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Uniaxial and triaxial compression tests are commonly used in rock engineering to achieve strength and shape variations parameters. However, loading conditions in real projects like excavation, underground reservoirs, and even high-magnitude earthquakes play a vital role in these experiments. Therefore, to represent the actual loading conditions on Earth, a true triaxial test ( σ 1 ≥ σ 2 ≥ σ 3 ) is performed on rock samples. In this regard, choosing the right shape of samples has significant effects on stress distribution. Cylindrical samples are more common in experimental tests because they represent better behaviour under stress and can compensate for some cubic drawbacks. Accordingly, this paper proposes a new simulation of a true triaxial test on cylindrical samples to present an efficient and economical procedure to understand the behaviour of rocks in a true triaxial state. Particle flow code in three dimensions (PFC3D) based on the discrete element method (DEM) and flat joint model were selected in this study. First, based on the data from experimental tests, a simulation of rock was performed in PFC3D software. Then, the micro-parameters were calibrated based on the macro-parameters of Inada granite test results, and the effect of those micro-parameters in this step was investigated. Next, the cylindrical true triaxial test machine was simulated in PFC3D, and loading conditions on the sample were simulated by applying various stresses in x-, y-, and z-directions. At last, the effect of unequal confinement pressure on the ultimate sample strength and failure has been investigated to predict the sample behaviour under true triaxial conditions. [ABSTRACT FROM AUTHOR]

The physical and mechanical properties of the surrounding rock on the geothermal wellbore will be deteriorated by high temperature, resulting in reduced stability. In order to investigate the tensile properties and damage mechanism of sandstone after the action of different temperatures, direct tensile tests were conducted on square sandstone after the action of 20 °C, 200 °C, 600 °C, and 1000 °C and tested the change rule of wave velocity in the process of rock gradual cracking and analyzed the influence of temperature on roughness. Meanwhile, based on the Weibull distribution and the statistical damage principle, a statistical damage constitutive model for square sandstone, considering high temperature and tensile effects, is constructed, and the parameter expressions are determined. The result showed that the tensile stress–strain curve of square sandstone has four-stage properties, and the tensile strength and elastic modulus of square sandstone gradually decrease with the increase of temperature. The tensile strength and elastic modulus of the square sandstone gradually decrease, and they decrease by 35.55% and 76.43% after the action of 1000 °C relative to that of 20 °C. The overall wave velocity of the rock also tends to decrease with strain and temperature. And roughness gradually decreases with increasing temperature. The comparison of the model curves reveals that the calculated curve and the test curve have a high degree of agreement, indicating that the constitutive model can reflect the tensile stress–strain process of the square sandstone and can reflect the reasonableness of the model. The coupled damage variables D after different temperature effects showed an S-shaped monotonic increasing trend with strain. The research results of this paper can provide some theoretical guidance for the stability assessment of geothermal mining engineering. [ABSTRACT FROM AUTHOR]

Abandoned coal mine roadways are used to construct compressed air energy storage (CAES) caverns. However, the surrounding rock of the roadway is subjected to high internal pressure, which changes the stress state of the surrounding rock and causes rock deformation that directly affects the roadway stability. In this study, numerical simulation software FLAC3D was used to calculate the displacement and plastic zone volume of surrounding rock under 5–10 MPa internal pressures. Effects of buried depth, surrounding rock grade, and in situ stress state on surrounding rock deformation under high internal pressure were analyzed. Results show that the displacement and plastic zone volume of the surrounding rock decrease with the increase in buried depth, and the deformation of shallow surrounding rock is greater than that of the deep surrounding rock. The deformation of grades IV and V surrounding rocks is greater than that of grades I, II, and III surrounding rocks. The surrounding rock deformation is the largest under σv > σH > σh, followed by σH > σv > σh, and the smallest under σH > σh > σv. The research results play an important role in the safe construction of CAES caverns on abandoned roadways. [ABSTRACT FROM AUTHOR]

Wellbore instability in shale formations has always been a concern in oil exploration field. According to the Mohr–Coulomb criterion, a new model for evaluating and classifying the collapse of horizontal wellbore in shale formation is established. In this model, the temperature field and the pore pressure field affected by hydraulic pressure, thermal potential, and chemical potential difference between the drilling fluid and formation fluid are considered. Using the model, the influences of the temperature and solute concentration of drilling fluid on the failure degree of horizontal wellbore in shale formation with weak planes are simulated and analyzed. Based on the research results, weak planes that exist in shale formation will cause a significant wellbore collapse and using cool drilling fluid with high solute concentration can reduce the collapse degree of horizontal wellbore. The model established may help drilling engineers in practice to take effective measures for reducing wellbore instability and avoiding possible wellbore collapse during drilling in shale formations. [ABSTRACT FROM AUTHOR]

The effect of the normal stress and joint persistency on the mechanical behavior of granite samples containing discontinuous joints was investigated in the laboratory through direct shear tests. The test results revealed three different failure modes occurring at the rock bridge: (a) shear failure, (b) compressive-shear failure, and (c) tensile failure. The failure mode depends on the joint persistency and normal stress level and influences the peak shear strength. Moreover, the joint persistency and normal stress level were also found to have a considerable influence on the post-failure behavior such as the fracture roughness and residual shear strength based on rock bridge area. The conducted investigation provided better understanding of the effect of a rock bridge on mechanical behavior of granitic discontinuous joints towards estimating the resistance capacity of rock bridges and hence to allow evaluation of stability of rock masses in both surface and underground excavations in rock engineering. [ABSTRACT FROM AUTHOR]

During transition from open pit extractions to underground mining of an orebody, often both the open pit and the underground workings operate simultaneously, before the former is closed. To avoid the risk of inundation, the underground workings connecting or driven closer to the open pit are isolated using bulkheads. In this paper, the authors reviewed some of the theoretical equations and norms followed worldwide for determining the safe dimensions of a bulkhead to withstand water pressure. It is found that the theoretical equations are insufficient to represent the actual mode of failure and the ultimate pressure-bearing capacity of a bulkhead, as they were developed based on only one mode of failure of the dam construction material. For better representation of the bulkhead failure and its strength determination, it is found prudent to conduct strain-softening numerical modeling simulating a real mining scenario. Mode of dam failure and effect of parameters such as dam thickness and roadway dimensions on the ultimate pressure-bearing capacity of an arched bulkhead are studied. Numerical modeling studies show that the failure initiates with tensile cracking of the dam surface, but the bulkhead ultimately fails in a combination of tension and shear yielding. On comparison, it is found that the tensile failure theories underestimate the pressure-bearing capacity of a dam, while the shear strength- and crushing strength-based equations overestimate the same. Further, an application of the numerical modeling technique for design of water-retaining dams at an underground mine for its safe isolation from the open pit is presented. [ABSTRACT FROM AUTHOR]

