Failure Process Of Rock Mass Research Articles

The field-enriched finite element method with gravity effects for simulating the cracking behaviors of large-scale engineering rock masses

The field-enriched finite element method uses a scalar field defined as a field variable to describe cracks and characterize their impact on the displacement field and stress field of the solution model. It is capable of avoiding remeshing and employing level set functions to describe cracks when simulating the propagation of cracks. In this work, a field-enriched finite element model with gravity effects is proposed to simulate the large-scale failure process of engineering rock masses, and several numerical cases of geotechnical engineering are successfully analyzed. First, by introducing the unified tensile fracture criterion into the numerical model, the large-scale failure process of the intact slope is simulated. Second, the sliding process of rock slopes containing en echelon joints is numerically investigated. Third, the cracking process of the concrete dam is analyzed. Finally, the effects of joint and bedding plane inclination angles on the stability of tunnel chamber in transversely isotropic rock mass are studied. The numerical results indicate that the numerical method proposed in this work can accurately solve the large-scale failure process of rock masses.

Study on Permeability Evolution Law of Rock Mass under Mining Stress

In order to study the stress–strain–permeability coefficient relationship of overlying strata in a fractured zone after coal mining, taking the Changcun coal mine in the Changzhi basin as an example, the permeability evolution law of coarse sandstone, fine sandstone, siltstone and mudstone during a stress–strain process was analyzed through a triaxial compression permeability test. The generalized model of the rock mass permeability evolution process under mining stress was summarized, and then a coupling model of the stress–water pressure–permeability coefficient of fractured rock was established based on the continuum model of rock mass. The results showed that the maximum permeability coefficient of different coal overburden types was quite different, and the peak strength of the rock mass preceded the maximum permeability coefficient during the rock mass failure process; the permeability coefficient first decreased and then increased, reaching its maximum value after the peak stress, which occurred during the strain-softening stage; the generalized model of rock mass permeability included the compaction stage, elasticity stage, stable fracture stage, unstable fracture stage, macroscopic failure stage and residual strength stage.

Modeling the failure process of rock masses using a 3D nodal-based continuous-discontinuous deformation analysis method

A three-dimensional nodal-based continuous-discontinuous deformation analysis method (3D-NCDDAM) is developed in this study for modeling the failure process of rock masses. In the 3D-NCDDAM, four-node tetrahedral elements which can be automatally generated are used to discretize the problem domain. To reduce computational cost, and effectively model the failure process of rock masses at concerned regions, the problem domain is devided into two types of regions, namely, the “non-destructive region” and “possible destructive region”. The non-destructive region is mainly used to reduce the boundary effects on the numerical results, and is regarded transitional zone in which the failure of rock masses is not considered. For the possible destructive region, artificial joint elements are inserted, and the failure of these joint elements is used to represent fracture initiation and propagation in rock masses. Additionally, the contact potential which is initially proposed in finite-discrete element method (FDEM) is incorporated into the 3D-NCDDAM to simuate the opening and slipping of discontinuous structural planes involved in rock masses’ failure process. Note that the difficultly in determing contact types in the traditional 3D discontinuous deformation analysis (3D-DDA) method is avoided in the 3D-NCDDAM. The proposed 3D-NCDDAM can be treated as a hybrid method, which inherite both the advantage of DDA method and FDEM. To validate the 3D-NCDDAM, seven numerical examples are conducted. The numerical results indicte that the 3D-NCDDAM can effectively model the failure process of rock masses in both experimental scale and engineering scale.

Damage Behavior Study of Specimens with Double-Prefabricated Cracks under Dynamic–Static Coupling Loads

