查看更多>>摘要：? 2022 The Author(s)Accurate predictions of room closure are important for hazardous waste repositories in rock salt formations, such as the Waste Isolation Pilot Plant (WIPP). When Munson and co-workers simulated several room closure experiments conducted at the WIPP during the 1980's and 1990's, their simulated closure curves closely agreed with the closure measurements. A careful review of their work, however, raised concerns and prompted the reinvestigation in this paper. To begin the reinvestigation, Munson's legacy Room D closure simulation was reasonably recreated in a current-day finite element code. Next, special care was taken to obtain numerically converged results, re-introduce the anhydrite strata intermittently ignored by Munson, and calibrate the Munson–Dawson (M–D) constitutive model for salt as much as possible from laboratory test measurements. When this new model was used to simulate Room D's closure, it under-predicted the horizontal and vertical closure rates by 2.34× and 3.10×, respectively, at 5.7 years after room excavation. As a result, the M–D model was extended to capture the newly established creep behavior at low equivalent stresses (<8MPa) and replace the Tresca with the Hosford equivalent stress. Simulations using the new M–D model over-predicted the horizontal closure rate by 1.15× and under-predicted the vertical closure rate by 1.08× at 5.7 years, averaged over three room closure experiments. Although further improvements could be made, the new model has a stronger scientific foundation than Munson's legacy model and appears ready for careful engineering use.
查看更多>>摘要：? 2021 Elsevier LtdLiquid nitrogen (LN2) fracturing, as an environmentally friendly waterless fracturing technology, is given more and more attention in coalbed methane (CBM) exploitation. In this paper, a series of microscopic tests and a thermal damage simulation were combined to evaluate the changes in the internal structure of the coal caused by the LN2 cooling, firstly. Then, Brazilian splitting tests were carried out on the bedding coal, and influences of cryogenic cooling fracturing and bedding orientation on mechanical properties and fracture morphology of the bedding coal were analyzed. The results show that the micro-structure of the coal is obviously damaged under the LN2 cooling due to the thermal tensile stress. The P-wave velocity, tensile strength, Brazilian splitting modulus, and brittleness index of the coal sample have significant anisotropic characteristics and their values are reduced after the LN2 cooling. The weakening degree in mechanical characteristics of the coal induced by the LN2 cooling is also closely related to the bedding direction of the coal. After the LN2 cooling treatment, the maximum and minimum reduction of the tensile strength is at the bedding angle 0° and 45°, respectively, with the reduction of 55.81% and 6.98%, respectively. The maximum and minimum changes of the Brazilian splitting modulus and brittleness index before and after the LN2 cooling locate at 0° and 60°, respectively. The LN2 cooling treatment can increase the ductility of the coal. The length and surface complexity of the induced fracture increase after the LN2 cooling. The increase in the fracture length is particularly evident at the high bedding angle, while the significant increase of the crack surface complexity is more likely to occur at the low bedding angle. The results contribute to the design of LN2 fracturing for coal beds.
查看更多>>摘要：? 2022 Elsevier LtdThis article proposes a breakable grain scheme and a method to achieve bi-modular elastic material behavior in grain-based models of rocks using the Hybrid Lattice/Discrete Element Method; a version of the Discrete Element Method that employs Rigid Body Spring Network interactions. First, the paper presents both contributions and two validation exercises regarding bi-modular elastic behavior. Subsequently, a set of comprehensive parametric analyses defines the impact that numerical input parameters have on macroscale results, including: crack initiation and damage stresses; unconfined compression and direct tensile strengths; and material's characteristic length. Of note, the analyses investigate the influence of heterogeneity, based on the application of the Weibull probability distribution to compressive and tensile Young moduli and inter and intragranular strength parameters. Afterwards, simulations based on three theoretical rock types show the capabilities of the Hybrid LDEM with the proposed contributions in representing typical macroscale properties of rocks observed in laboratory tests. Lastly, visual comparisons between grain breakage processes from numerical simulations and laboratory thin-section analyses indicate consistent correspondence, which evidences the potential application of the developed methods to future research focused on rocks’ microcracking phenomena. This work concludes that the proposed breakable grains and bi-modular material behavior schemes widen the range of macroscale properties that grain-based models can produce while preserving simplicity.
