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The intensities and locations of rockbursts directly relate to both the magnitude and direction of the in-situ principal stresses in the rock mass. Therefore, achieving an accurate estimation of the in-situ stresses is essential when making rockburst assessments. Typically, the cases in deep tunnels are located in hard rock in tectonic regions near steep valleys. This study focuses on evaluating the complete in-situ stress tensor in deep tunnels in hard rock starting from the geological structure, topography, and in-situ stresses measured in the tunnel area, using multivariate linear regression. By inverting the three-dimensional (3D) in-situ stress field, a novel criterion was proposed for assessing rockburst potential that takes into account the directions of the in-situ principal stresses with respect to the axis of the tunnel. The new integrated approach is supported by rockburst generation in a tunnel in southwestern China. If the tunneling axis intersects the direction of the major horizontal principal stress at a large inclined angle, the post-excavation maximum stress would increase by some times. Taking into account the Kirsch solution, considerable stress concentrates after the tunnel excavation. Even though the overburden is less than 300 m, rockbursts occur frequently during excavation. The main reason for their occurrence is thought to be the in-situ stress field in the valley. Numerical modeling demonstrates a significant concentration of in-situ stress on both sides of the faults present. Moreover, rockbursts are expected to occur over 63.52% of the total length of the tunnel. Specifically, these rockbursts are expected to be of weak–moderate intensity. [ABSTRACT FROM AUTHOR]

This study introduces a novel large three-dimensional physical model experimental system (PMES) for studying the geomechanical problems related to deep underground engineering. The apparatus is mainly composed of a counterforce frame, a hydraulic loading subsystem, a sample installation and transportation subsystem, an excavation subsystem, an integrated monitoring subsystem, and auxiliary facilities. During the design and development of the PMES apparatus, key technological issues were addressed. A high-stiffness frame structure with opening and closing function was developed. Uniform, gradient, and designed non-uniform stresses can be applied with high accuracy. An independent long-term load-holding unit and a surface dynamic disturbance unit were designed. The basic and innovative functions of the apparatus were verified through calibration experiments. A large physical model experiment was conducted to simulate the excavation of a deep-buried drainage tunnel in China's Jinping-II hydropower station. The experiment obtained the failure processes of the surrounding rock masses induced by high in-situ stress, specific engineering geological structures and excavation disturbance. The internal fracturing and strain evolution within the sample were successfully captured by the integrated monitoring subsystem. The results also validated the reliability of this novel apparatus. The development of this novel apparatus provides a new tool for the study of geomechanical problems in deep underground engineering. Highlights: A novel large three-dimensional physical model experimental system for studying the geomechanical problems related to deep underground engineering was established. The basic and innovative functions of the apparatus were verified through calibration experiments. A large physical model experiment was conducted to simulate the excavation of a deep tunnel in China's Jinping-II hydropower station. [ABSTRACT FROM AUTHOR]

An intense fault rockburst recently occurred in a deep tunnel excavated by tunnel boring machine in China, which was taken as a case to study the failure characteristics in detail. The temporal and spatial evolution of microseismic (MS) activity, the dynamic stress drop and the spatial distribution characteristics of rock fracture mechanism were analyzed. Stress-induced mechanism was studied using numerical simulation, and the development mechanism of the fault rockburst was revealed finally. The research results are as follows: (1) The development process of the fault rockburst is divided into six stages: calm stage, crack initiation stage, crack development stage, crack accumulation–slight fault dislocation stage, further crack accumulation stage and crack penetration–fault slip stage. (2) During the development of the fault rockburst, stress and energy accumulate in the deep of surrounding rock. When rockburst occurs, stress and energy are released in the shallow of the surrounding rock; at the same time, the dynamic stress drop increases significantly. MS energy is released intermittently during the development of fault rockburst. (3) Affected by the geological condition, the high in situ stress and the excavation unloading, horizontal shear slipping of the fault occurs, leading to fault rockburst. These research results can provide a reference for establishing the warning and mitigation methods of fault rockburst in deep TBM tunnels. [ABSTRACT FROM AUTHOR]

