Search

Grain-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. [ABSTRACT FROM AUTHOR]

This article aims to numerically investigate the mechanical behaviour of concrete Brazilian Disk (BD) samples when exposed to multi-level cyclic Brazilian loading. A numerical 3D particle-based model was successfully established to simulate the dynamic stress-controlled cyclic testing. A generalized model is proposed based on the time-dependent evolution of bond diameter. This model shows great effectiveness in terms of reproducing the stress-strain curves, axial strain evolution and displacement fields as observed during lab testing. An acoustic emission (AE) tracking algorithm was implemented in the model to characterize and visualize the released AE energy connected with micro fracturing. The AE energy is found to be stress-level dependent which is consistent with lab testing. The failure pattern in numerical simulation is also in line with lab results. The distinctions between cyclic compressive and cyclic Brazilian tests are discussed and summarized. [ABSTRACT FROM AUTHOR]

Tensile failure in brittle rocks is crucial for the stability of rock engineering applications. However, a comprehensive quantitative evaluation of continuous-discontinuous tensile failure characteristics and the effect of loading platen shape on the microcracking behavior induced by tensile stress in granite, as well as the corresponding variations in the evolution of Fracture Process Zone (FPZ) at the microscale, remain unclear. This study describes an effective acousto-optic-mechanical multi-physical field observation system to capture the real-time acoustic, optical and mechanical response of a coarse-grained granite during Brazilian splitting tests applying various loading platen shapes. An improved 3D acoustic emission (AE) location algorithm is introduced and validated to interpret the spatio-temporal evolution of crack source mechanisms. Three stages of microcracking, namely quiet stage (QS), stable stage (SS), and outburst stage (OS) are identified based on the AE characteristics. We found that a distinction between different loading conditions is easier to detect at relatively low loading rates. Digital Image Correlation (DIC) results show that the loading platen shape affects the initial position of the tensile stress-induced crack initiation and the corresponding deformation. High-Speed Photography (HSP) images captures the evolution of FPZ and visible crack opening behavior. Both, indirect tensile strength and splitting elastic modulus are found to be sensitive to loading rates, with varying degrees of sensitivity under different loading conditions. The fractal dimension results closely correlate with the monitored COD outcomes, highlighting a smaller Joint Roughness Coefficient (JRC) value on the failure surface under arc platen loading conditions compared with flat platen loading conditions. The evolution of b -values reveals the transition from microcracking to macrocracking, which is consistent with the stress and FPZ evolution. The influence of loading platen shape on indirect tensile strength and failure characteristics of granite is theoretically validated. Finally, correlations among the AOM characteristics under different loading conditions and their practical implications are investigated. • Established effective acousto-optic-mechanical system for observing the responses of granite samples in indirect tension. • Quantitative analysis of internal and external responses during the continuous-discontinuous failure using AOM monitoring. • Theoretical validation of loading platen on granite strength and failure evolution under different loading conditions. [ABSTRACT FROM AUTHOR]

This study presents numerical investigations about the role of micro-crack heterogeneity in hydraulic fracture (HF) development and directional hydraulic fracturing (DHF) creation. By comparing the results of heterogeneous models (named DFN models) with the associated homogeneous models, the behavior is categorized into three modes: DFN models, which consume more (A), comparable (B), and much lower (C) energy than the associated homogeneous models. Variation of micro-crack distributions, which determines HFs interaction in the stress shadow zone, contributes to the difference in HF morphologies and injection energy consumption. The connection process, where one HF connects the other, predominates the energy cost in the DHF. Offset-related interactions usually occur between HFs and DFN in case of favorable distribution of micro-cracks, which guides the HFs to propagate through the interaction zone, thus favoring the connection process and improving the DHF efficiency. Adverse distribution of local cracks (even a single crack) with dilation-related interactions near the connection zone can produce extremely high stress due to the opening of the cracks, which significantly deteriorates the connection process and reduces the DHF efficiency. Therefore, a limited variation of crack distribution near the connection zone could cause a pronounced effect on HF connection and the associated energy consumption. Due to the limited sizes and shear slips of the micro-cracks in these simulations, deformation energy and fracture energy dissipation dominate the energy conversion during the injection, while frictional energy consumption plays a secondary role. The two-stage injection strategy can be a promising method to weaken the stress shadow effect, i.e., subsequent HFs extend directly to the previous one by taking advantage of the stress state produced in the first stage. The frictional resistance of micro-cracks also plays a significant role in DHF creation. Weak cracks allow the HFs to go through the interaction zone and avert the strict connection process, consequently improving the DHF efficiency. [ABSTRACT FROM AUTHOR]

