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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]

During underground mining excavation, dynamic disturbances (e.g., blasting) may accelerate rock creeping and induce unstable failure. It is crucial to properly quantify the impact of dynamic disturbances on creeping behavior to understand the long-term effects of underground excavation on rock media. A damage-based creep model was established in this study and implemented in Fast Lagrangian Analysis of Continua in 3D (FLAC3D) to quantify the damage and unstable failure of creeping rock triggered by dynamic disturbance. This model reveals the mechanical behavior of rock under creep-impact loading. It was verified by calculating the viscoelastic deformation around circular excavation, by reproducing the damage process under static loading, and by simulating the rock damage and failure under creep-impact loading. The model was then used to examine the mining-induced creep deformation around an operational shaft in Xincheng Gold Mine, China, where both the mining-induced creep of the country rock mass and dynamic disturbance of blast-induced vibration were considered. Numerical simulation indicated that the dynamic disturbance could accelerate the damage of rock under creep stress, which may be an important factor to trigger the accelerating creep of rock, even though the unstable failure of rock lagged behind the dynamic disturbance. The case study on the shaft stability analysis indicated that the dynamic disturbance could accelerate the time-dependent deformation of the shaft, the first dynamic disturbance induced much more shaft deformation, while in long-term creep (e.g., 5 years), the effect of blasting vibration was not significant. The trend and magnitude of shaft deformation agreed with the in situ measurements, which indicates that the numerical simulation could be used to predict the long-term shaft deformation in future. [ABSTRACT FROM AUTHOR]

Numerical simulation is a commonly used method for investigating rock failure. However, the numerical model is usually insufficient to predict real rock damage and failure because of rock microstructural heterogeneity. In fact, rock damage can be quantified using acoustic emission (AE) data. The aim of this study is to simulate and predict the failure of Brazilian and uniaxial compression specimens using AE data recorded during experiments. An AE data-driven model, in which cracks are assumed to be tensile in nature, is developed. AE data recorded from the test start up to a fraction of the peak stress (e.g., 20%, 40%, and 60%) are input into the data-driven model to predict the evolution of failure pattern beyond that stress level up to failure. First, we quantified stress-induced rock damage with AE data based on the tensile model. The results indicate that most of damage source radii are less than one millimeter, and the corresponding damage degree is close to one. Then, the inversed damage is input as the initial conditions for the numerical simulation to predict the future damage and failure of rock. With the increase of damage elements driven by AE data, the inversed damage zone develops from diffuse to localized, and the dominant factor for rock failure transits from microstructural heterogeneity into stress-induced rock damage. The damage and failure pattern of rock is well predicted when sufficient AE data are taken into account as known conditions. [ABSTRACT FROM AUTHOR]

Abstract The reliable prediction of excavation responses for squeezing rock remains a challenging issue in rock engineering. This paper presents a numerical procedure for a strain-softening rock mass around highly deformed circular opening based on the finite strain theory, so the overall elasto-plastic analysis is a Lagrangian method which focuses on the motion of each material point and is well suited for the problems involving large deformation. The influence of axial in-situ stress is considered by introducing the initial axial stress into the elastic constitutive equations, and the axial plastic flow is realized by redefining the plastic internal variable and considering different stress distributions in the plastic zone. Thus, the numerical procedure enables prediction of the rock deformation and squeezing potential in a more rational and rigorous manner. The capability of the method for estimating the small strain and squeezing deformation is verified by being compared with the existing analytical results and the field measuring data. Negligence of the axial in-situ stress or the axial plastic flow underestimates the rock deformation, and the induced deviation essentially depends on the post-peak constitutive relation and the rock heterogeneity after rock yields. The higher level of the material homogeneity in the plastic zone, the lower effects of the axial in-situ stress on the rock responses. The stress distribution and strain-softening behavior of surrounding rock can be significantly affected by the axial in-situ stress and the critical plastic internal variable η *. Tunnel instability is independent of large deformation for the high η * but is closely associated with the squeezing problem when η * is small. As η * decreases, both of the plastic radius and wall convergence increase initially and remain constant afterwards. Lastly, the paper investigates the rock responses and ground-support interaction when the small strain theory is applied on the squeezing rock. [ABSTRACT FROM AUTHOR]

