查看更多>>摘要：The creep of shales affects the fracture conductivity, well production rate, and hence ultimate recovery of hydrocarbons from reservoir formations. This paper presents an experimental study of the creep characteristics of a shale softened by thermo-hydro-mechano-chemical (THMC) treatments that mimic the rock-fluid interactions affected by high temperature, high pressure, and hydraulic fracturing fluids. Microindentation was conducted to characterize the creep behavior of the THMC-treated specimens with the data analyzed by the Kelvin-Voigt (KV) and compliance methods, while X-ray diffraction and scanning electron microscopy were performed to analyze the compositional and microstructural changes respectively to aid interpretation of indentation measurements. Results show that the THMC treatments increase the creep rate by 46-162%, owing to the treatment-induced softening that varies with the treatment time and distance from the fluid-rock interface, while the Young's modulus and hardness decrease with both the THMC-treatment and creep durations. The softened zones become more viscous, as reflected by the decreased KV model's viscoelastic coefficients for the softened specimens. The higher creep rate of the softened zones is primarily attributed to the weakening in cementation bond and increase in microscale porosity due to the dissolution of carbonates. The findings help understand the mechanisms for creep deformation and pertinent proppant embedment in fractured shales, and hence can be used to predict shale gas production and optimize fracturing fluid additives and stimulants.
查看更多>>摘要：The cutting forces of a disc cutter are of great significance for advancements toward the design of rock-cutting tools. Studies often model rock-breaking through the 2D analysis of the cutterhead pressure or a linear cutting test of single-or double-disc cutters. However, neither approaches accurately reflect the actual cutting pattern. In this study, two models were constructed for the numerical simulation of continuous multi-cutter rotary rock cutting: one for the modelling of sandstone with soft-to-medium hardness, and the other for the numerical calculation of rotary cutting based on the actual cutting motion of the disc cutter. The results were compared with those of similarity tests conducted in a laboratory. The normal force required by the disc cutter for breaking intact rock surfaces was observed to be greater than that for breaking the broken rock surfaces. Unlike in linear cutting, in multi disc rotary rock-breaking, the cutter applies lateral compression to the rock on both sides, thereby promoting crack extension and penetration. As the mounting radius of the disc cutter increases, the normal and rolling forces, as well as the cutting coefficient of the rolling force, increase; the side force decreases. Moreover, a low thrust and high torque are maintained by the cutterhead.
查看更多>>摘要：This research adopts the four-dimensional lattice spring model (4D-LSM) to investigate the shear strength and failure mechanism of irregular rock joints and the influence of rock heterogeneity. The 4D-LSM has been used for a wide range of rock engineering applications, however its ability for simulating direct shear tests has not been investigated in detail. The material strength parameters were calibrated using uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) tests on granite specimens while the joint parameters were calibrated from direct shear tests on saw-cut UCS specimens and rock joints containing a single triangular asperity. From UCS tests, a calibration method is proposed to adjust the excessive machine deformation due to the high brittleness of granite, which is further applied to the direct shear test results. It was found that the strength of rock joints is strongly influenced by the intact rock properties. To investigate the influence of joint geometry and rock heterogeneity, three joint asperity angles and five heterogeneous models were simulated using the calibrated 4DLSM model. Simulation results were able to capture the influence of different joint asperity angles on the shear strength and failure mechanism as reported in previous experimental and numerical studies. With larger asperity angles, higher shear strengths were obtained. Meanwhile, rock heterogeneity represented by particles with lower strength will reduce the shear strength and alter the failure mechanism, especially at a high porosity. This strength reduction was observed to be greater for steep-angle asperities and becomes less significant for flatter asperities. Similar conclusions were also obtained based on the simulations of natural joint profiles. From these findings, it is suggested that the characterisation of rock heterogeneity is crucial to estimate the strength of rock joints. In addition to the roughness parameters, heterogeneity parameters such as porosity are fundamental for rock engineering applications.
