Rock Specimens Research Articles

A geometric variation method to extract rock strength parameters via uniaxial compression testing

Numerical modelling in rock engineering and mechanics often requires inputting multiple sets of strength parameters, such as the commonly encountered tensile strength, cohesion, and friction angle. Usually, obtaining these parameters requires performing different types of rock mechanics experiments on certain rock specimens, such as tension tests and triaxial tests, which are time-consuming and labor-intensive. Uniaxial compression testing is simpler and requires less demanding equipment. This paper proposes a solution that relies solely on uniaxial compression testing to obtain multiple sets of rock strength parameters. The basic idea is to treat the process of acquiring multiple parameters as a problem of solving a set of mathematical equations. The uniaxial tests with different geometric specimens provide known quantities and the corresponding equations that link these known quantities (e.g., peak loads) with unknown quantities (e.g., strength parameters). We established three-dimensional numerical tests to represent the relationship between input strength parameters and the peak load. By transforming it into an optimization problem, numerical modelling is embedded into the objective function of the optimization algorithm, thereby achieving the automatic extraction of strength parameters. This method is verified through uniaxial compression tests using red sandstone specimens with various geometries. The results show that by changing the geometry of the rock specimens, multiple sets of strength parameters can be obtained. These parameters are successfully used to predict (extrapolate) peak loads of other geometric specimens. This method holds promise for achieving the automated extraction of material parameters for the numerical modelling of rock.

Monash Rocks

This paper describes the development of the “Monash Rocks” app - designed to bring our landscape to life through augmented reality. We describe the highs and lows of the development process, the lessons we learned along the way, and our plans for further development of the app to showcase the Monash Earth Sciences Garden and extend the space into further innovative, immersive teaching and learning experiences. The creation of Monash University’s Earth Sciences Garden (MESG), a "living" geological map of Victoria collating nearly 500 rock specimens, gave us the perfect vehicle for an Augmented Reality (AR) experience. Students and visitors to the MESG can now use the Monash Rocks App on their phones to view a 3D display that overlays the live camera feed on the device enhancing the experience of the environment, taking it to another dimension. The value in augmenting a learning environment is in its ability to pull virtual objects into real scenes (Green & Chandler, 2014, p.549), in this case expanding the physical environment through time and space on a journey back millions of years. The rock now becomes alive, telling its story and supplying information that is missing in the “real life” walk through the garden.

End friction and its effect on crack propagation in fractured rock specimens

End friction and its effect on crack propagation in fractured rock specimens

Investigating the failure mechanism of X-shaped non-persistent joints under uniaxial loading: Experimental and numerical analysis

In this paper, rock samples containing non-persistent cracks of ‘‘X’’- shape were prepared and experimentally studied under uniaxial compression. Then, the numerical simulations of these tests were established using dynamic finite element software (LS-DYNA). Various joint configurations are assumed and their lengths are considered to change from the center of “X” in different rock specimens. The cracks propagation patterns, the failure processes, and energy absorptions during the tests are studied. The gypsum samples’ dimensions were 10 × 10 × 5 cm and one “X” shaped non-persistent crack was provided inside each specimen. The tensile strength of gypsum samples was 1 MPa. The notch's lengths are similar in some three cases while the length of one wing of the crossest joint varied in two other cases. In two residual cases, the length of the two wings of the crossest joint was varied. The opening of the crack was 1 mm. Angel between the notch trajectories was 90°. The model was subjected to an applied axial load rate of 0.05 mm/min. Results show that the failure mechanism of specimens was affected by notch length. The larger notch has a dominant effect on the breakage mode and failure strength. Newborn tensile cracks originated at the notch tip and propagated parallel to the loading axis to merge with the model boundary. Both the crack initiation stress and breakage stress were decreased by increasing the notch length. The absorbed energy has a minimum value when two large notches with a length of 6 cm exist in the model while it has a maximum value when notch lengths are equal to 2 cm. The increase in notch length resulted in a decrease in the difference between strains associated with crack initiation stress and final stress, indicating progressive breakage. Whereas the difference between strains associated with crack initiation stress and final stress was increased by decreasing the notch length, therefore delay failure occurred after crack initiation in the model. The results obtained from both laboratory experiments and numerical simulations show good agreement in the failure progression.

