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A micromechanical numerical model is proposed to simulate the hydro-mechanical coupled behaviors of Aue granite at the grain size level. The hydro-mechanical coupled behaviors of simulated granite samples under triaxial compression tests show similar observations related to stress, strain, and permeability evolutions found in laboratory tests. The proposed modeling strategy gives more in-depth insight into the damage pattern along grain boundaries under triaxial compression. It reveals the importance of tensile microcracks for enhanced permeability (Percolation) and can duplicate all hydro-mechanical phases observed in the lab except the macroscopic fracture evolution at the end of the post-failure region. The model reveals the development of highly inhomogeneous pore water pressure distribution and flow paths in a sample under triaxial compression and steady-state flow conditions. Linear correlations between volumetric strain and permeability are observed from both numerical and experimental results. [ABSTRACT FROM AUTHOR]

In this study, a heterogeneous discrete element model of granite was built considering different mineral contacts, different mineral grains, and the percentages of minerals which were obtained from analysis of the gray-scale values of the rock picture. First, the contacts and grains parameters were fixed by simulating triaxial compression and fracture toughness tests. Then, the permeability of the numerical model was calibrated on constant head tests. Finally, a coupled hydro-mechanical code was used to investigate the hydraulic fracturing under different conditions, considering the influences of permeability, fluid viscosity, fluid injection rate, boundary stress, pre-existing cracks, heterogeneity and weak layer. [ABSTRACT FROM AUTHOR]

The anisotropic behavior of a rock mass with persistent and planar joint sets is mainly governed by the geometrical and mechanical characteristics of the joints. The aim of the study is to develop a continuum-based approach for simulation of multi jointed geomaterials. Within the continuum methods, the discontinuities are regarded as smeared cracks in an implicit manner and all the joint parameters are incorporated into the equivalent constitutive equations. A new equivalent continuum model, called multi-joint model, is developed for jointed rock masses which may contain up to three arbitrary persistent joint sets. The Mohr-Coulomb yield criterion is used to check failure of the intact rock and the joints. The proposed model has solved the issue of multiple plasticity surfaces involved in this approach combined with multiple failure mechanisms. The multi-joint model was implemented into Fast Lagrangian Analysis of Continua software (FLAC) and is verified against the strength anisotropy behavior of jointed rock. A case study considering a circular excavation under uniform and non-uniform in-situ stresses is used to illustrate the practical application. The multi-joint model is compared with the ubiquitous joint model. [ABSTRACT FROM AUTHOR]

The prior aim of lab testing is the determination of parameters. However, the measurements do not only reflect the rock or soil properties, but contain also, at least to some extent, the influence of the test equipment. Even if measurements are designed to minimize the influence of the test equipment often some measurement values are effected by an unknown amount. Therefore, it is proposed to build detailed numerical models of the test device including the specimen and to duplicate the lab test completely. This paper presents experimental and numerical tests performed with a shear box device. Multi-stage direct shear tests were conducted on smooth plane joints in artificial rock-like material under constant normal loading (CNL) conditions. A 3-dimensional numerical model of the shear box device including the sample was built and the laboratory tests were simulated. The numerical model comprises the loading frame, the hydraulic pistons, the shear boxes and the sample and considers the measuring devices. The numerical model was able to reproduce the non-uniform stress distribution at the joint connected with sample rotation, which in turn produces unwanted frictional forces at the upper shear box. Consequently, only 91–94% of the applied and measured normal force really acts at the joint. Moreover, numerical simulations also shows, to what extend the different parts of the loading frame deform. [ABSTRACT FROM AUTHOR]

In this study pre-failure damage characteristics are investigated by conducting a series of conventional triaxial compression tests and corresponding numerical simulations for granite from Kirchberg (Saxony, Germany). First, lab test results are analysed. For evaluation and prediction of damage in the pre-failure range, damage indices are proposed considering increase of dissipation energy density and decrease of secant modulus. It can be concluded that the damage increases slowly before the reversal of volumetric strain and accelerates quickly afterwards. Then, a new failure criterion is deduced based on a correlation of maximum elastic strain energy density with uniaxial compressive strength and confining pressure. Finally, a micro-mechanical grain-based discrete element model using Voronoi blocks to represent minerals is set-up. It considers elastic grains and elasto-plastic contact deformations as well as inter- and intra-granular fracturing. The triaxial compression tests are simulated and the damage process including evolution of damage indices are investigated in detail. The proposed approaches can be used to predict and analyse limit and damage state of brittle rocks. [ABSTRACT FROM AUTHOR]