查看更多>>摘要：The surrounding rock of tunnels may suffer from dynamic disturbances caused by impact loading or blasting at different locations nearby, leading to the instability of fractured rock masses. To investigate the mechanical behaviour of the surrounding rock subjected to various impact loading directions, an improved drop weight system is introduced for experimental study. To better understand the mechanisms in fractured rock mass, the extended peridynamic (XPD) method is adopted for auxiliary analysis. Both the numerical and experimental results consistently show that the initiation time, initiation toughness and fracture toughness (K-I and K-II) of precracks are notably affected by the loading directions. As the crack propagates towards the tunnel, the crack growth speed gradually decreases and is inversely proportional to the fracture toughness. The average errors between the test results and numerical results for crack fracture time, average propagation toughness and crack propagation velocity are 4.85%, 3.76% (K-I), 4.08% (K-II) and 6.79%, respectively, demonstrating the effective-ness of the XPD model in experimental prediction. Moreover, by using the XPD model, the distribution of deformation is analysed and the failure trend of tunnels on the shadow side can be identified. The precracks exhibit a tensile failure mode shortly after initiation, but then change to a mixed tensile-shear failure mode because of the influence of different loading directions. When the cracks break through the upper side of the tunnel, the crack initiation points of the shadow side are notably different for different loading directions, mainly occurring at the corner (E2), roof (A), spandrel (C2) and tunnel arc-side 45 & nbsp; (B2), providing a valuable reference for tunnel engineering.
Banerjee, Bikram PratapSingh, Sarvesh KumarLato, Matthew J.Sammut, Claude...
17页
查看更多>>摘要：Laser scanning is an efficient approach for collecting rock mass point cloud scans to characterise structural discontinuities. However, developing computationally efficient and robust analytical workflows remains an open research problem. Existing semi-automated and automated approaches rely on point normals which are prone to mapping error when high variability exists in the local-support region. This study proposes a new automated algorithm that uses the spatial distribution of points on discontinuities to capture unique signatures in the form of sinusoidal waves. The discontinuities are then effectively characterised by clustering the amplitude and phase profiles of the sinusoidal waves. The presented amplitude and phase decomposition (APD) approach requires minimal pre-processing. Moreover, it can be applied directly to raw point clouds as filtering is inherently included through the fast Fourier transform (FFT) based decomposition of the signals. The method was evaluated on an underground tunnel dataset with exposed structural discontinuity planes. The efficacy of the developed approach was tested against manual segmentation using virtual compass plugin in open-source software (Cloud Compare), semi-automated open-source (discontinuity set extractor) and proprietary (Maptek PointStudio) software, and other automated algorithmic approaches based on point normal clustering and region growing. The APD approach produced the least error in estimating mean discontinuity dip angle and dip direction which was & PLUSMN;1.15 & nbsp; and & PLUSMN;1.39 with a dispersion error of & PLUSMN;2.24 and & PLUSMN;1.54, respectively.
Zhang, Q. B.Aligholi, S.Ponson, L.Torabi, A. R....
12页
查看更多>>摘要：Measuring the intrinsic fracture properties of quasi-brittle materials like rocks is of great importance and at the same time a major issue for engineers. In this study, we explore the ability of the Theory of Critical Distances (TCD) to determine accurately both the tensile strength and fracture toughness. To this end, we conduct ring tests and semi-circular bend tests on four rock types including a red sandstone, a white coarse-grained marble, a finegrained granite and a coarse-grained granite. This selection covers sedimentary, metamorphic and igneous rock types with different grain sizes. The experimental data are analysed using a new methodology developed from the so-called Point Method (PM), a particular form of the TCD, from which we infer the intrinsic tensile strength and the fracture toughness of the studied rock materials. Our results are compared with those obtained from ISRM suggested methodology that is modified to take into account the finite notch root radius used in our experiments. The comparison is successful, supporting that the newly developed methodology is suitable to determine the intrinsic tensile strength and fracture toughness of rock materials.
查看更多>>摘要：Understanding the temperature-dependent shear behavior of rock joints is significant for the design, construction and maintenance of deep rock engineering. However, the shear characteristics of healed rock joints under high temperatures are still poorly understood. In this paper, to capture the thermal effect on the shear behavior of healed rock joints, a series of direct shear tests are performed with the aid of X-ray diffraction and scanning electron microscope techniques. The testing results demonstrate that the shear resistance of the healed joints is slightly lower than that of the intact rock, but much greater than that of clean joints. Compared with intact rocks, the ductile failure characteristics of the healed joints after high temperature thermal treatment are more sig-nificant. The peak shear strength, shear stiffness and dilation angle of healed joints are greatly influenced by thermal treatment temperature, while the ultimate shear strength is almost independent on with temperatures. The influence of thermal treatment on shear resistance is attributed to the physical and chemical changes. The thermal decomposition of the filling minerals at around 400 ?& nbsp;is believed to be the main cause of the change in thermal-induced shear behavior of the healed joints. The findings of this work are useful to understand the mechanical behavior of jointed rock mass under high temperatures.
