查看更多>>摘要：? 2022 Elsevier LtdFoliation is a ubiquitous, sheet-like planar structure formed by differential pressure during the regional metamorphism history. Foliation causes metamorphic rocks to exhibit high anisotropy and heterogeneity, which strongly influence the mechanical properties. This study samples three types of slate and two types of schist to investigate the mechanical properties of foliation. A series of pull-off tests and direct shear tests are conducted on the foliations to obtain the tensile and shear strengths, and a failure criterion for foliation is proposed. The proposed foliation failure criterion exhibits a nonlinear trend in the low normal stress range, considers both tensile and shear strengths, and has three material parameters. The proposed criterion is extended to establish a failure criterion for foliated metamorphic rocks, considering various types of rocks under different orientation angles and confining pressures. For the foliated metamorphic rock failure criterion, a total of eight material parameters are required, which can be determined using the foliation pull-off tests, direct shear tests, and triaxial tests. The results show that the proposed criterion can accurately describe the failure envelope of a single foliation on various anisotropic rock, and its application can reasonably predict the strength of metamorphic rock under various conditions.
查看更多>>摘要：? 2022 Elsevier LtdIntersecting discontinuities are often encountered in rock engineering and sometimes associated with damaging geohazards. Our understanding of intersecting discontinuities instability remains vastly insufficient due to difficulties in comprehensively monitoring the failure process. Here we use microseismic (MS) monitoring to virtualize the MS events in the rock masses surrounding a powerhouse crown and investigate the effect of geological features on the occurrence of MS events. We subsequently build a three-dimensional numerical model and validate this model using the in-situ measurements by multipoint displacement meters. The numerical results demonstrate how the displacements of surrounding rock masses near the fault and the dyke increase and reveal possible causes, such as stress condition as well as geometry and orientation of rock discontinuities. We also discuss the correlation between the tempo-spatial distribution of MS events and the failure pattern of rock masses and confirm the weakened dyke as the main cause of the rock collapse. This study highlights that the stability of intersecting discontinuities can be controlled by both the geometrical and mechanical properties of individual discontinuities, and attentions should be paid to key properties favorable for rock instability.
查看更多>>摘要：? 2022 Elsevier LtdTo investigate frictional properties of the rupture surface of the carbonate Jiweishan rock avalanche, we conducted high-velocity-friction tests on the limestone and shale samples at different shear rates. The samples showed shear- and rate-strengthening of friction at low shear rates, but transformed to shear- and rate-weakening of friction at large shear rates. The critical shear rate was deduced to be between 0.05 and 0.2 m/s in our tests. The friction coefficient reduced to an extremely low value of less than 0.1 at a shear rate of 2.1 m/s. The sliding rock mass therefore could slide out of the rupture surface at a high velocity and consequently traveled a long distance as granular debris. Two mechanisms may have reduced the frictional resistance. Recrystallized calcite nanograins and CaO nanograins produced through calcite decomposition covered and lubricated the shear surface. Furthermore, the CO2 emitted through calcite decomposition reduced the effective normal stress and therefore the friction on the surface. Our conclusions for Jiweishan rock avalanche are also useful for understanding the high mobility of other widely distributed carbonate rock avalanches.
查看更多>>摘要：? 2022 Elsevier LtdThere are many studies on rock-breaking efficiency of TBM's disc cutters on homogeneous rocks; however, there are few studies on the cutter's rock-breaking efficiency on composite rocks. Specifically, research on the breaking efficiency of cutters on composite rocks with substantial differences in strength and composite ratios has not yet been systematically reported. The aim of this study was to numerically simulate the rock-breaking process of the TBM disc cutters to investigate the performance of the cutters on composite rocks. The underlying numerical simulation was validated by our full-scale laboratory tests. A series of numerical simulations were shown to lead to an accurate determination of the disc cutters performance on composite rocks of sandstone and granite in terms of the normal force, torque and specific energy. This indicates that there are some similarities between homogeneous and composite rocks, e.g., both show an optimal ratio of cutter spacing, s, to penetration depth, p, i.e., optimal s/p. Nevertheless, there are many more significant differences demonstrated in our study, including the composite ratio-dependent different ranges of the cutter force, torque and specific energy, changes of the optimum s/p, and the characteristic that s/p decreases linearly with the increase of the composite ratio. Although the optimal s/p is different for different composite ratios, it is possible to obtain a representative optimal s/p, e.g., 14, in our case, i.e. composite rocks of sandstone and granite, which may approximately optimise the composite rock-breaking, irrespective of different composite ratios. The paper also suggests that it is possible to study the rock-breaking efficiency of the disc cutters on composite rocks with appropriate consideration of composite UCS.
