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An understanding of shear behavior of fractured rocks subjected to different durations of groundwater immersion is essential for practical rock engineering. Direct shear tests were conducted on fractured granite samples after different immersion durations of 1, 3, 6 and 12 months, and the initially dry granite fracture samples were also tested as reference. The results indicate that the peak shear strength, residual shear strength, shear stiffness and dilation at peak shear stress decreased with increasing immersion duration, while the peak shear displacement increased with increasing immersion duration. The shear induced reductions in fracture surface parameter (Z2) and the sheared-off volumes of asperities increased slightly with increasing immersion duration. An empirical function was proposed to successfully describe the evolutions of the shear parameters and the volume of sheared-off asperities with immersion duration. In addition, micro-scale tests including atomic force microscope, X-ray photoelectron spectra, nanoindentation and so on were conducted to reveal the underlying mechanism of immersion-induced weakening of granite fractures, which is attributed to the formation of a thin water layer attached on granite surfaces and the decreasing mechanical properties of asperities. [ABSTRACT FROM AUTHOR]

A clear understanding of shear behaviors of granite fractures with high temperatures undergoing water-cooling treatment is important in enhanced geothermal system (EGS). Two types of fractured Gonghe granite specimens, containing either a Brazilian-induced artificial fracture and or a pre-existing natural fracture, respectively, were considered. Before direct shear tests, all specimens were suffered from slow heating and rapid water-cooling treatment, and the thermal treatment temperatures ranged from room temperature 25 °C (without thermal treatment) to 300 °C. Shear properties of natural fractures, including shear strength, shear stiffness and shear dilation, are lower than that of the artificial fractures, mainly ascribed to their lower surface roughness than the artificial ones. Better matching in the artificial fractures after thermal treatment contributes to the increase in peak shear strength. The roughness increment or reduction after thermal treatment is insignificant. The thermal treatment results in the larger sheared-off asperities damage volume after shear in the artificial and natural fractures than that without thermal treatment. There is a strong correlation between the shear stress and acoustic emission (AE) parameters in both artificial and natural fractures. The AE responses in natural fractures are weak compared to that in artificial fractures. AE events distribute uniformly near the fracture surface in artificial specimens but present dispersedly in the natural ones. [ABSTRACT FROM AUTHOR]

• The increasing thermal treatment temperatures can weaken fracture shear properties. • The distributions of thermal microcracks determine fracture shear properties. • Thermal over-closure causes a slight increase in fracture shear strength. In this study direct shear tests on a series of thermally-treated Beishan granite samples including a rough fracture were conducted, and the thermal treatment temperature was up to 400 °C. Particle mechanics models for Beishan granite were also built up to examine the characteristics of thermally-induced and shear-induced microcracks in granite samples. The results showed that fracture shear stiffness and peak strength decreased with increasing thermal treatment temperature. Thermally-induced microcracks are the main source of the change in fracture shear properties with increasing themral treatment temperatures. Generally the randomly distributed thermally-induced microcracks mainly play a role in reducing the fracture toughness of the asperities, which contributes to the decrease in fracture shear stiffness and peark strength. However, some thermally-induced microcracks may be at large angles to the fracture plane, and can blunt the crack tip and trigger the shear-induced cracks to propagate deeper into fracture walls, which can increase the fracture shear strength and surface damage. [ABSTRACT FROM AUTHOR]

Direct shear tests on a series of fractured Beishan granite samples were performed under different normal stresses, to investigate the characteristics of asperity degradation during shear and to quantify their effects on solute retardation in fractures. A three-dimensional laser scanner and a laser diffraction particle size analyzer were used to examine the shear-induced asperity volume loss and the size distribution of sheared-off fragments. The results identified two main modes of asperity degradation during shear, i.e., instantaneous failure of asperities occurring at the peak shear stress, and crushing of the generated fragments with further shear displacement. Based on the volume of asperity degradation, a new joint damage coefficient was proposed. The size of the sheared-off fragments under different normal stresses was found to follow a Weibull distribution. Combining the proposed joint damage coefficient and the Weibull size distribution of the sheared-off fragments can approximately predict the potential effects of sheared-off fragments on solute retardation coefficient in rock fractures. The results showed that the shear-induced asperity degradation significantly increases the values of the solute retardation coefficient, by offering more sorption surfaces in fracture voids. [ABSTRACT FROM AUTHOR]

