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Burden is one of the most important parameter which plays an immense role in rock breakage using explosive energy. It is very difficult to select the optimum burden under different geotechnical and geomining condition. The selection becomes more critical when the excavation method is underground as very few researches or studies have been carried out in such cases. Therefore, in this paper, numerical simulation-based approach has been employed to estimate the optimum burden for ring blasting. The numerical models were created under different scenario including blasthole diameter and strength of the rock. The RHT concrete constitutive model has been used in this paper to study the damage contour under different circumstances. The model output has been used to calculate the volume of rock damage along the free face. This damage volume is further correlated with various parameters. This correlation shows that the damage volume along the free face initially rises with the increase in burden and then starts decreasing after a certain value. The burden value which recorded the maximum volume of rock excavated along the free face has been considered as the optimum burden in this paper. The trial blast has been also carried out at the experimental site for twenty-four rings. The optimum burden achieved from the experimental trial for 102 mm blasthole diameter and rock having uniaxial compressive strength as 92 MPa lies between 2.4 and 2.65 m. However, the optimum burden predicted through developed empirical equation from the numerical model is 2.6 m. The study shows that the simulation-based prediction exhibits a minor deviation within 15% with the experimental result. Hence, to assess the optimum burden under various geotechnical and geomining circumstances, numerical simulation-based approach may be carried out using the proposed methodology. Highlights: Burden for ring blasting is optimised using different numerical simulation models. Volume of rock excavated along free face (EV) has been calculated in each case. The optimum burden from experimental trial and empirical data is 2.4–2.65 m. The optimum burden predicted from developed model is 2.6 m. [ABSTRACT FROM AUTHOR]

The shale oil reservoirs of the Lucaogou Formation in the Jimsar Sag developed a large number of natural fractures and thin interlayers. The expansion of hydraulic fractures is full of randomness and unknowingness due to this complex geological feature. Since the multi-cluster fracturing process with small well spacing is widely used, the growth of hydraulic fractures requires a combined consideration of competition between perforation clusters and interference between wells. From the perspective of engineering application, this paper summarizes the fracture propagation law under single-fracture, multi-cluster fracturing, and adjacent well interference from simple to complex based on three-dimensional discrete element methods and a large number of field data. Then the key controlling factors of fracture group morphology were quantitatively analyzed. Comparative analysis reveals that under complex geological conditions, a single crack can still be described by existing theories, but the expansion law of the fracture group is not a simple superposition of single fractures. The competitive expansion of multiple fractures is reflected in the Fracture Surface Area (FSA) differences between clusters, asymmetry, and irregularity. Moreover, under inter-well stress interference, the hydraulic fractures of the posterior-construction well will tend to expand in the opposite direction of the first-construction well. The optimal well spacing of the Jimsar shale oil reservoir is 200–300 m. Highlights: In this paper, there are five highlights worthy of attention: This paper discussed the fracture propagation law under actual multi-cluster fracturing conditions through three aspects: 1. Clarifying the difference between the expansion law of multi-fracture competition and traditional cognition by comparing the morphology of fracture group and single-fracture; 2. Competitive expansion modes were quantitatively analyzed through FSA curves and inter-cluster stress interference. 3. Based on inter-well stress interference, a more complex simulation of actual working conditions was carried out to elaborate further on the influence of inter-well stress interference on fracture expansion. This paper compared the penetration expansion ability of hydraulic fractures under the condition of single fractures and fracture group. We quantitatively analyzed the dynamic characteristics of multi-fracture competitive expansion through Fracture Surface Area (FSA) and evaluated the morphological attributes of fracture groups under inter-cluster and inter-well stress interference. The stress interference range and the increment of the minimum horizontal in situ stress caused by fracturing adjacent well under different well spacings were quantitatively analyzed. The research was based entirely on the actual working conditions of the oil field, and the microseismic data and simulation results were thoroughly compared and demonstrated, which ensured the reliability of the conclusions. [ABSTRACT FROM AUTHOR]

Burden is one of the major influencing blast design parameters which should be optimised for the extraction of narrow vein ore bodies. Numerical simulation-based approach has been used for the optimisation of burden in this paper. Different models were prepared under varying parametric conditions including width of the vein, blasthole diameter and burden. The RHT concrete constitutive model was used for the analysis of damage contour. The volume of the excavated rock along the free face (EV) was quantified from the model output in all the scenario. The study showed that EV initially increases with the increase in burden for a particular width of vein. Furthermore, EV starts decreasing after a point termed as pivot point. The value of burden at this pivot point was considered as optimal in this study. The best fit curve between EV and burden for three blasthole diameter and seven different width of vein were plotted. In all instances, the outputs of the model followed the second-degree polynomial equation. The study also suggests that the optimum burden reduces with the increase in the width of the vein. It has also been found that the optimum burden follows power trend with the increase in width of the ore body irrespective of blasthole diameter. Based on the optimum burden obtained from the output of simulation along with different blasthole diameter and width of the ore body, an empirical relationship has been established. The developed empirical relationship has a good agreement with the experimental trial data. Highlights: Numerical simulation technique has been used in this paper for the optimisation of burden while extraction of narrow vein stopes. The study suggest that the relation of volume of the excavated rock along the free face and burden is second-degree polynomial in nature. It has been found that the optimum burden for narrow vein is higher as compared to wider width ore body. Study also suggest that optimum burden follows power trend with the increase in width of ore body. The influence of width of the ore body in the selection of burden vanishes after certain width of ore body as per the observation of model output. [ABSTRACT FROM AUTHOR]

River valley contraction (RVC) has increased continuously at the Xiluodu Dam site since the completion of the slope excavation that commenced on December 1, 2008. Previous studies on RVC only analyzed the displacement after reservoir impoundment. Since deformation is affected by many factors, it is difficult to determine the leading causes of RVC by merely studying the displacement after impoundment. This study examines, for the first time, the complete displacement process before and after impoundment. Four models that include poroelastic, thermo-poroelastic, poroelastic-plastic, and poroelastic-creep are used to simulate the RVC displacement using COMSOL multiphysics. It is observed that the creep of the basalt formation is the leading cause of RVC. Sloped excavation causes RVC, and the subsequent reservoir impoundment accelerates the RVC by changing the creep parameters. A poroelastic-creep model with two-stage creep parameters was applied to the numerical simulation. The simulation results are in good agreement with the field observations. The predicted results show that the convergence value of RVC is 187 mm, and an additional closure of 21.5 mm will occur in the next 14.5 years. This research provides a further assessment of the RVC by providing a model that is validated by existing data and leads to an estimation of future RVC. Highlights The paper presents a comprehensive investigation of the river valley contraction behavior at the Xiluodu reservoir. 2D simulation was performed to reveal the mechanism of the river valley contraction. Four models that include poroelastic, thermo-poroelastic, poroelastic-plastic, and poroelastic-creep were applied to the numerical simulation. [ABSTRACT FROM AUTHOR]