In this work, an extremely intense fault-slip rockburst case, with a local magnitude of 2.3, observed in a deep tunnel in southwestern China was introduced as a case study. The rockburst caused a large economic loss and delayed the construction schedule for nearly 2 months. In-situ failure analysis, geological surveys, and microseismic (MS) monitoring were carried out to study the development and occurrence mechanisms of fault-slip rockburst. The fault-slip rockburst occurred in the hanging wall of a structure plane with dark green filling material and the rock mass slipped along the structure plane. It shows characteristic of intermittent occurrence, which results in great psychological pressure on workers. The dominant frequency of the rockburst MS event during the fault-slip rockburst is 13 Hz, which shows that the fault slip rockburst can be captured by the MS monitoring system. Extremely intense MS activity was recorded during the rockburst. The development and occurrence processes of the fault-slip rockburst were studied using this MS information. The stress evolution during the rockburst was studied by analyzing the dynamic stress drop. For the evolution mechanism of the fault-slip rockburst, many tensile fracture events occurred during the development stage, and a few shear fractures, corresponding to structure plane dislocation, occurred before the occurrence of the fault-slip rockburst. The presence of a large natural structure plane resulted in a higher stress and crack extension, and reduced the length of the penetration path, resulting in a stronger destructiveness. The results can be used for the further research on fault-slip rockbursts and planning appropriate warning and mitigation measures. Highlights: An extremely intense fault-slip rockburst with local magnitude of 2.3 in a deep tunnel was introduced. Occurrence characteristics of fault-slip rockburst were analyzed. The development and occurrence mechanisms of the fault-slip rockburst including the fracture type and evolution mechanism, were studied. [ABSTRACT FROM AUTHOR]

Rockbursts, stress-induced collapse, and spalling often occur in the excavation process of deep-buried hard rock tunnel, which affect the stability thereof. The early identification of these potential high-stress hazards during tunnel excavation can provide reliable basis for the selection of preservative and controlled measures. During the excavation of a deep-buried hard rock tunnel in southwest China (a railway tunnel with a maximum burial depth over 2000 m), rockbursts, stress-induced collapse, spalling, and cracking occurred frequently. In this study, the microseismicity of various high-stress hazards in the genesis process of the tunnel was analysed. The difference of microseismicity characteristics of these high-stress hazards was revealed. A matter-element extension mathematical model was established to quantify the microseismicity of the rock mass during tunnel excavation. Based on microseismic (MS) information, an identification method of potential high-stress hazards in deep-buried hard rock tunnels was established. The method was then applied to a real case, and reliability analysis thereof performed. Thirty test case samples were selected for reliability analysis and case verification. Through practical application, it is found that 90% samples were identified correctly. The results show that the identification method of potential high-stress hazards in the deep hard rock tunnel constructed in this study is both applicable and reliable, and can identify potential high-stress hazard types in the process of tunnel excavation by using MS information. [ABSTRACT FROM AUTHOR]

The occurrence of rockburst is related to the surrounding rock stress and geological rock mass strength. Rockburst occurs frequently during the construction at a deep tunnel in southwest China (a tunnel with a depth of approximately 2080 m, and a length of 13 km), leading to casualties, damage to construction equipment, and schedule delays. The application of the rockburst criteria based on the stress-strength/strength-stress ratio shows that they are not suitable for the construction stage. This paper suggests a simple and feasible rockburst intensity criterion based on the Geological Strength Index (GSI) for the construction stage. In this criterion, the strength of the rock mass exposed during excavation is estimated by the Hoek-Brown criterion and is used to reflect the geological conditions of the surrounding rock, considering the influences of the joint conditions and engineering disturbance. The maximum principal stress along the tunnel is obtained by the in situ stress inversion. The ratio of the rock mass strength to the maximum principal stress (σcm/σ1) is taken as the evaluation index. The criterion is as follows: σcm/σ1 > 1.2, none rockburst; 0.75 < σcm/σ1 ≤ 1.2, slight rockburst; 0.4 < σcm/σ1 ≤ 0.75, moderate rockburst; and σcm/σ1 ≤ 0.4, intense rockburst. The engineering applications show the reliability of this rockburst intensity criterion. The results of this paper provide a reference for rockburst intensity evaluation during the construction stage of deep tunnels. [ABSTRACT FROM AUTHOR]

