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The stability graph is an empirical approach to predicting stope performance based on the geometric dimensions of stopes and the geotechnical conditions of the surrounding rock mass. Stability graph is commonly used to design and estimate the stability of open-stope mines and underground openings. Most engineers calculate the rock stress factor by the stability graph method considering the maximum principal stress at the center of the critical face and the uniaxial compressive strength of the intact rock, whereas the spatial multidimensional stress state, compressive strength of the rock mass, and tensile strength of the rock mass are not taken into account. In this study, the maximum and the minimum principal stresses dominating the rock stress factor are introduced into the stability graph. The rock stress factor (A) is modified by the Mohr-Coulomb criterion with tensile cutoff. The modified stability graph method was validated by adopting Cavity Monitoring System (CMS) laser technology, and it was then successfully applied in a case study at the Alhada Lead-Zinc Mine in Inner Mongolia. Compared with Mitri's method, the proposed stability graph expands the scope of the narrow vein stope design dimensions. The modified rock stress factor in the stability graph provides a technical basis for the subsequent recovery of residual orebodies. [ABSTRACT FROM AUTHOR]

The AE energy radiated from rock microfractures is widely used to understand the rock failure process. However, the radiation energy of AE sources is usually inaccurately quantified because of imprecise knowledge of the radiation pattern associated with the tensile angle. Based on theoretical calculations, we quantify the radiation energy of the AE source more accurately by considering the influence of the tensile angle on the radiation pattern and then analyze the radiation energy evolution during the rock failure process. The energy radiation pattern and average energy radiation pattern coefficients of the P and S waves change significantly with the tensile angle. During the failure process of the granitic gneiss specimen, the radiation energy release of the rock specimen is characterized by a sudden intermittent increase in the time domain. The sudden increase is mainly due to the occurrence of large-energy AE sources rather than many low-energy AE sources in a short time. The main microfracture mechanism at the low stress level is shear compression; as the stress increases the main microfracture mechanism changes to shear. When the specimen is at failure, the shear microfractures account for > 70%. [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]

When translating digital roughness profiles of a closed rock joint along the shear direction, the overlapping translational areas can reasonably represent the main potential contact areas in shear tests. By translating Barton's ten typical roughness profiles, a series of statistical parameters, including the overlapping areas (S*), overlapping number (N*), cumulative overlapping projection length (Lsum∗), and the weighted cumulative overlapping projection length (Ssum∗), were utilized to quantify the 2D joint roughness. Based on the dimensional analysis, these parameters were combined to estimate the joint roughness coefficient. The correlation coefficient results show that the parameter combinations of S∗/L and N∗-1 or Ssum∗/STotal allow good estimation of the joint roughness coefficient of Barton's 2D roughness profiles. A conceptual joint model was used to extend the 2D translational overlapping statistical parameters to a 3D condition, revealing the suitability of the 3D translational overlapping volume parameter V∗/SL and the hourglass parameter to estimate the joint roughness coefficient of common rock joints. However, the overlapping number parameter N*−1 becomes unreliable and lacks practical significance for common rock joints because there are so many meaningless weak links among the overlapping volumes. Shear tests were conducted on 25 sandstone joints to verify the applicability of V∗/SL and Vsum∗/VTotal to estimate the joint roughness coefficient. The results indicated that these two parameters show good agreement with the experimentally back-calculated values, allowing the establishment of a new joint roughness coefficient fitting formula. [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]

Rocks with microdefects exhibit viscoelastic behavior during stress wave propagation. The viscosity coefficient of the wave can be used to characterize the attenuation as the wave propagates in rock. In this study, a long artificial bar with a readily adjustable viscosity coefficient was fabricated to investigate stress wave attenuation. The viscoelastic behavior of the artificial bar under dynamic loading was investigated, and the initial viscoelastic coefficient was obtained based on the amplitude attenuation of the incident harmonic wave. A one-dimensional wave propagation program was compiled to reproduce the time history of the stress wave measured during the experiments, and the program was well fitted to the Kelvin-Voigt model. The attenuation and dispersion of the stress wave in long artificial viscoelastic bars were quantified to accurately determine the viscoelastic coefficient. Finally, the method used to determine the viscoelastic coefficient of a long artificial bar based on the experiments and numerical simulations was extended to determine the viscoelastic coefficient of a short rock bar. This study provides a new method of determining the viscosity coefficient of rock. [ABSTRACT FROM AUTHOR]

During the excavation of underground opening, the rock may experience a complex loading path that includes the highly confined compression before excavation, unloading of confining stress and further disturbance of dynamic loading after excavation. By using Rock Failure Process Analysis for Dynamics (RFPA-Dynamics), the failure of rock sequentially subjected to this complex loading path is numerically simulated, in order to examine the rock failure mechanism induced by excavation. The RFPA-Dynamics is firstly used to reproduce the failure of rock under confined compression, followed by unloading of confining pressure, and it is validated against with the existing experimental observation. Then, the failure characteristics of rock specimen sequentially subjected to the quasi-static triaxial loading, unloading of confining pressure and dynamic disturbance are numerically simulated, where the effect of magnitude of axial loading and confining pressure, and duration and amplitude of the dynamic disturbance on the final failure patterns of rock are examined. The numerical results indicate that the arc-shaped spalling damage zone is prone to develop with the increase in the axial pressure and lateral pressure coefficient. As for the effect of dynamic disturbance, the contribution of duration and amplitude of dynamic disturbance on the energy input are similar, where the area of damage zone increases with the energy input into the rock specimen. In this regard, the area of the damage zone is influenced by both the magnitude of in situ stress and waveform of dynamic disturbance. This study denotes that it is of great significance to trace the complex loading path induced by excavation in order to capture the rock failure mechanism induced by underground excavation. [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]

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]