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Shale anisotropy characteristics have great effects on the mechanical behaviour of the rock. Understanding shale anisotropic behaviour is one of the key interests to several geo-engineering fields, including tunnel, nuclear waste disposal and hydraulic fracturing. This research adopted the finite discrete element method (FDEM) to create anisotropic shale models in ABAQUS. The FDEM models were calibrated using the mechanical values obtained from published laboratory tests on Longmaxi shale. The results show that the anisotropic features of shale significantly affect the brittleness and fracturing mechanism at the micro-crack level. The total fracture number in shale under the Uniaxial Compressive Strength (UCS) test is not only related to the brittleness of shale. It is also strongly dependent on the structure of the shale, which is sensitive to shale anisotropy. Two new brittleness indices, BIf and BICD, have been proposed in this paper. The expression for BIf directly incorporates the number of fractures formed inside of the rock, which provides a more accurate frac-ability using this brittleness index. It can be used to calculate the frac-ability of rocks in projects where there are concerns about fractures after excavation. Meanwhile, BICD links brittleness to the CD/UCS ratio in shale for the first time. BICD is easy to obtain in comparison to other brittleness indices because it is based on the Uniaxial Compressive Strength test only. In addition, it has been shown there is a relationship between tensile strength and the crack damage strength in shale. Based on this, an empirical relationship has been proposed to predict the tensile strength based on the Uniaxial Compressive Strength test. Highlights: Through the use of FDEM simulations, the fracturing mechanics of shale, fracturing behaviour, and brittleness of shale have been investigated from a micro-scopic viewpoint taking shale anisotropy into account. Two new brittleness indices are proposed. One relies on the number of fractures in the shale to reflect the shale's frac-ability, and the other uses strength parameters that can be easily obtained from UCS tests. This paper proposes an empirical relationship to predict the tensile strength of rocks based on UCS tests. [ABSTRACT FROM AUTHOR]

It is very important to predict the blasting fragmentation size of fractured rock mass. Based on the geological conditions of practical engineering rock mass, a plane analysis model of bench blasting in layered rock mass considering microcrack structure was established. And the distribution law of stress in the bench blasting of layered rock mass with different occurrence and layer thickness is analyzed by using the CDEM numerical simulation method. Combined with the theory of rock fracture mechanics and relevant test results, this paper quantitatively studied the change laws of the angle for the cracks to propagate, speed and crack initiation length at different particle points in bench blasting and realized the time history analysis of the crack development and rock fragmentation in bench blasting. And then, this paper derived the quantitative relationship between the post-expansion cranny ratio and the blasting fragmentation from a theoretical point of view, and proposed the prediction method of the median fragmentation in bench blasting of layered rock mass. Combined with the bench blasting productive test of Changjiu (Shenshan) limestone mine, the largest sand aggregate production mine in the world, the correctness of the theoretical results is verified. The test results demonstrate that in a vertically inclined rock bench, the degree of rock fragmentation intensifies in the horizontal direction, and the overall fragmentation effect can be optimized by adjusting the blasting hole spacing; the blasting fragmentation prediction method proposed in the paper can accurately predict the median fragmentation of layered rock, which provides a novel method for predicting the effects of bench blasting. Highlights: The prediction method for the median fragmentation of layered bench rock mass blasting is obtained The quantitative relationship between the post-expansion cranny ratio and the median fragmentation of bench blasting is theoretically derived The distribution law of microcracks in bench rock mass is simulated by discrete fracture network model. The internal force distribution characteristics are analyzed by continuous medium discrete element method The propagation law and rock breaking process of microcracks and layered cracks in bench blasting are studied At the same time, the correctness of the prediction method is verified by the field test and production of Changjiu (Shenshan) limestone mine [ABSTRACT FROM AUTHOR]

The method of backfill in underground mining is important for ground control as well as material recycling and energy efficiency. Even though extensive testing and field studies of backfill have been conducted, less is known about the detailed damage and fracturing that occurs directly at the rock/backfill interface. In this paper, cylindrical specimens containing an inner diameter of backfill and an outer diameter of rock (RB) were tested under triaxial compression. Acoustic emissions (AE) were used throughout testing, and X-ray computed tomography (CT) scanning was conducted before loading was applied and after the specimens had failed. The high-resolution CT images were then converted into point clouds to isolate the fractures and visualize them in three dimensions. The point clouds clearly show that fracturing occurred both in the rock and along with the contact between rock and backfill, while very little fracturing was found to occur in the backfill. Based on the point cloud and AE results, a unique evolution of fracturing is found to occur that includes two stages of shear fracturing in the rock, tensile fracturing along with the rock/backfill interface, and final tensile fracturing in the rock after delamination from the backfill, all of which contributed to the nonlinear stress–strain response. This paper presents a novel approach for investigating the initiation and propagation of 3D fractures in laboratory testing and can offer a useful reference for further studies on the mechanics of bi-material structures. [ABSTRACT FROM AUTHOR]

This paper presents a new scour model (SM) that is able to evaluate the rock scour development, shape, ultimate depth and probability of occurrence. The model takes into account two main scour mechanisms: (1) the maximum rock crack extension that mainly depends on the geology, rock characteristics/mechanics and hydraulic fracturing analysis; and (2) the isolated rock block stability that depends on the flow hydraulics and rock block dimensions. The former defines the ultimate extent of the fractured area in a rock matrix where the isolated rock blocks can be detected; only in this area, the rock block stability analysis can be performed afterwards. The isolated rock block stability has been studied in the literature; on the contrary, the information related to the rock crack propagation in dissipaters is limited, which is investigated in this paper. Here, the expected maximum rock crack extension is defined by a threshold on the stress intensity factor. The rock stress intensity factor is investigated via stochastic approaches, compatible with the stochastic nature of the turbulent bottom pressure field, with the support of physical hydraulic modelling results. The proposed SM is a physically based model that couples the hydraulics and geological aspects of scour phenomena, allowing a realistic evaluation of the rock scour. This is drawn out by the comparison between the maximum scour depth computed via the rock block stability analysis and the maximum depth of rock matrix fractured area where the isolated rock blocks can be formed by the incident flow; the smallest depth gives the expected scour. The ultimate scour could be significantly smaller than the one computed via the approaches in the literature, especially for compact rocks suitable for unlined dissipaters. This highlights the relevance of this study that takes into account the high-velocity flow characteristics such as aeration and turbulence effects as well as the main geomechanical characteristics of the whole rock mass obtained by geological surveys. Furthermore, the SM stochastic approach makes it suitable in the risk-based design of dams, hydropower outlet works and other hydraulic structures as well as dam stability assessment. The model is validated using well-known real-life scour cases in the literature such as Wivenhoe and Cabora Bassa dams. A design example is included in the paper demonstrating the evaluation of the rock scour downstream of a large dam. Highlights: The paper presents a novel probabilistic rock scour model applicable to hydraulic structures. The model is able to evaluate the rock scour development, ultimate depth and probability of occurrence in unlined dissipaters. This physically-based model couples the hydraulics and geological aspects of scour phenomena allowing a realistic evaluation of the rock scour. The model is suitable for the risk-based design of dams, hydropower outlet works and other hydraulic structures as well as dam stability assessment. The model is validated using knowing real-life scour cases in the literature such as Wivenhoe and Cabora Bassa dams. [ABSTRACT FROM AUTHOR]

In recent years, interest in the effect of positive temperatures on rock faces has increased, as evidenced by the extensive literature on the subject. The subsurface rock masses of rock slopes are submitted to uncontrollable natural temperature cycles which cause deformation, displacement, opening and closing of discontinuities and can lead to surface failure or more important rockfalls. In this paper we offer a review of the publications on this theme focusing on in situ observations and measurements. Measurements are described and analysed considering conditions of temperature and insolation, displacements, strains, focusing on two French sites (Les Rochers de Valabres and La Roque-Gageac). Mechanical analysis is then performed. The impact of temperature variation is studied from basic assumptions to much more complex ones. Thermoelasticity is first assessed, and analytical computation is suggested, fracture mechanism and fatigue are also considered. Finally, we outline some challenges to be addressed in the coming years. Highlights: The article reviews the accumulated knowledge on the effect of positive temperature changes on slope stability. Excluding the effect of water freezing and thawing. The different types of measurements are reviewed considering surface and in-depth measurements of temperature, displacements, fracture opening, and strain. Simple thermo-elastic analysis, alongside solutions from fracture mechanics are proposed. A focus is proposed on 2 French sites to illustrate interpretations. The conclusion summarizes the points of consensus, the aspects that are still under discussion among authors or should be more debated and open to pending challenges for the next years. [ABSTRACT FROM AUTHOR]