Uncertainty in the locations where rockbursts may occur with respect to a tunnel cross-section greatly increases the difficulty in monitoring and providing warnings of rockbursts and mitigating their effects. In this paper, we analyze the characteristics of 40 rockbursts that occurred in a railway tunnel in China. The rockbursts are divided into three cases depending on their position with respect to the tunnel cross-section and the prevailing geological conditions. An in situ microseismic (MS) monitoring system was established to determine the likely position of rockbursts with respect to tunnel cross-sections. The spatiotemporal evolution of the MS events that occurred during the development of rockbursts belonging to the three cases was subsequently obtained. The mechanism responsible for the changing positions of rockbursts with respect to tunnel cross-sections was further elucidated by analyzing the fracture type of the MS events and conducting numerical simulations using three-dimensional distinct element code (3DEC). Finally, support schemes, based on the monitoring and simulation results, are suggested. [ABSTRACT FROM AUTHOR]

A new, high-efficiency technology for fracturing and breaking rocks is required. Due to various advantages including high efficiency, energy-saving, and having no secondary pollution, the technology of microwave-induced fracturing of hard rock has been considered as a potential method for rock fracturing and breaking. Aiming at the realisation of two engineering applications: microwave-assisted mechanical rock breaking and stress release from rock masses in deep underground engineering works to prevent geological disasters caused by high-stress concentrations such as rockbursts, a novel (open-type) microwave-induced fracturing apparatus (OMWFA) for fracturing hard rocks was developed. On this basis, the two modes of microwave-induced subsurface fracturing and microwave-induced borehole fracturing of hard rocks were proposed. Due to removal of the restraint of the microwave cavity, OMWFA can be used to fracture large-size rock samples and engineering-scale rock masses. Using the apparatus, the fracturing effects of the two fracturing modes on different dimensions of cuboidal basalt samples were investigated. By combining the microwave-induced fracturing apparatus with a press machine to explore the influence of unidirectional stress on the fracturing effect of microwave treatment on basalt. Moreover, field tests were carried out on rock masses encountered in underground engineering works at Baihetan Hydropower Station in Sichuan Province, China, and the fracturing effects were evaluated by applying a digital borehole televiewer and conducting acoustic wave testing. The results show that the apparatus had favourable fracturing effects on the subsurface and borehole samples of basalt. When no stress was applied, the cracks radially expanded from the approximate centre of the radiant surface and unidirectional stress promoted fracturing. The number and depth of cracks increased with prolonged microwave exposure. After microwave treatment, the P-wave velocity of the samples declined, and the longer the microwave exposure, the more significant the reduction in P-wave velocity was. The results of field test reveal that borehole fracturing can exhibit a favourable effect around boreholes. The sound velocity around the borehole and between the boreholes both declined to some extent. Microwave-induced hard rock fracturing offers guiding significance to those exploring and developing new rock breaking and tunnelling methods, and generally enhances construction safety in deep underground engineering works. [ABSTRACT FROM AUTHOR]

Columnar jointed basalt (CJB), characterized by a hexagonal joint network, poses a great challenge for geotechnical designs due to its time-dependent propensity to deformation and damage. In this paper, the geometrical structure and failure modes of CJB exposed on a dam foundation is presented. The study was conducted during the unloading excavation of the Baihetan dam, China, aiming to provide a more in-depth understanding of the CJB damage mechanism. A systematic field test for the columnar jointed rock mass was also carried out on the dam foundation using ultrasonic P wave measurements. It was found that the excavation damage zone of the CJB not only showed time-dependent damage both in depth and degree but also showed spatially inhomogeneous distribution of the damage degree. Sequential regression analysis further demonstrated the time-dependent behavior of the CJB. Successive observations using digital borehole cameras in the field showed that the unloading damage mechanism of the Baihetan CJB was mainly related to the time-dependent opening of the initially closed joints. Several reinforcing methods for improving the stability of the damaged CJB, including the injection of liquid cement and the installation of the prestressed rock bolts, are discussed. [ABSTRACT FROM AUTHOR]

