Saceanu, M. CristinaPaluszny, AdrianaZimmerman, Robert W.Ivars, Diego Mas...
19页
查看更多>>摘要：This study presents a three-dimensional numerical analysis of multiple fracture growth leading to spalling around nuclear waste deposition boreholes. Mechanical spalling due to stress amplification after drilling is simulated using a finite element-based fracture growth simulator. Fractures initiate in tension based on a damage criterion and grow by evaluating stress intensity factors at each fracture tip. Tip propagation is multi modal, resulting in final fracture patterns that are representative of both tensile and shear failure. Their geometries are represented by smooth parametric surfaces, which evolve during growth using lofting. The corresponding surface and volumetric meshes are updated at every growth step to accommodate the evolving fracture geometries. The numerical model is validated by comparing simulated fracture patterns against those observed in the AECL Underground Rock Laboratory Mine-By Experiment. It is subsequently calibrated to simulate fracture initiation and growth around boreholes drilled in the Forsmark granodiorite, subjected to a far-field anisotropic triaxial stress that corresponds to the in situ stress model from the Swedish Forsmark site. The deposition tunnel is implicitly simulated by attaching the deposition borehole to a free domain boundary.Several geomechanical cases are investigated, in which fracture growth is numerically evaluated as a function of in situ stress state, tunnel orientation, borehole geometry, total number of boreholes and borehole spacing. Numerical results show that spalling occurs in all cases, given the underground conditions at Forsmark, with borehole geometry, spacing and stresses affecting the extent of fracture nucleation and growth patterns.The uncertainty in underground stress conditions is evaluated through varying stress magnitudes and orientations relative to the tunnel floor. Whereas tunnel orientation influences the relative locations where fractures initiate with respect to the tunnel floor, fracture growth and its final extent is determined by the relative magnitudes of the in situ stresses. Higher stress differential causes higher spalling depths, but in all cases, failure extent is localised to the borehole vicinity, not exceeding one borehole radius. The cylindrical borehole is modified at the top to provide an access ramp for the spent fuel canisters and fracture growth around several deposition boreholes is simulated for borehole tops having cylindrical, conical, and wedge shapes. The enlargement of the borehole top induces higher stress concentrations at the borehole-tunnel junction, increasing the severity of spalling closer to the tunnel floor. Massive failure occurs when a multiple borehole model is considered and the inter-borehole distance is small enough that adjacent "spalled"areas interact. At Forsmark, through-going fractures are predicted to develop when the borehole spacing is less than 4 m. The effect of spalling on the structural integrity of the deposition boreholes is illustrated for each test case and quantified in terms of maximum spalling depth, spalling width and total fractured surface area.
查看更多>>摘要：The relationship between velocity, acceleration, and dynamic fracture toughness during rapid crack propagation is difficult to effectively capture. In this study, a novel semi-circular holed disc bend is proposed to achieve a long fracture path. Hopkinson pressure rods with crack growth and strain gauges are used to impact the semi-circular holed disc bend specimens and to monitor the cracking process of a mode-I fracture. The experiments show that the crack growth speed increases to a critical value after crack initiation and then oscillates as a result of the reflected stress wave. The dynamic propagation fracture toughness (K-IC(d)) increases with increasing crack velocity for steady crack propagation; however, this does not apply for unsteady propagation. An experimental-numerical-analytical method is developed to determine the dynamic stress intensity factor, which initially increases with time, then decreases, and ultimately increases again. The acceleration (v) can be used to investigate the unsteady crack propagation process. K-IC(d) values for v < 0 are greater than those for v > 0. An energy release rate formula related to v and the crack propagation velocity (v) is proposed and verified by the experiments. A thorough understanding of v, v, and K-IC(d) is important to analyze high-speed fault rupture processes.
查看更多>>摘要：The first-generation automatically formed roadway without coal pillars by roof cutting (AFRCPRC) technology has achieved significant technical and economic benefits in the period 2009-2019. However, it adopts shaped charge blasting for roof cutting, which is not only unsafe but also troublesome with respect to explosive approval. Besides, the installation procedure of macro-negative Poisson's ratio (NPR) anchor cable is rather complicated. To overcome these drawbacks, the second-generation AFRCPRC technology was developed in 2020. It employs a non-explosive blasting method, i.e., instantaneous expander, forming a single fracture (IESF), and thus its safety is greatly improved. In addition, micro-NPR anchor cables, which are easy to operate and have the characteristics of large elongation, high strength, strong impact resistance and no obvious necking phenomenon, are used for roadway support. In this paper, the basic principles, key techniques and main processes of the second-generation AFRCPRC technology are introduced, and experiments that were conducted in a coal mine are described. The experimental results show that IESF has excellent directional roof-cutting ability which can replace the explosive energy-focusing blasting, and the micro-NPR anchor cable boasts superior supporting performance which can effectively control roof deformation. The automatically formed roadway deforms insignificantly and satisfies the requirement of the next working face, illustrating the success of the second-generation AFRCPRC technology. Compared with the first-generation AFRCPRC technology, the remarkably improved second-generation one will have greater application potential in the mining field.