This study analyzes the fracture patterns generated from the high-energy release caused by commercially available explosives and the current capability of numerical methods to replicate this. The mechanics of rock fracture and fragmentation are first studied using the Hybrid Stress Blasting Model (HSBM) at the lab-scale through comparison with experimental results available in the literature. Following this, an in-house experimental blast campaign was undertaken with a detailed examination of an unconfined, 605 mm diameter cylindrical sample and a pressurised (confined), 700 mm diameter cylindrical sample. The post-blast samples were dissected and fractures were visually mapped before comparing the fracture intensity results to the output of the HSBM, which captures stress-wave-induced damage but not gas loading, for these test cases. This paper makes contributions to the literature in three fundamental areas. Firstly, the capability of numerical methods to capture the phenomenology of blasting on rock fracture and fragmentation was validated and the limitations were discussed when looking to use this as a design tool for practical blasting operations. Secondly, the experimental campaign provides detailed insights into the response of cementitious grout materials to internal, blast-induced loading that can be applied in various fields such as mining and construction. Finally, the impact of confining pressure on blast damage was investigated and details were provided for how this can be captured in numerical predictions. It was found that certain aspects of the material response can be well predicted through numerical analysis, namely, the damage radius and the number of dominant fractures. However, it also indicated a shortcoming in the explicit comparison of fracture intensity measures such as P21, with the use of damage metrics appearing more appropriate. The grout-based testing methodology discussed in this work provides an efficient means for gathering data on the impacts of individually controlled aspects of a blast in both unconfined, and confined environments. Highlights: Development and outline of the methodology associated with a novel experimental setup for confined blasting tests. Investigated state-of-the-art numerical methods' ability to capture the phenomenology of blasting on rock fracture and fragmentation. Discussion of existing limitations of numerical modelling when considering industrial blasting applications. Experimentally and numerically studied the impact of confining pressure on material response to provide insight into in-situ blast conditions. Explicit comparison of experimental and numerical fracture patterns induced by blasting. [ABSTRACT FROM AUTHOR]

Pre-existing flaws often exist near horseshoe-shaped cavities in practical rock engineering projects. The adjacent flaws can affect the failure mode of horseshoe-shaped cavities. In this paper, we use a combination of laboratory experiments and numerical simulations to explore the effect of an adjacent flaw on the failure mode of a horseshoe-shaped cavity. In the experimental tests, the length and distance of the adjacent flaw are taken as the variables to prepare the specimens, which are then loaded under uniaxial and biaxial compression conditions. Our experimental results show that when an adjacent flaw exists near a horseshoe-shaped cavity, cracks are initiated from the vaults and arches of the horseshoe-shaped cavity, rather than from the adjacent flaw. Subsequently, this phenomenon is verified by the M-integral theory in fracture mechanics. Moreover, the testing results show that the propagation path of the new cracks is attracted by the adjacent flaw. As the length of the adjacent flaw increases or the distance between the flaw and the cavity decreases, the attraction from the flaw tip to the new cracks increases. Highlights: The effect of adjacent flaw on the failure mode of horseshoe–shaped cavity was studied The effect of adjacent flaw length and distance on the crack propagation was investigated The attraction of adjacent flaw and the crack initiation location were explained [ABSTRACT FROM AUTHOR]

Frost crack evolution induced by cyclic freeze–thaw is responsible for rock deterioration in cold regions and poses major threats to public safety, engineering structures, and alpine slope stability. This paper presents experimental and numerical works aimed at investigating the frost crack evolution in fissured rock masses, as well as the interaction between frost cracks. A series of laboratory freezing experiments are conducted on rock-like specimens with various pre-existing fissures. Experimental results show that frost cracks initiate at the pre-existing fissure tips and propagate under the freeze–thaw treatment. Moreover, the frost crack evolution is significantly influenced by external stress conditions and frost crack interactions, forming several typical propagation patterns (e.g., deflection, coplanar and butterfly shape, etc.). Then, numerical simulations with a low-temperature thermal–mechanical coupled model, where the water/ice phase transition and hence volume expansion are explicitly simulated, are conducted to reproduce the experimental observation. The numerical results are consistent with the experimental observations and help to reveal the underlying mechanisms of the frost crack growth and frost crack interaction. This experimental and numerical investigation helps to improve the understanding of frost cracking mechanisms that can inform engineering design in cold regions with fissured rock masses. Highlights: Frost crack propagation and interaction under freeze-thaw cycles are investigated. Frost heaving pressure induced by ice expansion is large enough to drive crack propagation. Confining stresses alter the stress field around the crack tips, inducing crack orientation. Mutual interaction between adjacent frost cracks significantly affects the cracking pattern. Novel low-temperature thermal-mechanical coupling model is developed for frost cracking problems. [ABSTRACT FROM AUTHOR]

A theoretical basis for evaluating safety and stability of underground engineering is to study changes in rock strength and failure. In the process of coal mining, the failure mechanical properties in coal present a complex change principle that, under various in situ stresses and in a hydrological environment, results in material failure because of its unstable state being driven by energy. Therefore, by establishing a relationship between energy change and deformation process of coal failure, it would contribute to reflecting the essential characteristics relating to coal strength and failure. In order to explore the mechanics and energy evolution characteristics of water-bearing coal under the triaxial compression process, triaxial compression mechanical experiments of coal under different water contents and confining pressures have been conducted in this paper, whose effects relating to different confining pressures, including water content on changes in total strain energy, elastic strain energy, and the dissipation energy of coal have been analysed. As coal is a porous medium containing a pore and fissure structure, the volume change law relating to coal during the deformation and failure process is characterised by porosity, consequently, a damage constitutive model for coal was established from the perspective of energy dissipation based on statistical damage theory. It is believed that this model could better reflect the deformation behaviour of coal under triaxial compression, in which the relationship between damage evolution characteristics and energy dissipation changes has been reflected in the model. Further, the methods of determining the model's parameters have been proposed to verify the rationality of the established model. Finally, the influence of confining pressure and water content on the brittleness in coal has been discussed from the position of energy evolution. Highlights: Triaxial compression experiments were conducted on coal specimens to investigate the effects of water content and confining pressure on energy evolution. Based on statistical damage theory, the constitutive model that combined energy dissipation and coal failure was established. The effects of water content and confining pressure on the coal brittleness were discussed from the aspect of energy evolution. [ABSTRACT FROM AUTHOR]

Crack initiation, propagation, and coalescence are important for the understanding of rock mass behavior. In this paper, we use an improved numerical manifold method (NMM) that can stimulate the initiation, propagation, and coalescence of many cracks simultaneously to model the continuous–discontinuous deformation of granite under uniaxial and confined compression. This new simulation method couples subcritical crack growth (SCG) theory and the Mohr–Coulomb strength criterion with tension cut-off to consider the propagation of virtual and real cracks, respectively, and uses a statistical Weibull distribution to study the influence of elastic modulus inhomogeneity on crack propagation and rock failure. The mechanical behavior and failure modes of the models are in good agreement with experimental data. Increasing elastic modulus inhomogeneity decreases strength and the strain required for macroscopic failure. The stress corrosion index (n) and crack growth constant (C) also have a strong influence on uniaxial compressive strength. Indeed, higher values of C and n result in faster virtual crack speeds, a lower uniaxial compressive strength, and a lower axial strain upon failure. Confining pressure also exerts an influence on crack propagation, compressive strength, and failure mode. Our study shows that the improved NMM code combined with the virtual crack model is therefore an effective and reliable method to simulate crack initiation, propagation, and coalescence in granite under uniaxial and confined compression. Highlights: A virtual crack approach combined with subcritical crack growth theory is implemented in numerical manifold method to simulate the initiation of multi-cracks. The Mohr–Coulomb criterion with tension cut-off coupled with subcritical crack growth theory is incorporated into numerical manifold method to simulate the propagation of micro-and macro-cracks. Three types of crack intersection are identified in numerical manifold method to allow the model to accurately simulate the propagation and coalescence of multi-cracks and large displacement. [ABSTRACT FROM AUTHOR]