The extensive bedding and cleats have a great influence on the mechanical properties of coal. The previous research mainly focused on the bedding effect, and few studies took the influence of both bedding and cleats into consideration. The scanning electron microscopy (SEM), uniaxial compression experiments, and acoustic emission (AE) monitoring were conducted on one hard coal specimen with different bedding and cleat angles. The experimental results show that for the bedding dip of 0° and face and butt cleat dip of 90°, the properties of coal specimens were characterized by the highest peak strength, the drastic failure process, the post-peak brittle behavior, the long initial quiet stage, and the short violent booming stage of AE hits number. For the bedding and face cleat dip of 45°, and the butt cleat dip of 90°, the properties of coal specimens were characterized by the lowest peak strength, the slip failure mode, the step-like drops of the post-peak stress-strain curve, and the several peaks of AE hits number. On the other hand, for the bedding and butt cleat dip of 90°, and the face cleat dip of 0°, the properties of coal specimens were presented as tensile failure mode with plate cracks, the short non-linear stage and the slight step-like drops of stress-strain curve, the short initial quiet stage of AE hits number, and the large value of elastic modulus. Then, a theoretical model of coal containing bedding and cleats was established in this paper. The simulation results of peak strength and failure mode verified the proposed model. The simulation of coal strength shows that bedding had the decisive influence on the uniaxial compressive strength of coal, while the weakening effect of cleats was non-negligible. When bedding and cleats were taken into consideration together, the minimum peak strength was reduced and the bedding dip related to the minimum peak strength changed. [ABSTRACT FROM AUTHOR]

Initially used as foundations for transmission towers, helical piles are now used for solar power plants, wind turbines, boardwalks, retaining walls, retrofitting works, and even residential buildings. Crucial progress has been made over the last few decades using various numerical solution approaches, field tests, and software analyses to improve the design of such piles, as they have numerous benefits over conventional piles. A few countries have developed codes for their design such as Acceptance Criteria 358 (USA), Australian Standard AS 2159-2009, British Standard BS 8004: 2015 (UK), and Canadian Foundation Engineering Manual CFEM 2006. Helical piling field practices are also done based on design manuals formulated by companies manufacturing them. However, the investigation is still in progress for perfecting their design and accurately predicting their uplift, lateral, and compressive capacities. In this paper, the most important and widely used methods for predicting the axial load carrying capacities and the essential design parameters influencing them have been discussed. A detailed review of the finite element analysis conducted on helical piles has been presented, which will be helpful for further analyses on such piles using software. A design scheme has also been outlined for the analysis of the behavior and capacity of helical piles. [ABSTRACT FROM AUTHOR]

This paper aims to utilize different stochastic numerical procedures for designing a rock tunnel lining, namely, strength classification (SCM) and random field (RFM) methods. The procedures are implemented in a finite difference scheme, while a stochastic process based on the Monte Carlo simulation technique is followed for rock mass characterization, considering its property variability. Moreover, a performance function is defined to investigate the lining serviceability based on the interactions between bending moment, thrust, and shear forces. The effects of coefficient of variation and correlation length on the results are also investigated. Finally, the probabilistic analysis results are compared with the deterministic ones and the in situ measurements. The results show that the effect of the coefficient of variation on the spatial variability is more significant than the effect of the correlation length. It can also be observed that the SCM models calculate lower support requirements than the RFMs when CoV is smaller than 30%. Moreover, the cross-correlated models lead to lower PoF, which can be related to the assumed negative cross-correlation between c and φ . [ABSTRACT FROM AUTHOR]

When using the Hoek–Brown strength criterion or Mohr–Coulomb strength criterion for tunnel collapse analysis, the effect of intermediate principal stress is always ignored. In order to study the collapse mechanism considering the influence of intermediate principal shear stress, the collapse mechanism of deep buried tunnel subjected to pore water pressure in conjunction with generalized nonlinear unified strength theory is established. The analytical solutions of two-dimensional detaching curve f x of circular and rectangular tunnels are deduced; therefore, the range and shape of collapsing blocks can be clearly obtained by substituting the practical rock mass parameters; the results obtained by the presented approach are approximate with the numerical results provided by finite difference code, which proves that the proposed method in this work is valid. Furthermore, the impacts of the influence factors are discussed in the analytical solution; the results show that the intermediate principal shear stress influence coefficient b has significant effect on the collapse of deep buried tunnel, which is one of the non-negligible parameters in collapse analysis, ignoring that the intermediate principal shear stress effect would underestimate the strength properties of the rock material. When setting the coefficient b to 0, the results show great consistency with other strength theory and verify the validity of present approach. In addition, the intermediate principal stress coefficient is functionally related to instantaneous friction angle in plane strain state, indicating that the instantaneous friction angle also has an impact on tunnel collapse, and the influence regularity is discussed accordingly. The results of this paper may provide workable guidelines for the prediction of potential collapse extent and support designs in minimizing rock detachment above tunnel roofs in practice. [ABSTRACT FROM AUTHOR]

The macroscopic mechanical properties of granites are affected by inhomogeneous factors such as mineral shape, size, and distribution. Microscopic parameters of granites from the Songliao Basin were calibrated in PFC2D using the grain-based model approach. The effects of mineral grain size on strength, deformation, and microcracking behavior were analyzed by conducting the uniaxial compression test and triaxial compression test under confining pressure of 50 MPa. Research showed that the strength of the rock gradually reinforced as the grain size increased and was positively correlated with the average grain-disk size ratio of 2.5~16.87. The results of numerical simulations on grain size effects conflicted with the results obtained in laboratory experiments. To reflect the true mechanism of the grain size effect, the relative percentage method of microscopic strength parameters was proposed, and the strength variation law after the discounted reduction matched well with the conclusion of the laboratory test, indicating that the method has considerable feasibility. Research on the microscopic parameters of granites will be of great guidance for geothermal research in the Songliao Basin. [ABSTRACT FROM AUTHOR]

With the demand for many engineering facilities on coral reefs, it is urgent to study the physical and mechanical properties of reef limestone to meet the needs of coral reef construction. In this paper, real-time high-resolution X-ray micro-CT was used to scan reef limestone sample experimentally under unconfined axial compression for the first time. The rock samples from 286 m below sea level in S Reef were made into cylindrical specimens with diameter of 3.871 mm and height of 8.443 mm to study progressive failure characteristics. Through 2D vertical slices and reconstructed 3D stereograms, the microstructure and pore morphology of the samples were extracted and quantitatively characterized corresponding to each loading stage. These data show that biological components, coral gravel, and algae bonding surface are weak parts in the reef limestone, which reduces its tensile strength and enhances the heterogeneity of the internal structure. In addition, the relationship between the void volume, solid volume, and total volume of reef limestone CT images with the axial stress-strain curve was discussed. [ABSTRACT FROM AUTHOR]

Landfill cover is one of the most important elements of any modern landfill, the integrity and stability of which is obviously a major issue in the design and construction of landfills. Tapered landfill cover is an attractive technique to enhance the stability of landfill cover. In most of design analyses of landfill cover, the simultaneous effects of reinforcement (root and artificial layer), seepage, and seismic forces are ignored, and only the impact of each of these forces on the landfill cover stability has incompletely investigated. Basically, simply neglecting the simultaneous effects of mentioned forces can lead to an overestimated factor of safety (FoS). A novel approach based on limit equilibrium method (LEM) was proposed to analyze the stability of tapered landfill cover, which is supported by artificial reinforcement layer in addition to the root system under seismic and seepage loading conditions. The results indicated that the reinforcement parameters (Kc/Kt and tR) remarkably affected the stability of tapered landfill cover system, specifically when the landfill cover is under extremely adverse conditions, and the slopes should remain steep and uninterrupted along their length to ensure optimal use of the landfill area; the application of proposed reinforcement mechanisms can significantly enhance the stability. [ABSTRACT FROM AUTHOR]

This paper designed a special-shaped cross-section hollow self-drilling bolt with discharge grooves, in which Mindlin's solution of displacement was used in the elastic solution of a wholly grouted bolt. Theoretically, the ultimate pulling force and the residual pulling force were analyzed by considering the average distribution assumption of residual stress. The bolts' performance with 2, 3, and 4 discharge grooves was compared with circular cross-section bolts, which experimental results are consistent with the theoretical results. The ultimate pulling force and the residual pulling force of the special-shaped cross-section bolt improved significantly according to the circular bolt. The bonding properties of special-shaped cross-section bolts present significant difference to circular section bolts. The residual pulling strength was contributed both by the bar-grout and grout-grout frictions. Upgrading the grout strength may further enhance the pulling strength of special-shaped cross-section bolts. [ABSTRACT FROM AUTHOR]