Traditional models of pull-apart basins usually develop along two strike-slip faults. However, there is an important variant that does not fit with the traditional pull-apart basins. This study presents this type of pull-apart basin called composite pull-apart basin formed in a strike-slip system consisting of multiple master fault segments. A set of two-dimensional discontinuum based, scaled, elastic models in upper crust are built to provide insights on composite pull-apart basin evolution. Three representative fault segment interaction geometries are modeled, showing underlapping, neutral, and overlapping releasing steps. They produce similar composite pull-apart basins consisting of two or three en echelon rhomboidal subbasins as offset increases. However, a change of initial fault geometry from underlapping to neutral and overlapping causes the fracture pattern to be more diffuse. The underlapping model produces a composite pull-apart basin with a throughgoing fault evolving from cross-basin faults in the subbasins whereas composite pull-apart basins produced in neutral and overlapping systems have no cross-basin faults and throughgoing faults. Modeling results show that local extension in underlapping systems with both pure strike-slip and 5° transtension is accommodated through development of R-shears, small subbasins, cross-basin faults obliquely cutting the subbasins, and throughgoing fault. The morphology and structural evolution of composite pull-apart basins are determined by factors including pre-existing master strike-slip fault geometries (underlapping, neutral, or overlapping), fault kinematics (pure strike-slip, transtension, or transpression), and strike-slip displacements corresponding to evolution stages. The composite pull-apart basin geometries and fracture patterns of our numerical models fit well with natural examples such as the Gulf of Aqaba and the northern part of the Marmara Sea. • A discontinuum based modeling is used to study composite pull-apart basin evolution. • Underlapping models produce composite pull-apart basins with throughgoing faults. • A change of initial fault geometry causes the fault pattern to be different. • Our models fit well with the Gulf of Aqaba and the Marmara Sea. [ABSTRACT FROM AUTHOR]

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]

The accuracy of rock property measurements in real-time heating loading tests has been considered as questionable due to the thermal effect on loading platens. In this study, the impact of steel platen performance at high temperatures (up to 1000 °C) on granite property measurements is analysed by numerical simulations. Simulation results show that a smaller stiffness of the loading platen can result in specific stress redistributions, increasing micro cracks, and peak axial stress reduction of the sample during uniaxial compression. This can subsequently lead to an underestimation of failure stress and strain. However, the decreasing platen stiffness with rising temperature does not produce any remarkable impact on the temperature-dependent property measurements of the selected Eibenstock granite (EG) heated up to 1000 °C. This is related to the progressively decreased strength of EG and the consequently negligible deformation of the softened platen at elevated temperatures. Therefore, the loading platen can still be treated as ideal stiff. Considering the big variations in strengths and thermal response of different rock types, the suggested procedure to obtain reliable rock properties at high temperatures (e.g. above 800 °C) is to perform an evaluation by replicating the lab tests via numerical simulations. [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]

• The abrasive parameters of three construction rocks are analysed. • Cerchar test was conducted by using a special instrumented West apparatus. • Cerchar abrasion ratio is a meaningful parameter to evaluate rock abrasivity. • Cerchar abrasive parameters and their effecting factors are summarized. The Cerchar test method and its index, the so-called Cerchar abrasivity index, are most commonly used to determine the abrasivity of rocks and other geo-materials for construction. Recently, with the aid of a new test device, various abrasive parameters can be obtained from the Cerchar test. In this article, several abrasive parameters such as scratching force, scratching energy and scratching specific energy are determined using a special instrumented West apparatus. Three typical rock materials for construction (granite, sandstone and slate) are selected to study the correlation of Cerchar abrasive parameters with rock intrinsic properties. The research results show that the Cerchar abrasion ratio is a meaningful parameter to quantify not only the rock abrasivity, but also the rock-stylus interaction, as well as the stylus wear and the scratching effectivity. [ABSTRACT FROM AUTHOR]

Wear occurs on mechanical tools during rock cutting and drilling, or any other kind of mechanical based excavation of rocks. The determination of rock abrasivity plays therefore a key role for estimating the tool's lifetime and for evaluating the economic efficiency. Due to simple design and convenient handling, the Cerchar abrasivity test is commonly used to assess the rock abrasivity and to predict the tool wear. However, the abrasivity index reflects only the abrasion of the stylus, and does not consider what happens on the tested rocks, as well as on how the stylus interacts with the rock during the scratching process. In this paper, an instrumented West apparatus is used to determine the applied horizontal force in the Cerchar test. The area under the force-displacement curve represents the consumed scratching energy. The material removed from the rock surface is measured using a digital microscope. Then, a new parameter called Cerchar specific energy, which is defined as the ratio of scratching energy to material removal volume, is proposed to assess the rock abrasivity, to describe the rock-stylus interaction, as well as to evaluate the efficiency of rock scratching, which can be used as indicator for rock cutting or drilling efficiency. • The applied scratching force is determined by means of an instrumented West apparatus. • The rock material removed in the scratch groove is measured using a digital microscope. • The Cerchar specific energy is used to describe rock-stylus interaction and to evaluate efficiency of rock scratching. • The abrasive mechanisms of the Cerchar scratch test are investigated for different types of rock. [ABSTRACT FROM AUTHOR]