This paper describes the conditions leading to a sustained, low-rate, heave phenomenon affecting a building founded on hard Keuper anhydritic rock. The building was located in an abandoned gypsum quarry. Monitoring data as well as vertical profiles of gypsum and anhydrite content indicate that swelling was associated with the presence of a shallow level of anhydritic clay rock. This paper concludes that the initial quarry excavation as well as the additional building foundation work modified the original stress state and contributed to opening fractures at depth. It also resulted in a facilitated access of water to the upper rock levels, immediately under the foundation footings. Measured heave rates are substantially lower than other rates recorded in a few recent cases. An explanation is provided for the difference. This paper describes a comforting solution for the building. [ABSTRACT FROM AUTHOR]

This paper presents a study on the stability of a mine entry of an innovative approach to non-pillar coal mining—gob-side entry retaining using cantilever beam theory (GERMC). This method is on the rise in China and it has shown to increase the coal seam rate of recycling and mining productivity. The method utilizes the caved-in material resulting from the mining activities to fill the gob area and ensure the stability of the roof of the already mined area. The caved-in material will form the gob-side wall of the retained entry. This characteristic is unique to this method. Thus, there are not many studies that analyze the stability of the filling material and of the gob-side entry wall under these circumstances, which is a vital element for the safety of the mining operations. This paper studies the stability of the gob-side retained entry of the Hongjingta underground coal mine in China. The authors perform the analysis using a discrete fracture network (DFN) model developed by the Massachusetts Institute of Technology (MIT), GEOFRAC, in combination with the discrete-element method (DEM) software UDEC. GEOFRAC estimates the rock blocks (fracture networks) in the filling body, from fracture traces measured along the gob-side wall of the entry. Statistical methods are used to estimate the fracture intensity and the mean fracture areas, which are inputs to the DFN. The generated fracture networks are then inputted into the DEM code, UDEC, to evaluate the stability of the mine-retained entry. For this study, we developed two types of models in UDEC, one considering the fractures generated by the DFN model and another considering the gob-filling material as a continuum. This methodology—combined DFN–DEM—is unique and provides a more realistic representation of the gob-filling material by considering the fractures in the filling body and by estimating those fractures (interfaces between rock blocks) using field data. In addition to this, we also considered the effects of uncertainties that are associated with estimating three-dimensional (3-D) fractures' networks/rock blocks from two-dimensional (2-D) field data. Finally, the results of the simulations were compared with the measurements from the mine and are in good agreement. [ABSTRACT FROM AUTHOR]

The common method for determination of the mechanical properties of the rock joints is the direct shear test. This paper aims to study the effect of boundary condition on the results of direct shear tests. Experimental studies undertaken in this research showed that the peak shear strength is mostly overestimated. This problem is more pronounced for steep asperities and under high normal stresses. Investigation of the failure mode of these samples showed that tensile cracks are generated at the boundary of sample close to the specimen holders and propagated inside the intact materials. In order to discover the reason of observed failure mechanism in experiments, the direct shear test was simulated using PFC2D. Results of numerical models showed that the gap zone size between the upper and lower specimen holders has a significant effect on the shear mechanism. For the high gap size, stresses concentrate at the vicinity of the tips of specimen holders and result in generation and propagation of tensile cracks inside the intact material. However, by reducing the gap size, stresses are concentrated on asperities, and damage of specimen at its boundary is not observed. Results of this paper show that understanding the shear mechanism of rock joints is an essential step prior to interpreting the results of direct shear tests. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study on the effects of microwave irradiation on the mechanical properties of hard rock. More specifically, the weakening effect of microwave heating induced damage on the uniaxial compressive and tensile strength of granite-like rock is numerically evaluated. Rock fracture is modelled by means of a damage-viscoplasticity model with separate damage variables for tensile and compressive failure types. We develop a global solution strategy where the electromagnetic problem is solved first separately in COMSOL multiphysics software, and then provided into a staggered implicit solution method for the thermo-mechanical problem. The thermal and mechanical parts of the problem are considered as uncoupled due to the dominance of the microwave-induced heat source. The model performance is tested in 2D finite element simulations of heterogeneous numerical rock specimens subjected first to heating in a microwave oven and then to uniaxial compression and tension tests. According to the results, the compressive and tensile strength of rock can be significantly reduced by microwave heating pretreatment. [ABSTRACT FROM AUTHOR]

This paper presents a novel method on the hydraulic brittle fracture propagation in a rock mass forced by high-frequency fluctuating pressures due to the turbulence phenomena that occur in unlined dissipation structures and spillways. This type of brittle fracture analysis allows the risk assessment related to the design of unlined emergency spillways and in pre-excavated plunge pools and stilling basins, with reference to small duration flood events that are larger than the design ones. These peaks in dam spillway discharge could be caused by the clime change, or imposed by new regulatory requirements for managing the Probable Maximum Flood (PMF). The fluctuating pressures propagating inside the rock fissures generate zones of alternative stress at the tip of fractures. If the stress exceeds the limited toughness of the rock, it generates brittle fractures. This increments the fracture length in a stable manner, with a finite crack velocity propagation, which differs from the catastrophic behaviours of tensile cracks in structural engineering as assumed in the previous studies on rock scour. The finite crack velocity propagation, for these peak flood events, implies the rock scour depth could be mainly defined by the profoundness of the brittle fractured area. The hydraulic brittle fracturing also has application in the oil and gas industry for waste injection and unconventional gas production wells. In the literature, these cases are analysed in quasi-steady conditions neglecting the inertial terms in the flux equations, because these terms are considered negligible in all cases for a fracturing liquid except at extremely early time. On the contrary, in this investigation, these terms become more and more important when the frequency of the fluctuating pressures increases and they cannot be neglected in the design of the hydraulic structures. This is because these terms outline the resonance conditions where the strongest stress in the rock matrix occurs. Modelling hydraulic fractures requires considerations of both fluid and solid mechanics. This study considers the fissure lengthening in time by coupling the geological and hydraulic aspects of the phenomena via reformulation of the classical water hammer equation. The water hammer equation takes into account both the inertial term and dissipative term in the fluid equation. It allows evaluating the advancement of the disintegration zone in a rock matrix, starting from the pre-existing rock fissure system inferred by geological surveys, during flood events. The outcomes of the paper include a sensitivity analysis changing the rock mechanical characteristics, fissure geometry and the air concentration in the air–water mixture inside the fissure. The results of this novel study also improve the Discrete Fracture Network Software for application in dams and hydropower engineering. [ABSTRACT FROM AUTHOR]