The surrounding rock of the deep-buried chamber contains high-ground stress and initial cracks. Under a dynamic load, cracks will develop and expand, leading to the fracture and collapse of the confining pressure. Therefore, it is essential to study the failure process of fissured surrounding rock under the joint action of static stress and a dynamic load. In this paper, samples with cracks are used to simulate the defective rock mass. Similar modeling tests and numerical simulation studies were carried out to reveal the damage process of cracked deep rock mass under dynamic disturbance and investigate the impact threshold of rock mass damage under a certain level of hydrostatic pressure. The model test investigated the damage behavior for specimens with double-prefabrication cracks under pressure from a dynamic–static coupling load. The influence of the mechanisms of the angle of a crack, the initial static pressure, and impact capacity on specimen damage was analyzed. It was perceived that, with an increase in the angle of the crack, the omen of specimen damage is less obvious, and the specimen is subjected to sudden damage. On this basis, the damage process of the specimen containing prefabricated cracks under combined dynamic and static loads is realized through numerical simulation, and tests verify the accuracy of the results. The analysis allowed us to come up with a variation rule for the single-disturbance energy threshold for specimens with a prefabricated crack angle and the initial static load level of the specimen containing double-prefabrication cracks. The study lays the foundation for the future analysis of any deep rock mass failure process under dynamic disturbance and the protection of a deeply buried chamber.

Discrete Element Study on Mesoscopic Shear Mechanical Behavior of Fully Weathered Coastal Red Sandstone Considering Different Rock Mass Stability Grades

The macro parameters such as cohesion and internal friction angle are usually measured on experimental test. The macro shear mechanical behaviors are determined by mesoscopic shear mechanical behaviors, which are difficult to be measured accurately on experimental test. This paper aims to study the mesoscopic shear mechanical behaviors of fully weathered coastal red sandstone (FWCRS) such as the force and displacement of rock particles, the force chain network, the microcracks and the coordination number, which can be systematically studied by discrete element software such as Particle Flow Code (PFC). This paper established models characterizing FWCRS with different rock mass stability grades by results obtained on experimental tests, and investigated the differences of their mesoscopic shear mechanical behaviors. The results show that the mesoscopic shear mechanical behaviors of FWCRS can be characterized by performing direct shear simulation and the differences among different rock mass stability grades were also revealed. As the stability increases from grade 1 to grade 5, the shear stiffness increases, the number of microcracks reduces by 48.07%, and there are significant differences in properties of shear band, development of microcrack, the coordination number and the force chain network. The research results can provide reference for the related research such as the evolution of mesoscopic shear mechanical behaviors in the failure process of rock mass in FWCRS and similar strata.

An XFEM-based hydro-mechanical model to consider the non-Darcy flow in complex fracture networks

Geotechnical instability and failure are closely related to the hydro-mechanical coupled behavior of fractured rocks, and the existing studies often regard the fracture seepage as linear Darcy flow. Due to the neglect of non-Darcy effect caused by rough rock wall and high flow velocity, the estimated rock deformation and seepage flow can lead to incorrect assessment of geotechnical risks. Hence, this study proposed an efficient and robust hydro-mechanical (HM) coupled model to consider non-Darcy flow in complex fracture networks. The fractured rock mass's mechanical response, including the fracture surface's frictional contact, was calculated using the XFEM solver, and the widely used Forchheimer's law was adopted to describe the non-Darcy flow along the fractures. The fracture segments cut by mechanical element edges are considered one-dimensional flow elements, and the Newton-Raphson iteration method is adopted to solve the Forchheimer equation directly. To consider the influence of fracture deformation on the conductivity of fractures, the Barton-Bandis model was adopted to determine the hydraulic aperture of the fractures when the fracture was closed. Subsequently, a staggering Newton-Raphson method can be developed to decouple the fracture deformation and fluid flow process. Experimental results were adopted to validate the accuracy of the numerical model, and several numerical cases were used to illustrate the efficiency and robustness of the proposed model. The effects of inlet pressure, confining stress, and non-Darcy effect on the HM-coupled behavior of fractured rock were investigated. Numerical results suggested that the distribution of fluid pressure and failure process of rock mass based on Darcy assumption could be dramatically changed after considering the nonlinear seepage behavior.