Quantitative characterization and fracture morphology in reservoirs with various lithologies: An experimental investigation

Accurately quantifying the development of hydraulic fractures is pivotal for further elucidating the relationship between reservoir physical properties and hydraulic fracture propagation. In this research, we centered on reservoirs featuring diverse lithologies, procured downhole full-diameter cores, conducted triaxial hydraulic fracturing experiments, and employed CT scanning and electron microscopy to characterize fracture morphology. The primary findings are summarized as follows: sandstones specimens typically exhibit bi-wing fractures inclined at a certain angle to σH, with internal microfractures predominantly comprised of corrugated shear fractures traversing mineral grains. Conversely, conglomerate specimens display a higher prevalence of branching fractures, with fracture zones forming in areas of gravel densities and weak heterogeneous matrices, along with the development of multiple microfractures. Volcanic rock specimens fractures primarily propagate via deflection following NF activation, showcasing two modes of microfracture propagation: through and alongside the NF. Quantitative parameters of fracture morphology indicate that the fractal dimension ranges from 2.02 to 2.08 for sandstone fractures, 2.19 to 2.22 for conglomerate fractures, and 2.11 to 2.13 for volcanic rock fractures. Notably, the NF filler in volcanic rock undergoes erosion by fracturing fluid, accumulating non-uniformly on the surface of rock crystals, thereby augmenting fracture tortuosity. Additionally, the fracture surface area of conglomerates surpasses that of sandstones and volcanic rocks by 1.3 times, with the stripping rate of conglomerate fracture surfaces exceeding 10%. Furthermore, the average normal width of conglomerate fractures surpasses 2 mm, whereas volcanic rock and sandstone measure 1.6 mm and 1.1 mm, respectively. Notably, sandstone fractures exhibit uniform normal width distribution, while conglomerate fractures, especially those around gravels and eroded by fracturing fluid, manifest larger widths. Moreover, NF influenced volcanic rock fractures also exhibit wider widths. Elucidating the quantitative attributes of fracture morphology in lithologically diverse reservoirs holds significant implications for formulating hydraulic fracturing and targeted optimization strategies aimed at enhancing fracturing efficacy.

The macroscopic and microscopic fatigue failure mechanisms of high-temperature thermally-damaged granite under cyclic impact loading

The rock surrounding wells for developing geothermal energy are in an extreme environment with high temperature and long-term dynamic disturbance. To study the influences of different temperatures and dynamic disturbances on the damage, mechanical properties, and fatigue failure mechanism of rocks, a series of microscopic thermal damage analysis and dynamic cyclic impact tests were conducted on granite specimens treated at 25 ∼ 900 °C. Results show that after being thermally treated to different extents, granite specimens are weakened in terms of the dynamic strength and deformation resistance under cyclic impact loading. As the temperature rises, the number of impacts bearable by specimens before failure decreases accordingly. Under the joint action of temperature and strain rate, 450 °C is found to be the threshold temperature for the significant reduction of dynamic strength of the rock. Under action of temperature and strain rate, the proportion of dissipated energy gradually reduces with rising temperature. Moreover, the dynamic failure mode of rock specimens at high temperatures gradually changes from splitting failure to crushing failure, the fractal dimension gradually increases, and the cumulative dissipated energy density decreases. Finally, the deterioration degree of elastic modulus was adopted to describe the progressive damage evolution of thermally-treated granite under cyclic impact loading. Furthermore, the fracture morphologies of rock specimens after failure were analyzed from the microscopic perspective to reveal the macroscopic and microscopic fatigue failure mechanisms of the rock under the coupling of temperature and strain rate.