Shale gas has recently gained significant attention as one of the most important unconventional gas resources. Shales are fine-grained rocks formed from the compaction of silt- and clay-sized particles and are characterised by their fissured texture and very low permeability. Gas exists in an adsorbed state on the surface of the organic content of the rock and is freely available within the primary and secondary porosity. Geomechanical studies have indicated that, depending on the clay content of the rock, shales can exhibit a brittle failure mechanism. Brittle failure leads to the reduced strength of the plastic zone around a wellbore, which can potentially result in wellbore instability problems. Desorption of gas during production can cause shrinkage of the organic content of the rock. This becomes more important when considering the use of shales for CO2 sequestration purposes, where CO2 adsorption-induced swelling can play an important role. These phenomena lead to changes in the stress state within the rock mass, which then influence the permeability of the reservoir. Thus, rigorous simulation of material failure within coupled hydro-mechanical analyses is needed to achieve a more systematic and accurate representation of the wellbore. Despite numerous modelling efforts related to permeability, an adequate representation of the geomechanical behaviour of shale and its impact on permeability and gas production has not been achieved. In order to achieve this aim, novel coupled poro-elastoplastic analytical solutions are developed in this paper which take into account the sorption-induced swelling and the brittle failure mechanism. These models employ linear elasticity and a Mohr-Coulomb failure criterion in a plane-strain condition with boundary conditions corresponding to both open-hole and cased-hole completions. The post-failure brittle behaviour of the rock is defined using residual strength parameters and a non-associated flow rule. Swelling and shrinkage are considered to be elastic and are defined using a Langmuir-like curve, which is directly related to the reservoir pressure. The models are used to evaluate the stress distribution and the induced change in permeability within a reservoir. Results show that development of a plastic zone near the wellbore can significantly impact fracture permeability and gas production. The capabilities and limitations of the models are discussed and potential future developments related to modelling of permeability in brittle shales under elastoplastic deformations are identified. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to provide a comprehensive state of the art of fatigue and cyclic loading of natural rock materials. Papers published in the literature are classified and listed in order to ease bibliographical review, to gather data (sometimes contradictory) on classical experimental results and to analyse the main interpretation concepts. Their advantages and limitations are discussed, and perspectives for further work are highlighted. The first section summarises and defines the different experimental set-ups (type of loading, type of experiment) already applied to cyclic/fatigue investigation of rock materials. The papers are then listed based on these different definitions. Typical results are highlighted in next section. Fatigue/cyclic loading mainly results in accumulation of plastic deformation and/or damage cycle after cycle. A sample cyclically loaded at constant amplitude finally leads to failure even if the peak load is lower than its monotonic strength. This subcritical crack is due to a diffuse microfracturing and decohesion of the rock structure. The third section reviews and comments the concepts used to interpret the results. The fatigue limit and S–N curves are the most common concepts used to describe fatigue experiments. Results published from all papers are gathered into a single figure to highlight the tendency. Predicting the monotonic peak strength of a sample is found to be critical in order to compute accurate S–N curves. Finally, open questions are listed to provide a state of the art of grey areas in the understanding of fatigue mechanisms and challenges for the future. [ABSTRACT FROM AUTHOR]

Proppant embedment is a serious issue that reduces fracture width and conductivity. The paper presents the results of experiments on embedment phenomena on a shale rock from the region of the Baltic Basin, which is regarded as an unconventional gas deposit. A novel laboratory imaging procedure was implemented to the proppant embedment visualization. The tests were performed for conditions corresponding to the average reservoir conditions occurring in the studied deposit formation. The parameters characterizing damage of the surface of the fracture faces by the grains of proppant material, after the application of axial compressive stress to two shale core samples with proppant placed in between, are presented. The tests were carried out for rock samples pre-saturated with fracturing fluid. The obtained results of relatively low total effective penetration depth of proppant grains into the walls of the fracture (0.293 mm), and high effective width of fracture with proppant material after hydraulic fracturing (87.9%), indicate the proper selection of proppant and fracturing fluid for the properties of the rock and the reservoir conditions. The results of the experiments present a range of embedment parameters, that have not been widely described before. The test procedure presented in the article is a good method for assessing the vulnerability of a deposit rock to embedment phenomenon. [ABSTRACT FROM AUTHOR]

Based on the research background of the damage of high-temperature jointed rock masses, this paper studies the mechanical properties of jointed granite under high-temperature conditions and analyzes the failure law of jointed rock masses with increasing temperature, decreasing temperature and types of joints. The results have important scientific and engineering value. The main research results are as follows: (1) high-temperature conditions change the mechanical properties of granite, especially when the temperature is higher than 400 °C, and the peak stress of granite decreases sharply; the change rate of the mechanical parameters of granite under the condition of water cooling is higher than that under the condition of natural cooling. (2) The uniaxial compressive strength of jointed granite increases with increasing angle α between the joint and the maximum principal plane, and the peak strength of intact granite is higher than that of jointed granite with either conjugate or echelon joints (referred to as conjugate or echelon jointed granite here). (3) The conjugate jointed granite exhibits three failure modes, in which the fractures are all tensile fractures; the echelon jointed granite exhibits three failure modes, most of which include tensile and shear fracturing. [ABSTRACT FROM AUTHOR]

To study the anisotropic mechanical properties and energy evolution of layered rock masses under the action of cyclic loading, uniaxial graded cyclic loading and unloading tests under different upper and lower stress limits were carried out on samples with different bedding angles (0°, 22.5°, 45°, 67.5°, and 90°) to explore the deformation and mechanical response characteristics of bedded phyllite under cyclic loading and the mechanism controlling the evolution of the total absorption energy, elastic strain energy, and dissipated energy. The damage variable D and the damage increment ΔD were introduced to characterize the damage accumulation state inside the layered rock mass, and an instability criterion of bedded phyllite under mechanical cyclic loading was proposed. The results show that when the bedding angle increases from 0° to 90°, the failure types of the sample change from splitting shear failure to slipping shear failure and then to splitting tensile failure, and the corresponding irreversible strain and deformation modulus first decrease and then increase. With increasing stress level, the cumulative energy density increases in three stages, and the rate of increase presents a change in characteristic of "accelerating-steady-decelerating." The energy storage coefficient (Ke) decreases with increasing cycle number and stress level and first increases and then decreases with increasing bedding angle. Meanwhile, the energy dissipation coefficient (Kd) exhibits the opposite trend. The damage increment increases ΔD sharply after the first cycle in the nonfailure stage, drops sharply in the next cycle, and gradually decreases and stabilizes to near ΔD = 0.01 in the subsequent cycle. After entering the failure stage, the change in the damage increment of the variation in phyllite samples with different bedding angles is closely related to the stress level and number of cycles. The damage variable D can be used to describe the failure and instability of bedded phyllite under graded cyclic loading: when D ≤ 2 × 10–4, the rock is in the stage of compaction and steady crack growth; when D ≥ 2 × 10–4, the rock is at risk of failure and instability. Highlights: The changes in the irreversible strain and elastic modulus were analyzed based on the results of uniaxial graded cyclic loading and unloading testing. The evolution and distribution rules of various energy indicators under uniaxial graded cycle load are discussed from the perspective of energy. An instability judgment index based on energy dissipation theory is proposed, considering the relationship of layered millennium rock macro–micro deformation and energy evolution under the cyclic load. [ABSTRACT FROM AUTHOR]

Core data from the hydraulic fracturing test site 2 (HFTS-2) show that a complex fracture network is created. In this work, a novel fracturing-reservoir simulator is applied to the HFTS-2 data to provide a thorough assessment of the impact of the fracture network on well performance. A methodology is also presented to effectively represent the dynamic propagation of hydraulic fractures in complex naturally fractured formations. First, data that characterize the natural fractures at HFTS-2 are used to create a realistic representation of the reservoir (a discrete fracture network, DFN). Then, a fracturing simulator that fully couples fluid flow, fracture mechanics, and a black oil reservoir simulator is used to first create the fracture network and then simulate flowback and production. A comparison of our simulation results with core data shows good agreement with the characteristics of the natural fracture network based on the post-frac core analysis. The production/flowback results are compared with the field results and are found to agree well with actual production data. While it is possible to use planar fractures to history match production, the results provide unrealistic fracture dimensions and reservoir drainage volumes. This directly impacts design and operational decisions related to well spacing and fracture size, which demonstrates the importance of incorporating realistic complex fracture networks into reservoir simulators for production evaluation and forecasting as well as fracture design and well spacing selection. Highlights: A novel integrated DFN-fracturing-reservoir model is developed that allows us to dynamically model hydraulic fracture propagation in a naturally fractured reservoir followed by multi-phase fluid flowback during production. The model allows us to assess the impact of the natural fracture network on well performance. Natural fractures are represented by a discrete fracture network (DFN), and the interaction of multiple hydraulic fractures with natural fractures is captured by the displacement discontinuity method (DDM). A methodology is presented to automatically calibrate pre-existing natural fractures with core data. [ABSTRACT FROM AUTHOR]