This study examines the impact of particle shape (ball, polygon, and trigon) on rock mechanical behavior using laboratory-scale tests and a field experiment with the soft bond model (SBM). Parametric analysis shows that the softening behavior in SBM significantly influences the strength ratio (R ct) between uniaxial compressive and tensile strength, but depending on bond strength ratio (R bs), softening factor (ζ), and particle shape. By adjusting these parameters, we achieve an R ct ranging from 8 to 80, suitable for most rock materials. Adjusting micro-parameters such as friction angle and coefficient allows us to obtain failure envelope slopes ranging from 30 to 60 degrees. Compared to the ball particle model (BPM) and trilateral particle model (TPM), the polygonal particle model (PPM) is more sensitive to friction coefficient due to enhanced interlocking mechanisms near macro shear bands. While all three models effectively replicate laboratory experiments on hard rocks, the PPM tends to exhibit unrealistically high dilation before peak stress, and BPM and PPM fail to capture non-linear failure envelopes at high confinement. Overall, TPM with SBM overcomes common shortcomings observed in traditional particle bond models. Simulations of the Mine-by tunnel demonstrate that TPM successfully reproduces the notch profile and the spalling mechanism, whereas both BPM and PPM cannot replicate the spalling. Detailed analysis reveals that TPM facilitates the creation of smooth pathways for tensile cracks and propagation of shear fractures, promoting the localization and coalescence of microcracks − features absent in the other two models. [ABSTRACT FROM AUTHOR]

Infrared radiation count (IRC) is considered as an indicator for damage evolution of rocks. This new indicator links damage intensity and corresponding infrared radiation temperature. As demonstrated exemplary for sandstone, the sudden change of IRC can be considered as a damage indicator for rocks under loading. All samples showed IRC sudden changes around the peak stress, and 75% of the samples showed sudden changes before reaching the peak stress. Therefore—considering that infrared camera covers only about 25% of sample surface—IRC can be used as an early warning signal for rock failure. IRC variations are mainly related to energy release during the energy transformation process from elastic strain energy into fracture energy, heat and seismic energy. IRC is randomly distributed over the whole sample surface with very low values in the quasi-elastic deformation stage, but with the onset of local cracking, sudden and significant changes occur. The use of IRC as a nondestructive failure precursor in laboratory tests is recommended. [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 new continuum-based constitutive model called the Transubi model is introduced. • The model considers both the stiffness and the strength anisotropy of bedded rocks. • Transubi model is calibrated against lab experimental data conducted on Opalinus clay. • Two tunnels are simulated as case studies in the URL, Switzerland. Opalinus clay has been extensively investigated, as it is considered as potential host rock for underground nuclear waste repositories. The behavior of the Opalinus clay is a transverse isotropic possessing anisotropic characteristics in both strength and stiffness. Also, the lab investigations conducted on the Opalinus clay found that it behaves as bi-linear elasto-plastic strain softened material in general and as brittle plastic in particular. Two case studies of tunnel excavated in shaly facies formation of the Opalinus clay have been numerically analyzed by applying a new continuum-based constitutive model called "Transubi model" providing an Instantaneous hydromechanical coupling short-term simulation by generating the pore pressure considering no fluid flow. Lab test results obtained from literature have been used to validate this model: triaxial experiments and direct shear tests. The Transubi model is able to duplicate the pre-yield hardening and the post-yield softening for loading perpendicular and parallel to bedding planes, S- and P- samples, respectively. Finally, two in-situ tests in the Mt. Terri URL are used to demonstrate the applicability of the Transubi model to predict the short-term hydro-mechanical coupled behavior. The results from numerical simulations for both excavations are compared to those from field observations showing better agreements than other previous numerical analyzes from literature. [ABSTRACT FROM AUTHOR]