Experimental study on I/II/III mixed mode fracture characteristics of a combined rock mass under creep loading

I/II/III mixed mode fractures of intersecting joint fissures often occur in natural rock masses, and jointed rock masses are prone to rockbursts in deep underground engineering when subjected to long-term crustal stresses. However, most studies of the mechanical mechanisms of these intersected joints have been conducted by simplifying two-dimensional joint model tests. Furthermore, the fracture mechanisms of two-dimensional intersected joints under tension and compression are completely different from those of three-dimensional joints. This paper presents a novel prefabricated specimen with combinations of intersecting joints capable of detecting the failure behaviours of rock I/II/III mixed mode fractures under creep loading. Uniaxial compression and multistage creep tests are performed on prefabricated sandstone specimens with intersecting joints of 0°/0°, 0°/30°, 0°/60°, and 0°/90°. The experimental results show that with the increase in the number of prefabricated intersecting joints, the uniaxial compressive strength and elastic modulus values of the sandstone specimens gradually decrease. In addition, the sandstone specimens experience relatively few AE events and minor axial strain variations in the first creep stage and the second creep stage of the multistage creep test. The axial strain increases sharply due to the sharp increase in the number of AE events in the third creep stage. The 0°/60° sandstone specimen undergoes accelerated creep failure, resulting in mixed X-shaped tensile‒shear rupture. The RA value is high based on the quantification of the creeping cracks using the acoustic emission parameters of the rise angle (RA) and average frequency (AF). The AF values of the 0°/0°, 0°/30°, and 0°/90° sandstone specimens are high. The experimental results show that a larger joint intersection angle leads to greater mutual restraints and greater effects of prefabricated crack propagation in the rock specimens, thus increasing the final failure strength. Finally, based on the acoustic emission count, a characteristic variable D suitable for characterizing the creep damage evolution of a joint rock mass is established. The findings of this paper can facilitate an effective understanding of the creep effect of I/II/III mixed mode fracture and its micromechanism. The research results will have a certain reference value for the detection and risk mitigation of instantaneous and time-delayed rockbursts.

Interaction and initiation mechanism of non-parallel multiple cracks by strain gauge method

The maximum stress intensity factors, KI,kmax(θ) and KII,kmax(θ), of three non-parallel oblique cracks were calculated to reveal the interaction mechanism of multiple cracks. The multiple crack initiation criterion was adopted to predict the non-parallel multiple crack initiation parameters. Uniaxial compression tests were performed on rock specimens (red sandstone) with three non-parallel oblique cracks, where the strain near the crack tips was monitored by the strain gauge measurement method to determine the crack initiation sequence. The results showed that the change of crack relative inclination angle had little effect on the fracture trajectory. Multiple crack tips were seldom initiated at the same time. The failure modes of red sandstone specimens with three non-parallel oblique cracks under uniaxial compression are divided into two types: wing crack penetration and wing crack–secondary crack penetration. All of the fracture mechanisms are Mode I (tensile) initiation. The predicted results coincided well with the test results, verifying the validity of the theoretical results.

Meso-mechanical behavior of crystalline rocks: An image-based discrete element method

This research examines the meso-mechanical behavior of rock materials under various loading conditions, with a focus on the influence of mesoscale heterogeneity in crystalline rock specimens. Traditional approaches often rely on a limited set of statistical parameters for constructing numerical models, failing to capture the complex mesoscale heterogeneities integral to rock mechanics. To overcome this limitation, the study utilizes an advanced image-processing technique to accurately identify mineralogical features within rock microstructures. The extracted mineralogical data were then integrated into a discrete element method (DEM) model to explore the mesoscale mechanical responses of crystalline rocks. The results of the simulations indicated that mesoscale heterogeneities play a critical role in stress concentration localization, which in turn dictates the direction of fracture propagation and leads to a sophisticated interaction among various fracture pathways.

