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Failure Mechanism of Highly Stressed Rock Mass during Unloading Based on the Stress Arch Theory
Abstract
During excavation unloading, highly stressed rock masses at great depth exhibit failure modes with different scales, for instance, zonal disintegration, rockbursts, and super-low friction. The root cause of the failure mechanisms of highly stressed rock masses differs. In recent years, research on the failure mechanism of highly stressed rock masses during unloading has become a frontier topic in geotechnical engineering. This paper relies on the deep mining process laboratory of the China Hongtoushan Copper Mine, where it has been observed that the deep rock unloading process causes macroscopic deformation and opens microscopic joints in deep rock. Analysis of monitoring data has shown that compressional deformation occurs in the surrounding rock masses subjected to unloading during mining. It has also verified the engineering phenomenon of local compression in rockbolts. Test materials similar to deep rock were used to simulate the unloading process of in situ rock mass and verify the physical phenomenon of shrinkage of the rock mass in the unloading area. Based on this phenomenon, the stress arch theory for rock mass failure mode has been established. Finally, numerical simulation analysis of the influence of different radii of the curvature fissures in surrounding rock and the displacement field distribution of the stress field was used to verify the stress arch theory for the failure mechanism of highly stressed rock mass during unloading.
... e overlying weight has been resisted by the pressure arch finally. e formation of a pressure arch could prevent the further deformation or movement of the rock masses [13][14][15][16]. ...
... Hence the numerical simulation method has gradually become the priority method to research the pressure arch. e finite difference method (FDM, e.g., FLAC, FlAC3D) and the finite element method (FEM, e.g., ABAQUS, ANSYS), which are based on the continuous medium, have been widely used to reflect the redistribution of the stress and the development of the pressure arch under the various parameters or the different engineering conditions [15][16][17][18], [24][25][26][27]. However, the continuous medium method could not well reflect the regularity of the movement of the rock masses. ...
The self-bearing capacity of the rock masses has been a hot pot in the research of supporting technology in underground engineering. The pressure arch has been considered the main hypothesis to explain the self-bearing capacity of the rock masses. In this paper, the engineering of the twin-parallel openings in the jointed rock masses was selected as the background of the research. The characteristics of the deformation or the movement of the rock masses above the openings were studied based on the pressure arch theory. The difference between the single opening and the twin-parallel openings has been revealed, and the mechanism has also been illustrated. Besides, the joints’ strength, the in situ stress ratio, the distance between the two openings, and the excavation sequence were selected as the variable parameters, and their influences have also been illustrated by the DEM method.
... Yue used numerical simulation to obtain the shape parameters of the stress arch in close-distance coal mining and thus obtained the distribution of additional stress in the floor roadway [6]. Some scholars have also studied this subject [7][8][9][10]. ...
In order to mater the reasonable layout basis of the terminal mining line position in the repeated mining of close-distance thick coal seams, taking Yan mine as the engineering background, we conducted theoretical analyses, numerical simulations, and field measurements to study the action mechanism of the stress arch. The results show that (1) with repeated mining, the shape of the left and right half arches of the stress arch changes in the order of time and space until it is stable; (2) the shape of the stress arch is highly related to the distribution of abutment pressure in the working face. If the shape is unchanged, the distribution of abutment pressure is unchanged; (3) in the final mining stage of repeated mining, when the shape of the right half arch is stable, the difference stagger distance of the terminal mining line has little effect on the distribution of abutment pressure of the working face where the front arch foot is located; (4) when the internal stagger distance between 3216 working face and terminal mining line of 4216 working face is greater than 22 m or the external stagger distance is greater than 30 m, 3216 working face is located in a relatively safe position. This study clarifies the key factors for the layout of terminal mining lines in close-distance thick coal seams, which can provide a scientific basis for similar projects.
... Natural disasters in deep underground areas occur frequently Kouame et al. 2017;Qiao et al. 2020;You et al. 2018). However, in existing literature, studies on the triaxial tensile tests of rock specimens under high ground stress and their fracture mechanism are lacking (Guo et al. 2013;Jin et al. 2018;Li et al. 2018;Liang et al. 2019;Ramsey and Chester 2004;Zeng et al. 2019;Zhou and Bi 2012). Therefore, an unloading device suitable for laboratory use is required to study the action path of triaxial compression followed by tensile unloading to failure. ...