Peak strength, coalescence and failure processes of rock-like materials containing preexisting joints and circular holes under uniaxial compression: Experimental and numerical study

The stability of rock masses is greatly influenced by the voids and joints present in rock masses; therefore, the peak strength, coalescence and failure process of rock masses with preexisting joints and holes should be systematically studied. To investigate the mechanical properties and failure mechanism of rock masses with circular holes and preexisting joints, rock-like materials were prepared, and uniaxial compression tests were performed. An analysis of physical test results showed that the coalescence patterns of rock bridges can be classified into 6 types, and the failure modes have 3 types. To further study the mechanical characteristics of rock-like materials with preexisting joints and circular holes, PFC2D numerical simulations were conducted, and the Bayesian optimization method was applied to obtain the microparameters of the PFC2D models. Through analysis of the numerical simulation results, it was concluded that the failure process of the specimen can be divided into four stages: stage I, stage II, stage III and stage IV. No cracks appeared in stage I, and few cracks appeared in stage III and stage IV. In stage IV, the specimen broke, and a large number of cracks appeared. These studies can serve as an experimental and numerical basis for the stability and fracture analysis of rock masses with preexisting joints and circular holes.

Analysis of Crack-Characteristic Stress and Energy Characteristics of Sandstone under Triaxial Unloading Confining Pressure

The deformation and failure of underground engineering are usually caused by unloading. In this work, triaxial unloading confining pressure tests are carried out to simulate the failure process of rock mass caused by unloading, analyze the crack-characteristic stress, and study the energy evolution of rock under unloading and the pre-peak and post-peak energy characteristics combined with the energy theory. The results show that, when the confining pressure increases from 5 MPa to 20 MPa, crack closure stress σcc, crack initiation stress σci, dilatancy stress σcd, and peak stress σp are 6.34 times, 2.75 times, 1.93 times, and 1.66 times higher than the original, respectively. By comparing the increase in crack-characteristic stress, it can be found that the confining pressure has a large effect on the crack closure stress and crack initiation stress, while the dilatation stress and peak stress have relatively little influence. From the perspective of energy evolution, the pre-peak axial absorption energy U1 increases exponentially, the elastic energy Ue is similar to U1, and the circumferential consumption energy U3 and dissipation energy Ud are small. After reaching the peak stress, the growth rate of U1 decreases slightly, Ue decreases rapidly, and U3 increases rapidly but only as a small fraction of the total energy, while Ud grows almost exponentially and rapidly becomes the main part of the energy. Under each crack-characteristic stress state, the energy characteristic parameters gradually increase with the increase in confining pressure, which is manifested by the increase in slope in the linear fitting formula of energy characteristic parameters. The release process of the releasable elastic energy after the peak stress can be divided into three stages of “slow–fast–slow”, and the energy release process shows an obvious confining pressure effect.

Nonlinear time series analysis of limestone rock failure process

Nonlinear analysis of the rock mass failure process is a powerful tool for the study of instability and prevention of disaster in rock mass engineering. Nonlinear analysis has rapidly developed in recent decades. We propose a new method to determine the fractal characteristics of the failure process using time series analysis based on an iterative cycle division of the box dimension of fractal theory. We use AE technology to carry out time series analysis of fracture sources in the failure process of rock mass. We show that the disordered failure of rock mass exhibits notable fractal characteristics in the time series. The advantage of this method is that only the time and frequency of AE monitoring are used, resulting in a better identification and classification of damage, compared with traditional natural sequence frequency distribution. In addition, MATLAB numerical simulation analysis shows that these time series fractal dimensions can accurately characterize the failure characteristics of rock mass. In brief, we show the potential for future orderly time analyses of the complex and disorderly process of rock mass failure.

Progressive failure and friction motion characteristics of contact surface of composite rock mass

The structural planes existing in natural rock mass can control the failure process of rock mass. Based on the progressive response-stability failure process of a single fracture interface, the relationship between progressive response and material failure of composite rock mass is discussed. The method of friction contact plane analysis in PANDAS numerical analysis software is applied to explore the correlation between the movement and mechanical properties of composite rock mass under external forces. The motion index is mainly represented by the sliding speed and distance of the contact surface, while the mechanical properties are mainly explained by the normal contact force, the friction of the contact surface, the friction coefficient and the material damage. The relationship between these six variables illustrates the progressive response relationship during the movement of the composite rock mass. Based on the static and progressive characteristics of the progressive response process of deep rock mass engineering, the failure evolution law and energy dissipation law of composite rock mass loading process and the synergistic failure characteristics of composite rock mass are discussed. The scientific problems that need to be studied in the structure, deformation and failure of deep rock mass are put forward. The reference is provided for the failure law of fractured rock mass in water inrush process under the influence of mining, as well as fault plate material and the study of mechanical state of rock mass in fault fracture zone.