Zonal disintegration refers to the phenomenon whereby fractured zones and intact zones appear alternately in surrounding hard rocks under high stress conditions. In this study, zonal disintegration within hard surrounding rocks of deep tunnels at the China Jinping Underground Laboratory Phase II (CJPL-II) was observed by using a digital borehole televiewer in pre-drilled boreholes. The results revealed that fractured zones within intact high-strength marbles were small and exhibited a higher degree of fracturing, whereas fractured zones within low-strength, low-integrity, marbles showed fewer, yet larger fractures. An evaluation method based on rock mass integrity index using digital borehole televiewer data (RMIBT) has been proposed for the analysis of zonal disintegration process in the deep hard rocks surrounding the tunnels. By using the method we were able to quantify the initiation and evolution of fractures during zonal disintegration. The formation mechanism of the second and third fractured zones induced by the existing fractures was further investigated using numerical analysis. The data provide an insight into the evolution of failures in, and a basis for support design and stability control of, deep engineering works in hard rock. [ABSTRACT FROM AUTHOR]

To study rock spalling in deep tunnels at China Jinping Underground Laboratory Phase II (CJPL-II), photogrammetry method and digital borehole camera were used to quantify key features of rock spalling including orientation, thickness of slabs and the depth of spalling. The failure mechanism was analysed through scanning electron microscope and numerical simulation based on FLAC3D. Observation results clearly showed the process of rock spalling failure: a typical spalling pattern around D-shaped tunnels after top-heading and bottom bench were discovered. The orientation and thickness of the slabs were obtained. The slabs were parallel to the excavated surfaces of the tunnel and were related to the shape of the tunnel surface and orientation of the principal stress. The slabs were alternately thick and thin, and they gradually increased in thickness from the sidewall inwards. The form and mechanism of spalling at different locations in the tunnels, as influenced by stress state and excavation, were analysed. The result of this study was helpful to those rethinking the engineering design, including the excavation and support of tunnels, or caverns, at high risk of spalling. [ABSTRACT FROM AUTHOR]

During the excavation of the underground powerhouse in the Baihetan hydropower station, which is currently still under construction, stress–structure controlled collapse has occurred frequently. In order to study the mechanism behind the evolution of this kind of collapse, an in situ experiment involving microseismic (MS) monitoring was carried out in the left main/auxiliary powerhouse. In this paper, the spatiotemporal characteristics of stress–structure controlled collapse are summarized and presented. A field survey, scanning electron microscopy and MS monitoring have been used to investigate a typical stress–structure controlled collapse that occurred during the monitoring period. These methods provided a consistent set of results, namely, that tensile fracturing is the rock-mass fracturing mechanism that is most active during the process of evolution of stress–structure controlled collapse. In addition, the evolution of the microseismicity during the development of the studied collapse was also obtained. The results provide a direct case history that will assist the prediction and support of stress–structure controlled collapse disasters and contribute to excavation of deeply-buried caverns in the field. [ABSTRACT FROM AUTHOR]

Microseismic (MS) monitoring techniques have been used to warn of rockbursts in tunnels to reduce rockburst risks and ensure safe construction with different degrees of success. In order to reveal the characteristics of MS activity in tunnel rockburst development processes, and lay a foundation for tunnel rockburst warning, MS sequences accompanying rockburst development processes in tunnels are studied in this paper. The study is based on tunnels (with a maximum burial depth of 2525 m) of the Jinping II hydropower station, China. Firstly, two MS parameters are put forward to express the comprehensive microseismicity and intensity of the MS events in the rockburst development process. Secondly, the laws governing the evolution of the two parameters are analyzed based on 44 moderate and intense immediate rockburst cases in the tunnels of Jinping II. Then, the MS sequences associated with immediate rockburst development processes in tunnels are divided into two types: fore-main and swarm shock MS sequences. The characteristics of the two types of MS sequences are different with each other. Finally, based on a theoretical analysis and verification using engineering examples, the mechanism of formation of the two MS sequences is analyzed. The fore-main MS sequences are mainly controlled by excavation unloading in the tunnel. The swarm type is mainly controlled by the combined effects of excavation unloading and the presence of structure planes. [ABSTRACT FROM AUTHOR]