Highly mineralized and acid/alkaline groundwater is critical to the short- or long-term stability of fractured rock masses in geological engineering. Understanding the impact of water-rock interactions on mechanical properties of fractured rock masses is important but challenging. This study investigated the physicochemical interactions between chemical solutions and granite fractures which can directly control the mechanical behaviors of fractured rock masses. Direct shear tests were performed on artificial granite fracture samples immersed in chemical solutions with different pHs for 30 and 150 days. The results show that the solution of pH = 2 has the most significant influence on fracture shear properties, followed by the solutions of pH = 12 and pH = 7. The deionized water has the minimal influence on fracture shear properties. The shear properties of fractures immersed in acid and alkaline solutions are sensitive to immersion time, whereas the shear properties of fractures immersed in neutral solutions are not significantly affected by immersion time. The weakening mechanisms of the water-rock chemical interaction are different in different chemical solutions. Dissolution mainly works in acid solutions, whereas the competing minerals dissolution and precipitation/crystallization occur in alkaline solutions. This is why alkaline solutions have less significant influence on fracture shear properties than acid solutions. • Shear behaviors of granite fractures immersed in different pH solutions are studied. • The mechanisms responsible for fracture weakening induced by physicochemical water-rock interactions are discussed. • A coefficient to evaluate the effect of physicochemical water-rock interactions on fracture peak strength is proposed. [ABSTRACT FROM AUTHOR]

• AE energy is an effective index to evaluate shear behavior of single granite fracture. • Characteristics of shear induced asperity degradation are correlated with AE energy. • A new method of calculating AE b -values based on AE energy is proposed. Shear induced asperity degradation and gouge formation significantly affect the mechanical and fluid flow behaviors of single rough rock fractures, but the asperity damage process during shear has not been entirely understood and requires further investigation. In this study, direct shear tests were performed on the artificial and natural fractured granite specimens, respectively. The evolution of asperity damage during shear was monitored using acoustic emission (AE) technique, and the asperity degradation zones and damage volumes caused by shear were evaluated with the 3D blue light scanner. The correlation between the asperity degradation and AE energy was examined. The results show that there is a strong correlation between shear stress and cumulative AE energy in both artificial and natural fractures. AE events are scattered in the pre-peak stage but are localized around the fracture surface in the post-peak stage. The shear induced asperity damage is represented well by the clusters of AE events with large amounts of energy, which initially occur after the shear entering the post-peak stage. Their distribution conforms to the destruction locations of surface asperities. The proposed new method of calculating AE b -values based AE energy is more suitable to predict the surface damage characteristics in the artificial and natural granite fractures. The consistency between AE energy and shear induced damage volume of asperities is found when the detection area increases up to a certain range of about 25 mm. [ABSTRACT FROM AUTHOR]

The behavior of fluid flow through rock fractures at varying temperatures is a critical issue in many subsurface geo-energy projects. Previous work has considered the thermal effects on fracture permeability, but not in isolation of the chemical effects as well. Therefore, to quantitatively assess the mechanical influences on fracture permeability, we present results from permeability tests of five cylindrical Beishan granite samples, each with a single artificial fracture, at different temperatures. Three samples were tested at a constant confining pressure of 5 MPa and temperatures of 22 °C, 90 °C and 150 °C for 22 days, to examine creep-induced changes in fracture permeability. Fracture permeability decreases with time until about 10 days, and the eventual magnitudes of fracture permeability reduction are much greater at 90 °C and 150 °C than at 22 °C. Two samples were subjected to three heating cycles (30 °C to 150 °C) at a constant confining pressure of 5 MPa. Fracture permeability decreases as temperatures increase from 30 °C to 150 °C, and then slight changes in permeability occur as the sample is cooled to 30 °C. A similar tendency appears in subsequent temperature cycles, while the magnitude of fracture permeability reduction decreases with increasing heating-cooling cycles. A coupled thermal-mechanical model considering asperity damage is developed to describe the thermally-induced changes in fracture permeability, which properly predicts the experimental results. The residual deformation of asperities and temperature dependent Young's modulus play an important role in thermally-induced changes in fracture permeability. [ABSTRACT FROM AUTHOR]