Conventional water-based fracturing treatments may not work well for many shale gas reservoirs. This is due to the fact that shale gas formations are much more sensitive to water because of the significant capillary effects and the potentially high contents of swelling clay, each of which may result in the impairment of productivity. As an alternative to water-based fluids, gaseous stimulants not only avoid this potential impairment in productivity, but also conserve water as a resource and may sequester greenhouse gases underground. However, experimental observations have shown that different fracturing fluids yield variations in the induced fracture. During the hydraulic fracturing process, fracturing fluids will penetrate into the borehole wall, and the evolution of the fracture(s) then results from the coupled phenomena of fluid flow, solid deformation and damage. To represent this, coupled models of rock damage mechanics and fluid flow for both slightly compressible fluids and CO are presented. We investigate the fracturing processes driven by pressurization of three kinds of fluids: water, viscous oil and supercritical CO. Simulation results indicate that SC-CO-based fracturing indeed has a lower breakdown pressure, as observed in experiments, and may develop fractures with greater complexity than those developed with water-based and oil-based fracturing. We explore the relation between the breakdown pressure to both the dynamic viscosity and the interfacial tension of the fracturing fluids. Modeling demonstrates an increase in the breakdown pressure with an increase both in the dynamic viscosity and in the interfacial tension, consistent with experimental observations. [ABSTRACT FROM AUTHOR]

Time-dependent rheological deformation of rocks affects the stability of underground stopes or constructions. It may also be altered by dynamic disturbances, such as rock blasting. To study such effects, a new stress relaxation-disturbance testing machine was invented, capable of loading conditions for stress relaxation and dynamic disturbance. Effects of testing machine stiffness on rock deformation behavior were examined to confirm that rocks had undergone stress relaxation. Stress relaxation tests on specimens under uniaxial compression were carried out over 6 or more days. Under single-stage stress relaxation, axial stress relaxed within 5-7 days, stabilizing at constant strain. During two-stage stress relaxation, larger stress decay was observed under the higher strain level. A dynamic disturbance from a pendulum hammer was applied to specimens under stress relaxation to evaluate their response. In these tests, stress decline and strain increase were related to residual strain induced by the dynamic disturbance. The strain variation in specimens was found to be within 1.5 % of values before the disturbance. Finally, a damage-based constitutive model for rocks subjected to stress relaxation and dynamic disturbance is proposed. It reproduces the stress relaxation behavior of rock observed in our experiments. The model was used to quantify stress relaxation of rocks and the effects of dynamic disturbance on this process. The larger stress decay of rocks under higher strain is likely related to damage in the rock. Clearly, increases in strain and decreases in stress induced by dynamic loading, coupled with rock damage, are over-estimated in our numerical simulations. [ABSTRACT FROM AUTHOR]

As jointed rocks consist of joints embedded within intact rock blocks, the presence and geometrical fabric of joints have a great influence on the mechanical behavior of rock. With consideration of the actual spatial shape of joints, a numerical model is proposed to investigate the fracture evolution mechanism of jointed rocks. In the proposed model, computerized tomography (CT) scanning is first used to capture the microstructure of a jointed sandstone specimen, which is artificially fabricated by loading the intact sample until the residual strength, and then digital image processing (DIP) techniques are applied to characterize the geometrical fabric of joints from the CT images. A simple vectorization method is used to convert the microstructure based on a cross-sectional image into a layer of 3-D vectorized microstructure and the overall 3-D model of the jointed sandstone including the real spatial shape of the joints is established by stacking the layers in a specific sequence. The 3-D model is then integrated into a well-established code [three-dimensional Rock Failure Process Analysis, (RFPA3D)]. Using the proposed model, a uniaxial compression test of the jointed sandstone is simulated. The results show that the presence of joints can produce tensile stress zones surrounding them, which result in the fracture of jointed rocks under a relatively small external load. In addition, the spatial shape of the joints has a great influence on the fracture process of jointed rocks. [ABSTRACT FROM AUTHOR]