Fluid-rock interactions during intermittent injection of supercritical CO2: An investigation for CO2 storage in the depleted Nini West oil field, Danish North Sea

CO2 that is planned to be stored in the depleted Nini West oil field will be delivered by ship which results in intermittent injection of supercritical CO2 (scCO2). It is important to investigate the effect of this injection scenario on rock specimens in the early phase of the storage program. In this regard, core flooding experiments have been conducted at current reservoir conditions of 200 bar and 60 °C. The state of plug samples pre- and post-scCO2 injection were compared by coupled scanning with the scanning electron microscope (SEM) and micro-CT scanning to track mineralogical changes at core-scale. The coupled approach shows partial dissolution of garnet, specifically almandine and carbonate minerals such as siderite, rhodochrosite and calcite in the glauconitic sandstone after 5 sets of scCO2 injection periods. Effluent brine ion content measured after each brine injection operation shows an increase in Fetot, Si+, and Mn2+ in the effluent brine compared with the injection brine, which agrees with dissolution of minerals captured by the SEM and micro-CT scan. ScCO2 effective permeability at the end of each scCO2 injection period ranges between 18 and 34 % of the initial brine permeability at 100 % water saturation (Sw). The conclusion is that CO2 permeability over these periods of injection is not affected by neither the dissolution of minerals, since these minerals are present only in low concentrations, nor the salt precipitation due to scCO2 injection.

Study on the Failure Process and Acoustic Emission Characteristics of Freeze–Thawed Sandstone under Cyclic Loading and Unloading

In order to investigate freeze–thawed red sandstone failure processes under cyclic loading and unloading conditions, real-time acoustic emission (AE) and scanning electron microscopy (SEM) techniques were used to reveal the fracture process of the saturated red sandstone after cyclic loading and unloading tests using uniaxial compression. The results show that the stress–strain curves of the freeze–thawed sandstones show signs of hysteresis and exhibit a two-stage evolution of “sparse → dense”. In the cyclic loading and unloading process, the modulus of elasticity in the loading process is always larger than that in the unloading process, while the Poisson’s ratio is the opposite, and the radial irreversible strain and cumulative irreversible strain are larger than those in the axial direction. As the number of freeze–thaw cycles increases, the rock specimens need more cycles of loading and unloading to make the crack volume compressive strain Δεcv+ reach the maximum value and tend to stabilize, while the crack volume extensional strain Δεcv− tends to decrease gradually. This study also shows that the growth phase of the cyclic loading and unloading process has more ringing counts and a shorter duration, while the slow degradation phase has more ringing counts with loading and less with unloading. In addition, the F-T cycle gradually changes the internal microcracks of the red sandstone from shear damage, which is dominated by shear cracks, to tensile damage, which is dominated by tensile cracks. This study’s findings contribute to our knowledge of the mechanical characteristics and sandstone’s degradation process following F-T treatment, and also serve as a guide for engineering stability analyses conducted in the presence of multiphysical field coupling.

Gas Fracturing Simulation of Shale-Gas Reservoirs Considering Damage Effects and Fluid–Solid Coupling

With the increasing demand for energy and the depletion of traditional resources, the development of alternative energy sources has become a critical issue. Shale gas, as an abundant and widely distributed resource, has great potential as a substitute for conventional natural gas. However, due to the low permeability of shale-gas reservoirs, efficient extraction poses significant challenges. The application of hydraulic fracturing technology has been proven to effectively enhance rock permeability, but the influence of environmental factors on its efficiency remains unclear. In this study, we investigate the impact of gas fracturing on shale-gas extraction efficiency under varying environmental conditions using numerical simulations. Our simulations provide a comprehensive analysis of the physical changes that occur during the fracturing process, allowing us to evaluate the effects of gas fracturing on rock mechanics and permeability. We find that gas fracturing can effectively induce internal fractures within the rock, and the magnitude of tensile stress decreases gradually during the process. The boundary pressure of the rock mass is an important factor affecting the effectiveness of gas fracturing, as it exhibits an inverse relationship with the gas content present within the rock specimen. Furthermore, the VL constant demonstrates a direct correlation with gas content, while the permeability and PL constant exhibit an inverse relationship with it. Our simulation results provide insights into the optimization of gas fracturing technology under different geological parameter conditions, offering significant guidance for its practical applications.