查看更多>>摘要：In this work, a three-phase coupled numerical model is developed for rock fracturing by integrating the lattice Boltzmann method (LBM) and the distinct lattice spring model (DLSM). The multiphase flow is handled by using the LBM based on the Shan-Chen potential, whereas the rock fracturing is solved by using the DLSM with a cohesive-like model. During the calculation, the solid and fluid models are executed in their own timeline independently, and the coupling is realized from boundary exchange within each step. The influence of fracturing on the multiphase distribution is considered by using a phase update operation. The three-phase model is verified against a few multiphase benchmark problems. Then, its ability to model the fracturing of rock is demonstrated through a comparison with the hydraulic fracturing test. Following this, the influence phase distribution along cavities on the hydraulic fracturing path was investigated by using the coupled model. Our results show that the three-phase coupled model can be a useful numerical tool for handling rock fracturing problems involving multiphase fluids.
查看更多>>摘要：Pretension is the pre-added axial tensile force on fully grouted bolt systems, which include rock bolts and cable bolts. Although a series of studies via lab testing and analytical modelling proved the importance of pretension in cable bolts, there is no systematic analysis to guide the optimal pretension values to be applied on cable bolts in different engineering geological conditions. Complicated geological conditions, such as anisotropic in-situ stress, bedding plane detachment and country rock deformation, cannot be fully represented in the lab or analytical work. In addition, field scale numerical models to investigate cable bolt performance and its interaction with various geological conditions are rare. This paper analyses the effect of pretension on the performance of cable bolts in underground coal mines via field scale numerical models, with considering different geological conditions (in-situ stress, bedding plane stiffness and rock mass properties). Numerical models were developed based on a case study mine in Australia with challenging ground conditions. The in-situ stress, properties of claystone and bedding planes are considered critical variables to be analysed. The results show higher pretension contributes to higher force along the cable bolt. Neutral points, where shear force in grout materials is zero, are used to describe the pretension-induced stress redistribution on cable bolts. Pretension also has a positive effect on roof and bedding plane management, resulting in a more stable roof condition. In addition, high pretension and high in-situ stress are more likely to result in cable-grout (CG) interface failure since the relative vertical displacement difference between cable bolts and the surrounding rock mass is higher. Once CG interface failure is generated, the pretension effect turns out to be negative, leading to severe roof sagging and bedding plane aperture. This paper provides a guide to the application of pretension in coal mining.
Thongprapha, T.Tengpakwaen, K.Daemen, J. J. K.Fuenkajorn, K....
11页
查看更多>>摘要：Uniaxial and triaxial compression tests have been performed on prismatic specimens of rock salt under confining pressures (sigma(3)) up to 40 MPa. The specimens contain different bedding plane orientations with respect to the major axis. Results indicate that transverse isotropic effects occur under low confinement where the salt strength is lowest when the normal to bedding planes makes an angle (beta) of 60 with the major principal axis. The lowest intrinsic elastic modulus is obtained at beta = 0 & nbsp;, and the highest is at beta = 90 & nbsp;. The apparent elastic parameters measured for 0 & nbsp; < beta < 90 & nbsp; agree well with those predicted by Amadei's solutions. The confining pressures rapidly increase the elastic and shear moduli normal to bedding plane strike, and eventually they reach those parallel to the bedding planes under sigma(3) about 30 MPa. Loading the salt under high confinement induces some plastic deformation, as evidenced by the evolution of Ramberg and Osgood's parameters with the confining pressures. The confining pressures gradually tighten the inter-crystalline boundaries along bedding planes, and hence the applied differential stress can no longer recognize their transverse isotropic textures. The salt strengths for all bedding orientations become equal under sigma(3) above 30 MPa. The proposed exponential equation is capable of describing the salt dilations and strengths under unconfined to highly confined conditions. Distortional strain energy (W-d) induced at dilation linearly increases with mean strain energy (W-m). The W-d-W-m relation for beta = 60 & nbsp; can represent the dilation of the salt from low to high confinements where the mechanical responses of the rock transitionally change from transverse isotropic to isotropic behavior. The W-d-W-m relations for beta &NOTEQUexpressionL; 60 & nbsp; are applicable only for confining pressures less than 30 MPa.
NSTL
Elsevier