Hydraulic stimulation allows to increase permeability, create fluid paths and raise the flow rate during injection or production in a reservoir. The success of a hydraulic stimulation campaign is coupled to preceding planning and a suitable design. The stress field surrounding the injection well has critical influence on the hydraulic fracture development. Numerical simulation of the in situ stress field is a key task for preparation of a hydraulic stimulation. Knowledge of the geology, geomechanics and hydraulics of the target rock is fundamental to create a feasible model.Accompanying hydraulic stimulation field tests in the Reiche Zeche underground laboratory in Freiberg, Germany, we conduct a numerical simulation of the in situ stress field. Therefore we create a detailed 3D geological model of the investigation area. A large scale model geometry of 2.5 x 2.5 km horizontal extent and 0.5 km depth incorporating 12 large ore veins, which act as discontinuities, is created based on mine maps and GIS data. Integrated into the large scale model is a local scale model, smaller by a factor of ten. The smaller model contains mining galleries and locally mapped fault structures which the injection borehole would penetrate.We examine the extent and influence of these local faults on the stress field around the injection borehole and determine the in situ stress at the fracking intervals. For numerical stress field simulation we use an integrated modeling approach and the DEM code 3DEC by ITASCA to consider shear movements in the strongly discontinuous geology. To represent the foliation of the Freiberger Graugneis, an anisotropic elastic constitutive law is applied.Additionally we estimate the tendency of the locally appearing faults to being reactivated by change of in situ stress during stimulation (enhanced slip tendency analysis). Therefore we create a MATLAB routine which can be used for any arbitrarily oriented stress field and discontinuity orientation. The output is visualized and color coded.The stress field model results fit the measured values of orientation and magnitude of the stress tensor components. Stress measurement data from the field test are used to calibrate the local stress model. The modeled stress field is anisotropic with a ratio of 1.3 : 1 : 0.8 and describes a strike-slip regime. From the local model results we assume that the fault structures mapped at the tunnel walls either do not proceed throughout the rock volume or are healed and have only minor influence on the stress field orientation. From the results of the slip tendency analysis we expect that the local faults are not prone to be reactivated by the hydraulic stimulation. [ABSTRACT FROM AUTHOR]

Disposal of high-level radioactive waste (HLW) is one of the most challenging subjects across the world. And it is reported that China intends to build an HLW repository in granite strata in Beishan, Gansu Province. In this research, to explore hydraulic properties of Beishan granite considering thermal damage, specimens were heated up to 300, 400, and 500 °C, respectively. Then, hydro-mechanical (HM) coupled tests were conducted after specimens were cooled to room temperature. It was observed that a large amount of acoustic emission (AE) is monitored during the initial compaction and elastic stage in the HM coupled tests. For mechanical response, under each fluid pressure, the peak strength and crack damage threshold gradually decrease when the treatment temperature is within 400 °C. However, they show increase after 500 °C treatment compared with that of 400 °C. In addition, peak strength and crack damage threshold reach their maximum under fluid pressure of 4 MPa for each treatment temperature. The change tendency of strain with treatment temperature is roughly the same as that for the strength. And it is noticeable that the crack damage threshold strain at 500 °C is even higher than that of specimens without heat treatment. As for the permeability of Beishan granite, the initial permeability does not change much after different high temperature treatment and maintains at about 10−18 m2 under confining pressure of 20 MPa. It is revealed that before peak stress is reached, the change of permeability with volumetric strain can be expressed by two linear relations, taking the expansion point as the dividing point. And a permeability evolution model of Beishan granite is proposed. The obtained results are of great importance for the safety of an HLW repository. [ABSTRACT FROM AUTHOR]

• Micro- and macroscopic behavior of granite under and after heating are compared. • Mechanical difference between RT and AT granite is not related to crack density. • Cooling effect improves the stress resistance and interlocking at the grain size level. • Real-time weakening and strengthening mechanisms of thermal-treated granite is revealed. The distinctive mechanical behaviors exhibited by granite under real-time temperature (RT) and after thermal treatment (AT) highlight a challenge in directly observing real-time cracking phenomena in RT granite within a laboratory setting. To overcome this limitation, a thermo-mechanical coupled grain-based model (GBM) was introduced for numerical investigations. The simulation outcomes reveal that disparities in mechanical responses between AT and RT specimens are attributed not merely to the count of thermally induced cracks, but rather to intricate grain-contact deformations arising from heterogeneous crystal expansions/contractions and the intricate interplay of direction-independent interactions along block edges. During elastic deformation, RT grain contacts with larger deformation reach the peak stress faster than AT contacts. Failed contacts have smaller residual cohesion under temperature with larger deformation due to the stronger slip-weakening effect. Moreover, shear displacements and grain rotations can create new interlocking, increasing granite stiffness after cooling. Consequently, the RT sample is softer to deform and fail, while the overall stress resistance of AT granite is higher due to cooling-induced micro-structure changes. These findings shed light on why the mechanical responses, including strength, stiffness, and plastic strain, of AT and RT granite are different. [ABSTRACT FROM AUTHOR]