... In particular, when mining deep coal resources Liu et al. 2016;Ranjith et al. 2017;Li et al. 2017; Wang and Li 2017;Song et al. 2020), under the combined influence of three high and one disturbance, super-large section, and close-distance, the stability of the chambers' surrounding rock becomes more prominent, which seriously affects safe production in deep coal mining. The main difficulties are: (1) the surrounding rock stress environment of the chambers becomes complex due to the combination of high geostress and strong tectonic stress in deep coal mines; and (2) the mechanical properties of coal and rock changed significantly, which has a greater impact on the deformation and plastic zone development of the surrounding rock (Shen 2014;Liu et al. 2018;Wang et al. 2019a;Zhao et al. 2018;Jin et al. 2018;Guo et al. 2018). However, it usually takes a long time for the rock that surrounds super-large section chambers to converge and stabilize. ...
The stability of surrounding rock for large or super-large section chambers is extremely hard to control in deep coal mines, as well as in close-distance chambers. Based on the on-site geological conditions of the super-large-section chamber group of a coal gangues separation system in Longgu Coal Mine, Heze City, Shandong, China, this paper will establish a FLAC3D numerical simulation model to research the failure evolution of the surrounding rock under different chamber spacings and thicknesses of the loosening circle. The results show that the interaction between chambers increased with the decrease of chamber spacing and the increase in the loosening circle thickness, the critical spacing (L2) range was 15-10 m, and the critical thickness of the loosening circle was 4-5 m. Based on this, the instability mechanical models of the surrounding rock that is controlled by chamber spacing and loosening circle will be established, respectively. Then, L2 and loosening circle radius without chain instability (R*a) will be obtained theoretically. Finally, the theoretical verification will be realized by on-site calculations. This research revealed the chain instability mechanism of the chamber surrounding rock and provided theoretical guidance and reference for the close-distance parallel chamber with a super-large section design under deep mining conditions.
... The exploration and exploitation of deep resources have been the strategic direction of medium-and long-term scientific and technological development in China (Yang et al. 2017(Yang et al. , 2018. As the buried depth increases, with the influences of high geostress, strong rheology, strong mining, crushing, and other factors, the surrounding rock of roadway gradually transfers from small, steady deformation in shallow parts to large nonlinear rheological deformation in deep parts, which then seriously restricts the stability of surrounding rock and safe production of coal mines (Jin et al. 2018). Thus, it is of great engineering significance and theoretical value to investigate the creep mechanical properties, damage evolution and failure mechanisms, and the corresponding constitutive model of surrounding rock in deep-buried roadways Fu et al. 2019;Li et al. 2017;Yang et al. 2014). ...
A typical sandy mudstone from the deep-buried roadway of the Kouzidong coal mine was taken as the research object. Based on the micro-mesoscopic experiments combining the acoustic emission (AE) and digital speckle correlation method (DSCM), as well as numerical simulations based on the discrete element method (DEM), both the micro-mesoscopic creep damage evolution and the failure mechanism of sandy mudstone were investigated. The experimental results show that the AE curve of sandy mudstone in the multilevel creep test is consistent with the axial strain curve. Three stages, including the decelerative, isokinetic, and accelerative creep stage, can be identified. The three-dimensional source location of AE events shows that the creep loading leads to more severe, homogeneous, and dispersed microdamage in sandy mudstone and a looser failure mode. It was also found by DSCM techniques that the microscopic deformation field evolution of sandy mudstone in the creep process well corresponds to the three stages of creep failure. A transition of microscopic deformation field from random, via symmetrical, to asymmetrical distribution, can be observed during the creep failure process of sandy mudstone. By introducing the time-dependent stress corrosion theory into the existing parallel-bonded model (PBM) in particle flow code (PFC), the microdamage evolution mechanism of sandy mudstone under creep loading was studied both theoretically and numerically, and the corresponding results revalidate our experimental findings.