This paper presents a composite element algorithm of coupled normal stress and fluid flow process for fractured rock mass, developed from the composite element method (CEM). The coupled relation between the fracture flow and normal stress makes use of the 'filled model', which examines the asperities in the fracture as a layer of granular medium having high porosity and being clipped by the two parallel plates. The existence of fractures is not considered in the mesh generation, but it will be considered explicitly in the mapped composite element. The coupled normal stress and fluid flow process has been simulated by applying a cross iterative algorithm between the two fields. The proposed algorithm considers not only the flow through the fractures, but also the flow exchange between fractures and the surrounding rock blocks. In addition, it can be used for both the filled and non-filled fractures. The verification of the proposed algorithm has been conducted through the illustration of three examples by comparison with the conventional finite element method (FEM), from which the advantages and reliability of the proposed algorithm have been shown clearly. [ABSTRACT FROM AUTHOR]

Geophysical site investigation techniques based on elastic waves have been widely used to characterize rock masses. However, characterizing jointed rock masses by using such techniques remains challenging because of a lack of knowledge about elastic wave propagation in multi-jointed rock masses. In this paper, the roughness of naturally fractured rock joint surfaces is estimated by using a three-dimensional (3D) image-processing technique. The classification of the joint roughness coefficient (JRC) is enhanced by introducing the scan line technique. The peak-to-valley height is selected as a key indicator for JRC classification. Long-wavelength P-wave and torsional S-wave propagation across rock masses containing naturally fractured joints are simulated through the quasi-static resonant column (QSRC) test. In general, as the JRC increases, the S-wave velocity increases within the range of stress levels considered in this paper, whereas the P-wave velocity and the damping ratio of the shear wave decrease. In particular, the two-dimensional joint specimen underestimates the S-wave velocity while overestimating the P-wave velocity. This suggests that 3D joint surfaces should be implicated to obtain the reliable elastic wave velocity in jointed rock masses. The contact characteristic and degree of roughness and waviness of the joint surface are identified as a factor influencing P-wave and S-wave propagation in multi-jointed rock masses. The results indicate a need for a better understanding of the sensitivity of contact area alterations to the elastic wave velocity induced by changes in normal stress. This paper's framework can be a reference for future research on elastic wave propagation in naturally multi-jointed rock masses. [ABSTRACT FROM AUTHOR]

The rapid and accurate measurement of in situ stress fields is one of the essential steps in geological engineering. Indoor acoustic emission tests and the Kaiser effect provide a promising prior peak stress measurement method. This paper aims to qualitatively explore the time delay mechanism of the Kaiser effect under uniaxial compressive stress. Acoustic emission tests were conducted with natural and water-soaked sandstone specimens to examine the time delay phenomenon of the Kaiser effect, and the typical Griffith fracture mechanics theory was adopted to explain the mechanism. The time delay of the Kaiser effect under uniaxial compressive stress was affected by the friction coefficient on the crack surfaces and the initial crack angle β . The time effect was indirectly induced by the changes in the friction coefficient on the crack surfaces, and the friction coefficient was controlled by the water content in the rock specimen. A higher initial friction coefficient led to an earlier and easier time delay of the Kaiser effect. Consequently, from the aspect of the Kaiser effect, it was suggested that when testing in situ stress, measures should be taken to maintain the original moisture contents in the rock cores after they are drilled underground. Otherwise, the in situ stress should be tested within 7 days. [ABSTRACT FROM AUTHOR]

Previous studies on the strength and deformability of pillars in hard-rock mines have demonstrated that the influence of natural fractures diminishes with increasing pillar width. Jointing conditions and fracture intensity/intersections play a critical role in the behaviour of slender pillars (width-to-height ratio less than 0.5). An important challenge is the characterization of rock mass damage as a function of the natural fracture network and loading conditions. In this paper, we present advanced pre-processing methodologies to measure fracture intensity and fracture density prior to loading and relate those measures to the evolution of damage (intensity and density) with increasing pillar load. A hybrid finite-discrete element method (FDEM) with fracture mechanics capabilities is used to simulate the mechanical behaviour of synthetic pillar models with different embedded discrete fracture traces of similar total length. Newly developed post-processing methodologies are used to capture the location of damage events, track rock mass damage intensity and density as the simulation progresses and relate those changes to the modelled pillar strength. We believe the results of this study provide useful insights on rock mass damage evolution. [ABSTRACT FROM AUTHOR]

The paper presents the results of experiments that were conducted to determine the influence of a triaxial stress state on the evolution of the permeability of an argillaceous limestone. The limestone rock is found in the Ordovician rock formations above the Precambrian basement located in southern Ontario, Canada, which is being considered as a potential host rock for the construction of a deep ground repository for storing low- and intermediate-level nuclear waste. The paper presents the results of an extensive series of hydraulic pulse tests and steady-state tests that were conducted to determine the permeability alterations in zones that can experience levels of damage that can be present in vicinity of an excavated underground opening. A 'state-space' relationship is developed to describe permeability evolution with the triaxial stress in the pre-failure regime. The permeability evolution in extensively damaged post-failure states of the rock is also investigated. It is shown that permeability alterations were four orders of magnitude higher as a result of significant damage to the material, which is an important consideration in establishing the efficiency of the host rock formation as a barrier for the long-term containment of radionuclide migration. [ABSTRACT FROM AUTHOR]

Estimation of representative elementary volume (REV) is significant to analyze fractured rock mass in the framework of continuum mechanics. Engineers can therefore simplify the analysis by using an equivalent rock block with an average property, and the influence of fractures can be neglected in finite element modelling. The indicators to determine the REV size based on the joint geometrical parameters include the volumetric fracture intensity ( P ) and the fracture tensor, but this type of calculation generally provides a lower bound evaluation. A novel conceptual framework of damage coefficient is introduced in this paper to consider the mechanical properties of fractures, such as joint aperture and roughness. A parametric study has been performed to establish the correlation between the proposed dimensionless damage coefficient and the traditional derived P value. The effectiveness of the developed method is demonstrated by a case study, where a larger mechanical REV size is indeed calculated based on the damage coefficient. [ABSTRACT FROM AUTHOR]

Flexible rockfall barriers are a common form of protection against falling blocks of rock and rock fragments (rockfall). These barriers consist of a system of cables, posts, and a mesh, and their capacity is typically quantified in terms of the threshold of impact (kinetic) energy at which the barrier fails. This threshold, referred to here as the 'critical energy,' is often regarded as a constant. However, several studies have pointed out that there is no single representative value of critical energy for a given barrier. Instead, the critical energy decreases as the block size decreases, a phenomenon referred to as the 'bullet effect.' In this paper, we present a simple analytical model for determining the critical energy of a flexible barrier. The model considers a block that impacts normally and centrally on the wire mesh, and rather than incorporate the structural details of the cables and posts explicitly, the supporting elements are replaced by springs of representative stiffness. The analysis reveals the dependence of the critical energy on the block size, as well as other relevant variables, and it provides physical insight into the impact problem. For example, it is shown that bending of the wire mesh during impact reduces the axial force that can be sustained within the wires, thus reducing the energy that can be absorbed. The formulas derived in the paper are straightforward to use, and the analytical predictions compare favorably with data available in the literature. [ABSTRACT FROM AUTHOR]

New developments of an in-house hybrid code, named Modified Discrete Element Method (MDEM) are presented in the paper. The new developments are on the treatment of pre-existing and propagating fractures in quasi-brittle materials. These developments are the embedment of Linear Elastic Fracture Mechanics (LEFM) and elastic-softening crack band model -based methodologies in the MDEM and their application in lab and reservoir scale. Using the first methodology, MDEM can calculate stress intensity factors, K I and K II using the internal contact forces of particles. K I and K II are calculated independent of boundary conditions and geometrical configuration with acceptable accuracy level. The methodology has been also used in reservoir scale to study the rupture likelihood of faults and fractures due to fluid injection. This methodology enables the code to model mode I and mode II failures and propagation direction based on the fracturing model proposed by Rao et al. (Int J Rock Mech Min Sci 40(3): 355–375, 2003). Using the second methodology, the MDEM can model nonlinear behavior of quasi-brittle materials including or excluding preexisting cracks based on fracture energy. A model was verified against an experiment of a three point bend test with a notch. The numerically obtained force-crack mouth opening curve was reasonably comparable to the experimental test. The analysis was repeated for three other mesh sizes and the results are less mesh size dependent. Finally, it was shown that MDEM has the potential in studying fracture mechanics of quasi-brittle materials both in lab and large-scale investigations. [ABSTRACT FROM AUTHOR]