In hydraulic fracturing for fractured wells, the pressure of cyclic loading and unloading in the casing can easily lead to integrity failure of the cement sheath in the fracturing section, representing a production safety hazard. The integrity failure evaluation experiments of cement sheath were carried out for fractured wells under the condition of alternating loading. The cause of integrity failure was analyzed based on the fatigue cumulative damage theory and failure criteria. The integrity failure law was obtained for cement sheath under a confining pressure of 30 MPa and alternating loading. The results demonstrate the following findings: firstly, under alternating pressure, the internal structure of the cement is damaged, and its mechanical properties (such as compressive strength) decrease with increases in the number of alternating pressure cycles; the maximum decrease can be 40%. Secondly, the bond strength of the casing–cement interface is low, concentrated between 0.45 and 0.65 MPa, and the plastic accumulation of the cement sheath during fracturing leads to inconsistency in the interface strain between the casing and the sheath. The interface separates and forms a micro-annulus when the interfacial tensile stress exceeds the interfacial bond strength during pressure relief. Thirdly, under the condition of a confining pressure 30 MPa and high alternating pressure, micro-annulus formation is the primary mode of failure regarding cement sheath integrity. Increasing the elasticity and deformation ability of the cement sheath helps prevent this failure. The conclusions of this research may provide a technical reference for improving the cement slurry and fracturing operation of fractured wells and help ensure the integrity of the cement sheaths of fractured wells. [ABSTRACT FROM AUTHOR]

In the Changning shale gas development blocks of the Sichuan Basin, casing deformation and uneven gas production rates between wells seriously restricted the efficient development of shale gas reservoirs. Heterogeneous distributions of in situ stresses are one of the important factors that influence the occurrence of these unfavorable engineering phenomena. This paper presents the assessment of present-day in situ stress distributions on one of the blocks in the Changning shale gas development area through field diagnostic fracture injection test (DFIT) and the development of a 3D in situ stress model. To accurately model the distributions of in situ stress in the Longmaxi shale gas formation, more than 11 DFITs were conducted on a 192-km2 shale gas development block. Minimum principal stress and pore pressure were interpreted from the DFIT data by using pressure transient analysis (PTA) methods. The 3D in situ stress model was constructed by using the finite element (FE) method based on a site-specific 3D geological model. The stress model was calibrated and constrained by the results from field DFIT stress measurements. The reservoir is highly over-pressured with pore pressure gradients ranging from 16 to 19 kPa/m within the development block. The distributions of in situ stress are highly variable within the reservoir, and the shear stresses in some areas are significantly higher than that in other areas. The authors compiled the casing deformation and well production data from more than 40 wells. It was found that (1) the casing deformation frequently occurred in the area with large shear stress, especially in the case of the non-uniform shear stress distribution along a horizontal well, and (2) the gas production rates were positively correlated with the shear stress level (SSL). The higher the SSL, the greater the gas production rates are. Because many previous studies about in situ stress modeling have been based on well log-interpreted stresses, this study emphasizes the importance of constructing in situ stress models based on direct field stress measurements and constraining the stresses through multiple sources and methods. The methodology and the data repository are potential sources for future geomechanical studies on similar projects. [ABSTRACT FROM AUTHOR]

Mixed mode fracture is quite common in rock structures. Numerous investigators have used the Brazilian disk specimens with a central crack for investigating modes I, II, and mixed fracture toughness in brittle materials. In this study, analytical, experimental, and numerical investigations were planned and performed on Central Straight Through Crack Brazilian Disk (CSCBD) specimens. Ranking of geometrical parameters effective on the value of stress intensity factors (SIFs) of CSCBD specimens were obtained using stochastic analysis. Furthermore, experimental tests were undertaken in order to evaluate the crack propagation in rock-like material of low brittleness. Finally, numerical modeling was performed to assess the effect of crack length on the failure mode of CSCBD specimens. Analytical analyses revealed that the inclination angle of the crack with respect to the diametrical load has the most important impact on the SIFs among the geometrical parameters of CSCBD specimen. Performed experimental and numerical analyses also confirmed the effect of inclination angle and crack length and their impact on the mode of failure of the tested specimen. [ABSTRACT FROM AUTHOR]

To investigate the characteristics of the acoustic emission and microseism occurring during rock shear failure, we conducted a series of direct shear tests on the cubic specimens under different normal stress conditions. First, the tested specimen shear strength, deformation characteristics, and crack during rock shear failure are investigated. Then, the waveform, fractal, and spectrum of microseism (MS) are analyzed. The deformation and stress characteristics of the tested specimen during the direct shear test are consistent with those obtained in the previous studies, indicating that the experimental results from our direct shear test are reasonable. In addition, a "quiet period" can be observed in the waveform of MS before the overall shear failure under low normal stress conditions for all the types of specimens. With the increase of normal stress, the relative length of the "quiet period" decreases, especially for the granite specimens, the "quiet period" will disappear. Otherwise, it can be obtained that before the overall fractal dimension exhibit a rapid increase. Furthermore, investigation on the frequency-amplitude distribution of MS indicates that high-amplitude MS signals occur during the rapid stress decreasing stage. Meanwhile, with the increases of the normal stress, more high-amplitude MS signals arise. The distribution of these high-amplitude MS signals changes from punctiform-shaped to bar-shaped and then clustered shape. It is concluded that the "quiet period" of MS waveform, the rapid increase of fractal dimension, and the appearance of high-amplitude MS before the overall shear failure can be taken as the precursor information to the overall shear failure. [ABSTRACT FROM AUTHOR]

The development and application of rock-breaking technologies, such as laser, hydraulic and acoustic, have provided more possibilities for traditional mechanical rock-breaking tunnel boring machine (TBM) to improve tunnel construction efficiency and realize construction in complex geological environment and large-scale tunnel excavation. At present, the research and development of high-performance TBM coupled with multiple rock-breaking technologies is still in the exploratory stage, and there is still a lack of understanding of the synergistic operation mode of different rock-breaking technologies. According to the differences in the implementation characteristics and scope of action of different technologies, the rock-breaking process can be classified into three modes: borehole treatment, slotting treatment and surface broken treatment of rock wall. And the synergistic operation characteristics of the three modes for rock breaking are analysed. Taking the slotting treatment mode as an example, the free surface is formed on the rock mass by prefabricated kerfs, and the penetration tests of the sandstone with slotting treatment are carried out with single and double indenters. The results show that the slotting treatment is of great significance for reducing the rock-breaking force and improving the rock-breaking efficiency of TBM mechanical disc cutter, and the feasibility of the application of slotting treatment mode in TBM for rock breaking is verified. It shows that the coupling of various rock-breaking technologies and synergistic operation with different treatment modes are of great significance to the renovation and upgrading of TBM equipment. [ABSTRACT FROM AUTHOR]

