查看更多>>摘要：? 2021 Elsevier LtdThe mechanical response of Cobourg limestone is anisotropic, which stems from heterogeneity of its fabric. In this work, a new fabric-dependent approach is developed for the description of strength and deformation characteristics of this rock. The quantification of fabric is carried out by employing the basic principles of stereology and the specific fabric descriptor used is the so-called mean intercept length. The results demonstrate that the material may be perceived as transversely isotropic and the mechanical response is affected by the volume fraction of argillaceous partings. The latter are spatially variable, so that the notion of Representative Elementary Volume (REV) is not, in general, applicable. A numerical study is included involving simulation of triaxial tests conducted on differently oriented samples of Cobourg limestone.
查看更多>>摘要：? 2021Preconditioning techniques are frequently used to reduce the seismic hazard induced by mining under high-stress conditions. However, the impact of preconditioning on the seismic rock mass response has not been completely quantified. This study evaluates the impact of preconditioning using an Epidemic-Type Aftershock Sequence (ETAS) model. This stochastic model simulates a process in which each event produces its own offspring events. For the analysis, the mining-induced seismicity recorded during the construction of the New Mine Level Project (NML) tunnels at the El Teniente mine was used. The data was divided into five scenarios depending on the type of preconditioning applied. Next, the parameters of the ETAS model are estimated for each scenario and compared. The results show that both hydraulic fracturing (HF) and destress blasting (DB) increase the speed of decay of seismicity after blasts. However, only HF has a significant effect on reducing the seismic hazard and the background levels of seismicity. The α parameter of the ETAS model, which measures the efficiency of a given magnitude in producing its offspring, is proposed to quantify and classify the seismic hazard amongst the NML tunnels. Additionally, a residual analysis based on the difference between the recorded seismicity and that estimated by the ETAS model is proposed to evaluate the seismic rock mass response to mining in calibration and application. Auxiliary graphs to better understand the fluctuations of the residuals are also provided.
Aribowo A.G.Wildemans R.van de Wouw N.Detournay E....
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查看更多>>摘要：? 2021 The Author(s)This paper extends bit/rock interface laws for drag (PDC) bits, originally formulated for homogeneous rocks, to the transition between two rock layers with distinct mechanical properties. It formulates a set of relations between the weight-on-bit, the torque-on-bit, the depth-of-cut per bit revolution, and the engagement parameter of the bit in the lower rock layer. This model enables us to extend the 2D E?S diagram for the homogeneous case to a 3D E?S diagram for the transitional case, where the third dimension is related to the engagement parameter. Moreover, this model is used to derive an expression for the drilling efficiency for the transitional phase. Examples are provided for describing the 3D E?S diagram and drilling efficiency under the condition of quasi-stationary drilling (i.e., constant angular velocity, constant weight-on-bit). These examples show that the drilling efficiency depends nonlinearly on the bit engagement between the two rock layers. This intrinsic dependency is closely related to the bit shape.
查看更多>>摘要：? 2021Plasma blasting technology (PBT) is a potential alternative to chemical blasting and mechanical cutting methods for fragmentation of natural rocks, concrete, geopolymers, and other rocks-like materials. We present an analytical model of PBT addressing currently inadequate understanding of the dynamics of shock waves generation and propagation versus the electric energy release conditions. The proposed model describes the operation of the electrical discharge circuit, plasma channel initiation and expansion, and the generation and propagation of shock and pressure waves in the destructible solid. The dynamics of the power generator energy conversion into the plasma channel and into the wave of mechanical stresses in the solid are considered and the main factors determining the efficiency of the method, namely the pulse generator circuit parameters, exploding wire length, and shock wave-transmitting media, are evaluated. Solid fracture efficiency is shown to depend on the pressure pulse wave shape which, in turn, is determined by the rate of electrical energy deposition into the plasma channel. Increasing the exploding wire length leads to an earlier formation of the tensile tangential stresses and to their higher magnitude and thus facilitates material's fragmentation. The use of acoustically stiff media for shock wave transfer marginally improves material's fracture efficiency. Preliminary verification of the functionality of the model was carried out using commercial concretes, with good agreement between the analytically derived and experimentally obtained values. The results demonstrate that the proposed model allows to simulate PBT fracture over a wide range of instrumental and process conditions and can therefore be used for PBT process design, thus realising environmental and economic benefits through significant savings in time and experimental confirmation costs.