The current study investigates hydraulic fracture stimulation for an Enhanced Geothermal System (EGS) in a petrothermal environment to evaluate stress shadowing and fracture interaction in a multi-fracture setup. Previous studies investigated the geothermal potential of the area around Freiberg (Saxony, Germany), which is therefore used as a case example. The commercial discrete element code 3DEC™ is applied to conduct the numerical simulations. The simulation results show that hydraulic fracture stimulation results in a strong stress field alteration, which significantly influences the propagation of subsequently stimulated fractures. The resulting deflection of fractures can be minimized applying an optimized stimulation concept. [ABSTRACT FROM AUTHOR]

Highlights: [•] A lifetime prediction scheme was developed for rock under constant loadings. [•] Numerical simulations were performed utilizing the developed modeling scheme. [•] The damage process and macroscopic fracture pattern of the models were studied. [•] Typical fracture patterns were observed in the numerical models. [•] Preliminary studies were performed through numerical modeling. [ABSTRACT FROM AUTHOR]

Abstract: Biot's coefficient, which is the key parameter of poroelasticity, strongly depends on the microstructure of the porous medium. Due to the complexity to conduct effective experiments at the microscale, there are very limited experimental data available, which document the influence of microstructure on Biot's coefficient. Therefore, numerical method was used in this paper to specify the influences of microstructural parameters on Biot's coefficient. Two-dimensional discrete element models with simplified microstructural geometries have been constructed with the purpose of obtaining a quantitative description of the influences of internal cavities (pores, fractures etc.) on the macroscopic deformability. Based on the numerical results some observations were revealed: 1) the Biot's coefficient is strongly dependent on the internal microstructure of the rock, and different porous media with the same composition of constituents may exhibit quite different poroelastic behavior due to their different microstructures; 2) the cracks, rather than the pores seem to have the dominant effect on Biot's coefficient; 3) the distribution of cracks and pores influences the Biot's coefficient and creates anisotropy. Two phenomenological equations were established by using the generalized mixture rule to describe the relationship between Biot's coefficient and rock microstructures, which give more accuracy results than the existing empirical equations. [Copyright &y& Elsevier]

Flooding induced uplift for abandoned coal mines is monitored mainly by satellite data and geodetic levelling. Predictions are based on empirical formulas, but nowadays numerical simulation procedures provide an interesting alternative, because it is physically based and allows to consider complex mining, geological and hydrological conditions. According to the key driving mechanisms, the uplift process can be classified as either confined/unconfined (in respect to the mine water), isolated/connected (in respect to ponds) and with/without faults (in respect to the tectonic situation). The paper summarizes the state-of-the-art of monitoring and prediction of flooding induced uplift of abandoned coal mines. A workflow for using numerical simulations as tools for prediction of flooding induced uplift is proposed. • Comprehensive review on flooding induced uplift for abandoned coal mines. • Summarizing empirical and numerical simulation approaches to study and predict flooding induced uplift. • Classifications of uplift processes. • Proposing a general flowchart to conduct numerical simulations to predict flooding induced uplift. [ABSTRACT FROM AUTHOR]