Microseismic Monitoring, Positioning Principle, and Sensor Layout Strategy of Rock Mass Engineering

Microseismic monitoring technology can start from the most initial stage of rock deformation and track and monitor the progressive failure process of rock mass from the fracture of rock blocks to the instability of rock mass. Thus, the scientific nature of monitoring work is greatly promoted, and the accuracy and advance of the prediction of engineering and geological disasters are improved. In this paper, the fracture and instability of rock can be analyzed by analyzing the microseismic signals produced by rock failure; The location of microseismic source can be determined by multipoint synchronous data acquisition to determine the time when each sensor (at least 5) receives microseismic signals; Combined with practical engineering experience for underground engineering in growth, heading tunnel, put forward only sensor arrangement to take mobile, follow the semienclosed layout network. We hope to give some reference to the researchers who are concerned with microseismic monitoring technology.

Crack Initiation Behaviors of Granite Specimens Containing Crossing‐Double‐Flaws with Different Lengths under Uniaxial Loading

Crack initiation is an important stage in the failure process of rock masses. In this paper, crack initiation behaviors (crack initiation model, crack initiation location, crack initiation angle, and crack initiation stress) of granite specimens containing crossing‐double‐flaws with different lengths were investigated using PFC2D software. Crack initiation models were all tensile wing cracks, which did not exactly initiate from the main flaw with a length of 30 mm. They can initiate from the secondary flaw with a length 20 mm at α of 30° (included angle between main flaw and horizontal direction) and β of 90° (included angle between main and secondary flaws) and from main and secondary flaws at α of 30° and β of 60°. These were mainly induced by the superposition of stress fields around the main and secondary flaws as β varied from 0° to 90°, especially the tensile force concentration zones superposition. The tensile forces concentration zone around flaw shrank towards flaw tips with the increase of flaw’s inclinations measured horizontally. Under stress field superposition effects, the crack initiation stress decreased firstly and then increased with β at α of 30° and 45°. Crack initiation locations were close to flaw tips but not restricted to them. The distances between crack initiation locations and flaw tips, and the crack initiation angles depended on the flaw where first macrocracks initiated from. Microdisplacement field distributions of granite specimens to reveal the mesomechanism of crack initiation behaviors were discussed.

Experimental and numerical study on mechanical and fracture behavior of rock-like specimens containing pre-existing holes flaws

Laboratory tests and numerical simulation are conducted to study mechanical behavior of rock containing two pre-existing holes in this paper. First of all, rock-like specimens containing two pre-existing holes were prepared to investigate mechanical and acoustic emission characteristics in the uniaxial compression. Different geometries were obtained by changing the angle of the center line of holes with respect to the direction of axial loading. Experimental results show that the stress-strain curves present obvious fluctuation, and evolution process of acoustic emission signal can be divided into four stages. There are three kinds of failure modes: only shear failure, mainly shear and auxiliary tensile failure, mainly tensile and auxiliary shear failure. Then, parallel bonded mode (PBM) based on the particle flow code in two dimensions (PFC2D) were established to study strength and failure modes in the biaxial compression. Finally, process of cracks initiation, propagation and coalescence, and fracture mechanism were further discussed. The most important conclusion reached is that the deformation and failure process of rock mass containing holes defects can be divided into two stages: rock bridge penetration and overall instability, which are sequential.

An Improved Discontinuous Deformation Analysis to Solve Both Shear and Tensile Failure Problems

This paper is mainly aimed to present an improved DDA (discontinuous deformation analysis) that can deal well with both shear and tensile failure problems. Firstly, the shear mechanism of DDA is detailed investigated. The results show that when handling the frictional interface, the critical shear resistance can be accurately determined only if the penalty value is carefully selected, however, when handling the cohesive interface, the critical shear resistance is significantly underestimated. The inaccurate prediction is due to the inconsistent distribution of normal force and shear force between the two vertex-to-edge contacts in one edge-to-edge contact. Here an edge-to-edge treatment is introduced into DDA. Secondly, to moderately reflect the tensile failure process of rock masses, a two-phase constitutive model is introduced into the DDA with edge-to-edge treatment, and the improved DDA is obtained. Finally, the improved DDA is used to simulate the failure process of gypsum centrifuge model. The results show the improved DDA can deal well with rock failure problems by shear or tension failure.