• Beishan granite thermally treated under high temperatures exhibits auxetic behavior. • Thermally-induced microcracks result in many block clusters in granite samples. • Block clusters rotating causes auxetic behavior under the low uniaxial loadings. Majority of minerals or rocks commonly display a positive Poisson's ratio between 0.05 and 0.45. However, granites that experience high temperature treatment can exhibit negative Poisson's ratio under low compressive stresses. The underlying mechanism responsible for the auxetic behavior of thermally-treated granites is poorly understood. Here we present experimental results of uniaxial compression tests on thermally-treated Beishan granite, which show auxetic behavior with increasing thermal treatment temperature and the Poisson's ratio is as low as −0.18 for a maximum thermal treatment temperature of 650 °C. To reveal the thermally-induced change in the microstructures of Beishan granite, particle discrete element models are built and calibrated using the experimental data. The numerical results show that intensively distributed microcracks appear in the Beishan granite specimens after high temperature treatments. The number and the widths of microcracks increase with increasing thermal treatment temperature. A number of isolated block clusters consisting of mineral grains form due to the connection of thermally-induced microcracks under high temperature thermal treatment. The deformation behavior of these block clusters is primarily responsible for the observed negative Poisson's ratio of the thermally-treated Beishan granite during compression. [ABSTRACT FROM AUTHOR]

Microwave radiation is pioneered for weakening and cracking rock in the fields of tunneling and mining engineering. However, there has been limited research on the application of microwave heating for geothermal energy extraction, and the precise damage mechanism remains poorly understood. To address this gap, a fully coupled electromagnetic-thermal–mechanical model, incorporating damage variables, was established and then verified by lab tests. Simulation results of coaxial microwave heating granite show that the damage mechanism is different from that of conventional rectangular waveguide heating. Coaxial microwave heating leads to the formation of two distinct hot spots in the granite. The differential tensile stress between the interior and exterior of the borehole contributes to the damage in the upper hot spot, while the interior compressive and exterior tensile stress state induces damage in the lower hot spot. Consequently, the deformation pattern of granite assumes a vase shape, with the damaged area exhibiting a canopy shape. When the initial crack trend is 135°, the crack connectivity efficiency is maximized to 61 % at 1.5 kW microwave power and 2.45 GHz frequency. These findings offer valuable theoretical insights for the potential application of microwave-assisted super-long gravity heat pipes in geothermal extraction. [ABSTRACT FROM AUTHOR]

• Shear stages correspond to the cumulative AE parameters-shear displacement curves. • Three characteristic stresses are defined for intact rock samples undergoing shear. • A correlation between shear stress and AE parameters exists in intact rock samples. Shear behaviors of core samples from a deep geothermal exploration well was investigated using direct shear tests. The fracturing process inside the intact rock samples during shear was monitored using acoustic emission (AE) technique. The results show that three shear stages including compaction, quasi-elastic and failure phases can be easily divided based on the shear stress-shear displacement or the cumulative AE parameters-shear displacement curves. Three characteristic stresses in terms of crack initiation stress, crack damage stress and peak strength are defined for intact rock samples undergoing shear. The crack initiation stress is about 0.80 times of the peak strength, and the crack damage stress is about 0.88 times of the peak strength. In addition, the different shear behaviors and AE characteristics between intact rocks and rock fractures are compared. AE events are scattered in the pre-peak stage but are localized around the fracture surface in the post-peak stage, while AE events are located along the shear zone in intact rock samples. [ABSTRACT FROM AUTHOR]

The conventional driving mechanisms including elevating the pore pressure or far-reaching poroelastic effects, may not necessarily explain the injection-triggered seismicity that can be up to 35–90 km far away from injection wells. In this experimental study, we examined the mechanism of water lubrication for the triggered seismicity where the fluid pressure was negligible compared to the in-situ stresses. The results show that fault slip can occur upon saturation when the ratio of the applied shear stress to the fault peak shear strength exceeds a critical threshold, which is defined to quantitatively assess the critical stress state for faults. The microscale slip test using atomic force microscopy reveals the mechanism responsible for the significant reduction of friction between granite surfaces from dry to wet state. A nanometer-thick water layer was attached on the granite surfaces, causing the shear plane shifting from the direct granite/granite interface to the water layer, and meanwhile the adhesive force between granite surfaces was significantly reduced by water intercalation, both of which played the dominate role in triggering fault slip. In practice, slip may be associated with in-situ fluid flow into far-field faults, which causes the increase of water and lubricates the fault zone. • The occurrence of fault slip triggered by saturation is dependent on the applied shear stress and surface roughens • The ratio between the shear stress on fault planes and fault peak shear strength is proper to assess the critical stress state for faults • The formation of thin water layer between the granite surfaces reduces the shear stress and adhesive force which induce friction reduction [ABSTRACT FROM AUTHOR]