In order to determine the reasonable width of a stopping coal pillar in close-distance coal seams, the evolution law of front abutment pressure of the working face with repeated mining was studied. Based on the actual engineering project, we conducted field measurement, theoretical analyses, numerical simulations and a physical similarity simulation test to study. The results show that: (1) according to field measurement, the influence range of front abutment pressure increases from 60 m to 75 m with repeated mining; (2) according to theoretical analysis, the arch height and span are negatively and positively correlated with the influence range of front abutment pressure, respectively; (3) with repeated mining, the arch height increased to 165 m, the arch span to 235 m and the influence range to 83.5 m by 14.5 m relative to that before repeated mining; (4) if it is necessary to ensure that the main roadway is less affected by the mining stress, the width of the stopping coal pillar in 2214 working face should be greater than 80 m. The influence range of front abutment pressure increases obviously with repeated mining in close-distance coal seams. The study provides a reference for similar engineering projects.
According to the different stress paths, similar model test and PFC simulation test of tunnel surrounding rock are designed to compare the failure mechanisms at macroscopic and mesoscopic scales. The following conclusions are drawn. 1) Excavation unloading will disturb the surrounding rock to form a certain excavation damaged zone. 2) Under the loading path, the stress of surrounding rock failure is 1.500 MPa; under the unloading path with initial stress of 60% σZmax and 100% σZmax, the failure stress is 1.583 and 1.833 MPa respectively in the model test. 3) In terms of the failure mode of rocks under different stress paths, tensile fractures first appear in two sides of the vertical walls; thereafter, the spandrel and arch foot are loosened due to the stress concentration. The fractures gradually coalesce with those occurring in the vertical walls. 4) In the process of excavation unloading, the proportion of shear cracks is 35.3%, and the rock is subject to strong shear effect. The final failure surface is approximately V-shaped. 5) The tangential peak stress on the vertical walls at the free face is the lowest; the vertical walls at the free face show the poorest bearing capacity and are easily subjected to tensile failure.
Uniaxial compressive tests were performed on Y-jointed granite specimens to explore how intersecting rough joints affect the strength, failure mode, and precursory damage characteristics of the jointed rock mass. The mechanical properties, joint dip angles, and joint roughness of the specimens were measured prior to the tests, then the loading stress, surface displacement fields, acoustic emission (AE) signals, and failure modes were recorded during the loading process. As the uniaxial compressive strength (σc) of the specimens increased, three different failure modes appeared: shear failure along the persistent joint with rock wedge falling, compressive failure with rock wedge ejection, and overall crushing accompanied with rock fragment splashing, the σc thresholds of the three failure modes are about 50 MPa and 90 MPa, respectively. The deformability of the specimens weakened gradually as σc increased. AE events spread from the joint surface to the interior of the rock block until final failure. For specimens that failed in shear mode, multiple sudden stress drops along with sharp displacement increases occurred during the tests but no entire failure occurred. Other specimens tended to present one eventual unstable failure. When the peak shear strength of the non-persistent joint was higher than or near to that of the persistent joint, the σc of a given Y-jointed specimen was mainly determined by the persistent joint. Otherwise, it was simultaneously controlled by the two joints. Both the joint dip angle and joint roughness were found to significantly influence the σc of the specimens. Each stress drop was accompanied by significant AE signal variations, though the AE indicators are difficult to effectively utilize for early warning of specimen failure.
Failure characteristics of rocks under different loading and unloading conditions have been an important subject of laboratory tests. This study aims to investigate the influence of the height to diameter (H/D) ratio on the failure characteristics of marble specimens under unloading conditions. Conventional triaxial unloading (TU) tests were performed on marble specimens of 50 mm diameter with different H/D ratios from 0.5 to 3.0. Moreover, for comparison, uniaxial compression (UC) tests and conventional triaxial compression (TC) tests were conducted on marble specimens with different H/D ratios. The results indicate that the failure strengths of the specimens, including the UC strength, TC strength, and TU strength decrease with the increase in specimen height, and the decreasing trend can be fitted with a function that is deduced from Weibull's theory. The specimens with lower H/D ratios (0.5 and 1.0) underwent tensile failures in the TU tests, whereas the specimens with higher H/D ratios (2.0 and 3.0) showed shear failures in the TU tests. In addition, the analyses of strain energy evolutions indicate that the unloading failures of the specimens with lower H/D ratios are an ongoing process and the unloading failures of the specimens with higher H/D ratios present a sudden process.