This paper examines the engineering background of the main shaft of Ruihai Mining Group Company in Laizhou City, with a focus on the loose permeable stratum located in the frozen section of the shaft. Field measurements and data collection, including brine temperature and surface subsidence values, were conducted using temperature and hydrological boreholes. The distribution of the frozen wall temperature field was then numerically simulated using finite element analysis, and the results were compared and analyzed with field data. Scanning electron microscopy (SEM) was used to qualitatively describe the microstructure of the soft rock in the frozen section under different freezing schemes. Based on the formation of the frozen wall, a new construction scheme for freezing and excavating the internal and external circles of the vertical shaft in the loose permeable stratum is proposed. This involves the implementation of "inner and outer double-circles of freezing holes" and a comparison of the freezing effect of the temperature field before and after the improvement. The results indicate that the new freezing scheme can accelerate the freezing rate of the surrounding rock of the shaft, and reduce the time required for closure by more than 10 days. After applying the improved scheme for 60 days, the temperature is lowered by 4–5 ℃ compared to the original scheme, and the thickness of the frozen wall is approximately 4.8 m, significantly thicker than before. These findings demonstrate the effectiveness of adding an inner circle of freezing holes in achieving the lowest temperature which contributes to subsequent shaft excavation. The new scheme holds significant implications for the safe construction of shaft excavation in complex hydrogeological areas. Highlights: The main shaft in the unique geological conditions and the impact of tidal activities on freezing performance are analyzed. The new construction scheme incorporating "inner and outer double-circles of freezing holes" significantly improves freezing efficiency and reduces closure time. The improved scheme results in a thicker frozen wall and lower temperature compared to the original scheme. The findings have important implications for the safe construction of shaft excavation in complex coastal hydrogeological areas. [ABSTRACT FROM AUTHOR]

The strain-hardening behavior of soils and rocks under compression is critical for evaluating the construction stability in geological and mining engineering. Numerical simulation is an effective approach and widely used in the stability analysis of constructions. To ensure the accuracy of the simulation results, it is necessary to select appropriate constitutive models and determine the correct model parameters. The double-yield model, built in FLAC3D, is often used to describe the strain-hardening behavior of geotechnical materials. On the assumption that there is only volume yielding in the material, a relationship between the cap pressure, plastic strain in the double-yield model, and the axial stress–strain in the compression test with axial loading and four lateral-face constraints was established. Then, the cap pressure and plastic strain could be calculated directly by a series of known axial stress–strain data. With this calculation method, the double-yield model parameters can be determined quickly and accurately. This method was derived based on the constitutive equation of the double-yield model and could quickly determine the model parameters by reverse calculation according to the expected results, so it was more efficient and credible than the more widely used trial-and-error method. Based on comparisons between the calculation and trial-and-error methods, some case studies also proved its efficiency and effectiveness. Moreover, there are no requirements for the source of known stress–strain data. Therefore, this method can be applied to the model parameter assignment based on the stress–strain data from experimental tests, theoretical analyses, empirical formulas, and other numerical simulations. Highlights: A relationship between the double-yield model parameters and the axial stress-strain in a uniaxial loading test with lateral constraints was established. An approach for accurate numerical modeling of strain-hardening materials using a double-yield model was developed. Some contrastive analyses with similar studies were conducted to verify the accuracy and efficiency of the approach in this paper. [ABSTRACT FROM AUTHOR]

Tunnels embedded in soft rock masses experience time-dependent behaviour, and designing an appropriate supporting system is a challenging task. This condition can be more problematic when a damaged zone is also developed (due to poor quality blasting). In this paper, a theoretical solution is presented to predict long-term tunnel convergence and long-term induced lining pressure, considering the existence of an altered zone due to excavation. The rheological behaviour of rock mass and the altered zone is simulated by viscoelastic behaviour obeying the Burgers model. It is also assumed that a continuous lining, i.e., a shotcrete layer, is placed (with elastic behaviour) to confine the convergence. A comprehensive parametric study is performed by the presented solution to evaluate the effects of various parameters on the induced lining pressures and the tunnel convergence. Then, to have more practical conditions, the elastic–viscoplastic rheological model (known as CVISC) is assigned to the rock mass to consider its failure. The problem is further studied by only numerical approach using FDM. The results show that the support installation time and the radius of the altered zone have remarkable effects on the output results. Highlights: A new analytical method is proposed to obtain the tunnel displacement over time. The interaction of the tunnel and rheological rock mass is taken into account. The Burgers rheological model is assigned to altered and unaltered zones. [ABSTRACT FROM AUTHOR]

To evaluate the ultimate capacity of the ground reinforced embankments, most of the research have been focused on high levels of impact energy. However, many existing embankments have been impacted by several blocks without collapsing. The residual characteristics and efficiency of post-impact embankments under low-energy impact are researched in this paper. Model tests of two types of embankments are compared to investigate the deformation performance and damage evolution of embankments. A simplified analytical approach, based on the test results and the energy method, is proposed to assess the residual efficiency of the post-impact embankments. Furthermore, the adverse impact conditions are analyzed by numerical simulation. The results show that the damage evolution of the embankments goes through three phases, uphill face compression, downhill face displacement, and collapse. Compared with experimental data, the stability calculation method is proved to be feasible. It is also found that the impact location and inclination of the blocks affect the deformation and stability of the embankments significantly. These points should be beneficial to the design of ground reinforced embankments. [ABSTRACT FROM AUTHOR]

In this paper the performance of a constitutive model for the description of the hydro mechanical behaviour of soft rock is evaluated with respect to the experimentally observed behaviour of Maastricht Calcarenite under different stress states that is presented in the companion paper. The mechanical model is elasto-plastic and consists of an associated yield surface, internal variables for the description of the hardening and softening behaviour and a non-local extension for the simulation of strain localization in form of shear bands and compaction bands. The model is implemented in the software ABAQUS and the laboratory results from the tests under dry condition with Maastricht Calcarenite are used for the calibration. The good agreement of the numerical results with the laboratory results is shown and the suitability of the model is discussed. To describe the effect of compaction bands on the permeability of soft rocks a simple analytical model based on the Kozeny–Carman equation is proposed and calibrated with the experimental results from drained tests under different stress states for Maastricht Calcarenite rock material. As the results are in good accordance with the experimental results, the model is implemented in the software ABAQUS and the numerical results are presented and discussed. Finally the performance of the model is evaluated and possible improvements are suggested. [ABSTRACT FROM AUTHOR]

Correct estimation of the dilatant behavior of a rock has an essential role in a realistic numerical simulation of the fracturing threshold during the rock deformation process and its post-failure response, based on experimental and field observations. This importance has been poorly treated in most of the numerical analyses dealing with the rock engineering common problems such as deep excavations. The dilatancy angle as a commonly used parameter for describing the dilatation response of a rock is mostly simplified to be a constant value. Contrarily, a literature review declares the inadequacy of this approach in those rock mechanics applications. In the present paper, a new formula for considering the variable dilatancy angle is presented which relates this parameter to the plastic shear strain in the course of a loading procedure. It is aimed at characterizing an evolution of the dilatant or contracting volumetric response of a rock as well as giving the possibility to simulate a dilatant/contracting transitional behavior. The model is applied to simulate the development of strain localization, in shear band mode, within the inevitable excavation damaged zone created around an underground opening in rock. It is illustrated that using the model of variable dilatancy angle in a deep excavation modeling could help to better reproduce the fractures development around the opening in the course of tunneling. [ABSTRACT FROM AUTHOR]