• The stress-strain curves and mesoscopic voids of shale were obtained their evolution exhibited staged characteristics. • The relationship between the damage variable and the axial strain follows the quadratic function. • The mesoscopic mechanism of the macroscopic response of shale was analysed. • A damage constitutive model of shale under uniaxial compression conditions was proposed. With the continuous development of shale gas engineering in China, an accurate and adequate understanding of shale damage mechanism has become a critical scientific problem addressed by rock mechanics. So far, many researchers have studied the fracturing process and mechanism of shale in multi-scales. The mesoscopic structural characteristics of shale during loading can be obtained using the in-situ X-ray computed tomography (CT) technology. Some scholars have observed the structure evolution of shale under in-situ uniaxial compression conditions, but the quantitative investigation of damage evolution and constitutive model of shale still requires further studies. Given this, the in-situ micro-CT device was adopted to study the macroscopic mechanical response and the mesoscopic structural evolution of Longmaxi shale under uniaxial compression conditions. The evolution law of the damage variable with the axial strain was analysed according to the characterisation of the damage variable based on the CT image grey variance. In combination with the macroscopic stress-strain curve shape, evolution stage of the mesoscopic void, evolution of the damage variable, mesoscopic mechanism of the macroscopic mechanical response was investigated, and a damage constitutive model of shale under the uniaxial compression test was proposed. The conclusion of this study can provide a foundation for further research into the multi-scale failure mechanism of shale and exert theoretical guidance significance on shale gas fracturing engineering practice. [ABSTRACT FROM AUTHOR]

The article presents a study which evaluates the failure mechanism of unbonded prestressed thru-anchor cables (UPTAC) based on a case study of underground caverns in Sichuan Province, China. The study involves observations of several external failure modes of UPTAC including failure depth, break face and working load and installation method. It indicates laboratory experiments and field investigations demonstrating that modes of failed anchor cables can be classified as tensile failure.

Based on the radiated energy of 133 rock bursts monitored by a microseismic technique at the Jinping II hydropower station, in Sichuan province, China, we analyzed the advantages and disadvantages of qualitative classification methods for the rock burst intensity. Then, we investigated the characteristics, magnitude, and laws of the radiated energy, as well as the relationship between the rock burst radiated energy and intensity. Then, we selected the energy as an evaluation index for the rock burst intensity classification, and proposed a new rock burst intensity quantitative classification method, which utilized the hierarchical clustering analysis technique with the complete-linkage method. Next, we created a new set of criteria for the quantitative classification of the rock burst intensity based on radiated energy and surrounding rock damage severity. The new criteria classified the rock burst intensity into five levels: extremely intense, intense, moderate, weak, and none, and the common logarithms of the radiated energy of each level were >7 lg( E/J), >4 lg( E/J) and <7 lg( E/J), >2 lg( E/J) and <4 lg( E/J), >1 lg( E/J) and <2 lg( E/J), and <1 lg( E/J), respectively. Finally, we investigated the factors influencing the classification, and verified its feasibility and applicability via several practical rock burst examples. [ABSTRACT FROM AUTHOR]

To accurately describe the damage creep properties of marble, especially during the acceleration creep phase, creep testing was performed on the marble in the Jinping II hydropower station in China. Based on the experimental results, a time-dependent damage constitutive model was proposed in terms of fractional calculus theory and damage variables to describe these time-dependent damage characteristics. The time-dependent constitutive equations were derived for constant loading levels below and above the marble's long-term strength. The robustness and parameter sensitivity of the proposed model were analysed by utilizing data of creep testing. The results of the analysis showed that the proposed model could describe not only the attenuation and steady-state creep of marble but also the acceleration creep characteristics and negative exponential attenuation law of the yield strength over time when the constant loading was above the long-term strength. These are crucial to failure prevention during rock engineering construction and operating periods. [ABSTRACT FROM AUTHOR]