Method for Generating a Discrete Fracture Network from Microseismic Data and its Application in Analyzing the Permeability of Rock Masses: a Case Study

The deformation and failure process of rock masses is accompanied by the initiation, propagation and connection of fractures. The behaviors of fractures under engineering disturbance can be effectively determined by microseismic (MS) monitoring, and such information is essential for the stability analysis of rock masses. Based on moment tensor theory, the geometric properties of focal planes and the failure mechanisms of the seismic source can be determined, and this study proposes criteria for extracting the reasonable fracture from focal planes associated with different failure mechanisms and provides a detailed example. The fracture identified by the proposed criteria is called MS-derived fracture, and a formula describing the aperture is derived from the moment tensor theory and the motion characteristics of the MS-derived fractures associated with different failure mechanisms. This work chose the Shirengou Iron Mine as a case and selected an area exhibiting seepage and rock mass failure as the study area. Based on a three-dimensional noncontact discontinuity scan, a natural discrete fracture network (DFN) was generated. With the help of the proposed fracture generation method, a new DFN based on MS data was generated and is referred to as the MS-derived DFN. With the aid of Oda’s theory, the changes in the permeability value, principal direction and anisotropy degree in the study area were analyzed based on the MS-derived DFN, and the seepage channels in the study area were also determined. Therefore, the research methods in this paper could be used to better understand the changes in the permeability of rock masses based on MS data.

Dynamic reduction of rock mass mechanical parameters based on numerical simulation and microseismic data – A case study

Rock mass mechanical parameters change dynamically over time and space, and this process is accompanied by the release of seismic signals. With the aid of microseismic (MS) monitoring, the temporal, spatial and intensity source parameters that can reflect the rock mass damage were extracted from MS data, and the rock mass damage coefficient based on MS multiple source parameters was established. Taking Shirengou Iron Mine as a test site, discontinuity scanning was carried out, and the size of the representative elementary volume (REV) was obtained by analyzing the change of the joint mass density P32 with different lengths of the statistical cube. According to the spatial-temporal variation of source parameters that were used to construct the damage coefficient, the failure process and mechanisms were analyzed and the failure period was divided into five stages. The damage coefficient of each REV was calculated on the basis of the MS data combined with the rock mass damage, and the damage process in the rock mass failure process was also analyzed. Then, the REV damage coefficients in different time periods were embedded into the FLAC3D computing program as inputs, and with the combination of the reduction formula, the rock mass mechanical parameters were dynamically modified. Taking shear strength as a representative property, the shear strength changes in different time periods were analyzed. In addition, the change of rock mass shear failure zones was analyzed. By combining the rock mass damage coefficient established by multiple source parameters with numerical simulation, the dynamic reduction of rock mass mechanical parameters can be realized, providing a new quantitative analysis method for dynamic evaluation of rock mass stability.

A Quantitative Strain Energy Indicator for Predicting the Failure of Laboratory-Scale Rock Samples: Application to Shale Rock

Strain energy is one of the key factors that control the brittle failure process of rock mass, such as the onset of accelerating displacement. The phenomenon of accelerating displacement is a necessary and important precursor to predict the brittle failure of rock, and it is attributed to the unstable propagation of micro-cracks when the load reaches or exceeds the crack damage stress threshold. Thus, for the design of underground rock engineering and to ensure proper safety practices, it is important to propose a suitable strain energy criterion that uses the strain energy at the crack damage stress threshold (Wcd) to predict the strain energy at final failure (Wucs). For this purpose, a quantitative strain energy indicator (Wucs/Wcd) was proposed based on two-dimensional renormalization group theory for the prediction of the brittle failure of laboratory-scale rock samples subjected to uniaxial pressure. The indicator is a function of the homogeneity index, m, of the Weibull distribution. The reliability of the strain energy indicator was verified by a series of uniaxial compression tests on 12 shale samples that had different homogeneity indices, m. The heterogeneity of the sample was dominated by different bedding dip angles. The experimental results showed that the quantitative indicator could be used to predict the brittle failure of samples of any type of rock at the laboratory scale using their m values. In addition, the effect of the bedding dip angle on the homogeneity index, m, and the strain energy indicator were discussed based on the experimental results to provide a better understanding of the relationships among heterogeneity, the strain energy indicator, and the other physical and mechanical properties of the shale samples that were tested. Also, the discussion included the effects of the homogeneity of the sample and the strain energy indicator on the physical and mechanical properties of the samples. As a result, some conclusions were obtained that might be applicable to laboratory-scale shale samples.