The mining and construction industries have long been faced with considerable attention and criticism in regard to the effects of blasting. The generation of ground vibrations is one of the most significant factors associated with blasting and is becoming increasingly important as mining sites are now regularly located near urban areas. This is of concern to not only the operators of the mine but also residents. Mining sites are subjected to an inevitable compromise: a production blast is designed to fragment the utmost amount of rock possible; however, any increase in the blast can generate ground vibrations which can propagate great distances and cause structural damage or discomfort to residents in surrounding urban areas. To accurately predict the propagation of ground vibrations near these sensitive areas, the blasting process and surrounding environment must be characterised and understood. As an initial step, an accurate model of the source of blast-induced vibrations is required. This paper presents a comprehensive review of the approaches to model the blasting source in order to critically evaluate developments in the field. An overview of the blasting process and description of the various factors which influence the blast performance and subsequent ground vibrations are also presented. Several approaches to analytically model explosives are discussed. Ground vibration prediction methods focused on seed waveform and charge weight scaling techniques are presented. Finally, numerical simulations of the blasting source are discussed, including methods to estimate blasthole wall pressure time-history, and hydrodynamic codes. [ABSTRACT FROM AUTHOR]

Tunnel excavation in squeezing ground exhibits large time-dependent and often anisotropic deformation. Within the context of the Fréjus road tunnel and its safety gallery excavated under the Alps between France and Italy, an interesting configuration of two parallel tunnels under squeezing ground conditions is observed. The special feature of this case study lies in the fact that both tunnels have been excavated in similar geotechnical conditions but with different excavation techniques. The road tunnel was excavated with conventional drill and blast methods in the 70s, whereas the safety gallery was excavated between 2009 and 2016 with a single-shield tunnel boring machine (TBM). This paper presents monitoring data processing and numerical simulations of both tunnels with the aim of studying the influence of the excavation method on the time-dependent tunnel response. A calibration of a visco-elasto-plastic anisotropic constitutive model based on the back-analysis of convergence measurements retrieved during the excavation of the Fréjus road tunnel is carried out. The identified ground behavior can be extrapolated to the parallel zones of the safety gallery. In particular, we are interested in the prediction of the stress state in the segmental lining of the gallery during its excavation and the comparison with in situ measurements. It is shown that the time-dependent behavior of the ground is affected by the excavation technique. Finally, an attempt to predict the long-term response of both tunnels is proposed. [ABSTRACT FROM AUTHOR]

Rigorous design of flexible rockfall protection systems requires an adequate understanding of the system performance. In recent decades, finite element simulations have complemented experimental testing for economical and practical reasons. However, complex modelling techniques are required to capture the dynamic response of such systems, which leads to considerable computational costs. This paper presents a new approach called "hybrid method" to improve the computational efficiency of FE models with minimum effects on the results accuracy. The method is based on the idea to model the exact architecture of the wire net in the vicinity of impact (zone undergoing plastic deformation and failure) and to model the mesh with less computationally expensive elements far from the impact (zones undergoing elastic deformation). To this aim, a three-dimensional model of chain-link mesh was developed using commercial finite element code ABAQUS. The real architecture of the wires is modelled and discretised using three-dimensional beam elements. Homogenised shell surfaces are used to model the chain-link away from the impact zone. The model was calibrated and validated against published experimental data. The results prove that the proposed hybrid method leads to considerable reduction in computational costs of the finite element analysis while producing accurate results. [ABSTRACT FROM AUTHOR]

Identifying the surrounding rock loads acting on the external shield surface is critical to the safe construction of deep and long tunnels excavated by TBMs (tunnel boring machines). However, it is rather difficult or even impossible to carry out monitoring outside the TBM shield to directly determine the surrounding rock loads. In the present paper, based on modified Newton iteration and numerical simulation, an inverse analysis approach to identify the surrounding rock loads is proposed. This inverse problem is solved by minimizing the objective function that incorporates the difference between the monitored and calculated strains of discrete points on the internal TBM shield surface. During the inverse analysis, the external TBM shield surface is discretized into rectangular regions, and the nodal loads of each region are taken as the inverse parameters. Then, the distributed loads are approximately calculated by the interpolation of the nodal loads of each region. The proposed approach is calibrated with two numerical examples and then applied to analyze in situ monitoring data. The results show that the proposed approach is robust and efficient for identifying the distribution and magnitude of surrounding rock loads, and it is possible to carry out timely countermeasures to avoid shield jamming according to the identified loads. [ABSTRACT FROM AUTHOR]

Multiscale continuous lab oratory observation of the progressive failure process has become a powerful means to reveal the complex failure mechanism of rock. Correspondingly, the representative volume element (RVE)-based models, which are capable of micro/meso- to macro-scale simulations, have been proposed, for instance, the rock failure process analysis (RFPA) program. Limited by the computational bottleneck due to the RVE size, multiscale high-resolution modeling of rock failure process can hardly be implemented, especially for three-dimensional (3D) problems. In this paper, the self-developed parallel RFPA code is employed to investigate the failure mechanisms and various fracture morphology of laboratory-scale rectangular prism rock specimens under unconfined uniaxial compression. The specimens consist of either heterogeneous rock with low strength or relatively homogeneous rock with high strength. The numerical simulations, such as the macroscopic fracture pattern and stress-strain responses, can reproduce the well-known phenomena of physical experiments. In particular, the 3D multiscale continuum modeling is carried out to gain new insight into the morphologic interpretation of brittle failure mechanisms, which is calibrated and validated by comparing the actual laboratory experiments and field evidence. The advantages of 3D multiscale high-resolution modeling are demonstrated by comparing the failure modes against 2D numerical predictions by other models. The parallel RVE-based modeling tool in this paper can provide an alternative way to investigate the complicated failure mechanisms of rock. [ABSTRACT FROM AUTHOR]

This paper aims to gain fundamental understanding of the microscopic mechanisms that control the transition between secondary and tertiary creep around salt caverns in typical geological storage conditions. We use a self-consistent inclusion-matrix model to homogenize the viscoplastic deformation of halite polycrystals and predict the number of broken grains in a Representative Elementary Volume of salt. We use this micro-macro modeling framework to simulate creep tests under various axial stresses, which gives us the critical viscoplastic strain at which grain breakage (i.e., tertiary creep) is expected to occur. The comparison of simulation results for short-term and long-term creep indicates that the initiation of tertiary creep depends on the stress and the viscoplastic strain. We use the critical viscoplastic deformation as a yield criterion to control the transition between secondary and tertiary creep in a phenomenological viscoplastic model, which we implement into the Finite Element Method program POROFIS. We model a 850-m-deep salt cavern of irregular shape, in axis-symmetric conditions. Simulations of cavern depressurization indicate that a strain-dependent damage evolution law is more suitable than a stress-dependent damage evolution law, because it avoids high damage concentrations and allows capturing the formation of a damaged zone around the cavity. The modeling framework explained in this paper is expected to provide new insights to link grain breakage to phenomenological damage variables used in Continuum Damage Mechanics. [ABSTRACT FROM AUTHOR]