Digital image correlation (DIC) and the strain gage method are both widely used in characterizing the fracture mechanisms of materials. However, the quantitative comparisons of rock fracture parameters between the two methods have been studied insufficiently. In this paper, dynamic three-point bending impact tests were performed on white marble specimens using DIC and the strain gage method simultaneously. The complete cracking process was recorded by two high-speed cameras situated on either side of the specimen. The crack propagation velocities were obtained and the dynamic stress intensity factors (DSIFs) were calculated by both methods for analysis and comparison. The experimental results show that the crack propagation velocities almost remain constant during the cracking process, and the evolutions of strain values recorded by gages are consistent with the modified strain variations obtained theoretically. DSIFs calculated by DIC show good agreement with the results obtained from strain gages #1 to #4, while the size effect and the multiple stress waves interfere with the strain signals sensed in strain gage #5, causing a relatively larger deviation in the DSIF. In addition, the complete fracture process (including the variations in successive displacement and strain fields) can be obtained by the DIC method, while a rough changing trend in the DSIF can be given by the strain gage method with a limited number of strain gages. [ABSTRACT FROM AUTHOR]

Knowledge of the fracture hydraulic aperture and its relation to the mechanical aperture and normal stress is urgently needed in engineering construction and analytical research at the engineering field scale. A new method based on the in situ borehole camera measurement and borehole water-pressure test is proposed for the calculation of the fracture hydraulic aperture. This method comprises six steps. The first step is to obtain the equivalent hydraulic conductivity of the test section from borehole water-pressure tests. The second step is a tentative calculation to obtain the qualitative relation between the reduction coefficient and the mechanical aperture obtained from borehole camera measurements. The third step is to choose the preliminary reduction coefficient for obtaining the initial hydraulic aperture. The remaining three steps are to optimize, using the genetic algorithm, the hydraulic apertures of fractures with high uncertainty. The method is then applied to a fractured granite engineering area whose purpose is the construction of an underground water-sealed storage cavern for liquefied petroleum gas. The probability distribution characteristics of the hydraulic aperture, the relationship between the hydraulic aperture and the mechanical aperture, the hydraulic aperture and the normal stress, and the differences between altered fractures and fresh fractures are all analyzed. Based on the effects of the engineering applications, the method is proved to be feasible and reliable. More importantly, the results of the hydraulic aperture obtained in this paper are different from those results elicited from laboratory tests, and the reasons are discussed in the paper. [ABSTRACT FROM AUTHOR]

Most accessible mineral deposits are nearly exhausted. Consequently, some resources that were previously abandoned are now being considered for remining. This paper describes a case study that aims at revealing the instability mechanism and selecting the optimal control countermeasures for a cataclastic roadway regenerated roof (CRRR) during the extraction of the remaining mineral resources. Based on the structural model of a cataclastic regenerated roof (CRR) of a remaining lower slice, a modified natural arch mechanical model for a CRRR was established by considering the lateral pressure and stress concentration coefficient of the bearing arch, through which the modified bearing arch curve equation and arch rise expression were deduced. The relationship between the modified arch rise and the lateral stress was obtained through a practical case. Next, a numerical model of a CRRR, which accounted for the cataclastic features, was built in the Universal Distinct Element Code (UDEC). The development of subsidence and fractures in the roadway roof was determined through modeling under various roof support scenarios, namely no support, rock bolt (RB) support, rock bolt and cable (RBC) support and a combined support of the rock bolt, cable and steel beam (RBCS). The theoretical analysis, numerical simulation and monitoring results of the RB and RBC stress all indicated that a bearing arch existed in the CRRR and that the arch rise underwent changes during the roof hanging time and with different support scenarios. The support structure should exceed the bearing arch rise in height and should be leak proof to avoid roof leakage. After a comparative analysis of the numerical and field support effects provided by various supporting scenarios, the RBCS support scenario was shown to be the optimal roof support choice, providing a good controlling effect for a CRRR and meeting the production requirements. [ABSTRACT FROM AUTHOR]

In this paper, the embedded discontinuity approach is applied in finite element modeling of rock in compression and tension. For this end, a rate-dependent constitutive model based on (strong) embedded displacement discontinuity model is developed to describe the mode I, mode II and mixed mode fracture of rock. The constitutive model describes the bulk material as linear elastic until reaching the elastic limit. Beyond the elastic limit, the rate-dependent exponential softening law governs the evolution of the displacement jump. Rock heterogeneity is incorporated in the present approach by random description of the mineral texture of rock. Moreover, initial microcrack population always present in natural rocks is accounted for as randomly-oriented embedded discontinuities. In the numerical examples, the model properties are extensively studied in uniaxial compression. The effect of loading rate and confining pressure is also tested in the 2D (plane strain) numerical simulations. These simulations demonstrate that the model captures the salient features of rock in confined compression and uniaxial tension. The developed method has the computational efficiency of continuum plasticity models. However, it also has the advantage, over these models, of accounting for the orientation of introduced microcracks. This feature is crucial with respect to the fracture behavior of rock in compression as shown in this paper. [ABSTRACT FROM AUTHOR]

The unconventional fracture model can simulate complex fracture network propagation in a formation with pre-existing natural fractures. The interaction between hydraulic fracture branches, or stress shadowing effect, could be modeled by 2D or 3D displacement discontinuity method (DDM). In this paper, we concentrate on a hydraulic fracture model that integrates 3D DDM for computing the induced 3D stress field around the propagating hydraulic fractures and incorporates the changes in induced stress into the fracture height calculations and propagation criterion. Examples show that for parallel fractures, the height growth may be promoted or suppressed depending on the relative fracture height. For fractures initiated from different formation layers, the fracture growth into the layer occupied by the other fractures is reduced due to the vertical stress shadowing effect. [ABSTRACT FROM AUTHOR]

Overhanging rock instability is a serious geological problem in mountainous regions. The initiation and propagation of a controlling crack are the fundamental causes of overhanging rock failure. In the view of fracture mechanics, the instability of toppling overhanging rock can be equivalent to the effect of pure shear and pure bending moment. This paper analyzes the evolution of displacement-load curves and failure modes of overhanging rock models at different crack lengths, crack angles, and load distances based on fracture tests of the toppling overhanging rock model. Maximum circumferential stress theory is used as the fracture criterion of mixed mode I-II fractures to calculate the stress intensity factor (SIF) of the crack tip and the theoretical failure load of the overhanging rock model. The fracture mode mechanism is analyzed by the calculated SIF. We use the extended finite element method to simulate the crack propagation path and analyze the change process from equivalent stresses of the numerical model using propagation steps. Comparisons between the physical tests, theoretical calculations, and numerical simulations verify the rationality of the experimental results. [ABSTRACT FROM AUTHOR]

In this paper, a fully coupled hydro-mechanical model is presented for the study of multizone hydraulic fracturing. The momentum balance equation of the bulk together with the mass balance and momentum balance equation of the fluid phase are employed in order to derive the hydro-mechanical coupled system of governing equations of the porous media known as the (u-p) formulation. The hydro-fracture inflow is modeled based on the Darcy law, where the fracture permeability is determined by using the cubic law. Provisions are made for the plausible closure as well as the frictional resistance of the fracture edges in the solid phase by means of Kuhn-Tucker inequalities embedded in an X-FEM penalty method. In addition, for the fluid phase, the zero leak-off constrain is imposed through the application of the large time increment-based contact algorithm in the case of crack closure. The cohesive crack model is employed to account for the nonlinear fracturing process at the hydro-fracture tips. Multiple crack growth patterns are determined by means of energy based cohesive stress functions. Based on the X-FEM, the strong discontinuities in the displacement field due to fracture opening as well as the weak discontinuities within the pressure field due to leak-off flow are incorporated by using the Heaviside and modified level-set enrichment functions, respectively. A consistent computational algorithm is proposed for the determination of the fracturing fluid flow distribution across the existing perforations. Finally, several numerical examples are presented to demonstrate the robustness of the proposed X-FEM framework in the study of multizone hydraulic fracturing treatments through saturated porous media. The results appear to accord with the field observations reporting numerous failed attempts of multistage multizone fracturing treatments, which provide a great insight into the complexities encountered in practice. [ABSTRACT FROM AUTHOR]