An exact approach is used to discuss the Love wave behavior in a composite reinforced structure. In this body, the reinforced medium is covered by an initially stressed isotropically half-space from above and an initially stressed orthotropically half-space from below. The generalized dispersion relation of the Love wave is obtained in the presence of associated initial stress and reinforced parameters, which approve the significant effect of these parameters on the propagation of the Love wave in a composite fiber-reinforced structure. The obtained dispersion equation is in closed form, which is in agreement with the classical Love wave equation. Moreover, some important peculiarities are observed in the graphs.
We investigated the statistical characteristics and probability distribution of the mechanical parameters of natural rock using triaxial compression tests. Twenty cores of Jinping marble were tested under each different levels of confining stress (i.e., 5, 10, 20, 30, and 40 MPa). From these full stress–strain data, we summarized the numerical characteristics and determined the probability distribution form of several important mechanical parameters, including deformational parameters, characteristic strength, characteristic strains, and failure angle. The statistical proofs relating to the mechanical parameters of rock presented new information about the marble’s probabilistic distribution characteristics. The normal and log-normal distributions were appropriate for describing random strengths of rock; the coefficients of variation of the peak strengths had no relationship to the confining stress; the only acceptable random distribution for both Young’s elastic modulus and Poisson’s ratio was the log-normal function; and the cohesive strength had a different probability distribution pattern than the frictional angle. The triaxial tests and statistical analysis also provided experimental evidence for deciding the minimum reliable number of experimental sample and for picking appropriate parameter distributions to use in reliability calculations for rock engineering.
ISRM suggested method for rock fractures observations using a borehole digital optical televiewer is reviewed. According to in situ observation of the evolution characteristics of fractures subject to excavation or rheological effect the EDZ in the rock mass is identified based on the new fractures observable via the precision of the digital optical borehole camera. The apparatus related to this in situ observation of rock mass fractures mainly consists of the probe, depth measuring device, integrating control box, data logger cables, and alternative measuring rods for horizontal or inclined boreholes. The borehole wall in the panoramic image is represented as a ring, in which the inner circle is the upper end of the borehole wall and the outer circle is the lower end. The position of a point on the borehole wall as displayed on the ring is related to the azimuth of this point. The integrating control box is the power supply for the whole testing system, which controls the collection, input and output of the panoramic video signal and depth pulse signal.
In the process of excavating seven parallel tunnels at the Jinping II Hydropower Station, several extremely intense rockbursts occurred, killing and injuring construction workers and damaging several sets of equipment. Based on the characteristics and mechanisms of these rockbursts, four typical events were selected and their temporal and spatial characteristics were here described in detail. The geological conditions revealed after the rockbursts were surveyed carefully. The responses of support elements were also analyzed. The details documented in each case provide not only an important reference for understanding the development mechanisms of rockbursts but also a basis for the selection and development of rockburst prevention measures in deep hard rock tunnels.
The propagation of Love waves in an orthotropic granular layer under initial stress overlying a semi-infinite granular medium is investigated in this paper. A Fourier transform method has been applied to find the dispersion equation. The dispersion equation is obtained in the form of a ninth-order determinantal expression. The standard equation of dispersion in two-layered isotropic media is deduced as a particular case from this study. The results obtained are displayed graphically and there physical meaning is explained.
A simple practical equivalent continuum numerical model for simulating the behavior of jointed rock mass has been extended to three-dimensional using FLAC3D. This model estimates the properties of jointed rock mass from the properties of intact rock and a joint factor (Jf), which is the integration of the properties of joints to take care of the effects of frequency, orientation, and strength of joint. A new FISH function has been written in FLAC3D specifically for modeling jointed rocks using the Duncan and Chang hyperbolic model. This model has been validated first with simple element tests at different confining pressures for different rocks with different joint configurations. Explicit modeling of the joints has also been done in element tests and results obtained compare well with the results of equivalent continuum model and also with experimental results. Further, this has been applied for a case study of a large underground power house cavern in the Himalayas. The analysis has been done under various stages of excavation, assigning a null model available in FLAC3D for simulating the excavation.