This paper presents a new approach to determine more robust shear failure criteria, focusing on rough concrete-rock interfaces. The proposed method could also be applied to rock joints. The new approach is based on the distribution and variety of interfaces numerically tested in terms of surface roughness. For this reason, random field simulations are performed using the turning bands method to generate an extensive database of synthetic rough rock surfaces. With this database of synthetic rough rock surfaces, numerical simulations of direct shear tests are carried out. Finally, analytical and neural network models are proposed using the database of shear strength obtained from the finite element simulations to estimate the peak shear strength of concrete-rock interfaces. The performances of the analytical and neural network models in estimating this peak shear strength are evaluated by computing the percentage error and the mean absolute error (MAE) between the predicted and the numerically obtained values. Both models lead to satisfactory predictions. Nevertheless, it is worth noting that the neural network model mildly outperforms the analytical model regarding the magnitude of the error. Furthermore, the neural network model reproduces the possible non-bijective aspect of the correlation roughness-peak shear strength. [ABSTRACT FROM AUTHOR]

The goal of this paper is to provide an overview of the development of a novel pulsed combustion-based wellbore fracturing technology by reporting on 2D and 3D dynamic fracture modeling along with the associated full wellbore scale experimental demonstration fracturing of a high-strength concrete formation surrogate. With this technique a high pressure, gaseous mixture is rapidly combusted to produce repetitive, controllable wellbore strain rates that can induce complex fracture patterns around the wellbore. The technology can be used as a pre-conditioning and stimulation tool for in situ recovery and cave mining, enhanced geothermal systems, and unconventional hydrocarbon reservoirs. As part of this study, full-scale experimental testing on a rock-like material was conducted together with advanced numerical simulations based on the finite-discrete element method (FDEM). FDEM is a numerical approach capable of explicit consideration of rock fracturing processes and dynamic phenomena. Experimental results indicate that the fracturing tool can successfully apply a circumferentially and axially uniform high pressurization rate pulse to the wellbore, ultimately producing a complex fracture network. These results were also used to validate 3D numerical simulations, which showed good overall qualitative agreement in terms of fracturing modes and extent, as well as fragment size and shape. A 2D numerical parametric study on the effect of borehole pressurization characteristics, including gas-in-fractures effects, and geostatic confinement highlighted the influence of these factors on fracture complexity and radial extent. An in-depth analysis of the extent of the crushed zone and radial distributions of fracture specific surface area was carried out. Borehole pair configurations were simulated to gain insights into borehole spacing and loading sequence effects. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing is the prime technology for enhancing production from unconventional reservoirs. Design and operation of hydraulic fracturing requires adequate knowledge of formation properties, in situ stresses and fluid properties, due to its complex nature. One of the challenges, that have been the subject of many past studies, is the interaction mechanism when hydraulic fracture intersects a natural interface (e.g., natural fracture or interbed). Crossing, arresting and opening are the known interaction mechanisms. The formation properties, state of in situ stresses, stress contrast, strength properties of the interface, angle of intersection and injecting fluid characteristics affect the interaction mechanism to different extents. The use of numerical models is widespread in simulation of hydraulic fracturing. The use of methods based on the continuum (e.g., FEM) and discontinum (e.g., DEM) representation of media is subjected to some limitations in predicting the correct geometry of the propagating fracture. Lattice method, which is based on the bonded particle approach, has shown promise in simulating hydraulic fracturing. In this paper, lattice modeling is used to simulate the interaction mechanism of lab experiments carried out on mortar samples but at larger scales. The cubical mortar samples of 10 cm size had two synthetically built natural fractures with different angles of approach and filled with different types of glue. The effect of intersection angle and the mechanical properties of the interface were simulated numerically under conditions of viscosity-dominated fracture propagation regime as occurred in the lab. The results demonstrate the advantages of lattice simulation to study the impact of each parameter on the fracture interaction. A larger angle of approach and stronger glues are shown to promote the crossing mode. The shear strength and stress anisotropy are the main parameters influencing the interaction mechanism. [ABSTRACT FROM AUTHOR]

The paper investigates groundwater inflow into a longwall working face of the Xiegou Coal Mine located in the northwest of the Shanxi province, China. Three modelling approaches, including basic Darcy modelling, full poroelastic modelling using Biot's classical theory, and the full Biot poroelastic modelling that takes into account permeability alterations of the coal seam and strata due to alterations in the mining-induced stress state, are used to estimate the fluid flow into the underground longwall coal mine as the excavation progresses. Based on plausible assumptions of hydrogeological structure, boundary conditions and mining conditions, a three-dimensional finite element model of the simplified geological setting is developed using the COMSOL software. The permeability changes in both the strata and the coal seam due to the mining are associated with the mining-induced stress changes and water pressure, and the correlation between them accords with the exponential relationship. The differences in the modelling results of three approaches were analyzed, and the simulation results are compared with the field measurements. [ABSTRACT FROM AUTHOR]

The main constrain for effective gas drainage in coal mines is the low permeability nature of coal reservoirs. As coal mining activities are extending to deeper subsurface, the ever-increasing in situ stress conditions is anticipated to result in much lower permeability and more challenges for gas emission control in coal mines. In recent years, hydraulic slotting using high-pressure waterjet along underground gas drainage boreholes, as a general solution to stimulate low permeability coal seams, has become increasingly favourable. This paper presents a systematic investigation into the sensitivity of borehole slotting performance to a number of field and operational parameters. A wide range of geomechanical properties, in situ stress conditions, slot geometry and spacing of multiple slots were considered in a series of numerical simulations. The relations between these key parameters and the failure zone size/volume induced by slotting were quantified. The effect of different parameters in improving slotting performance has also been ranked, which provides theoretical base for mine operators to optimise slotting operations. [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]

Hydraulic fracturing is a complex operation which is influenced by several factors including the formation properties, state of stresses in the field, injecting fluid and pumping rate. Before carrying out the expensive fracturing operation in the field, it would be useful to understand the effect of various parameters by conducting physical experiments in the laboratory. Also, laboratory experiments are valuable for validating numerical simulations. For this purpose, laboratory experiments may be conducted on synthetically made samples to study the effect of various parameters before using real rock samples, which may not be readily available. To simulate the real stress conditions in the field, experiments need to be conducted on cube-shaped samples on which three independent stresses can be applied. The hydro-mechanical properties of a sample required for modelling purposes and the design of a scaled hydraulic fracturing test in the laboratory can be estimated by performing various laboratory experiments on cylindrical plugs. The results of laboratory experiments are scaled to field operation by applying scaling laws. In this paper, the steps to prepare a cube-shaped mortar sample are explained. This follows a review of the sample set-up procedure in a true tri-axial stress cell for hydraulic fracturing experiments. Also, the minimum tests on cylindrical plugs required to estimate the hydro-mechanical properties of the rock sample are explained. To simulate the interaction mode when a hydraulic fracture approaches an interface in the laboratory, the procedure for producing samples with parallel artificial fracture planes is explained in this paper. The in-fill material and the angle of fracture planes were changed in different samples to investigate the effect of interface cohesion and the angle of approach on the interaction mechanism. [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]