查看更多>>摘要：Investigations of the flow and diffusion mechanism of grout slurry in underground fractured rock mass are of critical significance to guarantee the grouting reinforcement of deep soft rock tunnel and to ensure the construction safety. This paper reports a series of permeation grouting tests in fractured sandstone samples based on the low-field nuclear magnetic resonance (LF-NMR) technique. During the grouting process, the NMR signal characteristics of the grout were monitored in real-time, and the parameters such as the slurry injection volume, effective grouting time and grout filling speed were comprehensively analyzed under various temperatures, confining pressures, flow velocities and numbers of fractures. The results show that the final injection amount of the grout decreases with increasing temperature and confining pressure. The effective grouting time is inversely proportional to the flow velocity, while it is positively related to the temperature. The filling speed decreases with increasing the temperature and confining pressure, while it increases with increasing the grout flow velocity. Compared to the single fractured sample, the sample with more fractures is injected with more grout slurry in a shorter effective grouting time and a higher filling speed. Under the conditions of high confining pressure (e.g., 15 MPa) or high flow velocity (e.g., & GE;5 ml/min), the grout slurry preferentially transports into the micropores, and then the mesopores and macropores. However, the temperature and number of fractures have equal effects on the grout flow and diffusion in the three types of pores. The grouting filling performance becomes worse when the temperature and confining pressure are higher. The findings in this study provide meaningful guidance for the grouting reinforcement of deep soft rock tunnel.
Tran, Nam HungDo, Duc phiVu, Minh NgocNguyen, Thi Thu Naga...
20页
查看更多>>摘要：The investigation of safe mud pressure window of horizontal wellbore drilled in the saturated rock by accounting for the combined effect of anisotropy and uncertainty is the main purpose of this work. To this aim, the deterministic solution of collapse and fracture initiation pressures are firstly presented in three cases that describe the behavior of wellbore: (1) immediately after drilling (i.e., undrained problem); (2) at long term due to the steady-state fluid flow (permeable boundary case) and (3) by neglecting the variation of initial pore pressure (i.e., impermeable boundary condition case). Based on these deterministic solutions, the key parameters of different sources of anisotropy (initial stress state, poro-elastic and strength properties of rock mass) are highlighted through sensitivity analysis. Then, the famous Monte Carlo Simulation (MCS) is undertaken to quantify the uncertainty effect on the probability of success of safe mud pressure window of the wellbore. We also present an adaptation of the Kriging metamodeling technique to study the stability of wellbore. In comparison with the referent solution of MCS, the Kriging metamodel provides a high accuracy and can be used as a performant tool for the probabilistic assessment of wellbore. The consideration of anisotropy combined with uncertainty, and the hydraulically boundary condition around wellbore in this work allows us to complete the contributions in the literature and confirm their strong effect on the design of wellbore.
查看更多>>摘要：To study the meso-damage evolution in coal under loading conditions, the industrial CT scanning tests under the uniaxial compression condition were carried out using an industrial CT scanner equipped with a loading control system. The gray level co-occurrence matrix (GLCM) theory was applied to quantitatively analyze the meso-damage evolution and the fracturing characteristics using the acquired CT images at each scanning stage. Four statistical features (i.e., contrast, energy, correlation, and homogeneity) of the CT images were extracted to evaluate the internal damage evolution in coal. The results show that the contrast generally took a changing trend of first decreasing and then increasing with the closure, initiation, and expansion of the fractures, that is, it decreased slowly at the compaction stage, slightly increased at the elasticity stage, and drastically increased at the post-peak stage. On the contrary, the energy, correlation, and homogeneity showed a changing trend opposite to that of the contrast. The changing tendency of mean values of the four statistical features could be depicted by the Boltzmann function, and in addition, the statistical features followed Gaussian distribution during the complete stress-strain process. The maximum damage cross-section of the coal sample could be transferred with the increase of deformation in the coal sample, indicating that the final failure location may not be the maximum initial damage location. The fracture ratio measured by the binarization and K-Means clustering algorithm methods typical showed three changing stages (i.e., a slow reduction process, a slight increase process, and a dramatic increase process) and could be used to reflect the meso-damage development process in coal at different deformation stages.