To evaluate the influence of confining pressure on the rock-breaking force required by the cutter and provide a new possible method for ground stress prediction, the rock breakage mechanism under TBM cutters is studied through theoretical analysis and numerical simulation. A theoretical model for peak penetration force required for rock breakage prediction is established with confining pressure considered. The rock breakage by cutters under different confining pressures is simulated by PFC and compared with the theoretical analysis results. The numerical simulation results are in good agreement with the theoretical analysis, which verifies the validity of the established mechanical model. The peak penetration force increases significantly with the increase of the confining pressure that is perpendicular to the cutting direction and increases slowly with the increase of the confining pressure that is parallel to the cutting direction. Considering the effect of biaxial confining pressure, the impact factor is introduced to modify the theoretical formula for peak penetration force prediction. Besides, we discuss the variation of specific energy and found that increasing confining pressure improves rock fragmentation efficiency at high confining pressure though the effect is less evident when it exceeds a critical value. [ABSTRACT FROM AUTHOR]

The rock mass of underground engineering is often faced with tensile–shear failure disaster due to excavation. However, the failure mechanism of flawed rocks under tensile–shear stress state is not well understood at present and there is no relevant experimental investigation due to the difficulty of test technology. To reveal the crack propagation mechanism of flawed rocks under tensile–shear stress, experimental investigation using a developed tension–shear auxiliary device and numerical simulation were conducted in this study. The results obtained from direct shear tests of single-flawed sandstone specimens under constant normal tensile stress indicate that the flaw inclination angle and normal tensile stress significantly affect the shear strength and crack pattern. The cracks are mainly subjected to tensioning for different flaw inclination angles. It is easy to develop secondary cracks to produce the nucleation failure with the primary cracks when the flaw inclination angle is an acute angle. Digital image correlation (DIC) analysis suggests that the strain concentrations are greater gradually near the two tips of the flaw and the cracks grow along the strain concentration areas. Furthermore, the crack propagating process and stress field evolution in single-flawed and double-flawed specimens were examined by DEM simulations. The primary cracks are initiated at the tensile stress concentration areas of flaws, and then the concentration areas transfer to the crack tips and impel the crack propagating. The nucleation failure formed by two wing cracks is a main rock bridge failure pattern in double-flawed specimens. The nucleation failure area becomes small and maybe disappeared due to the increase in normal tensile stress. The conclusions can provide the theoretical reference for the stability evaluation of underground rock mass excavation. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing of tight shale gas reservoir is considered an emerging high-efficiency production method, which is especially suitable for extremely low permeability reservoir. A complex crack network intertwined with the natural crack in the unconventional shale reservoir can be generated by hydraulic fracturing techniques, so as to form a high-permeability domain in the low-permeability reservoir. Vertical well fracturing is one of the typical approaches for increasing production of unconventional reservoirs in recent years. It is widely implemented in coalbed methane (CBM) extraction, geothermal development, unconventional oil and gas production, and marine hydrocarbon development engineering. In this paper, more attention is paid to whether the artificial echelon cracks will develop of a high-quality crack network in hydraulic fracturing through hybrid finite element method (FEM) and discrete element method (DEM). This problem is profoundly investigated through engineering-scale numerical simulation and the establishment of a simple theoretical echelon model. Through the quantitative description of the ultimate crack area, we find that the appropriate spacing, such as the actual engineering spacing mentioned in this paper, i.e., 67.6 m, is regarded as the optimal spacing. It is conceivable that there will also be corresponding "dessert spacing" in the practical engineering. In addition, the microseismic inversion algorithm is used to obtain the location and magnitude of microseismic events with different crack failure types, including contact slip events and damage events. [ABSTRACT FROM AUTHOR]

The objective of this work is to develop a new constitutive model based on a modified Mohr–Coulomb (M-C) criterion in order to reflect the brittle–ductile transition behavior of crystalline rock. The innovative contributions rest on that the post-peak strain softening and brittle–ductile transition behaviors of rock materials can be described by integrating an exponential function of damage variable and confining pressure into the yield criterion. Then, the numerical integral formulation of the model is deduced in detail based on the return mapping algorithm. And the iterative updating of plastic deformation and damage evolution is numerically calculated by the plasticity and damage decoupling correction algorithm (PDDC). Besides, the method of model parameters identification is presented based on laboratory test data. Finally, the consistency between numerical simulation and experimental results from crystalline marble validates the performance of the proposed model. The constitutive model is also versatile to describe the fundamental mechanical characteristics of the like crystalline rock. [ABSTRACT FROM AUTHOR]

High plastic clays (fat clays) instigate failures like rutting and fatigue cracks in flexible pavements and sometimes make the pavements unserviceable when subjected to wet-dry cycles of the environment. This study focuses on the strengthening potential of polypropylene fiber (PP) on the engineering properties of fat clays. Different fiber percentages (0.2, 0.4, 0.6, and 0.8% by oven-dried soil weight) were used in clay samples and then compaction properties, unconfined compressive strength (UCS), elastic modulus (E50), California bearing ratio (CBR), and swell-consolidation properties of reinforced samples were investigated. Also, desiccation tests were carried out to assess the efficacy of fibers in enhancing the cracking resistance of clay. The test results show remarkable strength improvement, as the UCS value of 0.4% reinforced soil increased by 356%, E50 by 109%, and soaked CBR by 225%, resulting in a better-quality subgrade for pavement construction. The compression and rebound indices were decreased by 49.5% and 78.8% for fiber content of 0.8%, respectively, as well as the percent swell and swell pressure were also lowered by 75.4% and 70%, respectively, shifting the medium swelling soil to a low swelling class. Furthermore, the scanning electron microscopic (SEM) studies revealed that the frictional interface between fiber-clay particles appears to be the primary mechanism influencing the reinforcing effect. Economically, stabilizing fat clay reduces the required pavement thickness by an average of 38%. Hence, PP fiber as a low-cost material provides a long-term stabilizing solution for widespread fat clays. [ABSTRACT FROM AUTHOR]

Aiming at the frequent occurrence of borehole wall collapse and other accidents in the drilling process of deep shale horizontal wells in the Luzhou block, the effects on mechanical properties of brittle shale were studied and analyzed by carrying out plugging test of drilling fluid and loading and unloading test of confining pressure. Considering the weak plane of bedding fracture and according to the shape of borehole caving, a mathematical model of ground stress inversion was established. The research results show that the deep shale in the Luzhou block has a high content of brittle minerals, and the bedding and microfractures are relatively developed. The plugging performance of the drilling fluid has a significant impact on the compressive strength and elastic modulus of deep brittle shale. The good sealing performance of drilling fluid can effectively improve the failure strength of borehole wall rock. In a complex downhole pressure environment, a too fast tripping speed will cause the effective bottom hole pressure to drop, reduce the effective support of the drilling fluid on the wellbore, and induce the damage of the wellbore rock along the bedding and microfractures. Under the condition of formation heterogeneity, the collapse shape of the wellbore is no longer the ideal round or oval shape but more of a four-leaf shape. With the known shape of the borehole after the collapse, a mathematical model of inverse in situ stress was established, and the accuracy of the mathematical model of inverse in situ stress was verified through the true triaxial test. The research results provide a certain theoretical basis for the safe and efficient development of deep shale gas. [ABSTRACT FROM AUTHOR]

To explore the rule of pore-fracture propagation in the course of hydraulic fracturing of a single pore shale under the action of fluid–structure coupling, a numerical model was established for 8 groups of single pore shales present in the Niutitang Formation of the northern Guizhou area. Different bedding dip angles and different hydraulic pressures were used with the numerical software RFPA2D-Flow. The results showed that: 1) the elastic modulus and compressive strength of shale showed pronounced anisotropy due to the influence of different water pressures and bedding dip angles; 2) for the same water pressure, the curve for compressive strength was V-shaped as the bedding dip angle rose; 3) with the same bedding dip angle, the compressive strength of single pore rock samples was negatively correlated with the increase in hydraulic compressive strength; 4) based on the distribution of cracks in samples with different bedding inclination angles and hydraulic pressures, the samples were divided according to three final failure modes, namely inclined type I (0°/30°/60° 10 MPa, 60° 8 MPa), inclined type N (0°/30°/ 90°, 8 MPa) and inclined type H (90°, 10 MPa). The results of this paper provide crucial theoretical guidance for exploitation of Lower Cambrian black shale with hydraulic fracturing in the Feng'gang No. 3 block of northern Guizhou. [ABSTRACT FROM AUTHOR]