The goal of this paper is to show how a recently developed advanced chemo-hydro-mechanical (CHM)-coupled constitutive and numerical model for soft rocks can be applied to predict the temporal evolution of settlement damage to buildings on cavities subject to weathering. In particular, a Building Damage Index (BDI) and its evolution with time is proposed. The definition of the BDI is inspired by the work of Boscardin and Cording (J Geotech Eng 115:1-21, 1989) and uses the surface differential settlements obtained by finite element (FE) analyses to assess how far a building is from a non-acceptable service condition. By modelling the reactive transport of chemical species in 3D and using a coupled CHM constitutive and numerical model, it is possible to simulate weathering scenarios and monitor the temporal evolution of surface settlements making the BDI time dependent. This approach is applied to evaluate the damage evolution of two buildings lying on two anthropic caves in a calcarenite deposit belonging to the Calcarenite di Gravina Formation. Standard and advanced experimental tests are performed on the in situ material, and the results are used to calibrate the constitutive model. The soundness of both constitutive relationship and reactive transport solver is subsequently tested by simulating two laboratory-scale boundary value experiments. The first is a model footing test on dry and wet calcarenite, while the second is a small-scale pillar that, after the saturation-induced short-term water weakening, fails due to a long-term dissolution weathering process. Finally, both 2D and 3D coupled FE analyses simulating different weathering scenarios and corresponding settlements affecting the buildings above the considered cavities are presented. Particular attention is placed on assessing the BDI and its temporal evolution. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing is a widely used stimulation method to enhance the productivity of unconventional resources. The hydraulic fracturing operation in naturally fractured reservoirs or when it is expected to intersect a natural interface, such as an interbed is subjected to complexity. The induced fracture may cross, get arrested by or open the fracture plane upon its arrival at the natural interface. Besides other parameters, this depends on the natural interface mechanical properties, including the cohesion and friction angle of the interface. Several analytical criteria have been developed to predict the interaction mechanism of induced and natural fracture. While these analytical solutions have been developed based on some simplified assumptions, they can provide a good understanding of the effect of different parameters. The first part of this paper summarizes the available criteria for interaction of hydraulic and natural fractures. Important factors will be mentioned and illustrations will be given to present the limitations of each criterion. The second part discusses the development and validation of an extension to Renshaw and Pollard criterion in the form a single analytical formula for non-orthogonal cohesive fracture. This includes the contribution of the strength of the in-fill material to the bonding of the two sides of a fracture, hence its effect on the interaction mechanism. The proposed criterion was validated using published laboratory data. Finally, a methodology is proposed to help the design of interaction experiments in the laboratory, which can also be used for prediction of interaction mode in numerical simulations. [ABSTRACT FROM AUTHOR]

Block theory has been successfully applied to solve practical problems in rock engineering associated with the stability of discontinuous rock masses. Traditional methods of block theory, which have not proposed detailed schemes for characterizing geometric features of arbitrary blocks with complex geometries, are difficult to simultaneously visualize the locations of blocks. To overcome the shortcomings of these traditional methods, this paper introduces the concept of block morphology to systematically represent the morphological characteristics of blocks. Morphological visualization analysis involves the classification, characterization and visualization of various blocks and the determination of their removability using the stereo-analytical method. The proposed morphological visualization method provides a basis for computer modeling and mechanical calculations of large-scale rock mass systems. Furthermore, examples are presented in this paper to verify the validity and demonstrate the exceptional performance of this method. [ABSTRACT FROM AUTHOR]

Numerical modeling is an important tool in rock engineering. The choice for a particular numerical method is based on its capacity to represent the problem boundary conditions, the material behavior, and the pertinent rock mass failure mechanisms. It follows that there could be more than one numerical method suitable for a particular problem. In this respect, the selection of a particular method will be dictated by a series of technical and practical trade-offs. This paper addresses issues associated with the simulation of jointed rock masses and ground support by employing both continuum and discontinuum numerical methods. The focus is on the explicit simulation of a jointed rock mass and the implementation of ground support using a three-dimensional numerical code based on the distinct element method. 3DEC was used to generate continuum and discontinuum rock mass models, which were calibrated based on field instrumentation data from the George Fisher mine, in Australia. In particular, the investigation focused on the response of the ground support system, consisting of both reinforcement and surface support elements, to stress changes associated with the excavation of nearby stopes. The main difference between the constructed continuum and discontinuum 3DEC models was the representation of the rock mass at the location of the instrumented drift. In the continuum model, an equivalent continuum rock mass model, with the ubiquitous-joint (UJ) constitutive model, was used, whereas in the discontinuum model, joints were simulated explicitly. In this case study, it was observed that the adopted discontinuum modeling approach provided more realistic results than the continuum model with the UJ constitutive model, in terms of the drift convergence, support behavior, and depth of yielded zone due to the excavation of stopes near the instrumented section of the drift. [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]

Entry driven along goaf-side (EDG), which is the development of an entry of the next longwall panel along the goaf-side and the isolation of the entry from the goaf with a small-width yield pillar, has been widely employed in China over the past several decades . The width of such a yield pillar has a crucial effect on EDG layout in terms of the ground control, isolation effect and resource recovery rate. Based on a case study, this paper presents an approach for evaluating, designing and optimizing EDG and yield pillar by considering the results from numerical simulations and field practice. To rigorously analyze the ground stability, the numerical study begins with the simulation of goaf-side stress and ground conditions. Four global models with identical conditions, except for the width of the yield pillar, are built, and the effect of pillar width on ground stability is investigated by comparing aspects of stress distribution, failure propagation, and displacement evolution during the entire service life of the entry. Based on simulation results, the isolation effect of the pillar acquired from field practice is also considered. The suggested optimal yield pillar design is validated using a field test in the same mine. Thus, the presented numerical approach provides references and can be utilized for the evaluation, design and optimization of EDG and yield pillars under similar geological and geotechnical circumstances. [ABSTRACT FROM AUTHOR]

A discrete element code with impact model has been developed and calibrated to simulate the dynamic behavior of rock materials, with special regard to rock fragmentation upon impact during rock-fall analysis. The paper summarizes the discrete element code, the calibration algorithms developed to identify the model microparameters, and the impact model. Experimental work on drop tests is then used to validate the code on modeling impact fragmentation. It has been found that the developed discrete element code and impact model can reasonably simulate rock fragmentation in drop tests. The use of the discrete element code and impact model can provide good reference results in evaluating impact fragmentation in rock-fall analysis. [ABSTRACT FROM AUTHOR]