It is well known that acoustic emission (AE) and microseismic (MS) events are indicators of rock fracturing or damage as the rock is brought to failure at high stress. By capturing the microseismic events, underground excavation induced rock mass degradation or damage can be located. The use of microseismic method has been shown as a valuable tool in a number of nuclear waste repository research programs to monitor the extent of the excavation damaged zone (EDZ), but most of the works are limited to a qualitative assessment.This paper presents a study on the quantification of the degree of damage, in terms of crack density calculated from the crack length, and the extent, in terms of crack density distribution, from microseismic event monitoring data. The approach builds on the finding that a realistic crack size corresponding to a microseismic event can be established by applying a tensile cracking model instead of the traditional shear model, commonly used in earthquake data analysis. It can be shown that brittle rock failure is the result of tensile crack initiation, propagation, accumulation, and interaction. Tensile stress can be generated in a confined rock with heterogeneous material properties. When a crack is formed by tensile cracking in this fashion, its orientation tends to become parallel to the direction of maximum compressive stress. A method is developed to take microseismic event monitoring data as input to determine the damage state and the extent of the EDZ by crack distribution. Based on the crack orientation and crack density information, the rock is modeled by a micro-mechanics based constitutive model which considers the anisotropic material properties. Numerical examples are presented using field monitoring data from a tunnel in granite to demonstrate how microseismicity can be quantitatively linked to dynamic rock mass properties.
Rock mass damage assessment is required for many applications in rock engineering practice including support design, contamination transport control, stope design, amongst others. While various methods such as displacement measurement, seismic refraction, and direct observation using borehole camera have been used, relatively few efforts have been made to use microseismic monitoring to quantify the rock mass damage. From laboratory tests, it is well known that microseismic events are indicators of fracturing or rock damage as the rock mass is brought to failure at high stress. By capturing the microseismic events, underground excavation induced rock mass degradation or damage can be located but can the amount of damage in terms of changes to strength or deformation properties be measured?
Observations of brittle failure at the laboratory scale indicate that the brittle failure process involves the initiation, growth, and accumulation of micro-cracks. Around underground openings, observations have revealed that brittle failure is mainly a process of progressive slabbing resulting in a revised stable geometry that in many cases take the form of V-shaped notches. Continuum models with traditional failure criteria (e.g. Hoek–Brown or Mohr–Coulomb) based on the simultaneous mobilization of cohesive and frictional strength components have not been successful in predicting the extent and depth of brittle failure. This paper presents a continuum modelling approach that captures an essential component of brittle rock mass failure, that is, cohesion weakening and frictional strengthening (CWFS) as function of rock damage or plastic strain.
Based on the mechanical tests of fine siltstone which were conducted under different confining pressures, the effect of confining pressure to peak strength and failure form was analyzed, and the law between tension-shear deformation and confining pressure of whole process was researched. The study results showed as follows. The relationship curve of peak strength and confining pressure of fine siltstone is a polygonal line which has a inflexion. When confining pressure is less than the σ 3L of inflexion, the failure of fine siltstone is controlled by shear deformation, and its strength is influenced by tension deformation, so it will be less than the strength calculated by the Coulomb rule, and its rupture angle is larger than (45°+φ/2). When the confining pressure is higher than the σ 3L, the failure of fine siltstone is only controlled by shear deformation, its strength and rupture angle are as same as the Coulomb rule gives. Based on this, the index f t/f γ which is a reflection of tension deformation and shear deformation in the failure process was given, and the strength rule of fine siltstone under different confining pressures was constituted, which accorded with fact.
This chapter reviews anisotropy in rocks and distinguishes between its induced and inherent forms. After an examination of features and the nature of anisotropy, the influence of confining pressure and strength parameters are considered mathematically for both rock types, and finally rock mass analysis is detailed. -R.Gower
A physical model test using artificial rock material with two joint sets is under uniaxial compression. The anisotropy of failure strength for rock mass is shown.
A non-Euclidean continuum model for the descriptions of the elastic stress-field distributions and fractured zones in the
surrounding rock masses around the deep circular tunnels subjected to nonhydrostatic pressure are established. In the non-Euclidean
continuum model, the elastic stress-field distribution of the deep surrounding rock induced by compatible deformation of non-fractured
zones and incompatible deformation of fractured zones is determined. The wavy behavior of the stress components based on the
non-Euclidean model are obviously different from that of the stress components which have extrema on the working contour and
tend monotonically to the value of the in-situ stress at infinity in rock masses within the framework of the classical model.