Effect of unloading rate of normal stress on the frictional slip behaviour of fractured rocks

Unloading disturbances encountered during excavations in fractured rock masses significantly affect the safety of deep underground structures. However, the influence of the unloading rate on the frictional slip behaviour of fractured rocks is not fully understood. In this study, a series of fracture slip tests induced by unloading normal stress were carried out, where the effects of unloading rate, initial normal stress, and initial shear stress were considered. Acoustic emission (AE) technique was employed to monitor damage in the fractured rock specimens. The results reveal that the unloading rate played a significant role in the frictional slip behaviour and failure mode of the fractured rock specimens. At lower unloading rates, the stress path approached the failure envelope horizontally, whereas at higher unloading rates, the stress path followed a diagonal trajectory. The near-critical stress state and near-critical stress ratio were introduced to describe the normal and shear stresses during the fracture activation slip. The results show that lower unloading rates (≤0.010 MPa/s) consistently placed the near-critical stress state closer to the failure envelope compared with higher unloading rates. In addition, the near-critical stress ratio decreased with an increase in the unloading rate. The AE count exhibited a distinct peak before failure of the fractured rock. This initial peak in the AE signals corresponds to the violent rupture of the surface asperities, making it a valuable indicator for monitoring and predicting potential failures in fractured rock masses. An empirical model was developed to describe the relationship between the unloading rate and near-critical stress ratio.

Experimental investigation of the intensity and conditions for strainburst triggering in intact hard rocks

Strainburst occurs during deep hard rock excavations. A series of single-free-face true triaxial compression (TTC) tests were conducted on seven hard rock types to investigate strainburst intensity and triggering conditions. Five of the seven rock types failed by strainburst, and two failed by spalling. The differences between the strainburst intensities are attributed to the mineral composition and structure. High-hardness minerals and mosaic-type microstructures facilitate stress conduction, which promotes rock microcrack initiation and propagation, causing severe rock fragmentation and fragment ejection, as observed for the Jinzhou granite. However, if the proportions of high- and low-hardness rock minerals are high and the microstructure is stable, such as in the triangular microstructure of the Chifeng basalt, then microcrack initiation and propagation are inhibited, decreasing the likelihood of rock fragmentation. The triggering conditions for strainburst were further analysed by considering the source of kinetic energy. For intact rock specimens, the kinetic energy for rock fragment ejection mainly originates from the intrinsic potential energy (Wbk) released by the specimen itself; thus, strainburst triggering requires that the applied maximum principal stress exceeds the bearing capacity and that the rock Wbk exceeds zero. However, a high Wbk is needed to trigger strainburst for structural rock specimens. Based on these findings, the possibility of triggering strainburst during excavation is greater when the Wbk (>9.1 kJ/m3), rock quality designation (>75%), and in situ stress of the rock are all high.