With the deepening of the development of natural resources, the difficulty of rock breaking is increasing. Novel efficient rock-breaking technology and matching rock-breaking tools are urgently needed. The high-voltage pulsed fragmentation (HVPF) method has high efficiency and great development potential. So, it has attracted wide attention. However, the unclear understanding of the relationship between the mechanism of HVPF, the design and parameter optimization of HVPF tools, and rock-breaking energy consumption also impede the commercial progress of this technology. In this paper, the electrical crushing is subdivided into "partial electrical breakdown" (PEB) and "complete electrical breakdown" (CEB) from electrical breakdown process (EBP), and the mechanism of PEB is proposed. Finally, a method, which is named as voltage partitioned method (VPM), for designing and optimizing parameters of the HVPF tools is provided. The results of numerical simulation and electrical breakdown experiment can well support the mechanism of PEB. The mechanism of PEB provides a basis for the application of multi-pulse electrical breakdown technology. The VPM establishes the relationship between voltage loading parameters, electrode structure parameters and rock electrical parameters, which can provide reliable help for the design and parameter optimization of HVPF tools. [ABSTRACT FROM AUTHOR]

A coal and gas outburst is an extreme hazard in underground mining. The present paper conducts a laboratory simulation of a coal and gas outburst combined with acoustic emission analysis. The experiment uses a three-dimensional stress loading system and a PCI-2 acoustic emission monitoring system. Furthermore, the development of a coal and gas outburst is numerically studied. The results demonstrate that the deformation and failure of a coal sample containing methane under three-dimensional stress involves four stages: initial compression, elastic deformation, plastic deformation and failure. The development of internal microscale fractures within a coal sample containing methane is reflected by the distribution of acoustic emission events. We observed that the deformation and failure zone for a coal sample under three-dimensional stress has an ellipsoid shape. Primary acoustic emission events are generated at the weak structural surface that compresses with ease due to the external ellipsoid-shaped stress. The number of events gradually increases until an outburst occurs. A mathematical model of the internal gas pressure and bulk stress is established through an analysis of the internal gas pressure and bulk stress of a coal sample, and it is useful for reproducing experimental results. The occurrence of a coal and gas outburst depends not only on the in situ stress, gas pressure and physical and mechanical characteristics of the coal mass but also on the free weak surface of the outburst outlet of the coal mass. It is more difficult for an outburst to occur from a stronger free surface. [ABSTRACT FROM AUTHOR]

In this paper, the time evolutions of the mechanical characteristics of a discretely mechanically or frictionally coupled (DMFC) rockbolt and surrounding rockmass are studied. A conceptual model is developed to analyze the interaction between the DMFC rockbolt and rockmass, and analytical solutions are presented for a tunnel supported with DMFC rockbolts. The formulation is based on the following assumptions: (1) a deep tunnel with a circular cross-section; (2) an axisymmetric problem, i.e., having a lateral pressure coefficient K = 1; (3) homogeneous and isotropic ground; (4) small deformations; (5) rockbolts simulated as two cylindrical surfaces and oppositely distributed forces, i.e., one at the head and the other at the bolt bond; and (6) viscoelastic material assumptions, i.e., a one-dimensional viscoelastic model for the rockbolt and a three-dimensional viscoelastic model for the rockmass. The analytical solutions of the rheological model are obtained by assuming the constitutive models of the rockmass and rockbolt in terms of a Maxwell model and Maxwell model (M-M), Kelvin model and Kelvin model (K-K), and Kelvin model and Maxwell model (K-M). The analytical solutions of the M-M model are compared with the results obtained from numerical simulations with FLAC3D. The analytical solutions are in good agreement with the numerical results in the case of small rheological rock deformation and close bolt arrangement. Furthermore, the variation of the axial force along the rockbolt and the evolution of stress and displacement in the rockmass are closely related to the rheological parameters and support parameters. [ABSTRACT FROM AUTHOR]

Presence of an excavation damage zone (EDZ) around a tunnel perimeter is of significant concern with regard to safety, stability, costs and overall performance of the tunnel. For deep-buried tunnel excavation by drill and blast, it is generally accepted that a combination of effects of stress redistribution and blasting is mainly responsible for development of the EDZ. However, few open literatures can be found to use numerical methods to investigate the behavior of rock damage induced by the combined effects, and it is still far from full understanding how, when and to what degree the blasting affects the behavior of the EDZ during excavation. By implementing a statistical damage evolution law based on stress criterion into the commercial software LS-DYNA through its user-subroutines, this paper presents a 3D numerical simulation of the rock damage evolution of a deep-buried tunnel excavation, with a special emphasis on the combined effects of the stress redistribution of surrounding rock masses and the blasting-induced damage. Influence of repeated blast loadings on the damage extension for practical millisecond delay blasting is investigated in the present analysis. Accompanying explosive detonation and secession of rock fragments from their initial locations, in situ stress in the immediate vicinity of the excavation face is suddenly released. The transient characteristics of the in situ stress release and induced dynamic responses in the surrounding rock masses are also highlighted. From the simulation results, some instructive conclusions are drawn with respect to the rock damage mechanism and evolution during deep-buried tunnel excavation by drill and blast. [ABSTRACT FROM AUTHOR]

This paper proposes a method for improving the design of rockfall protection fences and accounting for the variability of loading cases. It is based on a probabilistic reliability analysis and can combine loading cases from rockfall propagation simulations with numerical simulations of the structure response to the block impact. The advantage of such a reliability-based approach is that statistically relevant results can be obtained concerning the fence's efficiency in stopping the block with a limited number of simulations. This method was employed in a study case, involving a low-energy tree-supported fence placed on a forested slope. The trajectory simulations were conducted using Rockyfor3D and the fence was modelled using a three-dimensional discrete element method model. For demonstration purposes, two parameters were considered: block velocity and the block's angle of incidence before impact. The probability of the fence stopping the block was evaluated accounting for the variability of these two parameters separately and together, either considering these variables as non-correlated, or as correlated. The value of this approach is demonstrated in terms of computation cost. In addition, the results revealed the importance of accounting for both these parameters in designing the structure as well as in estimating the residual hazard downslope from the protective structure. [ABSTRACT FROM AUTHOR]

The diametrical compression of a circular disc (Brazilian test) or cylinder with a small eccentric hole is a simple but important test to determine the tensile strength of rocks. This paper studies the failure mechanism of circular disc with an eccentric hole by a 3D numerical model (RFPA3D). A feature of the code RFPA3D is that it can numerically simulate the evolution of cracks in three-dimensional space, as well as the heterogeneity of the rock mass. First, numerically simulated Brazilian tests are compared with experimental results. Special attention is given to the effect of the thickness to radius ratio on the failure modes and the peak stress of specimens. The effects of the compressive strength to tensile strength ratio ( C/T), the loading arc angle (2 α), and the homogeneity index ( m) are also studied in the numerical simulations. Secondly, the failure process of a rock disc with a central hole is studied. The effects of the ratio of the internal hole radius ( r) to the radius of the rock disc ( R) on the failure mode and the peak stress are investigated. Thirdly, the influence of the vertical and horizontal eccentricity of an internal hole on the initiation and propagation of cracks inside a specimen are simulated. The effect of the radius of the eccentric hole and the homogeneity index ( m) are also investigated. [ABSTRACT FROM AUTHOR]