This study aims to experimentally assess repeatability and reproducibility of direct tensile strength (DTS) tests with deformability measurements on two types of rocks: Blanco Mera granite (Spain) and Cotta sandstone (Germany). The tests were conducted in four rock mechanics laboratories located in different countries (Canada, Germany, Spain and Sweden). A total of 51 tests were performed on cylindrical specimens of the two rocks, using different test equipment and measuring devices. Mean and standard deviation DTS values were determined in the four laboratories for the granite (5.70 ± 0.32, 6.06 ± 0.11, 3.84 ± 0.50 and 6.76 ± 0.10 MPa) and for the sandstone (1.88 ± 0.07, 1.96 ± 0.06, 1.15 ± 0.32 and 1.74 ± 0.19 MPa), together with Young's moduli and Poisson's ratios in tension, being statistically analysed to evaluate the variability and compare the main results obtained from the participating laboratories. The findings indicate that the DTS test with deformability measurements on cylindrical rock specimens is operationally feasible. However, certain shortcomings have been identified during the course of the experiments with the existing methodologies, such as the one suggested by the ISRM for DTS tests. The results have also shown to be sensitive to appropriate test and strain measurement configurations. The objective of this study was to shed light on these issues and provide new insights for potential future improvements of the existing testing methods. Highlights: An experimental assessment of reproducibility and repeatability of direct tensile strength tests was carried out. Tests were conducted on two rocks (granite, sandstone), in four laboratories located in Canada, Germany, Spain and Sweden. Direct tensile strength test with deformability measurements on cylindrical rock specimens was found operationally feasible. Some shortcomings in current methodologies have been found, such as inappropriate loading rates or no recommendations for deformability measurements. This work provides new insights for potential future improvements of the existing testing methods. [ABSTRACT FROM AUTHOR]

Information regarding the 39th Geomechanics Colloquium held at the Technische Universität (TU) Bergakademie in Freiberg, Germany on October 8, 2010 is presented. It features professor Stephansson from the Research Centre for Geosciences, who delivered the plenary lecture on the state of knowledge for the determination of tectonic rock stresses. It discusses other topics including geothermal energy and rock mechanics, storage of salt mechanics, and testing in rock and rock mass mechanics.

A numerical model is presented to reproduce the stress induced fracturing process of brittle rocks. Material heterogeneity at the zone level of the model is considered, and distinct initial microcrack lengths and orientations are also included. The rock material is prescribed with a Weibull distribution, and the microcrack lengths and orientations are defined by distinct distribution functions. Stress intensity factor approach is adopted to characterize the stress condition of the microcracks, and is used as a failure criterion for the zones of the model. The mechanical response of rock specimens is investigated on models under different external loading conditions. Obvious nonlinear characteristics before the peak strength can be observed in models under uniaxial compression. Consistency can be found between simulation results and lab test data. The influence of microcrack conditions and heterogeneity levels on the simulation result is also investigated. The macroscopic failure modes of models are studied with different distributions of initial microcracks, and correlations are found between the microcrack conditions and fracturing processes. Especially, scattered zones and obscure fracture patterns are produced in the model defined by exponential and uniform distribution, compared with clear shapes of macroscopic fractures observed in models defined by normal or lognormal distributions. The simulations also show that the differences in the mean value of microcrack orientations obeying certain distribution result in distinct macroscopic fracture patterns, from shear bands at lower mean orientations to tensile fractures at higher mean orientations. Typical fracture patterns observed in laboratory are successfully reproduced in the model, which demonstrates the capability of the model in reproducing faithful rock behaviors through prescribing specific microscopic characteristics. [ABSTRACT FROM AUTHOR]

Based on dissipated energy approach (DEA), the energy dissipation characteristics of concrete samples subjected to uniaxial cyclic loading have been investigated. The effect of different cyclic load levels on energy dissipation is quantitatively analyzed by different damage indicators during cyclic fatigue tests. It is concluded that the cumulative speed of energy dissipation and increasing growth-rate of damage indicators in Continuum Damage Theory (CDT) follow an exponential function in relation to the maximum cyclic load level and follow a logarithmic function in relation to the minimum cyclic load level. The loading strategy has an effect on total energy dissipated during fatigue tests, in other words: Energy dissipation and damage evolution are stress path dependent. [ABSTRACT FROM AUTHOR]