In this paper, the dynamic fracture characteristics and mechanism of the medium with compound defect under blast loading are studied by varying the relative positions of the void and pre-crack in the compound defect. A transmission caustics experimental system was used, while polymethyl methacrylate (PMMA) was selected as the experimental material. The void and crack were integrated into the same defect form, defined as a compound defect. The experimental results indicate that the end of the pre-crack is the weakest point of the defect, while the cracks are more likely to appear in the stress concentration area. By combining the interaction mechanism between the explosion stress wave and the end of the pre-crack, two modes of action for the medium containing compound defect under the blast loading were identified. One is that the stress wave directly acts at the end of the pre-crack, and the other is that the stress wave acts at the end of the pre-crack after circling around the void. As the relative positions of the void and the pre-crack change, the two forms of initiation and non-initiation were observed at the end of the pre-crack. The difficulty of initiating cracks at the end of prefabricated cracks is constantly changing which is indicated by the differences in crack tip initiation time and the stress intensity factor (the peak stress intensity factor when the crack had not initiated). At the same time, combined with the LS-DYNA finite element numerical simulation, the JH-2 material damage model is used to reproduce the dynamic damage evolution process of the composite defect, and the experimental results of the model are highly restored. The pressure values of the end element of the pre-crack under different defect structures are extracted from the numerical simulation results, and the possible reasons for the defect initiation in the composite are explained from the numerical simulation perspective. In future research, the JH-2 damage model can be widely used to solve issues pertaining to brittle materials such as PMMA. In practical blasting design processes, the relative position of the blasthole and compound defect should be considered as the basis of blasting parameter design. [ABSTRACT FROM AUTHOR]

This work presents the experimental results of the fracture behaviors for Tibet hornfels rock obtained from an open pit with different natural fracture ratios under multi-level cyclic shear loads. A newly developed rock dynamic shear testing apparatus was employed to perform the increasing-amplitude cyclic loads on the samples; the digital image correlation (DIC) technique was used to monitor the full-field displacement and the principal strain evolution for all tested samples. The results show the shear strength, deformation, full-field displacement, and the final failure pattern are influenced by the inherent natural fractures. The DIC results reveal the full-field displacement evolution and the characteristics of strain concentration. The stimulation of the natural fractures is impacted by the relations between shear direction, natural fracture orientation, and its cement characteristics. High strain concentration and large displacement occur near the natural fractures. For the sample with low natural fracture volume or the sample whose natural fracture orientation is parallel to the shear direction, the morphology of failure surface is the simplest. It is interesting to notice that for the sample whose fracture orientation is almost vertical to the shear direction, the multiple strain concentration zones were identified. Overall, this study confirms the pronounced effect of rock structure on the deformation and fracture evolution of the natural rock mass exposed to cyclic shear loads. [ABSTRACT FROM AUTHOR]

Jointed rock masses contain complexly distributed fractures that have significant impacts on the mechanical behavior of the surrounding rock, especially during the tunneling process. In this paper, a field test was carried out to record the vertical displacement and deformation of the constructing tunnel passing through jointed rock masses. To further investigate the deformation characteristics of tunnels excavated in jointed rock masses, numerical simulations utilizing discrete element methods (DEM) were conducted. Numerical models simulated different sizes and shapes of blocks in jointed rock masses. The results show that during the process of tunnel excavation, the vertical displacement of surrounding rock undergoes the rapid growth phase and the gradual stabilization phase, and the time interval of these two stages is different due to the different depths of the tunnel. In the same monitoring section, the closer to the vault of the tunnel, the larger the vertical displacement of the monitoring point. The displacement value of the section closer to the tunnel face is larger than that far away from the tunnel face, while the tunnel section in shallower buried depth performs relatively larger liner deformation. For the same size of blocks, jointed rock masses formed by tetrahedral blocks perform relatively larger vertical displacement of surrounding rock, deformation of the tunnel, and positive and negative moments of liner structures than those formed by cubic or polyhedral blocks. [ABSTRACT FROM AUTHOR]

Confined water inrush disasters occur frequently in Hanxing mining area in China. In order to deal with mine water inrush disasters effectively, it is necessary to master the point and speed of water inrush from floor. According to the water inrush problem of mine with confined water in Hanxing mining area, a generalized model of mined space with concealed water-bearing structures was established from perspective of combined action of mining disturbance and confined water. The calculation results showed that, under the geological conditions of Hanxing mining area, the floor stress disturbance range was within 60 m after mining, and the main influence range of floor water pressure was 40 m above the interface of ordovician limestone. Furthermore, based on a simplified plane strain mechanical model of crack tip, the variation law of permeability coefficient of cracks along the path under different stress field of floor was obtained. The permeability coefficient of the cracks increased non-linearly along the direction of seepage, and increased sharply within 30 m of the working face floor. The closer it was to the stope, the smaller the seepage velocity was. According to the distribution law of principal stress difference, the conclusion of floor damage ahead working face and water inrush lagging working face can be drawn. The variation law of confined water permeability coefficient and permeability velocity obtained in this paper can provide theoretical basis for disaster control. [ABSTRACT FROM AUTHOR]

Excavation of a rock slope will form an excavation disturbance/damage zone (EDZ) near the excavation boundary. This paper proposes an EDZ analysis method based on point safety factor. The ratio of the shear stress to shear strength of each point in the slope is defined as the point safety factor, and the expression of the point safety factor under the Mohr–Coulomb and Hoek–Brown criteria is derived. The point safety factor before and after the excavation of a highway rock slope is calculated, and the influence of the Hoek–Brown parameters on the point safety factor is studied. The change rate of the point safety factor before and after slope excavation is used as a quantitative index to study the scope of EDZ, and the influence of excavation angle on EDZ is discussed. The results show that the point safety factor first increases and then decreases from the slope surface to the slope interior. After the slope is excavated, the point safety factor at the toe of the excavation face is significantly reduced, and its change is related to the stress adjustment of the rock mass in the slope. The point safety factor increases with the increase of the geological strength index GSI and uniaxial compressive strength σci of the rock mass, and decreases with the increase of the material constant mi and the disturbance factor D. As the excavation angle increases, the range of EDZ becomes smaller. The maximum point safety factor change rate occurs on the excavation boundary with a height of 1.8 ~ 2.0 m, which first increases and then decreases with the increase of the excavation angle. [ABSTRACT FROM AUTHOR]

The dynamic load in thick coal seam often results in roadway cracking and large deformation. Grouting support is an effective measure to reinforce the broken surrounding rock. This study presents a case of repairing shaft inset with pretensioned grouting support. The mine shock intensity of Yili No. 1 Coal Mine did not reach the level of percussive ground pressure, but such impact occurred several times after excavation, resulting in large deformation of roadway. After mining tremors, the concrete sprayed on the shaft inset cracked. In this paper, the repair technology of fractured roadway is explored. Firstly, the characteristics of impact failure are analyzed based on site monitoring data. Regional factors, roadway layout, and impact disturbance are the main causes of surrounding rock failure. In order to obtain the yield range of surrounding rock, dynamic analysis is carried out by Flac3D. Under dynamic load, the stress of surrounding rock near the surface of inset soon strides the compressive strength, reaching tensile yield state. And a wider range of shear yield is generated in the inner zone of surrounding rock. Corresponding grouting restoration scheme is designed according to the stress state. The shaft inset is reinforced with pretensioned anchor grouting support and the effect is tested by borehole observation. The results show that the pretensioned anchor grouting support has application value in the repair and reinforcement of roadway with impact damage. [ABSTRACT FROM AUTHOR]