Damage characteristics of sandstone subjected to freeze–thaw cycles under different stress paths

The damage characteristics and failure modes of rock specimens subjected to different freeze–thaw (FT) cycles are vital to rock engineering in cold regions. Hence, in this study, the physical and mechanical parameters of sandstone specimens under different FT cycles were measured, and their deterioration laws were analyzed. The damage features of the rocks in different FT states under loading along various stress paths, including the main-frequency variation and crack propagation path evolution, were statistically analyzed. Two damage variables, one related to the acoustic emission (AE) and the other related to the rock strength, were combined to reveal the AE characteristics and damage evolution laws in the different damage states. The results showed that FT cycling led to a decrease in the peak stress, an increase in the number of microcracks, an increase in the AE activity, a shift in the main-frequency distribution toward the low-frequency range, and a shift in the damage mode from shear damage to a mixed damage dominated by tensile damage. The degree of strength deterioration in the rock specimens was affected by FT cycling under the different stress paths in the following influence order: conventional uniaxial loading > equal-amplitude cyclic loading> multistage cyclic loading; cyclic loading was more likely to induce tensile rupture than conventional uniaxial loading. The AE characteristic parameters were consistent with the stress–strain curve variations, which could reflect the damage evolution process of the rock specimens. The main frequencies showed a band-like evolution trend with the loading process and could be divided into three bands: a low-frequency interval (0–50 kHz), a medium-frequency interval (50–150 kHz), and a high-frequency interval (150–300 kHz); the main frequencies were concentrated in the low-frequency interval. With the increase in the number of FT cycles, the main frequency in the high-frequency interval and the number of corresponding microcracks gradually decreased.

An extended discontinuous deformation analysis for simulation of grouting reinforcement in a water-rich fractured rock tunnel

Grouting has been the most effective approach to mitigate water inrush disasters in underground engineering due to its ability to plug groundwater and enhance rock strength. Nevertheless, there is a lack of potent numerical tools for assessing the grouting effectiveness in water-rich fractured strata. In this study, the hydro-mechanical coupled discontinuous deformation analysis (HM-DDA) is inaugurally extended to simulate the grouting process in a water-rich discrete fracture network (DFN), including the slurry migration, fracture dilation, water plugging in a seepage field, and joint reinforcement after coagulation. To validate the capabilities of the developed method, several numerical examples are conducted incorporating the Newtonian fluid and Bingham slurry. The simulation results closely align with the analytical solutions. Additionally, a set of compression tests is conducted on the fresh and grouted rock specimens to verify the reinforcement method and calibrate the rational properties of reinforced joints. An engineering-scale model based on a real water inrush case of the Yonglian tunnel in a water-rich fractured zone has been established. The model demonstrates the effectiveness of grouting reinforcement in mitigating water inrush disaster. The results indicate that increased grouting pressure greatly affects the regulation of water outflow from the tunnel face and the prevention of rock detachment face after excavation.

Experimental study of sulfate crystallization damage to glutenite rock in the Maijishan Grottoes

Salt crystallization is one of the most important factors causing weathering in grottoes. Cumulative crystallization of salts causes damage such as flaking and peeling of the rocks and accelerates the weathering processes of the grottoes. The accumulated crystalline salts cause spalling, skinning and other damage and accelerate the weathering process of the grotto rock body. It is necessary to study the existing glutenite rock grottoes. This paper took the glutenite rock of the Maijishan Grottoes as a case study, and Na2SO4 or MgSO4 solutions were applied to glutenite rock specimens subjected to different deterioration cycles. The crystallization patterns of the two different salts and their damage to the glutenite rock were analyzed and studied, the mechanism for salt crystallization damage to the glutenite rock was explored, and the crystallization pressures of the two salts in the glutenite rock were derived with theoretical calculations. The results showed that both Na2SO4 and MgSO4 crystallization damaged the glutenite rock, and the former different sulfate solutions changed at different rates, and the changes in the wave caused faster damage than the latter. The physical indices of the rock samples in the velocities and tensile strengths were consistent. Na2SO4 was mainly accumulated on the surface of the specimen and damaged the glutenite rock centripetally via pulverization and exfoliation. MgSO4 mainly crystallized inside the glutenite rock, which created internal fissures and reduced the strength of the rock. The theoretical maximum crystallization pressures of Na2SO4 and MgSO4 in the glutenite rock specimens reached 33.00 MPa and 9.94 MPa, respectively. This study provides a theoretical basis for studies of salt crystallization in glutenite rock grottoes and provides a method for protecting the stones in cultural heritage sites against weathering.