In this paper a numerical method for the simulation of the steady-state fluid flow in discrete fracture networks is described. It is based on the use of non-conforming meshes, enrichment functions and an optimization procedure. The meshing process is performed on each fracture independently of the other fractures, i.e. without geometrical conformity at the intersections (traces). The slope discontinuities due to the flux exchange at the traces are then captured with the enrichment functions of the extended finite elements, and finally a functional is minimized by resorting to an optimization procedure. The method can be easily implemented for parallel computers being based on many small independent problems. In order to show the effectiveness of the method and the quality of the results, simulations of fluid flow in simple networks are illustrated. [ABSTRACT FROM AUTHOR]

Homogenization in fractured rock analyses is essentially based on the calculation of equivalent elastic parameters. In this paper, a new numerical homogenization method that was programmed by means of a MATLAB code, called HLA-Dissim, is presented. The developed approach simulates a discontinuity network of real rock masses based on the International Society of Rock Mechanics (ISRM) scanline field mapping methodology. Then, it evaluates a series of classic joint parameters to characterize density (RQD, specific length of discontinuities). A pulse wave, characterized by its amplitude, central frequency, and duration, is propagated from a source point to a receiver point of the simulated jointed rock mass using a complex recursive method for evaluating the transmission and reflection coefficient for each simulated discontinuity. The seismic parameters, such as delay, velocity, and attenuation, are then calculated. Finally, the equivalent medium model parameters of the rock mass are computed numerically while taking into account the natural discontinuity distribution. This methodology was applied to 17 bench fronts from six aggregate quarries located in Tunisia, Spain, Austria, and Sweden. It allowed characterizing the rock mass discontinuity network, the resulting seismic performance, and the equivalent medium stiffness. The relationship between the equivalent Young's modulus and rock discontinuity parameters was also analyzed. For these different bench fronts, the proposed numerical approach was also compared to several empirical formulas, based on RQD and fracture density values, published in previous research studies, showing its usefulness and efficiency in estimating rapidly the Young's modulus of equivalent medium for wave propagation analysis. [ABSTRACT FROM AUTHOR]

Fracture network modelling plays an important role in many application areas in which the behaviour of a rock mass is of interest. These areas include mining, civil, petroleum, water and environmental engineering and geothermal systems modelling. The aim is to model the fractured rock to assess fluid flow or the stability of rock blocks. One important step in fracture network modelling is to estimate the number of fractures and the properties of individual fractures such as their size and orientation. Due to the lack of data and the complexity of the problem, there are significant uncertainties associated with fracture network modelling in practice. Our primary interest is the modelling of fracture networks in geothermal systems and, in this paper, we propose a general stochastic approach to fracture network modelling for this application. We focus on using the seismic point cloud detected during the fracture stimulation of a hot dry rock reservoir to create an enhanced geothermal system; these seismic points are the conditioning data in the modelling process. The seismic points can be used to estimate the geographical extent of the reservoir, the amount of fracturing and the detailed geometries of fractures within the reservoir. The objective is to determine a fracture model from the conditioning data by minimizing the sum of the distances of the points from the fitted fracture model. Fractures are represented as line segments connecting two points in two-dimensional applications or as ellipses in three-dimensional (3D) cases. The novelty of our model is twofold: (1) it comprises a comprehensive fracture modification scheme based on simulated annealing and (2) it introduces new spatial approaches, a goodness-of-fit measure for the fitted fracture model, a measure for fracture similarity and a clustering technique for proposing a locally optimal solution for fracture parameters. We use a simulated dataset to demonstrate the application of the proposed approach followed by a real 3D case study of the Habanero reservoir in the Cooper Basin, Australia. [ABSTRACT FROM AUTHOR]

Different failure modes during fracture shearing have been introduced including dilation, sliding, asperity cut-off and degradation. Several laboratory studies have reported the complexity of these failure modes during shear tests performed under either constant normal load (CNL) or constant normal stiffness (CNS) conditions. This paper is concerned with the mechanical behaviour of synthetic fractures during direct shear tests using a modified shear cell and related numerical simulation studies. The modifications made to an existing true triaxial stress cell (TTSC) in order to use it for performing shear tests under CNL conditions are presented. The large loading capacity and the use of accurate hydraulic pumps capable of applying a constant shear velocity are the main elements of this cell. Synthetic mortar specimens with different fracture surface geometries are tested to study the failure modes, including fracture sliding, asperity degradation, and to understand failure during shearing. A bonded particle model of the direct shear test with the PFC2D particle flow code is used to mimic the tests performed. The results of a number of tests are presented and compared with PFC2D simulations. The satisfactory results obtained both qualitatively and quantitatively are discussed. [ABSTRACT FROM AUTHOR]

Viscous boundaries are widely used in numerical simulations of wave propagation problems in rock mechanics and rock engineering. By using such boundaries, reflected waves from artificial boundaries can be eliminated; therefore, an infinite domain can be modeled as a finite domain more effectively and with a much greater accuracy. Little progress has been made, thus far, with the implementation and verification of a viscous boundary in the numerical, discrete element, discontinuous deformation analysis (DDA) method. We present in this paper a new viscous boundary condition for DDA with a higher absorbing efficiency in comparison to previously published solutions. The theoretical derivation of the new viscous boundary condition for DDA is presented in detail, starting from first principles. The accuracy of the new boundary condition is verified using a series of numerical benchmark tests. We show that the new viscous boundary condition works well with both P waves as well as S waves. [ABSTRACT FROM AUTHOR]

Sweden has more than 190 large hydropower dams, of which about 50 are pure embankment dams and over 100 are concrete/embankment dams. This paper presents results from conceptual analyses of the response of typical Swedish rock mass to the construction of a hydropower embankment dam and its first stages of operation. The aim is to identify locations and magnitudes of displacements that are occurring in the rock foundation and grout curtain after construction of the dam, the first filling of its water reservoir, and after one seasonal variation of the water table. Coupled hydro-mechanical analysis was conducted using the two-dimensional distinct element program UDEC. Series of the simulations have been performed and the results show that the first filling of the reservoir and variation of water table induce largest magnitudes of displacement, with the greatest values obtained from the two models with high differential horizontal stresses and smallest spacing of sub-vertical fractures. These results may help identifying the condition of the dam foundation and contribute to the development of proper maintenance measures, which guarantee the safety and functionality of the dam. Additionally, newly developed dams may use these results for the estimation of the possible response of the rock foundation to the construction. [ABSTRACT FROM AUTHOR]

In order to protect infrastructures against rockfalls, civil-engineered mitigation measures are widely used. Flexible metallic fences are particularly well suited to stop the propagation of blocks of rock whose kinetic energy can reach 5000 kJ before impact. This paper focuses on the design of highly flexible rockfall fences under the new European guideline ETAG027. The experimental testing and the numerical modeling using the discrete element method (DEM) of a new metallic rockfall fence are presented. Several scales of study were considered; the mesh, the net and the entire structure. The calibration of the DEM models is described and a parametrical study is proposed. The latter aims to underline the type of information that can be obtained from numerical simulations of such a system to enhance its design. [ABSTRACT FROM AUTHOR]