查看更多>>摘要：? 2022This study reveals the important role of hydromechanical responses of large hydraulic fractures in the cause and mechanisms of induced seismicity associated with the Mw 5.5 earthquake at the Pohang geothermal site, South Korea. Fractures intersecting two stimulation wells, PX-1 and PX-2, were identified through a comprehensive analysis of various data produced from drilling and hydraulic stimulations, including injection flow-pressure and seismicity data. Coupled hydromechanical numerical modeling of five stimulations performed in the PX-1 and PX-2 wells, showed that injection-induced hydraulic jacking and fracture shearing induce immediate stress transfer that plays a significant role in understanding seismic response at the Mw 5.5 fault. The friction coefficient times fluid pressure change by the PX-2 stimulations accounts for only 20% of the total CFS change at the location of the Mw 5.5 earthquake. The absence of seismic events at the Mw 5.5 fault by the PX-1 stimulations is attributed to the transfer of reduced shear stress at the PX-1 fracture due to shear slip. Kaiser effects in the seismic response observed during the PX-1 stimulation is explained by the irreversible nature of shear slip of the PX-1 fracture for the first time. In addition, post-injection seismicity, observed during the PX-2 stimulations, is attributed to a low permeability of the PX-2 fracture. The current modeling study suggests that coupled hydromechanical analysis with appropriate consideration of fractures is a powerful approach in understanding the mechanism of injection-induced seismicity in fractured geological media.
查看更多>>摘要：? 2022 Elsevier LtdGround surface uplift was detected during longwall mining in the Lazy Mine, a hard coal mine in the Czech part of the Upper Silesian Coal Basin, Czech Republic. The greatest uplift was 218 mm, based on surface point levelling by comparing surface points with heights of non-mined areas. This uplift occurred during subcritical mining at an average depth of 740 m below the surface. The mined coal seam is situated below the previously mined part of the multi-seam deposit. This study aims to provide a geomechanical explanation for ground surface uplift observed during longwall mining. The source of surface uplifts were deep inside the rock mass, therefore, enabling only the examination of geological, geomechanical, seismic, and technical mining data for uplift occurrence analysis. Therefore, the spatiotemporal evaluation of the observed vertical displacement and induced seismicity data was processed, and the observed ground surface was compared with empirical subsidence. Two time periods with surface uplifts during longwall mining were evaluated. Thick, competent rock layers, together with specific geomechanical conditions, were identified as the main characteristics responsible for the uplift.
查看更多>>摘要：? 2022 Elsevier LtdThe salts contained in water can gradually accumulate in pore networks of rocks under wetting-drying cycles, which could cause rock deterioration. The salt-induced damage under this cyclic process can result in dynamically evolving mechanical and hydraulic properties of the rock, which is relevant to many rock-related engineering activities and geological hazards. In this study, the structure damage and permeability evolution of Three Gorges Reservoir (TGR) sandstone subjected to wetting-drying cycles with salt solution were investigated by a series of multiscale experiments. The results showed that the salt solution has a significant impact on the physical properties of TGR sandstone during the wetting-drying process. With increasing wetting-drying cycles, two typical macroscopic damage modes in form of surface peeling and internal cracking were observed. Various physical parameters indicated that the internal structure of the TGR sandstone experienced two stages: 1) Salt accumulation and new crack formation stage and 2) Crack propagation stage. This variation also profoundly affected the permeability of the TGR sandstone. Specifically, the permeability of TGR sandstone decreased slightly at first due to blocked seepage channels by crystals before cycle 5, followed by a slow growth until the 15th cycle and a rapid increase after the 15th cycle. Cubic and exponential functions showed good performances to describe the evolution features of permeability with porosity and wetting-drying cycles, respectively. The deterioration mechanism was further elucidated by an evaporation test. This test indicated that the salt solution was more likely to crystallize in narrow pores and expectedly exert pressure enough to crack the pores, contributing to the structure damage feature and the permeability enhancement. These findings and results in this study deepen our understanding of salt-induced rock deterioration mechanism under the wetting-drying cycles and would be meaningful for rock engineering safety and geological disaster forecast in complex groundwater environments.