Ground surface movement shows different features during mining and post-mining period. If the mine goaf is dry, surface subsidence is observed in both periods. If the mine goaf is flooded by groundwater, however, ground surface uplift can be observed although much smaller in amplitude compared to the preceding subsidence. A numerical modeling strategy is proposed to predict uplift during the flooding process of an excavated coal mining area under complex geological conditions. The modeling approach considers three phases: virgin state before mining, stage immediately after mine closure, flooding phase. The model considers elasto-plastic material behavior including faults represented by interfaces. In-situ measurements are used for calibration of the model. The Oelsnitz abandoned coal mine is used for model validation and prediction. The measured subsidences up to 17 m were reproduced as well as the uplift within the last 10 years with values between 0.5–2.0 mm/year. Spatial distribution of uplift areas is well reproduced. Predictions up to the year 2051 show an increase in uplift velocity between 0.5–3.0 mm/year and additional total uplift of up to 300 mm. • A modelling approach is proposed to predict ground surface movement of a coal mine considering complex geology. • The model uses equivalent elastic moduli for the mine goaf during the active mining period and the flooding phase. • The simulations are calibrated on geodetic measurements, satellite data and groundwater observation wells. • The numerical model reproduces subsidence and uplift in the past, and allows predictions for the next decades. [ABSTRACT FROM AUTHOR]

A continuum based numerical approach is used to investigate the effects of slow and high-speed heating, including the ISO 834 fire curve, on thermal cracking of granite. During slow heating, thermal stress variations are mainly controlled by the heterogeneous thermal expansion of mineral grains. As a result, the stress concentrations are more isolated and dispersedly distributed throughout the sample. High-speed heating produces large thermal gradients especially at the outer surface of the samples, inducing big thermal strain increments and subsequent high compressive stresses. Larger cracks are formed due to a wider range of concentrated tensile stresses inside the sample during high-speed heating. At low temperatures, slow heating has a stronger influcence on micro-cracking. Due to larger cracks and newly induced micro-cracks, high-speed heating has an increasingly important impact on crack pattern in granite at high temperatures. Although shear failures show a significant increase at 800 °C, tensile cracks are still the majority of total failures in both, slow and high-speed heating scenarios. The final crack pattern in granite under fire conditions is a result of different cracking behaviors caused by various heating rates. This consequently affects strength and final failure pattern of granite. [ABSTRACT FROM AUTHOR]

This work proposed a progressive-sectional failure model (PSFM) based on Particle Flow Code (PFC) to simulate top coal drawing process in longwall top coal caving (LTCC) mining. The progressive-sectional fracturing of top coal was reproduced in simulation. 4 drawing schemes are numerically performed by considering drawing sequences and strategies. The results of simulations were compared in terms of top coal drawn out mass, drawing body, top coal recovery ratio (TCRR), top coal loss, etc. The results show that TCRR in PSFM is closer to average TCRR based on 9 LTCC coal mines measured in-situ. The top coal loss in PSFM is larger than loose granular model (LGM). The top coal boundary (TCB) in PSFM shows a "step" subsidence and is different from that in LGM. The factors to influence initial drawing body (IDB) are comprehensively analysed. A new mathematical model was proposed to specifically characterize IDB in LTCC. The results show that new model is well consistent with simulation results and is much more effective and accurate than former models. [ABSTRACT FROM AUTHOR]

A new methodology combining Weibull functions and the real mineral composition of granite is proposed, which overcomes the shortcomings of using existing methods for the heterogeneity characterization of rocks at the crystal-size or grain-size level. The proposed Weibull-mineral based model can simulate thermal cracking of rocks in a more realistic way compared with previous methods such as the mineral-based model. The study also reveals the importance of thermo-mechanical property distributions at the crystal-size level for the overall thermo-mechanical behaviours at larger scale. Supported by lab tests, the new model can reproduce the mechanical behaviour of heterogeneous materials. The thermal cracking of granite at elevated temperatures is also possible to be predicted. [ABSTRACT FROM AUTHOR]

A 3D particle-based numerical approach to simulate the longwall top coal caving (LTCC) process was proposed, which for the first time the effect of continuous support advance and the face-end support were considered. The shape of drawing body and top coal boundary and the distribution characteristics of coal loss were analysed. The top coal flow trajectories and velocity fields were visualized and consistent with the assumptions in Bergmark-Roos model. The vertical forces on the drawing support beams were continuously monitored during the process of simulation. It was found that both the drawing procedure and the support advance will lead to a reduction of the vertical forces on support beams. Results of the numerical simulations and laboratory testing were compared and good consistency was obtained. [ABSTRACT FROM AUTHOR]