查看更多>>摘要：? 2021 Elsevier LtdAn elastic constitutive model is developed to predict the mechanical unloading response for high porosity sandstone. The resulting model is sufficiently complex to represent known mechanical response, yet reasonably straightforward to calibrate. A mechanical dataset from a suite of true triaxial tests on Castlegate sandstone is used to develop, calibrate, and validate the model, which incorporates the experimentally observed evolutions of the elastic bulk and shear moduli with stress and plastic strain. Furthermore, to properly represent the hydrostatic unloading response, a coupled term is required, in which the plastic volume strain (accumulated during both hydrostatic and deviatoric loading) modifies the nonlinear stress dependence of the bulk modulus. Omission of this coupling term causes significant over prediction of the plastic volume strain and bulk modulus after complete unloading. However, the coupled term is not required to model deviatoric response and shear modulus evolution. While a suite of 28 tests was used to develop the model, only one carefully controlled test is required to calibrate the model for a chosen high porosity sandstone.
查看更多>>摘要：? 2021 Elsevier LtdAccurately predicting the location of a fault in front of a tunnel face is an essential part of deep-buried tunnel excavation. This paper proposes a modified g-λ (0<λ<1) model with in situ stress to investigate the propagation and attenuation of a P-wave across jointed rock masses, including a single joint or multiple parallel joints. Based on the modified g-λ model, the differential expressions of the transmitted wave's particle velocity reflecting the interactions between the P-wave and the joints are deduced by using the characteristics method and the displacement discontinuity model. Parametric studies examining the in situ stress, the value of λ, the incident wave frequency and amplitude, and the number of joints are conducted to evaluate the application of the modified g-λ model. This provides support for predicting the location of the fault during the in situ stress tunnel construction when ignoring the shape and width of the fault. The results show that the effective wave velocity of the jointed rock masses increases with increasing in situ stress when a normally incident P-wave impinges upon a single joint. Considering multiple parallel joints, the magnitude and time of the maximum peak of the transmitted wave are decreases and delayed correspondingly with the increase in the number of joints, respectively. Moreover, multisource seismic interferometry is used to predict the location of the fault at different surrounding rock wave velocities and dip angles of the fault, which contribute to the identification of unfavorable geological bodies on the tunnel advanced geological predictions.
查看更多>>摘要：? 2021 Elsevier LtdUltra-high pressure water jets (UHP-WJ) rock breaking is a promising technology for developing unconventional natural gas reserves. In this study, rock-breaking experiments were conducted using typical reservoir rock samples such as sandstone, shale, and coal. The failure patterns in rocks after impact by UHP-WJ were analyzed. Moreover, the 3D damage field model of the rock was reconstructed based on CT scanning, and the internal crack distribution was visualized and examined. Furthermore, the fracture mechanisms of three types of reservoir rocks under the UHP-WJ impingement were studied. The results indicate that intergranular fractures produced due to shear stress are the dominant type of fracture in sandstones, tensile failure due to stress wave was the main type of fracture observed in shale, and coal exhibited the double breaking characteristics of stress wave effect and water wedge effect. The fragments size distribution of samples shows the largest fragments in shale, followed by coal, and the smallest fragments are observed in sandstone. Finally, the evolution of rock pore structure before and after the impact of UHP-WJ was monitored by nuclear magnetic resonance (NMR) methods. The results reveal that the micropores transform into mesopores and macropores in the sandstone, where the total porosity increased by 14.3%–24.8%. As the jet pressure exceeds 100 MPa, the porosity of shale increases significantly, such that the total porosity of the shale sample increased by 7.6%–37.1%. The mesopores transform into macropores and fractures with the influence of the water wedge effect in coal samples, resulting in the reduction of mesopores and the increase of macropores, where the total porosity of the coal sample increased by 14.6%–40%. The complexity and heterogeneity of pores were reduced under UHP-WJ with the increasing jet pressure. The findings provide a theoretical reference for optimizing the recovery from unconventional natural gas reservoirs.