Study of the Rock Mass Failure Process and Mechanisms During the Transformation from Open-Pit to Underground Mining Based on Microseismic Monitoring

To quantitatively understand the failure process and failure mechanism of a rock mass during the transformation from open-pit mining to underground mining, the Shirengou Iron Mine was selected as an engineering project case study. The study area was determined using the rock mass basic quality classification method and the kinematic analysis method. Based on the analysis of the variations in apparent stress and apparent volume over time, the rock mass failure process was analyzed. According to the recent research on the temporal and spatial change of microseismic events in location, energy, apparent stress, and displacement, the migration characteristics of rock mass damage were studied. A hybrid moment tensor inversion method was used to determine the rock mass fracture source mechanisms, the fracture orientations, and fracture scales. The fracture area can be divided into three zones: Zone A, Zone B, and Zone C. A statistical analysis of the orientation information of the fracture planes orientations was carried out, and four dominant fracture planes were obtained. Finally, the slip tendency analysis method was employed, and the unstable fracture planes were obtained. The results show: (1) The microseismic monitoring and hybrid moment tensor analysis can effectively analyze the failure process and failure mechanism of rock mass, (2) during the transformation from open-pit to underground mining, the failure type of rock mass is mainly shear failure and the tensile failure is mostly concentrated in the roof of goafs, and (3) the rock mass of the pit bottom and the upper of goaf No. 18 have the possibility of further damage.

New models to predict rock failure by linking the volume dilatant point with the peak stress point

The rock mass failure process can be divided into several distinct deformation stages: the compaction stage, elastic stage, stable failure stage, accelerated failure stage, and post-peak stage. Although each stage has been well studied, the relationship among the stages has not been established. Here, we establish two models which are the Strain model Q and Energy density model S by using the renormalization group theory and investigate the mechanical relationship between the volume dilatant point and peak stress point on the rock stress-strain curve. Our models show that the strain ratio (e f /e c ) and energy ratio (E f /E c ) at the volume dilatant point and peak stress point are solely functions of the shape parameter m. To verify our models, we further studied the failure process of rock specimens through several uniaxial compression experiments and found that the relationship between e f /e c or E f /E c and m shares a notably similar pattern to that from our theoretical model. However, the e f /e c and E f /E c values in our experiments are slightly smaller than those predicted by the models. In brief, we demonstrate that our models can be used to predict the failure process of the laboratory-scale hard brittle rock samples.

Experimental Study on Cracking, Reinforcement, and Overall Stability of the Xiaowan Super-High Arch Dam

The Xiaowan super-high arch dam has faced challenging construction problems. Here, we provide a scientifically-based reference for applying geomechanical model testing to support the nonlinear design of super-high arch dams. We applied experimental similarity theory and techniques. Based on four 3D geomechanical model tests, the dam stress characteristics, deformation distribution, and the safety factors of the dam foundation were identified and compared. We also analyzed cracking characteristics of the up- and downstream dam surfaces and induced joints in the dam heel, the rock mass failure process of the dam-foundation interface, and the abutments. We propose foundation reinforcement measures for weak rock masses, alteration zones, and other faults in the abutments based on the 3D and plane tests each at a different elevation. The results show that all dam deformations remained normal with no yielding or tensile cracking under a normal water load. The reinforced rock mass increased the crack initial safety in the dam heel and toe by ~20 %. The minimum crack initial safety factor (K 1) of the dam heel was 1.4. The induced joint in the dam heel contributed to a reduction in tensile stress at the upstream dam heel, improving K 1. Compared with similar projects following reinforcement measures, the abutment stiffness and overall stability of the Xiaowan arch dam satisfy operational requirements. Four years of monitoring operations show that key areas near the dam remained normal and the dam foundation is functioning well. Our results may also be applicable to the design and construction of similar projects worldwide.