Mohr-Coulomb criterion is applied to research the occurrence of disintegration zones. Disintegration zones appear when the
stresses in deep rock masses reach a certain critical value. It is found from the numerical results that the magnitude and
site of fractured zones depend on the value of in-situ stress and non-Euclideanness parameters.
Incremental development of a rock mechanics, rock fracturing, and excavation stability knowledge base has been ongoing at Atomic Energy of Canada Limited's (AECL's) Underground Research Laboratory (URL) since shaft excavation commenced in 1982. Excavation response experiments conducted as part of shaft construction recorded displacements, shaft convergence, stress changes, and microseismic events in the rock as excavation progressed. An excavation response test at the 240 Level of the URL involved similar monitoring, as well as pore pressure measurements, of a horizontal tunnel excavated through a subvertical water-bearing fracture. These precursor studies led to a series of experiments in the more highly stressed rock at the 420 Level of the URL to investigate the formation of rock damage around tunnels, and to assess the factors that influence the stability of excavations. The first of these experiments was the Mine-by Experiment, an excavation response study involving a mechanically excavated cylindrical tunnel in a pre-instrumented rock volume. The Heated Failure Tests were subsequently conducted in the same rock volume to assess the influence of thermal loading on damage development. These investigations were complemented by studies of borehole breakouts in adjacent excavations. The Excavation Stability Study, which involved excavating ten tunnel segments of different geometry to assess the effects of excavation design on stability and damage development, followed these experiments. The Tunnel Sealing Experiment, focused on developing sealing technologies, also provided insight into the excavation response of the rock mass. The excavation response experiments at the URL culminated in the Thermal-Mechanical Stability Study (TMSS), a comprehensive study to link characterization, numerical modeling, monitoring, and design of underground excavations. This paper provides an overview of the various experiments and studies leading up to the TMSS, highlighting the advances in our fundamental understanding of rock mechanics related to underground excavations, and in our means of designing stable underground openings with minimal excavation damage.
This paper presents the methodology in which two computer codes—TOUGH2 and FLAC3D—are linked and jointly executed for coupled thermal–hydrologic–mechanical (THM) analysis of multiphase fluid flow, heat transfer, and deformation in fractured and porous rock. TOUGH2 is a well-established code for geohydrological analysis with multiphase, multicomponent fluid flow and heat transport, while FLAC3D is a widely used commercial code that is designed for rock and soil mechanics with thermomechanical and hydromechanical interactions. In this study, the codes are sequentially executed and linked through external coupling modules: one that dictates changes in effective stress as a function of multi-phase pore pressure and thermal expansion, and one that corrects porosity, permeability, and capillary pressure for changes in stress. The capability of a linked TOUGH-FLAC simulator is demonstrated on two complex coupled problems related to injection and storage of carbon dioxide in aquifers and to disposal of nuclear waste in unsaturated fractured porous media.
One of the critical design problems involved in deep tunnelling in brittle rock with continuous excavation techniques, such as those utilizing tunnel boring machines or raise-bore equipment, is the creation of surface spall damage and breakouts. The mechanisms involved in this process are described in this paper. The onset and depth of damage associated with this phenomenon can be predicted, as a worst case estimate, using a factored in situ strength value based on the standard uniaxial compressive strength (UCS), of intact test samples. The factor applied to the UCS to obtain the lower bound in situ strength has been shown repeatedly to be in the range of 0.35–0.45 for granitic rocks. This factor varies, however, across different rock classes and must be determined or estimated for each class. Empirical guidance is given for estimating the in situ strength factor based on the UCS for different rock types and for different descriptive parameters. Laboratory testing procedures are outlined for determining both this lower bound strength factor and the upper bound in situ strength. This latter threshold is based on the definition of yield based on crack interaction. These techniques are based, in part, on theoretical principles derived from discrete element micromechanical experimentation and laboratory test results. The mechanisms that lead to in situ strength drop, from the upper bound defined by crack interaction and the lower bound limited by crack initiation, are described. These factors include the influence of tunnel-induced stress rotation on crack propagation, interaction and ultimately coalescence and failure. A case study illustrating the profound impact of near-face stress rotation is presented.