查看更多>>摘要：? 2021 Elsevier LtdThis contribution presents an integrated and automated methodology for the computational analysis of permeability alterations in natural rocks under varying stress states, taking explicitly into account the complexity of the rock microstructure. The capacity of the methodology is highlighted on a subset of the microCT scans of a Vosges sandstone. After the generation of a high quality conformal mesh of the subset, isotropic compression at the scale of the microstructure is applied through FEM simulations. The adoption of non linear elastoplastic constitutive laws allows considering the local stress redistributions within the specimen. The mechanical loading of the subset highlights pore closures by local plastification. Permeability is evaluated at different confining pressures using the Lattice-Boltzmann method. Such a procedure allows analysing the impact of the pore space morphology (i.e. total porosity, pore size distribution, connectivity of the pore space, etc.) as well as the mechanical properties (i.e. stiffness and shear strength) on the evolution of the permeability under loading.
查看更多>>摘要：? 2021 Elsevier LtdThe design experience and the shallow tunnel theory, which emphasize the plane strain assumption, two-dimensional (2D) strength theory, and indoor or back analysis-based parameters of rock mass, should not be directly adopted for deep tunnel design. This paper reports the development of a digital acquisition technique to quickly, automatically, and accurately obtain the in-situ strength parameters (e.g., GSI and D), followed by the establishment of a three-dimensional (3D) numerical model for deep tunneling for a typical tunnel in western China to study the 3D and nonlinear spatial effects and mechanisms during excavation with incorporation of the 3D Hoek-Brown (HB) strength criterion. The reliability and correctness of the developed methods were verified by the field monitoring data, and the results of our study showed that: (1) The deformations based on 2D strength criteria (e.g., HB and MC) and 3D criterion (e.g. GZZ) are close for shallow tunnel, but significantly different for deep tunnel because of the widely distributed plastic zone in deep tunneling, thereby indicating the obvious effect of intermediate principal stress (σ2) on rock deformation; (2) The face extrusion deformation exhibits obvious nonlinear trends with an increasing buried depth, and the σ3 of the core rock reflects a tensile value in deep tunnel, thereby indicating the presence of an extremely unstable state. Reinforcement could increase the overall stress inside the core rock and reduce the stress unloading effect caused by the strong disturbances of deep excavations, thereby promoting a gradual change from tensile stress to compressive stress; (3) A uniform relationship was found between pre-extrusion and pre-convergence deformations, indicating that the pre-convergence deformation of the surrounding rock could be controlled by constraining the pre-extrusion deformation of the core rock. This digital technique and such 3D HB-based (e.g., GZZ) numerical modeling may be promising for conducting the 3D forward and dynamic analyses of tunnel stability. Additionally, the core rock extrusion control can be treated as an active method to render the rock itself a natural support material and to realize its strength potential, thereby making a valuable contribution to the construction of deep and ultra-deep rock tunnels.
查看更多>>摘要：? 2021 Elsevier LtdA novel method is presented that is capable of more accurately extracting rock surface features based on the geometrical characteristics of a 3D point cloud obtained from a survey. The core feature of the algorithm is a newly established method that can provide a robust estimation of point normals, while excluding statistical outliers from the calculation. A resilient multivariate mean and covariance estimator, termed Deterministic Multivariate Mean (DetMM) estimator, is used, which is capable of removing outlier effectively when calculating normals. To ensure the efficient operation of the estimator, a hybrid method is proposed where a simple and efficient method to estimate normal vectors is implemented first to filter the data before the robust DetMM method is applied for more accurate estimation of difficult cases. By using this strategy, the efficiency is significantly improved while the same level of accuracy is maintained. In addition, a novel segmentation algorithm, a region growing method, is introduced to handle complex geological features, which is capable of accurately detecting very small rock surfaces. The robustness and reliability of the developed method are compared with those of the well-known normal estimation method using principal component analysis (PCA). Finally, the method is validated on two case studies where the 3D point datasets were gathered from scanning two very different rock faces. The results have demonstrated that the proposed method outperforms significantly the conventional techniques in terms of accuracy with an acceptable increase in computation cost.
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