During underground mining, rock deformation may be time-dependent and is reflected in rheological behavior. These changes may be also accelerated by dynamic disturbances, such as rock blasting, which can trigger mining-induced hazards such as rockburst and a falling roof. A combined double-rock sample was analyzed to clarify the rockburst mechanism resulting from interaction between roof and pillar. In this work, a double-rock sample was subjected to a combined creep and dynamic loading condition. Based on the results, an ideal mechanical model was developed to describe the interaction between double rock samples loaded in series to simulate interaction between the pillar and roof. To test the model, a double-rock sample composed of granite and sandstone was loaded in series under creep and dynamic loading and the effect of dynamic disturbance on the creep process was quantified. Under the same creep stress, a higher impact dynamic disturbance resulted in shorter failure time. Under the same dynamic disturbance, the specimen failed faster with increased strain energy of rebound of the granite sample under increasing creep stress. The energy dissipation rate and energy transfer efficiency are defined to quantify the energy dissipation of double-rock sample and energy transfer between strong and weak sample. Based on the quantification of energy transfer efficient, it confirms that the ratio of stiffness of the double-rock sample should be a indicator for the energy transfer between strong and weak samples. The granite sample had higher uniaxial compressive strength, so it did not fail during the loading process. For the plaster-granite sample, the granite released most energy to accelerate the unstable failure of plaster. The granite-granite sample failed simultaneously and released most strain energy violently. The results can provide a reference to evaluate the delayed rockburst of rock subjected to creep stress. [ABSTRACT FROM AUTHOR]

Rock is a typical porous and heterogeneous geological medium, and water imbibition has a great influence on its mechanical properties. The degree of water imbibition in rock heavily depends on the internal fissures of rock and the soaking time. Here, sandstone specimens were soaked in a red ink solution for different amounts of time, and then cross sections were photographed to obtain RGB images of the water distribution in the rock. Next, the correlation between the water distribution in the rock specimens and the RGB values of the images was established to characterize the water distribution in the sandstone specimens. Finally, the water imbibition in rock was quantified with the diffusion equation, which was applied to determine the diffusivity of rock when the quantitated water distribution was matched to the observed wetted ring in the laboratory. This study quantifies the evolution of unsaturated moisture distribution in sandstone with soaking time and the variation of uniaxial compression strength as it is affected by the water distribution. The results from this study can be used as the scientific bases to study the rock weakening mechanism in a water environment. [ABSTRACT FROM AUTHOR]

When underground openings are excavated in weak rocks under a high overburden, large deformation and squeezing problems may take place. This paper generalizes the finite strain theoretical solution of Vrakas and Anagnostou (2014) for the ground squeezing response to elastic-brittle-plastic deformed rocks subjected to both the in-plane hydrostatic stress and the arbitrary axial in-situ stress. The ground behavior is analyzed based on the motion of material points in deformed configuration, thus the geometric non-linearity of rock mass is considered. Three different types of the plastic region can develop which depends on the magnitude of the axial in-situ stress, and the effects of axial in-situ stress and axial plastic flow on the squeezing response of rock mass are clarified in the case of large deformation. Comparisons among the large strain formula, the small strain theory and the numerical simulation results are made by using some examples, and the applicability of small strain method and some simplified models neglecting the axial in-situ stress is investigated. Finally, an engineering application in the severe squeezing Yacambú-Quibor tunnel in Venezuela is introduced to show the validity of the present large strain method in practical engineering. It is therefore very helpful from a practical standpoint for the preliminary design of cavities in squeezing rock mass, where the large deformation may exist and the classical small strain elasto-plastic analyses are inappropriate. [ABSTRACT FROM AUTHOR]

In problems involving large deformation, one of main concerns in rock engineering is the occurrence of a heavily reduction in tunnel section and even shield jamming. In this paper, a rigorous theoretical model is established to characterize the process of tunnel re-profiling works in heavily squeezing ground to re-construct the desired profile, based on the finite strain approach and Lagrangian material description. The post-peak behavior of rock mass and elasto-plastic strains in plastic zone during each excavation process are taken into account. Therefore, the proposed method enables to quantify the repetitive disturbances to a plastified surrounding rock and accumulated degradation of material properties. A numerical procedure is presented, in which the history-dependent behavior and path of material point in strain-softening rock mass during the entire excavation process are determined via our previous work, in combination with incremental algorithm and spatial interpolation. The discrepancy from the accurate solution induced by the negligence of elastic strains in all plastic zones is investigated by comparing the present results with the existing study. An extensive computations are then carried out to clarify some practical questions, including the effects of post-peak behavior, ground condition and old temporary support on the repetitive re-profiling, the variation of rock pressure developing on a rigid support after excavation, and the required minimum over-excavation size to achieve the desired profile. Also the effectiveness of advancing a pilot tunnel is investigated under different post-peak and geometrical conditions, in order to illustrate the applicability of the construction method in squeezing ground. [ABSTRACT FROM AUTHOR]