In order to evaluate the overall stability of the underground powerhouse at the proposed Baihetan hydropower station in China, the shear behaviour of a weak interlayered soil in the host rock and the soil/host rock interface was investigated in the laboratory by direct shear testing under different degrees of saturation. Shear creep tests were also undertaken on the interlayered soil. Typical results obtained from the multi-sample method (MSM) and single-sample method (SSM) are presented, which show that the shear behaviour of both the interlayered soil and the soil/rock interface is elastic perfect-plastic without dilatancy. It was not possible to determine the shear strength parameters using the multi-sample method due to the heterogeneity of the interlayered soil in terms of mineralogical composition and physical properties. Creep of the interlayered soil was found to be insignificant. The shear behaviour of the soil/rock interface depends strongly on the interface morphology: the rougher the interface the higher the shear strength. [ABSTRACT FROM AUTHOR]

• Brittle fracture of deep hard rock was simulated by physical model tests. • The evolution process of brittle fracture in deep tunnels was captured and analysed. • The onset conditions and mechanism of spalling failure in deep tunnels were revealed. • The spalling characteristics were verified by three research methods. Brittle fracture in deep hard rock tunnels in the Jinping laboratory, China, can often endanger the stability of engineering structures. This study describes physical model testing and real-time observations on the evolution process of v-shaped brittle fracture, failure characteristics, onset conditions, and influencing factors in hard rock tunnels. Real-time observations are presented integrated a high-speed camera, acoustic emission system, a digital image correlation, and distributed fiber optic strain sensing technologies. The results show that the process of spalling failure can be divided into two main stages, namely, the instantaneous cracking process in the sidewall surface, and the spalling process of the thin rock chips developing progressively from the excavation boundary to the far field, finally forming stable v-shaped notches. The deviatoric stress of spalling failure in the physical model test satisfies the constant deviatoric stress criterion. The critical strain at spalling failure is consistent with that calculated by the Hoek–Brown strength criterion. Spalling is a form of brittle failure caused by deferential tensile strain. The working face effect is observed in the physical model excavation process. The highest strain caused by overloading occurs near the free surface of the sidewall. The relationship between stress and geometric similarity constants in deep rock engineering is derived. The failure characteristics revealed in the physical model tests are verified by numerical simulations based on cellular automaton theory, which agree well with the experimental results. The research findings can provide theoretical support for the prevention and control of spalling disasters in deep engineering practice. [ABSTRACT FROM AUTHOR]

Proppants hold fractures open and increase fracture conductivity but must survive and remain functional during pressure drawdown. The shale reservoir usually suffers a high effective stress during gas depletion whilst most previous experiment works are conducted under a relative low stress level. In this work, permeability evolution was explored in a proppant-supported natural fracture of Longmaxi shale from the Sichuan Basin, China under a large effective stress range (1.5–59.5 MPa). Proppant performance was examined via continuous permeability measurements and by optical microscopy and laser-classifier measurements of particle size distributions (PSD) recored both pre- and post-loading. The permeability of the propped shale fracture is two orders of magnitude higher than that of the non-propped fracture and strongly controlled by the proppant behaviour. Surprisingly, overall permeability of the proppant pack decreases with an increase in thickness of the enclosed proppant. The decrease in the permeability with high stresses is largest for unpropped fractures and decreases with an increase in the number of layers. Most important, the mean compressibility of the non-propped and propped fracture is not constant but reduces with an increase in confining stress. This indicates that the compaction, crushing, embedment and repacking of the proppant particles, because of high effective stress, resulting in a decrease in the porosity of the proppant pack further reducing the compressibility and permeability of the supported fracture. • Permeability evolution was explored in a proppant-supported natural fracture under high (1.5–59.5 MPa) effective stress. • Proppant performance was examined via permeability measurements and particle size distributions (PSD) recored. • Overall permeability of the proppant pack decreases with an increase in thickness of the enclosed proppant. • The mean compressibility of the non-propped and propped fracture reduces with an increase in confining stress. [ABSTRACT FROM AUTHOR]