The alumina-magnesia castables is one kind of monolithic refractories for steel ladle, whose thermo-mechanical properties vary with temperature during the lining pre-heating process and metallurgical service. The understanding of their variation tendency and sensibility to temperature fluctuation is important for the improvement of high-efficiency pre-heating strategy and real-time service performance. In this study, the influence of heat treatment temperature and heating/cooling rate on the thermo-mechanical properties and fracture behaviour of alumina-magnesia castables is investigated using uniaxial compressive tests equipped with an induction heating system and Brazilian test combined with digital image correlation and acoustic emission. The brittleness and mechanical strength of alumina-magnesia castables increase with heat treatment temperature due to the better sintering and formation of calcium hexaluminate and spinel phases. Without full relaxation of residual stresses, the stiffness increases while the strength decreases after fast heating of 200 °C/min. Meanwhile. the same castables after different thermal treatments show disparate mechanical responses to cooling rates. [ABSTRACT FROM AUTHOR]

This paper describes lab test results of artificial rock-like material samples having a plane joint. Cyclic shear tests were performed under different nonnal loads and different shear displacement amplitudes. For this purpose, multi-stage nonnal loading tests (30 kN, 60 kN, 90 kN, 180 kN, 360 kN and 480 kN) with cyclic excitation at frequency of 1.0 Hz and different shear displacement amplitudes (0.5 mm, 1.0 mm, 2.0 mm, 4.0 mm, 5.0 mm, and 8.0 mm) were conducted using the big shear box device GS-1000. Experimental results show, that shear forces increase with the increase of normal forces and quasi-static friction coefficient is larger than dynamic one. With the increase of nonnal loads, approaching the peak value of shear forces needs larger shear displacements. During each cycle the normal displacements increase and decrease (rotational behavior in every cycle). Peak angle of inclination increases with the increase of normal load. A phase shift between maximum shear displacement and maximum shear force is observed. The corresponding time shift decreases with increasing normal load and increases with increasing shear displacement amplitudes. [ABSTRACT FROM AUTHOR]

Previous studies on thermal induced damage of rocks concentrated mainly on the analysis of macro-mechanical properties. This paper documents physical and microstructural characteristics of sandstone after high-temperature treatment by using several testing methods at the micro scale. Results indicate that physical including mechanical parameters of tested sandstone show some strengthening effect up to a temperature of about 100 °C, which is related to the evaporation of free water and the thermal expansion of mineral particles. Beyond 100 °C, the physico-mechanical properties of sandstone are weakened significantly. Mass loss and porosity show a continuously growing with increasing temperature, while P-wave velocity, splitting tensile strength, compression strength, and elastic modulus are decreasing. Nuclear magnetic resonance (NMR) imaging tests were carried out to capture the variations in porosity and pore size distribution characteristics of sandstone during the heat treatment. With increasing temperature, the number of macropores and microcracks increase and coalesce. There is a linear relationship between rock porosity and tensile as well as compressive strength. Differential thermal analysis (DTA) shows that the α-β phase transition of quartz minerals occurs at a temperature of 573.4 °C. Additionally, between 400 and 600 °C, more transgranular microcracks are observed detected by optical microscope and scanning electron microscopy (SEM) observations, which proves that quartz phase transition is an important factor to induce microcracks. Thermal treatment also has a significant influence on the variation of the brittle index of sandstone, which makes thermal-induced plastic deformations of the sandstone more obvious. [ABSTRACT FROM AUTHOR]

Thermo-mechanical performance of carbon-bonded alumina filters is essential for guaranteeing their structural stability and filtration capability. This work investigates the Mode I fracture of Al 2 O 3 -C porous ceramic as well as the influence of reticulate structure and coatings (alumina and carbon-bonded calcium hexaluminate). The cylindrical splitting test is validated for evaluating the strength of foam and individual strut. DIC and AE techniques were applied to monitor tensile failure induced by compressive loading. The results showed central crack propagation and continuous AE signal release caused by localized damage. Generally, the mechanical strength increases with pore density and the application of alumina coating. This improvement results from more continuous stress transmission and better sintered struts. However, calcium hexaluminate coating is less effective for strengthening foams at room temperature. Both functional coatings and the continuity of reticulate foams show positive effect on the deformation resistance to temperature and load. [ABSTRACT FROM AUTHOR]

