Search

Granite is a viable host for deep nuclear waste disposal because its low permeability and high strength enable the stable and safe operation of the repository. We examined the evolution of the permeability and triaxial mechanical behaviour of granite after high-temperature treatment. First, the effect of the high temperature on the physical behaviour and permeability evolution of granite was analysed in detail. The mass, P-wave velocity, and thermal conductivity of granite decrease, but the volume increases with increasing temperature. The permeability of intact granite increases by four orders of magnitude as the cycled temperature increases from 25 to 800 °C. Subsequently, the effect of high temperature on the triaxial deformation and acoustic emission (AE) behaviour of granite was investigated. Under uniaxial compression at T ≤ 300 °C, the stress decreases before the peak strength is reached, corresponding to a significant AE event, which is due to the development of multiple splitting tensile fractures along the loading direction. At T ≥ 450 °C, AE event is observed once a minor stress is applied, which results from failure is controlled by thermally induced cracks. However, under triaxial compression, the temperature has little effect on the AE characteristics. The granite fails along the shear fracture plane, which becomes wider with increasing confining pressure. At T ≥ 600 °C, it is easier to form intragranular cracks and the stress quickly decreases after the peak strength is reached. The shear plane is smoother under high confining pressure. Third, the effect of high temperature on the peak strength and crack damage threshold of granite was further analysed. Generally, under uniaxial compression, the peak strength and crack damage threshold first remain relatively constant at T ≤ 300 °C, begin to decrease at T = 450 °C, and decrease more rapidly at T = 600 °C. The confining pressure notably reduces the effect of the temperature on the peak strength and crack damage threshold. Finally, the effect and mechanism of high temperature on the triaxial strength parameters of granite were further discussed. At T ≤ 300 °C, thermally induced cracks are not notable and the temperature has little effect on the strength. At 450 °C≤ T ≤ 600 °C, thermally induced cracks are more notable and the temperature has a significant effect on the strength behaviour. Because of the thermal stress released by thermal macrocrack formation, the continuous increase in the temperature has little impact on strength. [ABSTRACT FROM AUTHOR]

Understanding thermo-mechanical properties of granites with different grain sizes is of great significance for the site selection of high-level radioactive waste disposal. In this paper, a cluster model was used to investigate the uniaxial compressive behaviors of granite specimens with different grain sizes. It turns out the results of simulation match experimental results. Grain size has a significant effect on the mechanical behaviors of granite. Macro-cracks can be easily observed in the coarse granite specimens after high-temperature treatment. However, the number of inter-cracks in the granite specimens decreased with increasing the grain sizes. When T ≥ 600 °C, main cracks were not observed in the compressed specimens, and the ultimate failure modes were mostly dominated by thermally induced cracks. The isolated grains can be observed in the coarse granite specimens. The reduction in the uniaxial compressive strength of the coarse-grained granite specimens was lower than that for the fine-grained granite specimens. [ABSTRACT FROM AUTHOR]

In tunnel engineering, due to the effect of excavation disturbance, the surrounding rock mass can produce an excavation damage zone with different damage extents. Therefore, knowledge of rock deformation and damage behavior is especially significant for the design of deep tunnel support. However to date, a few experiments and numerical simulations have been conducted to investigate the deformation and mechanical failure behavior of damaged rocks. Therefore, in this research, multi-stage triaxial compression test was used to investigate the mechanical behavior of mudstone specimens with different damage extents by experiment and two-dimensional particle flow code. First, a group of micro-parameters was calibrated by single-stage triaxial compression experiments of mudstone, and the numerical results agree very well with the experimental results. Then, multi-stage triaxial compression experiment and discrete element modeling of mudstone specimens were carried out. The more axial strain the specimens sustained, the less strength they had (because the degree of damage increased). A damage variable was defined by the ratio of the area of micro-cracks to the total area of the specimen. As the post-stress reducing ratio increases, the damage variable increases rapidly until the post-stress reducing ratio reaches 0.4; then, it remains constant. The force field were analyzed to reveal the damage evolution mechanism in the mudstone specimens under multi-stage triaxial compression. [ABSTRACT FROM AUTHOR]