查看更多>>摘要：? 2022 Elsevier LtdA new constitutive model is developed in this paper for clay-rich rocks. This model describes elastic, instantaneous and time-dependent plastic deformations, and induced damage. The dependency of mechanical behavior of clayey rocks on water saturation degree is taken into account. Further, the structural anisotropy commonly exhibited by sedimentary rocks is also incorporated in the proposed model. Moreover, differently with most full phenomenological models, the impact of micro-structural composition on the macroscopic mechanical behavior is explicitly described by the presented model. More precisely, the macroscopic elastic properties and plastic criterion are determined by using analytical homogenization techniques. These properties explicitly depend on mineralogical compositions and porosity. The damage process is physically related to the interface debonding between clay matrix and mineral particles. The proposed model is applied to the Callovo-Oxfordian claystone investigated in the context of geological disposal of radioactive waste in France. Its efficiency is assessed through comparisons between numerical predictions and experimental data from laboratory tests under different loading paths and water saturation degrees.
查看更多>>摘要：? 2022 Elsevier LtdFine material migration is one of the main challenges in the block/panel caving method because its presence can result in mud rush, inrush events, and early dilution. Sources of fines in block caving mines can include previous exploitation levels, presence of weaker rock, and secondary rock fragmentation during ore draw, among others. Then, one of the main tasks of mining design and planning is to identify, avoid and/or control when and in what magnitude this finer material could be extracted at drawpoints. Studies have been done regarding fine material migration in mining; however, recent studies have challenged current understanding about the main variables that control its mechanisms. This research aims to clarify some of the main variables controlling fine material migration through laboratory experiments of a block cave column during flow, scaled 1:50. Here, the variables studied are the ratio between the size of coarse and fine material, the particle-size-distribution of coarse material, the type of draw, and amount and location of fine material. Experimental results show that the ratio between coarse and fine material size is one of the more relevant variables: when this ratio increases, fine material migration also increases. Additionally, the coarse particle-size distribution has an important influence on fine material migration. A higher coefficient of uniformity (d60/d10) will delay fine material migration. For example, when the coefficient of uniformity changed from 1.5 to 3.8, the amount of coarse material extracted before the fine material changed from 69% to 143%. Additionally, the presence of fine material does not appear related to changes in the extraction zone's final geometry as long as coarse material is drawn first. The flow experiments conducted quantify key variables that influence dilution entry into caving environments improving our understanding of the phenomenon of fine material migration for caving mines.
查看更多>>摘要：? 2022 Elsevier LtdThe resin anchoring process is a critical aspect of coal mine roadway support, but presently faces several serious challenges. For example, resin s cannot be properly broken and mixed, often leak, and their density and circumferential thickness values frequently deviate from the design values. This results in a reduced effective anchorage length and prevents the correct application of a high pretension force. To address these issues, this study presents a systematic analysis of the anchoring characteristics of roadway bolts including theoretical calculations, a detailed analysis on the influence of mixing density, mixing parameters, and pretension force on the anchoring performance, and a primary focus on the components and mechanism within synergistic anchoring technology. Numerical simulations are performed following the orthogonal experiment method to optimize and determine the synergistic component structural parameters, and laboratory tests are conducted to compare and analyze the modes of the combined synergistic component use. The working performance of the synergistic anchoring technology is comprehensively verified using field tests. The results show that the anchoring system's bearing strength, yield displacement, bearing time, and energy absorption capacity are greatly improved when using a combination of the synergistic components. Bolts applied in the field using the proposed synergistic anchoring technology produce significantly higher anchoring force and pretension moment values than ordinary bolts, exceed the engineering requirements, and produce a good overall anchoring effect. A high pretension force can be applied to enhance the support capacity, ensure the bolt resin anchor quality for coal mine roadways, and improve the roadway bearing capacity under static and dynamic loading, which have high practical significance and application value.