• In-situ drilling rate and bit wear of an electrical drill hammer are measured. • Relations between drilling performance and rock mechanical properties are investigated. • Dependency of bit wear on rock abrasive parameters is discussed. Estimation of drillability is very important in rock drilling. Drilling performance and tool wear are two indispensable parameters for time and cost evaluation of an operation. In this article, by using an electrical drill hammer and two types of drill bits, in-situ drilling tests were conducted on three rock types to estimate the drilling performance and tool wear. Experimental results showed that the highest performance and the greatest bit wear occur in drilling sandstone, while the lowest values of penetration rate and wear are observed in amphibolite and basalt. Moreover, the penetration rate of drilling is related to rock mechanical properties. Regression analysis indicated that the correlation of performance with uniaxial compressive strength, Brazilian tensile strength and point load index, respectively, is significant and reliable. Regarding to the tool wear, equivalent quartz content showed a reasonable correlation with bit wear, compared to Cerchar abrasivity index and LCPC abrasivity coefficient. [ABSTRACT FROM AUTHOR]

• AE behavior and P-wave speed of concrete are revealed in cyclic Brazilian test. • Deformation behavior of concrete is revealed in cyclic Brazilian test. • DIC measurement is performed in cyclic Brazilian test. This work aims to investigate the fatigue behaviour of concrete subjected to indirect cyclic tensile loading. A series of laboratory tests were performed based on Brazilian disk-shaped (BD) concrete samples. Two schemes of cyclic loading tests were designed by considering complex variations of maximum and minimum stresses. The effect of loading parameters, such as loading method, maximum and minimum stress, on sample deformation, acoustic emission (AE) characteristics, P-wave speed evolution and failure patterns were analysed. The features of AE behaviour and P-wave speed were compared between monotonic loading and indirect cyclic loading. Insight on the specific fatigue behaviour was obtained from AE hit counts, AE energy and P-wave speed degradation. The failure patterns of BD concrete samples are governed by the loading method as well as the stress paths. The stress paths have a direct and pronounced impact on the distribution of main and sub cracks on the surface of BD sample at failure. [ABSTRACT FROM AUTHOR]

Lots of in-situ experience indicate that water spraying or injection to rock mass before excavation is an effective method for rock-burst prevention in deeply buried hard rock engineering projects. To investigate the underlying mechanisms of rock-burst prevention by watering, a series of uniaxial compression laboratory experiments were performed on three types of sandstone samples under oven-dried and water-saturated conditions. A high-speed camera was used to record the fracture processes of the samples during testing. Experimental results show that the presence of water plays a dramatic weakening role on the uniaxial compressive strength, Young's modulus, and energy storage capacity of the three sandstones. After water saturation, the failure pattern of rock sample becomes much gentler and less dynamic. Moreover, a simple mechanical model of a roadway containing zonal water-weakening rock mass was established to illuminate the local geo-stress regulation by water penetration. The mechanisms for rock-burst prevention by water are mainly due to the declining energy storage capacity and the lowered stress near the working face. [ABSTRACT FROM AUTHOR]

Freeze-thaw-fatigue (FTF) testing was carried out on Tibet marble at lab-scale by considering various Freeze-Thaw (FT) cycles and multi-level cyclic loading. The aim is to investigate the evolution of deformation and damage related parameters as well as the hysteresis behavior. The test results show that an increase of FT cycles and fatigue load level both accelerate the damage rate of marble. A rock suffering more FT cycles shows a much larger axial strain rate, radial strain rate and increasing Poisson's ratio. The relation of axial strain rate, radial strain rate and Poisson's ratio rate versus fatigue load level can be fitted by an exponential function. A warning level is defined according to the evolution of radial strain and Poisson's ratio which can inform before dilation starts. Moreover, the hysteresis behavior of stress-strain is investigated. It has been proven, that the proposed two indexes, "advance ratio" and "lag ratio" are effective precursors for rock failure prediction if subjected to FTF actions. The drastic drop of the "advance ratio" and the increase of the "lag ratio" are the indicators for forthcoming failure. The recommended warning limits for "advance ratio" and "lag ratio" for the tested marble are 5% and 70%, respectively. [ABSTRACT FROM AUTHOR]