A proper evaluation of the perturbations of the host rock induced by the excavation and the emplacement of exothermic wastes is essential for the assessment of the long-term safety of high-level radioactive waste disposals in clay formations. The impact of the thermal transient on the evolution of the damaged zone (DZ) has been explored in the European Commission project TIMODAZ (thermal impact on the damaged zone around a radioactive waste disposal in clay host rocks, 2006-2010). This paper integrates the scientific results of the TIMODAZ project from a performance assessment (PA) point of view, showing how these results support and justify key PA assumptions and the values of PA model parameters. This paper also contextualises the significance of the thermal impact on the DZ from a safety case perspective, highlighting how the project outcomes result into an improved understanding of the thermo-hydro-mechanical behaviour of the clay host rocks. The results obtained in the TIMODAZ project strengthen the assessment basis of the safety evaluation of the current repository designs. There was no evidence throughout the TIMODAZ experimental observations of a temperature-induced additional opening of fractures nor of a significant permeability increase of the DZ. Instead, thermally induced plasticity, swelling and creep seem to be beneficial to the sealing of fractures and to the recovery of a very low permeability in the DZ, close to that of an undisturbed clay host rock. Results from the TIMODAZ project indicate that the favourable properties of the clay host rock, which guarantee the effectiveness of the safety functions of the repository system, are expected to be maintained after the heating-cooling cycle. Hence, the basic assumptions usually made in PA calculations so far are expected to remain valid, and the performance of the system should not be affected in a negative way by the thermal evolution of the DZ around a radioactive waste repository in clay host rock. [ABSTRACT FROM AUTHOR]

Boom clay can be considered as a transversely isotropic geomaterial. However, due to lack of experimental evidence and data base, it is still difficult to describe the transversely isotropic plastic behavior of this argillaceous rock. In this paper, we present first, by means of an experimental approach, the main features of the mechanical properties of Boom clay. Then, combining the transversely isotropic elastic model and the modified Mohr-Coulomb criterion, a suitable constitutive model is introduced so as to fully describe the mechanical behavior of the studied material, in which, an elastic damage law which takes into consideration, the transversely isotropic effect, a plastic hardening law and a plastic damage law were introduced to describe the nonlinear elastic, hardening and softening behavior of Boom clay. As a preliminary step, the evolution law of both elastic moduli and Poisson's ratio during the elastic stage was obtained by direct analysis of the test data. The synchronism of the elastic damage in both transversal and axial directions was proved by this method. Some of the parameters of the model in the elastic stage were also determined by direct analysis method and further verified by back analysis. Other unknown parameters in the model were determined by back analysis. [ABSTRACT FROM AUTHOR]

In this paper, the feasibility of a thermally assisted drilling method is investigated. The working principle of this method is based on the weakening effect of a flame-jet to enhance the drilling performance of conventional, mechanical drilling. To investigate its effectiveness, we study rock weakening after rapid, localized flame-jet heating of Rorschach sandstone and Central Aare granite. We perform experiments on rock strength after flame treatments in comparison to oven heating, for temperatures up to 650 ∘C and heating rates from 0.17 to 20 ∘C/s. The material hardening, commonly observed at moderate temperatures after oven treatments, can be suppressed by flame heating the material at high heating rates. Our study highlights the influence of the heating rate on the mechanism of thermal microcracking. High heating rate, flame treatments appear to mostly induce cracks at the grain boundaries, opposed to slow oven treatments, where also a considerable number of intragranular cracks are found. Herewith, we postulate that at low heating rates, thermal expansion stresses cause the observed thermal cracking. In contrast, at higher heating rates, thermal cracking is dominated by the stress concentrations caused by high thermal gradients. [ABSTRACT FROM AUTHOR]

Laboratory experiments are conducted in this study to investigate hydraulic fracture initiation and propagation in porous test specimens in the presence of multiple fluid injection sources and anisotropic far-field stresses. Numerous laboratory observations of fracture growth are documented via clear, high resolution images and digital image correlation analyses. It is clearly shown that injection-induced stresses can appreciably affect hydraulic fracture trajectories and fracturing pressures. We show that induced hydraulic fractures, under our laboratory conditions, are attracted to regions of high pore pressure. Induced fractures tend to propagate towards neighboring high pore pressure injection ports. The recorded breakdown pressure in the fracturing experiments decreases significantly as the number of neighboring injectors increases. When a hydraulic fracture is induced near an injection port that is maintained at a relatively high injection pressure, a secondary fracture can grow at a markedly lower fluid pressure than that of the main fracture. The influence of an adjacent fluid injection source on the hydraulic fracture trajectory can be minimized or suppressed when the applied far-field differential stress is relatively high. Multiple fractures induced from closely spaced injection ports tend to grow towards neighboring ports and perpendicularly to the applied maximum far-field stress. The experimental observations in this paper can serve as benchmark evidence for numerical hydraulic fracturing simulators that are used for modeling field applications. [ABSTRACT FROM AUTHOR]

The anti-sliding stability analysis of a high concrete gravity dam under a complex geological condition is a difficult problem to solve. To depict the discontinuous characteristics of a dam foundation fractured rock mass in a numerical model, a multi-scale coupling 3D geoengineering modelling method is presented. Facial-based geometric models of rock strata and fractures are established with the NURBS-based B-Rep and then coupled together after being converted to the voxel-based model based on TEN. Using the partial differential equation method, the interaction of fluid and solid is realized. Thus, the plastic zone and pore water pressure distribution are obtained. The results show that the plastic zone is formed in some fractures, and the dam stability against sliding of this cross section is diminished. Because of the existence of interlaced fractures, the potential sliding surface may be produced once the fractures are connected. Considering the complex geological structures in a dam foundation, the spatial variability and uncertainty of the rock mass parameters cannot be ignored. The random field of the elastic modulus is generated and applied to the fluid-solid interaction. The range of the plastic zone is different when the elastic modulus is assigned a logarithmic random distribution. The proposed methods in this paper improve the stability analysis by enhancing the finite element numerical modelling by considering discontinuities of the rock mass and simulating the random distribution of the elastic modulus, which has significant application value for other projects. [ABSTRACT FROM AUTHOR]

This paper investigates fragmentation in an aggregate quarry in the light of the fragmentation-energy fan concept. Six blasts were monitored, located one behind the other in the same quarry area. The rock structure was blocky, the water level and the usage of explosives were variable within and between the blasts; however, the distribution of explosive energy in the blocks was relatively uniform and the performance of both explosives per unit mass appeared to be similar. The powder factor above grade was between 0.28 and 0.44 kg/m3. Fragmentation of the plant feed was measured using an online digital image analysis system and three belt scales in the crushing plant. The oversize material that was not directly fed to the plant after the blast corresponds to 3-9.3% of the total. Data from image analysis was used to build the size distribution in the coarse range (fragments above 120 mm), and the passing fractions at 120 and 25 mm obtained from belt scales data were used in the range 120-25 mm. The non-dimensional percentile sizes between 10 and 90% for each blast are described through power functions of the powder factor above grade; the model coefficients (i.e. nine prefactors and nine exponents) are statistically significant. From them, using the principles of the fragmentation-energy fan and the Swebrec distribution properties, the fragmentation can be expressed in terms of the powder factor by means of five parameters: the fan focus coordinates and the three parameters of a function of the exponents versus the percentage passing. The model shows a good capability to describe fragmentation from 10 to 90 percentile sizes, with an expected error below 6% and a maximum likely error less than 15%. [ABSTRACT FROM AUTHOR]