Two extremely intense rockbursts took place during the excavation of the drainage tunnel and headrace tunnel No. 4 at the Jinping II hydropower station in southwestern China. The geological conditions, damage, and failure of the surrounding rock mass at the sections where the rockbursts occurred were surveyed and are described here with a focus on the development of the damage to the rock mass and the mechanism by which it occurred. The numerical method based on the Cohesion Weakening and Friction Strengthening model and various indices, including the failure approaching index and the local energy release rate, were adapted for these purposes. The results shown here may help us understand the generation and process of development of rockbursts and evaluate the degree of failure in the rockmass. This information may be valuable in the study and design of rockburst-resistant measures in deep tunnels. [ABSTRACT FROM AUTHOR]

Abstract: We present a procedure for technically auditing rock mechanics modelling and rock engineering design. As rock mechanics modelling becomes more sophisticated, with the ability to include more parameters and to be coupled with other disciplines such as hydrogeology, and as the requirements on rock engineering projects become ever more challenging, it is prudent, if not essential, to have some form of procedure for checking that the modelling and design are suitable within the limits of current knowledge. Accordingly, we present here an appropriate auditing procedure, and demonstrate its use with the examples of measurement of rock stress (as a soft audit) and modelling for the design of hydropower caverns in China (as a semi-hard audit), the auditing questions being tailored to the nature of the work being audited. The types of procedures demonstrated can be used from the initial project concept, through site investigation and modelling, to construction, monitoring and back analysis, allowing the whole process to be concurrently checked providing a transparent and traceable audit trail of results and associated decisions made. [ABSTRACT FROM AUTHOR]

Microwave heating fracturing is potentially a green stimulation technology for gas shale recovery. However, the mechanism together with permeability evolution of microwave irradiated reservoir remain unclear. To fill this knowledge gap, the responses of Longmaxi shale from the Sichuan Basin, southwest China, to both continuous and intermittent microwave stimulation along variable microwave heating paths were explored. A complex thermally-induced fracture network can be formed gradually without sudden collapse under intermittent microwave irradiation. Changes in the petrophysical parameters of the shale including wave velocity, weight and volume at different intermittent microwave irradiation steps were measured together with temperature variation. The evolution of permeabilities for the two shale samples with alternately parallel and vertical beddings at different effective stresses was analyzed both before and after microwave irradiation. After the last step of intermittent microwave irradiation in this study, the shale permeability increased by two to four orders of magnitude for the shale sample with flow parallel to bedding and one to two orders of magnitude with flow perpendicular to bedding. Microwave treatment accentuates the anisotropy between bedding-parallel and bedding-normal permeabilities. Evolving pore size was measured by high-pressure mercury porosimetry and thermal-induced fracture characteristics and the changes of mineral composition were characterized by SEM combined with Energy Dispersive Spectroscopy (EDS). Thermally- and chemically-induced swelling stresses are mainly responsible for the development of fractures and micro-porosity in the shale. A permeability model with variable compressibility coefficient was adopted to fit the experimental data for shale permeability across a wide range of effective stresses from 2.5 MPa to 59.5 MPa. Shale fracture compressibility decreases in the later stage of microwave irradiation, suggesting the hardening of thermal-induced fractures. • Response of Longmaxi shale to variable microwave heating paths were explored. • The evolutions of permeabilities were analyzed both before and after microwave radiation. • A complex heating-induced fracture network was generated under the intermittent microwave radiation. • After microwave irradiation, the shale permeability increased by two to four orders of magnitude. • Microwave treatment accentuated the anisotropy between bedding-parallel and bedding-normal permeabilities. [ABSTRACT FROM AUTHOR]