Granite is commonly seen in tunnels, historical buildings, or energy generation/storage facilities, where repeated heating-cooling cycles can occur during regular operation or fire accidents. In this study, physical and mechanical properties as well as the cracking behaviour of Eibenstock granite during single and repeated heating-cooling cycles are described. Generally, repeated thermal cycling has only a small influence on properties and the cracking behaviour of granite. Thin section observations show that the main minerals of granite do not show big changes up to 1000 °C. P-wave velocity and open porosity measurements reveal that thermal induced microcracks mainly develop before quartz transition temperature (573 °C), and after this temperature these earlier-formed microcracks will be widened rather than newly formed. A hardening behaviour of granite after heat treatments can be observed in the temperature range of 25 °C–400 °C due to the evaporation of water. At higher temperatures, further developed or extended microcracks will lead to a reduction of both, uniaxial compression strength (UCS) and stiffness. Back-calculated parameters show that the thermal induced cracking might reduce the interlocking between the crystals by weakening or removing the cohesion rather than the friction angle. Tensile strength shows a continuous decrease with the development of thermal induced cracks, and no obvious difference can be observed between single and repeated heating-cooling scenarios. Additional testing on samples with different dwelling time shows that thermal time has a minor influence on the thermal cracking behaviour of granite. The slight difference at 1000 °C in terms of P-wave velocity and open porosity between single and repeated thermal cycles is mainly caused by repeated shrinkage and widening of cracks or closing and reopening of some microcracks. All in all, repeated thermal cycling has a negligible influence on physical and mechanical properties. The same holds for the cracking behaviour of granite. [ABSTRACT FROM AUTHOR]

• Loading frequency has almost no influence on dissipated energy for a single cycle. • Total dissipated energy is related to the total cycle number. • Two hysteresis indexes are proposed to predict fatigue failure of coal sample. • The damage evolution of coal sample subjected to cyclic loading is inhomogeneous. This paper documents investigations of the fatigue behaviour of strong burst-prone coal in terms of energy dissipation, hysteresis and micro-seismicity. The energy dissipation characteristics of the coal subjected to multi-level-frequency cyclic loading were studied. Two hysteresis indexes were proposed according to the stress-strain relation during cyclic loading. These two indexes are effective as twin precursors to predict the fatigue failure of coal samples. The effect of frequency on energy dissipation rate was studied. With the aid of an AE monitoring system and a high speed camera, 3D distribution of AE events and failure patterns were obtained. The damage evolution of coal samples subjected to cyclic loading is not homogenous due to the stiffness effect of the loading platen. The failure patterns captured by high speed camera are consistent with the results characterized by the spatial distribution of the AE events. [ABSTRACT FROM AUTHOR]

• The axial strain for cyclically loaded concrete specimens is inhomogeneous. • The energy dissipation for cyclically loaded concrete specimens is inhomogeneous. • A new definition of P-wave ratio is proposed as an effective fatigue failure precursor. • Monotonically loaded concrete show larger cracks than cyclic loading case. The mechanical behaviour of different parts (top, middle and bottom) of concrete exposed to monotonic and cyclic loading is investigated through laboratory testing. Strain evolution and energy dissipation are found inhomogeneous at different parts of the sample during compressive cyclic loading. The middle part of sample shows larger residual strain rate and dissipated energy than the top and bottom parts when larger load levels are applied. The disparity between the middle part and top/bottom parts reaches the peak value at peak strength. Based on two different cyclic loading strategies, it is concluded that the maximum load level has more pronounced effect on energy dissipation than the minimum load level. The evolution of P-wave speed during cyclic loading is also found inhomogeneous, a new definition of P-wave ratio is proposed as an effective fatigue failure precursor. The failure patterns of concretes subjected to monotonic and cyclic loading are presented, and it is shown that size and type of cracks is directly related to the loading methods. [ABSTRACT FROM AUTHOR]

• Stability of closed mines for energy storage in underground hydro plants is analyzed. • 3D numerical model is used to analyze the stability of the underground infrastructure. • Excavation of underground caverns and tunnels is technically feasible. • Excavation and operation stages are studied and approved for two mine scenarios. • Air pressure in the lower reservoir may cause stability problems in the tunnels. Electricity storage systems are necessary to increase the efficiency of variable renewable energies. Mine water in closed underground coal mines can be used for underground pumped-storage hydropower plants. Subsurface energy storage systems require the excavation of a powerhouse cavern and a network of tunnels as lower water reservoir. To prevent build-up of high air pressures in the network of tunnels during the water filling process it is necessary to excavate ventilation shafts. In this paper, fluid dynamics and geomechanical behavior are combined in the lower reservoir in order to know the feasibility of the underground infrastructure. Depending on the diameter of the ventilation shafts and water flow rate, air pressures up to 420 kPa can be reached. The stability of the powerhouse cavern and the effect of air pressure on the tunnels and shafts during the operation phase of the turbine are analyzed. Coal mines in Northern Spain have been selected as a case study to investigate the behavior of the underground infrastructure of a hydropower plant. Three-dimensional numerical models were developed and analyzed for different rock mass scenarios. The results obtained show that the effect of air pressure on the tunnels and shafts is moderate. The direction of air flow switches during operation time and long-term fatigue damage could be produced to the contour of excavations. The excavation of the powerhouse cavern is technically feasible in case of the application of a proper designed support system. [ABSTRACT FROM AUTHOR]