查看更多>>摘要：? 2022 Elsevier LtdA wellbore drilled in pre-stressed formations concentrates the far-field stresses. These concentrations often lead to borehole instability producing breakouts, drilling-induced fractures, or complete failure. In isotropic rock masses this problem has been extensively studied. However, fewer studies attempt to understand borehole failure in more realistic rocks characterized by both elastic and strength anisotropy. A program to calculate the failure pattern in elastic and strength anisotropic rocks is developed. The program combines a recent algorithm for calculating stress concentrations around a borehole within an anisotropic rock mass with anisotropic rock strength as applied through differing failure criteria. Shear failure in the rock matrix and the weak plane are examined based on Jaeger's single plane of weakness theory. Results from test models clearly indicate that failure in the weak plane complicates the failure pattern around the wellbore, resulting in that the breakout orientation cannot correctly represent the minimum horizontal stress orientation as commonly suggested for a vertical wellbore aligned with the principal stresses. The complexity of failure patterns due to formation anisotropy suggests that additional care must be taken when borehole image logs are interpreted for stress direction indication and stress magnitude constraint.
查看更多>>摘要：? 2022Grain-based models (GBM) are widely used to reproduce the heterogeneous characteristics of natural rocks at the grain size level. To incorporate the spatial distribution of the constituent minerals into the GBM, the digital image processing (DIP) technique and the random distribution of property (RDP) method are frequently adopted. However, an analysis of the differences, advantages and disadvantages of the named two popular methods in simulating the mechanical response of rocks under loading has so far not been undertaken. This study considers Brazilian and uniaxial compression tests using DIP and RDP models, showing distinct differences in stress-deformation behavior (especially tensile strength) and final failure patterns. Fracture initiation, accumulation, propagation, and the corresponding stress-deformation behavior of polycrystalline rocks are sensitive to the crystallographic spatial distribution and the micro-strength parameters of mineral contacts. The clustering of mineral grains tends to induce more significant local stresses while the stress fluctuations in RDP models are more widely distributed and isolated, leading to different microcracking behavior furtherly affected by the superimposed effect of heterogeneous contact properties. Therefore, it is essential to use DIP methods to model heterogeneous rocks if the grain-scale mechanical behavior is of interest. If a heterogeneous model is used to study the overall macro-mechanical behavior of rocks, using the RDP method may be appropriate with the advantage of reduced effort for model set-up.
查看更多>>摘要：? 2022 The Author(s)Although similarity simulation experiments have been widely used in geotechnical engineering, their application in soft rock mechanics is still limited due to the complex water–rock interaction of such rocks. The purpose of this study was to synthesise red mudstone–like materials by the method of mineral composition and proportion are completely similar to that of natural rock samples, using sodium silicate content, compression time and vertical stress to investigate the mechanical properties and composition of soft rocks. The physical and mechanical properties of the synthetic red mudstone–like materials were compared with those of natural red mudstone by the density, acoustic characteristics, and microstructure investigation as well as uniaxial and triaxial compression tests. The results show that the synthetic red mudstone–like materials can reproduce the physical and mechanical properties of natural red mudstone. They show that sodium silicate content is one of the most important factors governing mechanical properties, such as internal friction angle, of the synthetic red mudstone–like materials. Importantly, the synthesis method developed in this study could potentially be widely implemented in rock mechanics and engineering.
查看更多>>摘要：? 2022 Elsevier LtdEnhanced geothermal systems can provide a substantial share of the global energy demand. There exist several hurdles in the engineering implementations of such geothermal systems. One such hurdle is the accurate monitoring of the fracture networks created in subsurface through hydraulic stimulation of these systems. Micro seismicity associated with the stimulation is the primary means to locate the event hypocenters for estimating the stimulated rock volume. Existing methods for location the hypocenters are restricted to only the highest amplitude impulsive signals that are simultaneously detected on several sensors. Consequently, a large portion (usually ～99%) of the measurements are left unused. In this paper, an unsupervised manifold-approximation followed by clustering of 3-component accelerometer data is used to analyze the seismicity recorded on a monitoring well. With this method, a larger portion of the measured signal is used for the monitoring of the hydraulic fracture network. We analyze the EGS Collab experiment 1 microseismic data, recorded at the Sanford Underground Research Facility, South Dakota. Using the data from a single three-component accelerometer, the polarization features viz. Azimuth, incidence, rectilinearity, and planarity are used as inputs for the unsupervised manifold approximation followed by clustering. Our study shows that density-based clusters in the projected 3D space correspond to distinct types of hydraulically fractured zones around the injection point. We show that the temporal evolution of these clusters can be used to track fracture creation and propagation.
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