The deformation failure characteristics of marble subjected to triaxial cyclic loading are significant when evaluating the stability and safety of deep excavation damage zones. To date, however, there have been notably few triaxial experimental studies on marble under triaxial cyclic loading. Therefore, in this research, a series of triaxial cyclic tests was conducted to analyze the mechanical damage characteristics of a marble. The post-peak deformation of the marble changed gradually from strain softening to strain hardening as the confining pressure increased from 0 to 10 MPa. Under uniaxial compression, marble specimens showed brittle failure characteristics with a number axial splitting tensile cracks; in the range of σ = 2.5-7.5 MPa, the marble specimens assumed single shear fracture characteristics with larger fracture angles of about 65°. However, at σ = 10 MPa, the marble specimens showed no obvious shear fracture surfaces. The triaxial cyclic experimental results indicate that in the range of the tested confining pressures, the triaxial strengths of the marble specimens under cyclic loading were approximately equal to those under monotonic loading. With the increase in cycle number, the elastic strains of the marble specimens all increased at first and later decreased, achieving maximum values, but the plastic strains of the marble specimens increased nonlinearly. To evaluate quantitatively the damage extent of the marble under triaxial cyclic loading, a damage variable is defined according to the irreversible deformation for each cycle. The evolutions of the elastic modulus for the marble were characterized by four stages: material strengthening, material degradation, material failure and structure slippage. Based on the experimental results of the marble specimens under complex cyclic loading, the cohesion of the marble decreased linearly, but the internal friction angles did not depend on the damage extent. To describe the peak strength characteristics of the marble specimens under complex cyclic loadings with various deformation positions, a revised strength criterion for damaged rocks is offered. [ABSTRACT FROM AUTHOR]

To understand the fracture mechanism in all kinds of rock engineering, it is important to investigate the fracture evolution behavior of pre-fissured rock. In this research, we conducted uniaxial compression experiments to evaluate the influence of ligament angle on the strength, deformability, and fracture coalescence behavior of rectangular prismatic specimens (80 × 160 × 30 mm) of brittle sandstone containing two non-coplanar fissures. The experimental results show that the peak strength of sandstone containing two non-coplanar fissures depends on the ligament angle, but the elastic modulus is not closely related to the ligament angle. With the increase of ligament angle, the peak strength decreased at a ligament angle of 60°, before increasing up to our maximum ligament angle of 120°. Crack initiation, propagation, and coalescence were all observed and characterized from the inner and outer tips of pre-existing non-coplanar fissures using photographic monitoring. Based on the results, the sequence of crack evolution in sandstone containing two non-coplanar fissures was analyzed in detail. In order to fully understand the crack evolution mechanism of brittle sandstone, numerical simulations using PFC were performed for specimens containing two non-coplanar fissures under uniaxial compression. The results are in good agreement with the experimental results. By analyzing the stress field, the crack evolution mechanism in brittle sandstone containing two non-coplanar fissures under uniaxial compression is revealed. These experimental and numerical results are expected to improve the understanding of the unstable fracture mechanism of fissured rock engineering structures. [ABSTRACT FROM AUTHOR]

To investigate the role of fissure angle and heat treatment temperature on the mechanical properties and deformation failure behavior, uniaxial compression tests were carried out on granite specimens containing a single fissure. Using stress–strain curves, the peak strength, peak strain, and elastic modulus of the one-fissured granite specimens were analyzed in detail. The mechanical parameters are closely related to the fissure angle and the high temperature. As the fissure angle increases from 0° to 90°, the peak strength and elastic modulus first decrease and then increase, while the peak strain increases slowly. However, the peak strength and elastic modulus first increase and then decrease, while the peak strain first decreases and then increases with increasing treatment temperature. During the experiments, the crack evolution process and acoustic emission (AE) counts were obtained using real-time photography and the AE monitoring technique. In the granite specimens containing a pre-existing fissure, large AE counts are clearly observed before the peak strength, which indicates crack initiation and propagation. The accumulated AE count first increases slowly, but is followed by a sharp increase, with increasing deformation. The AE events in the one-fissured specimen also depend on the heat treatment temperature. As the temperature increases, the rate of increase of the accumulated AE count curve is reduced. Finally, using a digital image correlation method, the full fields of surface deformation were obtained for the entire testing process. In addition, the local strain around the pre-existing fissure was measured using strain gauges. The full strain field and local strain concentration are discussed to describe the fracture mechanism of brittle granite. [ABSTRACT FROM AUTHOR]

Microwave-assisted mechanical rock-breaking can effectively solve the problems of low efficiency of hard rock-breaking and severe wear and tear of equipment. This study used granite as the research object and analysed electromagnetic–thermal characteristics and dynamic tensile characteristics of microwave-irradiated granite. The microwave irradiation problem was decomposed into the electromagnetic–thermal unidirectional coupling and thermal–mechanical unidirectional coupling problems for numerical solutions, and the weakening mechanism of microwave irradiation was discussed at the microscopic level. The results showed that the thermal stress generated by the rapid heating and selective heating characteristics of microwave irradiation is the dominant factor in microwave fracturing. The presence of thermal cracks affects the crack extension path, making the failure more complicated. Tensile failure is the primary mechanical mechanism that causes the macroscopic fracture of the specimen, and intergranular failure dominates the overall failure of the specimen. Microwave irradiation reduces the shear resistance of the specimen. Microscopically, the proportion of shear cracks gradually increases with the increasing of irradiation duration (TM). Macroscopically, the area of the wedge-shaped failure zone gradually increases. [ABSTRACT FROM AUTHOR]