查看更多>>摘要：? 2022The shearing characteristics of rock joints govern the mechanical performance of discontinuous rock masses. The present study investigated the normal compression and shear behavior of rock joints through cyclic compression tests and direct shear tests. The irreversible relationship between the normal stress and the normal closure was confirmed by cyclic compression tests on rock joints. An elastoplastic model was proposed incorporating the aperture variation. A modified version of the critical state framework was developed for modeling the shearing and dilation behavior. Specifically, the initial state of the rock joints was determined according to the stress history. The proposed model for the mechanical behavior of rock joints was validated by predicting the experimental results. A parameter analysis was also performed to highlight the difference in the shear behavior of the rock joints due to the difference in the initial apertures.
查看更多>>摘要：? 2022Poroelasticity and gas slippage on the walls of nanopores are two major mechanisms that regulate shale permeability concurrent with hydrocarbons depletion. Nonlinear Klinkenberg plots reported in experiments indicate that gas slippage has a complex relationship with the variation of effective stress; however, a theoretical framework that explains the underlying physics is still missing. In the present contribution, a series of experiments were designed and conducted on intact Utica shale samples to study the simultaneous effects of gas slippage and poroelasticity under constant confining and constant effective stress conditions. In addition, an apparent permeability model was developed considering the effects of rock poroelasticity and a non-constant velocity gradient for gas flow at the pore wall. The model was calibrated and tested against the presented experimental results for Utica shale as well as previously published data. Results show that the concavity in Klinkenberg plots is caused by two separate parameters, each dominant within different ranges of pore pressures: second-order gas slippage and effective stress coefficient. The model demonstrates that gas slippage is inherently coupled with effective stress at low pore pressures. However, the evolution of apparent permeability with gas pressure deviates from the classical Klinkenberg theory only when the pore diameter is sufficiently small. When the effective stress coefficient is smaller than unity, the permeability evolution deviates from the linear Klinkenberg trend only at high pore pressures. In addition to the pore size, gas slippage is found to be closely correlated with the variation of gas viscosity with pressure and temperature.
查看更多>>摘要：? 2022 Elsevier LtdTo investigate the static mechanical properties and failure characteristics of damaged rocks after impact under realistic in-situ stress, dynamic triaxial compression tests with different strain rates and confining pressures were conducted on diorite by a modified split Hopkinson pressure bar. The damage factor was calculated according to the change of the longitudinal wave velocity of rocks before and after the impact, and the residual strengths of the damaged diorite were obtained by uniaxial compression and Brazilian tensile tests. In addition, three brittleness indexes based on the residual strength were adopted to evaluate the brittleness of damaged specimens, and fractal dimension was introduced to study the fragmentation characteristics. The results show that the dynamic peak stress, strain, elastic modulus, damage factor and plastic deformation of diorite all increase with increasing strain rate, i.e., rate dependence, and the increasing rate decreases with increasing confining pressure. The increment in confining pressure leads to the increase of dynamic peak stress, elastic modulus, static residual strength and brittleness, and the increasing rate increases with increasing strain rate, while other physio-mechanical parameters decrease with confining pressure. The post-peak behavior of dynamic stress-strain curves is divided into two classes according to the strain rate and fracture state of rocks under dynamic loading. Upon the strain rate increases above 87 s?1, the brittleness of damaged specimens generally decreases to moderately brittle or even low brittle, and the corresponding drillability becomes much easier. The fractal dimension of damaged specimens increases with the increase of confining pressure and decreases linearly with the increase of strain rate.
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