查看更多>>摘要：During the exploitation of deep resources, rock masses are under three-dimensional anisotropic stress conditions owing to tectonic conditions and engineering disturbances. Owing to the advantages of high heating efficiency and selective volume heating, microwaves have broad application prospects in fields such as unconventional low-permeability reservoir fracturing. Therefore, it is of great significance to study the mechanical behavior of rocks under true triaxial stress after microwave irradiation. In this study, true triaxial tests of sandstone under different microwave irradiation conditions were conducted. The results showed that the strength of the irradiated sand-stone first increased and then decreased. The strength of the irradiated sandstone was significantly lower than that of the unirradiated sandstone. The pre-peak brittleness of the sandstone gradually increased with an increase in sigma 2. The brittleness of the irradiated samples was lower than that of the unirradiated samples. For the post-peak brittleness index of the sandstone, with the increase of sigma 2, the microwave irradiated and unirradiated sandstone both gradually decreased first and then slowly increased. The residual strength of the unirradiated sandstone showed a trend of first increasing and then stabilizing, while the residual strength of the irradiated sample showed a greater dispersion and fluctuation. And, the residual strength of sandstone first increased and then decreased with the increase of microwave duration. In addition, a new double logarithmic linear strength model was proposed, which is in good agreement with the experimental results. The true triaxial stress induced anisotropic damage and deformation of sandstone are critical to rock fracture. The stress difference induced by water vapor expansion and mineral thermal expansion is the main factor for microwave breaking water-bearing sandstone. This work is of great significance to the research and application of microwave assisted breaking hard rock and fracturing deep low-permeability reservoir rocks.
查看更多>>摘要：The rock mass can be unstable due to the coupling effect of stress and seepage field, adversely affected by the geological field conditions. In this regard, this study implemented a 3D digital image correlation (3D-DIC) technique to conduct triaxial compression tests on sandstone samples under different seepage pressures. We analysed the evolution of strength and permeability during the rock failure process and space-time evolution of the surface radial strain. The results show that the strength and brittleness of the rock decrease with increasing seepage pressure. Further, the mechanical properties of the rock sensitive to were highly dependent on the seepage pressure. We also established a function model of permeability and axial strain consistent with the test results. By analysing the radial-strain-field of the rock surface at specific time points, we observed a gradual process of strain localisation instead of a sudden occurrence before failure. We extracted the spatial coordinates of the first 5% radial strain and subsequently conducted the spatial relation analysis by average-nearest-neighbour-analysis (ANNA). In addition, the approximate failure surface was fitted using the first 5% radial strain points in space, and the distance between the radial strain points and the fitted failure surface in space was analysed. This study shows that points with large strain values gradually converged towards the location of the fracture surface during the gradual failure process of the rock. Finally, this study provides a new method for predicting the failure zones of rock.
查看更多>>摘要：In recent years, supercritical carbon dioxide (SC-CO2) has been attempted in hydraulic fracturing in shale reservoirs as a new type of fracturing tool to overcome the drawback of traditional water-based fluids. Because of the unique physical and chemical properties of SC-CO2, its fracturing mechanism is more complicated than traditional fluids and still unclear at present. In this paper, we hydraulically fracture a shale reservoir rock using SC-CO2 and monitor the fracturing process using acoustic emission (AE) data. The results show the fractures stimulated by SC-CO2 composite of both shear and tensile fractures. In the initiation stage, SC-CO2 activates the natural fractures around the wellbore and induces shear fractures. In the propagation stage, SC-CO2 permeates the fracture tips quickly, results in a dynamic propagation process, and generates plenty of tensile fractures. The phase change of CO2 could be observed during the fracture propagation process, which is accompanied by a rapid pressure change and local stress shock formations in the fractures. Additionally, the experiments also demonstrate that the existence of bedding structures in shale could constrain the propagation of fractures, thus leading to a smaller volume of fracture network and limiting the complexity of the generated fractures. This research may help understand the fracturing mechanism of SC-CO2 and shed light on the development of hydraulic fracturing technology in shale reservoirs.
查看更多>>摘要：Multistage constant-amplitude-cyclic (MCAC) loading experiments were conducted on Tibet interbedded skarn to investigate and characterize fatigue mechanical behavior of the tested rock. Rock volumetric deformation, stiffness change, and fatigue damage evolution were analyzed along with the macroscopic failure morphology. The experimental results demonstrated that the volumetric deformation of the tested skarn was influenced by the interbed structure. Rock damage presented a two-stage pattern. The rock damage increased quickly at the beginning and subsequently became steady for long periods of time within a cyclic loading stage. A new damage evolution model was proposed on the basis of axial strain. Macroscopic failure morphology analysis revealed different fracture mechanisms, combining a tension-splitting mode, shear-sliding, and mixed shear-tension. In this study, the understanding of the anisotropic mechanical properties of interbedded skarn was highlighted, and this could contribute to the ability to predict the stabilities of rock engineering structures.
NSTL
Elsevier