Wellbore instability may lead to wellbore collapse, the sticking of drill tools, borehole sloughing, and burying accidents. Theoretical analyses of wellbore stability issues often assume the wellbore to be in the shape of a circle. However, the shape of the wellbore is not a standard circle and may deviate from this assumption, resulting in higher collapse pressures. To solve this issue, a new analytical model for collapse pressure was established, which treats the wellbore in an inclined well as an ellipse. The aim of this study is to analyze the change law of collapse pressure in an elliptical wellbore. Meanwhile, the formation fluid that flows from the formation to the wellbore was taken into account. Based on the rock mechanics, elastic mechanics, complex variable method, and Mohr–Coulomb failure criterion, a new wellbore collapse pressure analytical model was derived. The analytical results of this study by utilizing MATLAB, based on field data, show that an elliptical wellbore is more unstable than a circular wellbore, collapse pressure will increase with increases in the ratio of the major to the minor axis, and that seepage will ultimately increase the collapse pressure. At the same time, well trajectory has a great impact on the collapse pressure: (1) when the well-circumferential angles are 90° and 270°, the wellbore is the most stable; when the well-circumferential angles are 0° and 180°, the wellbore collapses more easily; (2) when the deviation angle increases from 0° to 90° and an invariable azimuth angle and well-circumferential angle are maintained, the collapse pressure will gradually increase; (3) when the deviation angle is invariable, the equivalent collapse pressure density attains a higher value if the azimuth angle is equal to 0° and 180°, which reveals that the wellbore is more unstable, and when the azimuth angle is equal to 90° and 270°, the wellbore is stable comparatively. The results of this study verified that it is of importance to treat the shape of the wellbore as an ellipse and provide a positive guide for the analysis of wellbore stability. [ABSTRACT FROM AUTHOR]

Cavity is one of the many interference mechanisms of hydraulic fracture propagation. In this paper, reservoir models with different positions, different sizes, and different numbers of caves were developed based on extended finite element method. The investigation results show that caves have an attractive effect on hydraulic fracture extension; caves have a certain influence on the change of hydraulic fracture width, but the degree of influence is different When the angle of cave to fracturing fluid injection point is 0°, 5°, 25°, 45°, 60°, and 67°, the horizontal offset of fracture end is 5.00 m, 3.75 m, 2.25 m, 1.25 m, and 0.75 m, and the maximum fracture width is 1.04 cm, 0.95 cm, 0.89 cm, 0.86 cm, and 0.83 cm. When the cave radius is 1.25 m, 2.5 m, 3.75 m, and 5 m, the horizontal offset of fracture end is 2.5 m, 4.3 m, 7.5 m, and 11.3 m, and the maximum fracture width is 0.78 cm, 1.04 cm, 1.21 cm, and 3.09 cm. When there are 1, 2, and 3 caves in the reservoir, the horizontal offset of fracture end is 2.75 m, 4.25 m and 6.25 m, and the maximum fracture width is 0.78 cm, 0.88 cm, and 0.94 cm. In short, as the distance between the cave and the fracturing fluid injection point decreases, the cave size increases, or the number of caves increases, the attraction capacity and hydraulic fracture width increases to varying degrees. Moreover, caves have a controlling effect on the hydraulic fracture propagation capacity and on the direction of hydraulic fracture propagation. [ABSTRACT FROM AUTHOR]

This work aims to reveal the influence of rock bridge length on fracture and energy evolution of granite containing a circular hole and two fissures exposed to uniaxial increasing-amplitude decreasing-frequency (UIADF) cyclic loads. The rock bridge length was designed to be 0, 5, 10, and 15 mm between the hole and fissure. The testing results show that rock strength, deformation, lifetime, energy dissipation, and failure morphology are all influenced by rock bridge length. Increasing rock bridge length leads to the increase of rock volumetric deformation, stiffness, and dissipated energy. The rock stiffness decreases with the increase of loading cycle, and its decreasing rate becomes faster. The dissipated energy increases with decrease of loading frequency, and an exponential relation is revealed between them, a damage evolution model that can describe a fast increase of accumulative damage is proposed based on the dissipated energy. Three failure modes of single tensile coalescence, single shear coalescence, and double shear coalescence were revealed from failure morphology. It is suggested that much more energy needs to be consumed to form a double shear coalescence pattern; the interaction between the hole and fissure is the weakest for a sample having a high bridge length. [ABSTRACT FROM AUTHOR]

Microseisms have appeared frequently in the Three Gorges Reservoir Area since the record. For assessing the long-term stability of rock slopes subjected to repeated microseisms, through indoor cyclic dynamic load tests, combined with nuclear magnetic resonance technology and particle flow code (PFC), the damage mechanics characteristics and meso-damage evolution mechanism of sandstone subjected to cyclic loads were systematically studied. The research results show that when the upper limit stress exceeds the fatigue threshold, the rock samples' fatigue life decreases as the upper limit stress and stress amplitude increase and increases when the loading frequency increases. Nuclear magnetic resonance (NMR) tests were performed on sandstone specimens at various loading times, and the porosity, T2 spectra, and NMR imaging results under different working conditions were obtained. The findings of NMR studies revealed the development characteristics of microcracks in sandstone at three stages: initial deformation, low-speed deformation, and accelerated deformation. When comparing results of NMR and numerical experiments, it was discovered that the results obtained by the two methods were consistent in the progression of meso-damage, and numerical calculations further explained the reason for the uniform distribution of pores in magnetic resonance imaging (MRI). [ABSTRACT FROM AUTHOR]

Dynamic compressive properties of rock material under cyclic freeze–thaw weathering are the basis of engineering construction in cold regions, while the effect of crack angles and freeze–thaw cycles on dynamic compressive properties has not been figured out yet. This paper adopted rock-like specimens containing crack with different angles as objects, and conducted cyclic freeze–thaw treatment, nuclear magnetic resonance test, and dynamic impact test on specimens. The influence of freeze–thaw cycles and crack angles on pore structures, dynamic mechanical properties, energy evolutions, and fractal dimension were obtained and analyzed. Results show that spectrum areas of all pores and macro-pores present accelerating growth trends with freeze–thaw cycles. With the increase of freeze–thaw cycles, dynamic peak strains and the ratio of reflect energy to incident energy increase nonlinearly, and fractal dimension of fragments increases linearly. Dynamic peak stress and the ratio of dissipate energy to incident energy decrease nonlinearly, and dynamic elastic modulus and the ratio of transmit energy to incident energy decrease linearly. With the increase of crack angle, dynamic peak stress, dynamic elastic modulus, the ratio of transmit energy to incident energy, and the ratio of dissipate energy to incident energy show a V-type variation trend. While dynamic peak strain and the ratio of reflect energy to incident energy show a reverse V-type variation trend, the fractal dimension of fragment shows an M-type variation trend. This study is of importance to engineering construction in cold regions. [ABSTRACT FROM AUTHOR]