Computational modelling of microwave-induced fractures in igneous rocks using phase field method

This study introduced a simplified thermodynamic model based on the regularized phase field method for simulating the thermally induced fractures of rocks pretreated by microwave. A thermo-mechanical model comprises three main parts developed to evaluate the fractures: the heat generation of the rock specimen is determined via the heat balance equation; the calculation of thermally induced stress is performed in the second part by taking into account the thermal stress due to temperature change; the third part calculates the fracture induced by microwave heating using the phase field equation. Combining these three parts enables simulations of rock fractures due to thermally induced stress. The first simulation is the quenching test of ceramics to validate the robustness of the phase field method in modelling the thermo-mechanical fracture. Then, examples of microwave-induced fracture are presented and discussed in comparison with microwave test results. A good agreement is achieved from the results of numerical analysis and experiments, which proves the feasibility of the proposed thermo-mechanical coupling fracture (phase field) model for the simulation of thermal-mechanical fractures. It is further found that the power level is crucial in influencing the fractures given the same energy input. More specifically, a higher power level leads to an increased non-uniformity of a thermal gradient, which, as a result, leads to a higher stress gradient and more fractures generated eventually. Furthermore, the energy utilization ratio also benefits under a more intensive microwave power level. The developed model quantitatively enriches the understanding of fractures produced by microwave irradiation and advances the application of the microwave-assisted rock breakage technique in industry and academics.

Energy evolution mechanism of structural surfaces in sandstones with different dips based on the energy principle.

A uniaxial compression test was conducted on sandstone specimens at various inclination angles to determine the energy evolution characteristics during deformation and damage. Based on the principle of minimum energy dissipation, an intrinsic model incorporating the damage threshold was developed to investigate the mechanical properties of sandstone at different inclination angles, and the energy damage evolution during deformation and damage. This study indicated that when the inclination angle of the structural surface remained below 40°, sandstone exhibited varying mechanical properties based on different inclination angles. The peak strain was positively correlated with the inclination angle, whereas the compressive strength and modulus of elasticity showed negative correlations. From an energy perspective, the deformation and damage of sandstone under external loading entail processes of energy input, accumulation, and dissipation. Moreover, higher inclination angles of the structural surface resulted in a smaller absorbed peak strain and a reduced proportion of dissipated energy relative to the energy input, thereby affecting the evolution of energy damage throughout the process. As the inclination angle of the structural surface increased, the absorbed total strain at the peak value decreased, whereas the proportion of the dissipated energy increased. Additionally, the damage threshold and critical value of the rock specimens increased with the inclination angle. The critical value, a composite index comprising the peak strain, compressive strength, and elastic modulus, also increased accordingly. These findings can offer a novel perspective for analyzing geological disasters triggered by fissure zones within underground rock formations.

A thermal stress loading technique for large‐sized hot dry rock mechanical tests

AbstractTesting of large‐sized specimens is becoming increasingly important in deep underground rock mechanics and engineering. In traditional mechanical loading, stresses on large‐sized specimens are achieved by large host frames and hydraulic pumps, which could lead to great investment. Low‐cost testing machines clearly always have great appeal. In this study, a new approach is proposed using thermal expansion stress to load rock specimens, which may be particularly suitable for tests of deep hot dry rock with high temperatures. This is a different technical route from traditional mechanical loading through hydraulic pressure. For the rock mechanics test system of hot dry rock that already has an investment in heating systems, this technology may reduce the cost of the loading subsystem by fully utilizing the temperature changes. This paper presents the basic principle and a typical design of this technical solution. Preliminary feasibility analysis is then conducted based on numerical simulations. Although some technical details still need to be resolved, the feasibility of this loading approach has been preliminarily confirmed.