In laboratory fracture toughness studies, the crack growth resistance of rock materials may be influenced by different factors such as specimen geometry, loading conditions, and also the type of pre-notch cut in the test sample. In this paper, a large number of mode I fracture toughness experiments are conducted on an Iranian white rock “Harsin marble” with six different mode I specimens. The selected test specimens are in the shape of cylindrical rod, rectangular beam, and circular Brazilian disk containing either chevron notch or straight crack. The effect of specimen geometry and pre-notch type was investigated statistically, and it was found that the average fracture toughness values of notched specimens were higher than those of the similar specimens but containing straight crack. Meanwhile, the scatters of fracture toughness data for chevron notched specimens were smaller than those for the straight cracked samples. For each set of experimental fracture toughness results, probability of fracture was investigated using two- and three-parameter Weibull statistical distributions. Comparison of the Weibull fitted curves for chevron notched and straight cracked samples with the same geometries demonstrated that the discrepancy between the corresponding curves can be described with a good accuracy by a simple shift factor. In addition, using the extended maximum tangential strain criterion which takes into account the influence of both KI and T-stress terms, the statistical fracture toughness data of chevron notched specimens were predicted in terms of the Weibull distribution parameters of the straight cracked specimens. [ABSTRACT FROM AUTHOR]

Grouting plays a crucial role in dam safety. Due to the concealment of grouting activities, complexity of fracture distribution in rock masses and rheological properties of cement grout, it is difficult to analyze the effects of grouting. In this paper, a computational fluid dynamics (CFD) simulation approach of dam foundation grouting based on a 3D fracture network model is proposed. In this approach, the 3D fracture network model, which is based on an improved bootstrap sampling method and established by VisualGeo software, can provide a reliable and accurate geometric model for CFD simulation of dam foundation grouting. Based on the model, a CFD simulation is performed, in which the Papanastasiou regularized model is used to express the grout rheological properties, and the volume of fluid technique is utilized to capture the grout fronts. Two sets of tests are performed to verify the effectiveness of the Papanastasiou regularized model. When applying the CFD simulation approach for dam foundation grouting, three technical issues can be solved: (1) collapsing potential of the fracture samples, (2) inconsistencies in the geometric model in actual fractures under complex geological conditions, and (3) inappropriate method of characterizing the rheological properties of cement grout. The applicability of the proposed approach is demonstrated by an illustrative case study—a hydropower station dam foundation in southwestern China. [ABSTRACT FROM AUTHOR]

Reliable prediction of fracture process in shale-gas rocks remains one of the most significant challenges for establishing sustained economic oil and gas production. This paper presents a modeling framework for simulation of crack propagation in heterogeneous shale rocks. The framework is on the basis of a variational approach, consistent with Griffith’s theory. The modeling framework is used to reproduce the fracture propagation process in shale rock samples under standard Brazilian disk test conditions. Data collected from the experiments are employed to determine the testing specimens’ tensile strength and fracture toughness. To incorporate the effects of shale formation heterogeneity in the simulation of crack paths, fracture properties of the specimens are defined as spatially random fields. A computational strategy on the basis of stochastic finite element theory is developed that allows to incorporate the effects of heterogeneity of shale rocks on the fracture evolution. A parametric study has been carried out to better understand how anisotropy and heterogeneity of the mechanical properties affect both direction of cracks and rock strength. [ABSTRACT FROM AUTHOR]

The net penetration rate of hard rock tunnel boring machines (TBM) is influenced by rock mass degree of fracturing. This influence is taken into account in the NTNU prediction model by the rock mass fracturing factor (ks). ks is evaluated by geological mapping, the measurement of the orientation of fractures and the spacing of fractures and fracture type. Geological mapping is a subjective procedure. Mapping results can therefore contain considerable uncertainty. The mapping data of a tunnel mapped by three researchers were compared, and the influence of the variation in geological mapping was estimated to assess the influence of subjectivity in geological mapping. This study compares predicted net penetration rates and actual net penetration rates for TBM tunneling (from field data) and suggests mapping methods that can reduce the error related to subjectivity. The main findings of this paper are as follows: (1) variation of mapping data between individuals; (2) effect of observed variation on uncertainty in predicted net penetration rates; (3) influence of mapping methods on the difference between predicted and actual net penetration rate. [ABSTRACT FROM AUTHOR]

The large diameter bored tunnel passing through rock and alluvial deposits subjected to seismic loading is analyzed for estimating longitudinal deformations and member forces on the segmental tunnel liners. The project site has challenges including high hydrostatic pressure, variable ground profile and high seismic loading. To ensure the safety of segmental tunnel liner from the seismic demands, the performance-based two-level design earthquake approach, Functional Evaluation Earthquake and Safety Evaluation Earthquake, has been adopted. The longitudinal tunnel and ground response seismic analyses are performed using a three-dimensional quasi-static linear elastic and nonlinear elastic discrete beam-spring elements to represent segmental liner and ground spring, respectively. Three components (longitudinal, transverse and vertical) of free-field ground displacement-time histories evaluated from site response analyses considering wave passage effects have been applied at the end support of the strain-compatible ground springs. The result of the longitudinal seismic analyses suggests that seismic joint for the mitigation measure requiring the design deflection capacity of 5-7.5 cm is to be furnished at the transition zone between hard and soft ground condition where the maximum member forces on the segmental liner (i.e., axial, shear forces and bending moments) are induced. The paper illustrates how detailed numerical analyses can be practically applied to evaluate the axial and curvature deformations along the tunnel alignment under difficult ground conditions and to provide the seismic joints at proper locations to effectively reduce the seismic demands below the allowable levels. [ABSTRACT FROM AUTHOR]

Natural fractures have a significant influence on the propagation geometry of hydraulic fractures in fractured reservoirs. True triaxial volumetric fracturing experiments, in which random natural fractures are created by placing cement blocks of different dimensions in a cuboid mold and filling the mold with additional cement to create the final test specimen, were used to study the factors that influence the hydraulic fracture propagation geometry. These factors include the presence of natural fractures around the wellbore, the dimension and volumetric density of random natural fractures and the horizontal differential stress. The results show that volumetric fractures preferentially formed when natural fractures occurred around the wellbore, the natural fractures are medium to long and have a volumetric density of 6–9%, and the stress difference is less than 11 MPa. The volumetric fracture geometries are mainly major multi-branch fractures with fracture networks or major multi-branch fractures (2–4 fractures). The angles between the major fractures and the maximum horizontal in situ stress are 30°–45°, and fracture networks are located at the intersections of major multi-branch fractures. Short natural fractures rarely led to the formation of fracture networks. Thus, the interaction between hydraulic fractures and short natural fractures has little engineering significance. The conclusions are important for field applications and for gaining a deeper understanding of the formation process of volumetric fractures. [ABSTRACT FROM AUTHOR]

Fracture and mechanical properties of the water saturated sedimentary rocks have been experimentally investigated in the present paper. Three types of sandstones and one type of shale were saturated in water for different periods of time. They were then tested for their index geomechanical properties such as Brazilian tensile strength (BTS), Young's modulus (YM), P-wave velocity and all pure and mixed-mode fracture toughness (FT). FT was measured using semicircular bend specimens in a three-point bend set-up. All the geomechanical and fracture properties of the saturated rocks were compared together to investigate their interrelations. Further, statistical methods were employed to measure the statistical significance of such relationships. Next, three types of fracture criteria were compared with the present experimental results. Results show that degree of saturation has significant effect on both the strength and fracture properties of sedimentary rock. A general decrease in the mechanical and fracture toughness was noticed with increasing saturation levels. But, t-test confirmed that FT, BTS, P-wave velocity and YM are strongly dependent on each other and linear relationships exist across all the saturation values. Calculation of the 'degradation degree' (DD) appeared to be a difficult task for all types of sedimentary rocks. While in sandstone, both the BTS and mode-I FT overestimated the DD calculated by YM method, in shale BTS was found to give a closure value. [ABSTRACT FROM AUTHOR]

Block-flexure is the most common type of toppling failure in rock slopes. In this case, some rock blocks fail due to tensile bending stresses and some overturn under their own weights. In this paper, first, a literature review of toppling failures is summarized. Then, a theoretical model is proposed for rock slopes with a potential for block-flexure toppling instability. Next, a new analytical approach is presented for the stability analysis of such slopes. Finally, a special computer code is developed for a quick stability assessment of the failures based on the proposed method. This code receives the rock slope parameters from the user as the input data and predicts its stability, along with the corresponding factor of safety against the failure, as the output. In addition, two case studies are used for practical verification of the proposed approach and the corresponding computer code as well. [ABSTRACT FROM AUTHOR]