Different failure modes during fracture shearing have been introduced including normal dilation or sliding, asperity cut-off and degradation. Attempts have been made to study these mechanisms using analytical, experimental and numerical methods. However, the majority of the existing models simplify the problem, which leads to unrealistic results. With this in mind, the aim of this paper is to simulate the mechanical behaviour of synthetic and rock fracture profiles during direct shear tests by using the two-dimensional particle flow computer code PFC2D. Correlations between the simulated peak shear strength and the fracture roughness parameter D recently proposed by Rasouli and Harrison () are developed. Shear test simulations are carried out with PFC2D and the effects of the geometrical features as well as the model micro-properties on the fracture shear behaviour are studied. The shear strength and asperity degradation processes of synthetic profiles including triangular, sinusoidal and randomly generated profiles are analysed. Different failure modes including asperity sliding, cut-off, and asperity degradation are explicitly observed and compared with the available models. The D parameter is applied to the analysis of synthetic and rock fracture profiles. Accordingly, correlations are developed between D and the peak shear strength obtained from simulations and by using analytical solutions. The results are shown to be in good agreement with the basic understanding of rock fracture shear behaviour and asperity contact degradation. [ABSTRACT FROM AUTHOR]

Loading arcs, which create a definite contact range and have a radius equal to the test specimen, have been recently introduced into Brazilian disc tests, which are common indirect tensile strength tests. Inspired by the testing apparatus, a new kind of loading pattern acting on the disc is assumed: the vertical uniform load. This load can be resolved into radial and tangential surface forces on the loaded rims of the specimen. Under the assumptions of isotropic, linear-elastic, and homogenous disc materials, the explicit closed-form expressions for the full-field stresses in the disc are derived by the complex variable method. The analytical solutions satisfy the stress boundary condition, and the analytical results are in good agreement with the numerical simulations. Based on the closed-form solutions, it is analytically proven that the maximum tensile stress occurs at the disc center. Then, the suggested formula for the tensile strength, which is related to the contact range, the external load, and the size of specimen, is provided. [ABSTRACT FROM AUTHOR]

Summary  Numerical Manifold Method (NMM) is an extension of Discontinuous Deformation Analysis which contains and combines FEM and joint/block oriented DDA in a unified form. At present, application of NMM is mainly limited to two-dimensional analysis which is not realistic for many underground excavation works. In the present paper, three-dimensional NMM based on tetrahedron and hexahedron elements is proposed. The corresponding three-dimensional cover contact detection algorithms which have some fundamental properties different from the corresponding two-dimensional analysis are also developed. Based on the present formulations, engineering problems with arbitrarily oriented discontinuities can be modeled and realistic modeling of construction works becomes possible. [ABSTRACT FROM AUTHOR]

Summary  Fractal descriptions have been proved to be a valuable tool in characterising irregular natural shapes. In particular, the discontinuity network of a real rock mass shows such a property. Therefore, the fractal geometry could help in describing the in situ state of the rock mass, which is an important task to be conducted before the practical implementation of any project involving rock mechanics problems. The present paper presents the results of an investigation conducted to estimate the box-counting fractal dimension using 3D stochastic simulations of discontinuity networks generated by the SIMBLOC program. The first part presents the adopted algorithm whereas the second part is devoted to the application of the algorithm to three quarry sites: Klinthagen (Sweden), Eibenstein (Austria) and El Alto (Spain). The computed fractal dimension parameter is used as a measure of the irregularity of the discontinuity network at each site and to assess the differences between the three rock masses with regards to the discontinuity network density and joint size distribution. [ABSTRACT FROM AUTHOR]

The current research scrutinizes the effects of axial loading width and immediate roof thickness on the failure mechanism of a notched roof in room and pillar mining by experimental test and numerical simulation. Gypsum specimens with dimensions of 150 mm × 150 mm × 50 mm containing two rooms and a middle immediate roof (with and without the presence of an edge notch) were prepared for this purpose. The immediate roof has three different thicknesses, i.e., 30 mm, 45 mm, and 60 mm. The length of the edge notches was 1 cm, and their angles were 45°, 90°, and 135°. The upper room was loaded under axial loading with a width of 50 mm. A total of 12 physical specimens were tested. Concurrent with experimental tests, 36 numerical models were simulated. The model geometry was similar to the experimental one, but the upper room was loaded under three different axial loadings, i.e., 10 mm, 30 mm, and 50 mm. The results show that the notch angle, roof thickness, and loading width affect the failure pattern and compressive strength of models. The immediate-edged roof with an angle of 45 has a critical condition compared to other configurations. Also, models under a loading width of 50 mm have less compressive strength compared to other loading widths. The experimental test and numerical simulation show similar outcomes. Highlights: The effects of axial loading width and immediate roof thickness on the failure mechanism of a notched room by experimental test and numerical simulation are investigated. Gypsum specimens with dimensions of 150 mm × 150 mm × 50 mm containing two rooms and a middle immediate roof (with and without the presence of an edge notch) were prepared. The results show that the notch angle, roof thickness, and loading width affect the failure pattern and compressive strength of models. The immediate-edged roof with an angle of 45 has a critical condition compared to other configurations. [ABSTRACT FROM AUTHOR]

Due to the challenge of measuring rock bridges in the field and the negligence of progressive damage and changes in stresses within a rock mass when defining rock bridges, it is questionable to evaluate mechanical properties of rock bridges using only geometric parameters. A demonstration is the scale effects of rock bridges, because the same geometric parameter may refer to different sizes and numbers of rock bridges, leading to erroneous equivalent rock mass responses. In this context, in-plane rock bridges in rock slope engineering were equivalent to rock bridges subjected to direct shear by conducting numerical simulations employing the Universal Distinct Element Code (UDEC) and described by constant geometric parameters, i.e., joint persistence, while the sizes and numbers of rock bridges were variant. In this way, the scale effects of rock bridges were investigated from the perspective of load–displacement curves, stress and displacement fields, crack propagations and AE characterizations. The results revealed that the mechanical properties of rock bridges deteriorated with decreasing scales. More specifically, the shear resistance and the area and value of stress concentration decreased with decreasing scale. Furthermore, an uneven distribution of displacement fields in an arc manner moving and degrading away from the load was observed, indicating the sequential failure of multiple rock bridges. It was also found that the propagation of tensile wing cracks was insensitive to scale, while the asperity of macro shear fracture mainly formed by secondary cracks decreased with decreasing scale. In addition, increasing the dispersion of rock bridges would overlap the failure precursors identified by intense AE activities. Based on the abovementioned results, the scale effects of rock bridges were characterized using existing rock bridge potential (RBP) index and degree of persistence (DoP) index. Interestingly, a scale threshold to possibly identify a rock bridge was found. Highlights: The mechanical properties of rock bridges deteriorated with decreasing scales. The scale effects of rock bridges were appropriately characterized using existing indices. A possible scale threshold to identify a rock bridge was found. [ABSTRACT FROM AUTHOR]