Data collection and classification for earthquake damage to tunnels and the analysis of the influencing factors are important for the anti-seismic design of tunnels. The first purpose of this review is to briefly review the seismic damage types of mountain tunnels from the perspectives of both the tunnel body and the tunnel portal. Then, damage influencing factors for mountain tunnels are explored from the perspectives of earthquake parameters, geology conditions, and tunnel structure characteristics. Finally, some suggestions are made to reduce the earthquake disasters of mountain tunnels. It is believed that this study will contribute to the earthquake disaster control and prevention design of future mountain tunnels. At the same time, it can always remind us that the tunnel earthquake disaster cannot be ignored and requires continuous in-depth research. [ABSTRACT FROM AUTHOR]

Several properties of shale reservoirs change after absorbing water due to hydration, which increases the difficulty for shale gas exploration and exploitation. The Permian marine-continental transitional shale with different hydration degrees in Guizhou Province, China was collected to clarify the characteristics and mechanism of shale hydration in this study. The deformation induced by moisture adsorption, X-ray diffraction (XRD), and other experiments were conducted. The results show that the high-degree hydrated shale cores are seriously disaggregated. The larger the moisture adsorption capacity means the higher hydration degree. The shale swells after absorbing moisture, and the deformation ranges from 808 to 9145 uε. The swelling of the high-degree hydrated sample can reach 10 times of that of the unhydrated sample within the same time. While the pore volume and specific surface area decrease, and the maximum reduction of specific surface area is 26.69%. Compared with the unhydrated transitional shale, the essential reason for shale hydration is the higher illite-montmorillonite mixture content. When replacing clean water with 3% (mass fraction) KCl solution, the swelling reduces to between 633 and 8634 uε. The reduction in the swelling of the high-degree hydrated samples is greater than that of the other samples, which demonstrates that shale hydration prevention may be achieved by adding a certain concentration KCl solution to the drilling fluid. Highlights: 1) The disaggregation characteristics of transitional shale with different hydration degrees in the western Guizhou Province, China, were compared. 2) The shale hydration characteristics were demonstrated using reproduced samples, which has practical significance for gas exploitation. 3) Compared with the unhydrated transitional shale, the reason for shale hydration was illustrated. 4) The measures for shale hydration preventionwere discussed. [ABSTRACT FROM AUTHOR]

The natural hazard of rockfall might be caused by earthquake, freeze/thaw action, rainfall, weathering and the erosion of rocks. Due to the limitation by site conditions, field tests cannot be conducted adequately. To acquire a better knowledge of the rockfall mechanism, the three-dimensional (3D) energy tracking method (ETM) is used to investigate the influence of collision property and fragmentation on the trajectories of rock blocks. According to the numerical results, when the rock blocks collide with the gentle slope, their velocities and total kinetic energy decrease sharply. The fragmentation phenomenon mainly occurs at the gentle slope, which leads to the obvious decreasing of velocity and total kinetic energy. When the collision restitution of rock blocks increases, more rock blocks are able to pass through the gentle slope. When the friction coefficient increases or the fragmentation phenomenon occurs, more rock blocks stop at the gentle slope. Additionally, the increasing of collision restitution and fragmentation phenomenon leads to the increasing of invasion zone width. The present 3D ETM–based simulator is potentially useful to optimize the design of protection measures and control geo-hazard. [ABSTRACT FROM AUTHOR]

Deep buried tunnel engineering is in the high ground stress environment, shotcrete, and surrounding rock jointly bear the huge pressure of surrounding rock mass. In order to ensure the stability of tunnel structure, it is necessary to study the mechanical characteristics of rock-shotcrete structure. In this study, two kinds of granite with different mechanical properties were selected as research materials, and the beam of granite-shotcrete composite structure was made after pouring the same shotcrete. Based on the particle flow theory, numerical simulation was carried out by using the discrete element software PFC3D. After coupling the macro and micro mechanical parameters of each material, a model fitting the actual specimen was established. By comparing the experimental and numerical simulation results of four-point bending deformation and failure of specimens, it can be found that the fracture process can be divided into three stages: elastic deformation, the specimen cracks rapidly, and the specimen was unstable. Combined with the mechanical analysis, the bending failure mechanism was obtained. The bearing capacity of the composite structure is mainly affected by the tensile strength of shotcrete; secondly, the granite with higher elastic modulus can make the composite specimen show higher bearing capacity and better deformation resistance; when the granite stiffness is constant, the granite strength has little effect on the bending failure process of the composite specimen. The research results of this paper have certain reference value to ensure the normal and safe use of deep buried tunnel lining. [ABSTRACT FROM AUTHOR]

Study on the change laws of surrounding rock stress field is helpful to solve the problem of roadway destruction and deformation. The principal stress trajectories diagram is a method of visualizing the surrounding rock stress field. This method analyzes the stress field topology based on the distribution of the principal stress trajectories and the locations and types of degenerate points. Taking the lateral pressure coefficient as an independent variable, through FLAC simulation, the principal stress vector cross-line diagram of the surrounding rock stress field with different lateral pressure coefficients is obtained. Then, the principal stress trajectories diagram is depicted based on the cross-line diagram. Based on the principal stress trajectories diagram, the distribution characteristics of principal stress trajectories can be summarized, and then, some suggestions to maintain the stability of surrounding rock can be summarized from these distribution characteristics. In a practical engineering case, the deformation characteristics of the roadway before and after support are analyzed by FLAC simulation. The analysis results are used to verify the correctness of the visualization method and suggestions. Research shows that the principal stress trajectories can effectively display the topology of the stress field. The distribution characteristics of principal stress trajectories have a clear guiding role in judging the stability of surrounding rock, and the suggestions of maintaining the stability of surrounding rock are applicable to the actual support situation. [ABSTRACT FROM AUTHOR]

The sprayed concrete and surrounding rock will be impacted by seismic waves, air shock, and flying rock created by the drilling and blasting process used in mine and tunnel construction. The dynamic compression behaviors of concrete–rock combined body with six different inclination degrees (0, 30°, 45°, 60°, 75°, and 90°) were numerically simulated using LS-DYNA and the Holmquist–Johnson–Cook constitutive model in this work. The effect of the confining pressure (ECP) and strain rate (ESR) on the responses of the 3D meso-combined body were analyzed. Results indicated that the compressive strength of the combination can be improved, while the peak strain is lowered by increasing the contact inclination angle. The inclination dip determines the eventual failure mechanism of the specimens, and the final failure modes of the composite specimens with different dip angles can be divided into splitting failure, shear-compression composite failure, and shear failure. Finally, calculation results were compared with theoretical data published by R.E. Sheriff (1996). The research results are helpful to understand the dynamic failure law of lining confining pressure structure. [ABSTRACT FROM AUTHOR]

Preparation of laboratory samples that have surfaces with complex but repeatable roughness distributions to study the mechanical behavior of jointed rock mass has been a challenge for experimental studies. In this paper, an innovative rough discrete fractures network (RDFN) model was proposed. Three-dimensional (3D) printing technology was used to generate synthetically jointed rock specimens while considering joint roughness and geometry. The size effect of shear behaviors under direct shear tests was discussed on the synthetic specimens described as rough discrete fracture network (RDFN) model, discrete fracture network (DFN) model, and intact specimen. According to the results, the shear strength of the 3D-printed jointed rock specimens was apparently lower than that of the intact rock specimens. The shear strength of the RDFN models, which considered joint roughness, was higher than that of the DFN models. The shear strength decreased with increasing specimen size for both the RDFN and DFN models and the fracturing patterns were greatly influnced by the joint geometry and distribution. The joint roughness is found important for the shear behaviors and should not be neglected when modeling the fractured rock mass. [ABSTRACT FROM AUTHOR]

