strength of the specimens with unilateral cracks is much higher than that of the specimens with bilateral cracks. The shear strength of the specimens is affected not only by the crack length but also by the crack distribution. The acoustic emission (AE) count peak occurs when the shear stress drops sharply and has an inverse “S”-type variation trend with the increase of the crack length. The inclination angle of the fracture decreases, the roughness of the fracture surface decreases, and the proportion of the wear area on the fracture surface increases gradually with the increase of the crack length. The AE source decreases with the increase of the crack length, and their locations are obviously asymmetric. This work can greatly contribute to the insight into the shear failure mechanism of rock discontinuities with nonpersistent joints.

Modeling shear behavior of rock joints: A focus on interaction of influencing parameters

One of the principal issues in the design of surface and underground structures on/in rock is the shear behavior of rock discontinuities that irreparable damages are caused when neglecting it. Several parameters are involved in the shear behavior of rock discontinuities which can interact with each other. The purpose of this study is to model the shear behavior of rock joints with regard to the individual impact of the influencing parameters and their interaction. To do so, a number of direct shear tests was performed where the values of independent parameters including the compressive strength (σc), normal stress (σn), modulus of elasticity (E), joint roughness coefficient (JRC) and basic friction angle (ϕb) were varied. Then, the response surface methodology (RSM) was used to incorporate the individual influence and interaction of the parameters into constitutive relationships for the peak shear strength, peak shear displacement, and dilation angle. For the shear strength, the most significant interaction was found to be between E and ϕb, for the peak shear displacement between JRC and E, and for the dilation angle between σc and E. The results show that the proposed model performs better in estimating the shear behavior characteristics than the model obtained from the nonlinear regression and the Barton-Bandis model. Therefore, the application of RSM is a winning approach for modeling the rock shear behavior which is capable of modeling the most complex interactions and provides a reliable performance.

Experimental and Numerical Studies on the Evolution of Shear Behaviour and Damage of Natural Discontinuities at the Interface Between Different Rock Types

Recent research has paid little attention to the shear damage of discontinuities with different joint wall material (DDJM). In this paper, we present an investigation on the evolution of the shear behaviour and the damage of three typical types of natural DDJM in a sliding-prone stratum of China. Experimental direct shear tests were performed on 14 pairs of natural DDJM specimens to examine the changes in the shear strengths and surface damages of the DDJM with increasing normal stresses and an increasing number of shear cycles by evaluating surface damages via damage zone distribution, damage area percentage, and variation of joint roughness coefficient (JRC). The results indicate that the differences in the shear damage between the two halves are closely related to the difference in strength of the two joint walls of the DDJM specimens with similar initial JRC values of the two joint surfaces. Simultaneously, parallel numerical direct shear tests were conducted in PFC3D. The performance of the numerical modeling was examined by comparing the parameters of shear strength, damage area and damage depth of DDJM specimens with those obtained in the experimental direct shear tests. Then the validated models were used to explore the evolution of the damage depth of DDJM specimens during the shearing process. The results demonstrate that the proposed numerical approach has the ability to reproduce the shear behavior and damage of DDJM reasonably and could be used to examine the internal damage of DDJM which are not easy to investigate via laboratory experiments.