Optimization of a soft rock replacement scheme for a large cavern excavated in alternating hard and soft rock strata is a complicated non-linear mechanical problem having a large parameter search space. Obtaining a global optimum solution is the key to the problem. A hybrid intelligent method is proposed for this purpose. It is an integration of an evolutionary neural network and finite element analysis using a genetic algorithm. The non-linear relation of the soft rock replacement scheme with the displacement and damage zone of the cavern due to excavation in the given geological conditions is learnt and represented by a forward neural network whose structure and connection weights are global optimally recognized by using the genetic algorithm. The learning samples are obtained from finite element calculations. The optimal soft rock replacement scheme, having the minimal displacement and damage volume induced by cavern excavation, is searched in a global space using the genetic algorithm. The new methodology is used to evaluate soft rock replacement schemes for the Shuibuya cavern in China excavated in strata consisting of alternating soft and hard rocks. The results indicate that the new methodology can recognize the optimal soft rock replacement scheme for a large cavern in such complicated geological conditions and the neural network model can provide a solution which is close to the finite element analysis for the same geological and construction conditions. [Copyright &y& Elsevier]

Sliding in a dam foundation along potential sliding paths is generally caused by two kinds of external factors: one is the overloading of the designed upstream hydrostatic load due to flooding; and the other is the gradual degradation of the shear strength of joints due to seepage, deformation, damage, and geochemical reactions between water and joint surface minerals. Based on the conceptualized geomechanical model of the Three-Gorges Dam, described in the Part I paper, in this Part II paper the limit equilibrium method and finite element method are used to study the effects of gradual degradation of the shear strength of joints on the stability of the dam foundation. The numerical modeling focuses on the stability conditions of the no. 3 powerhouse-dam section which are estimated to be the most critical. The constraint influences from the adjacent no. 2 and no. 4 powerhouse-dam sections are also included. The failure mechanisms, factors of safety and critical displacements of these dam sections are derived numerically as the measures for stability evaluation. The factor of safety is defined as the ratio between the combined shear strength of joints and intact rock bridges, and the mean shear stress along potential sliding path required for limit equilibrium under the designed external loads. All the results obtained from these different numerical models, together with the results of physical model tests as presented in Part I, are compared in this paper. The comparisons show that both the numerical modeling and physical modeling results support each other and demonstrate the stability of the Three-Gorges Dam foundation as designed. Nevertheless, considering the overall engineering and social–economical importance of the Three-Gorges Dam complex, some additional treatment and reinforcement measures are recommended in this paper. [Copyright &y& Elsevier]

Both coal and uranium resources are found in the Dongsheng area which is located in the northern part of Ordos Basin, China with the coal seam below the uranium deposit. In this paper, we attempt to study the feasibility of co-exploitation of the coal seam and the uranium deposit. The permeability evolution in strata around the coal seam are selected as the key parameter indexing the separate and safe concurrent recovery of two deposits and a novel strain-dependent permeability model is proposed covering the full deformation range of rock. The proposed model is first verified with experimental data with good fitting of the results. Then an upscaled numerical model is established considering the geology conditions of Dongsheng area. This is implemented using COMSOL Multiphysics; and the interface of COMSOL with MATLAB is used to study the damage to the rock mass. We show that (a) compared with the stress-dependent permeability model, the strain-dependent permeability model proposed in this work has the advantage that each strain value is associated with a unique permeability value; (b) The feasibility of co-exploitation of the coal seam and the uranium deposit depends on the relative distance between two resources. When the caved and fractured zone encroaches on the uranium deposit the coal seam should be abandoned. Conversely, when the uranium deposit lays in the constrained zone, the recovery of the coal seam would benefit recovery of the uranium deposit; (c) The feasibility of the co-recovery of the two resources can be optimized by the choice of mining approach. The top-caving mining approach and room-and-pillar mining approach can reduce the height of the caved and fractured zones and make the co-exploitation of the uranium and coal feasible. This work provides a new approach to investigate the feasibility of co-exploitation of uranium and coal. [ABSTRACT FROM AUTHOR]