A large-scale shaking table test and a series of numerical simulations were performed to clarify the seismic response of the slope reinforced by frame beam and anchor cable by applying Wenchuan-Qingping and El Centro seismic records. The horizontal acceleration responses and response spectrum characteristics of slope, the axial force of anchor cable, and the displacement of frame beam were mainly studied by shaking table test. The influence of the excitation direction of seismic waves was investigated in numerical simulation. The results show that the amplification effects of acceleration and response spectrum along the slope height are independent of the type of earthquake, while the amplification effects of acceleration and response spectrum with excitation amplitude are related to the type of earthquake. There are obvious differences in the acceleration responses between the lower and upper weak sliding layers, and a stronger nonlinear behavior occurs in the upper weak sliding layer, which is explained by the time-frequency spectrum calculated by synchrosqueezing wavelet transform (SWT). The axial force of anchor cable and the peak displacement of frame beam increase with the increases of slope height and excitation amplitude. The dynamic response and damage range of the slope under bi-directional earthquake excitation are significantly larger than that under one-directional earthquake excitation. [ABSTRACT FROM AUTHOR]

As a kind of newly emerged excavation equipment, the TBMs play an increasing important role in underground coal mines due to their high penetration rates, while the failure and large deformation of large cross-section TBM assembly chambers need to be controlled under combined action of high ground stresses and live loads generated by heavy TBM parts. In this research, the mechanical parameters of rock specimens and rock quality designation (RQD) were used to estimate mechanical parameters of rock masses. The numerical tests were conducted and the micro-parameters of surrounding rocks in UDEC were calibrated. A numerical model of the TBM assembly chamber was established and the disturbance characters under the original support scheme after excavation were studied. Moreover, the influences of live loads generated during the hoisting and assembling operations of TBM parts were taken into account. An optimized support scheme was proposed and examined by numerical simulation. The in situ monitoring works were conducted to verify the simulation results. The monitoring results are in good agreements with the simulation results and the optimized support scheme satisfied the demands of stability controlling of the TBM assembly chamber. [ABSTRACT FROM AUTHOR]

This study aims to control the deformation of a large section of tunnel excavated in steeply inclined layered phyllite under high geo-stress. To this end, anisotropic mechanical properties, which are factors influencing the deformation and stress characteristics of the phyllitic structures, were studied by combining rock tests with field tests based on Yangjiaping Tunnel on the Chengdu-Lanzhou Railway, China. Moreover, measures to control the surrounding rock deformation are proposed. The main conclusions are as follows: mechanical properties of chlorite-phyllite present significant anisotropy and its strength and failure modes are related to the azimuth of the applied load. There are significant differences in the failure modes of the surrounding rock at different positions in the tunnel excavated in steeply inclined layered strata. The rock surrounding the sidewalls is dominated by tension and the bending failure of beddings, whilst the vault and inverted arch are characterised by shear failure. Furthermore, the surrounding rock deformation of the tunnel excavated in steeply inclined layered phyllite under high geo-stress is affected by factors such as tectonic stress, strata occurrence, a soft interlayer and construction disturbance. The surrounding rock deformation and stress of the support structures are distributed non-uniformly. Large displacements and deformations are found on sidewalls, where targeted control measures should be taken. The measures that can be taken include optimising the cross-sectional profile, strengthening support rigidity, simplifying the excavation steps, enhancing advanced pre-support measures, adjusting the angle at which anchor bolts are applied and using mechanised construction. They can timeously provide support to the surrounding rock and limit the development of plastic zones, which not only controls tunnel deformation, but also improves the progress of construction by around 50%. [ABSTRACT FROM AUTHOR]

In this research, a new rock failure precursor is proposed that can be used effectively for an early rock failure indicator (RFI). This RFI is based on the characteristics of mudstone and due to the acidic coal mining environment; the tests were performed on the acidic samples under uniaxial loading in the presence of infrared radiation (IR). Three IR characteristics indexes, i.e., variance of infrared radiation temperature (VIRT), average infrared radiation temperature (AIRT), and variance of differential infrared image temperature (VDIIT), were used for predicting rock failure precursors. The effects of acidity on these indexes were also evaluated, and the results indicate that the aforementioned indexes give failure precursors nearly at the failure point of the samples. Thus, for an early rock failure precursor, the absolute rate of ratio (ratio of AIRT and VDIIT) was used for the first time in the research. This ratio gives the average time of 78 s before the failure compared to 52 s of AIRT and VDIIT. The precursor point stresses in the absolute rate of ratio is lower (0.87 σpeak) compared to AIRT and VDIIT (0.94 σpeak). It is revealed from the results that the absolute rate of ratio gives 26 s an early RFI than VIRT and VDIIT. Hence, the new index is proposed to be used as an early indicator of rock failure in an acidic environment for the safe and efficient execution of underground projects. [ABSTRACT FROM AUTHOR]

In the process of underground coal mining, the energy released by fracture of roof beam is the main source of rock burst energy. This part of energy is affected by its size and material properties. This study redefined the geometric parameter (GEO) and material parameter (MAT) that characterize the properties of rock beams. Three point bending tests were designed and carried out on account of the two parameters. The parameter representing the energy release capacity of rock beams was proposed, and the effects of GEO and MAT on the parameter were quantitatively analyzed by numerical simulation. Finally, a new method for predicting the energy released by rock beam fracture was proposed. The results show that with the increase of MAT and GEO, the total energy stored in the rock beam and the energy released during fracture both showed an upward trend. MAT is the main parameter that affects the load–deflection curve and AE parameter distribution characteristics, while the influence of GEO on the test curve was only reflected in the numerical change. The energy release ratio ranges from 0.5 to 0.7, which exhibits a quadratic function relationship with the MAT of the rock beam. The proposed new method for predicting the energy released by rock beam fracture was successfully used in the 10,301 working face of Jining No. 2 Coal Mine. The prediction results of the basic roof and the direct roof beam were consistent with the field microseismic test results, and the errors were only 10.88% and 11.50%, respectively. [ABSTRACT FROM AUTHOR]

This study aims to ensure the safe and efficient mining of No.69 tantalum–niobium ore bodies in an underground metal mine in Jiangxi Province. Brazilian splitting tests were conducted on disc samples of two types of surrounding rocks (sandy slate and granite) from the + 417 m middle section of the mine at different loading rates (0.002, 0.005, and 0.01 mm/s). The tensile strength, failure mode, acoustic emission (AE) characteristics, and fractal features of disc samples during splitting failure were analyzed in relation to the loading rate. The study demonstrates that with an increase in the loading rate, the duration of splitting failure of both sandy slate and granite significantly decreases, and the corresponding ultimate load at the time of failure significantly increases. The tensile strength of the sandy slate and granite increases linearly with the loading rate, and the growth rate of the tensile strength of the granite samples was slightly larger than that of the sandy slate. The loading rate has a significant effect on the failure pattern of the splitting process of rock disc samples. The AE ringing counts and energy of the sandy slate and granite both show an increasing trend with the loading rate. Under low loading rate, the AE ringing count signal is generally of "solitary vibration" type, and under high loading rate, the ringing count curve is generally of "group vibration" type. The peak mean values of ringing counts and energy of the sandy slate were larger than those of granite under different loading rates. The maximum values of the AE fractal dimension of both sandy slate and granite appeared between 60 and 80% of the peak stress. The research results can provide a reference for the analysis and prediction of rock stability during the design and construction of ore body mining. [ABSTRACT FROM AUTHOR]