Geotechnical Engineering Online Library

There are a large number of abandoned coalmines in China, and most of them are located around major coal-fired power stations, which are the largest emission sources of carbon dioxide (CO2). Considering the injection of CO2 into abandoned coalmines, which are usually in the flooded condition, it is necessary to investigate the effect of CO2–water–coal interaction on minerals and pore structures at different pressures, temperatures and times. It reveals that the CO2–water–coal interaction can significantly improve the solubility of Ca, S, Mg, K, Si, Al, Fe and Na. By comparing the mineral content and pore structure before and after CO2–water–coal interaction, quartz and kaolinite were found to be the main secondary minerals, which increased in all samples. The structures of micropores and mesopores in the range of 1.5–8 nm were changed obviously. Specific surface areas and pore volumes first increased and then decreased with pressure and time, while both increased with temperature. By using the Frenkel–Halsey–Hill model, the fractal dimensions of all samples were analyzed based on Ds1 and Ds2, which reflected the complexities of the pore surface and pore volume, respectively. The results show that fractal dimensions had very weak positive correlations with the carbon content. Ds1 had a positive correlation with the quartz and kaolinite contents, while Ds2 had a negative correlation with the quartz and kaolinite contents.

The mechanical properties and fracturing mechanism of shale containing beddings are critically important in shale gas exploitation and wellbore stability. To investigate the effects of shale bedding on crack behavior and fracturing mechanism, scanning electron microscope (SEM) with a loading system was employed to carry out three-point bending tests on Longmaxi outcrop shale. The crack initiation and propagation of Longmaxi shale were observed and recorded by taking photos during loading. The cracking paths were extracted to calculate the crack length through a MATLAB program. The peak load, fracture toughness and fracture energy all increase with the bedding angle from 0° to 90°. The crack length and energy were also found to increase with the bedding angle in the range of 0°–60° and then drop slightly. The fracturing mechanism of shale includes the main crack affected by the bedding angle and disturbed by randomly distributed particles. The main cracking path was accompanied by several microcrack branches which could form an interconnected crack system. When the main crack encounters larger sedimentary particles, it will deflect around the particles and then restore to the initial direction. A numerical technique using extended finite element method (XFEM) coupled with anisotropic cohesive damage criteria was developed, which is able to capture the dependence of crack propagations on bedding angle and sedimentary particles. This study sheds light on understanding and predicting mesoscale fracture behavior of shale with different bedding angles.

Understanding the stress–strain relationship and permeability change for contact compression fracture at closing stage has been a hot issue for a long time. Previous investigations of this topic were mainly focused on experimental tests; however, theoretical approaches were rarely reported. Based on this, this paper focuses on the contact fracture at closing stage when rock is uniaxially loaded, and then a theoretical model is proposed. Based on the change of fracture elasticity modulus, it shows that as crack apertures are gradually reduced in the loading process, the permeability of rock sample will decrease progressively. This scenario shows that theoretical computation matches well with the experimental results. Finally, the effects of ratio of sample size to fracture aperture (n), pore pressure (P), and initial aperture (b) on stress–strain relationship and permeability change for contact compression fracture at closing stage are analyzed.

It is always desirable to know the interior deformation pattern when a rock is subjected to mechanical load. Few experimental techniques exist that can represent full-field three-dimensional (3D) strain distribution inside a rock specimen. And yet it is crucial that this information is available for fully understanding the failure mechanism of rocks or other geomaterials. In this study, by using the newly developed digital volumetric speckle photography (DVSP) technique in conjunction with X-ray computed tomography (CT) and taking advantage of natural 3D speckles formed inside the rock due to material impurities and voids, we can probe the interior of a rock to map its deformation pattern under load and shed light on its failure mechanism. We apply this technique to the analysis of a red sandstone specimen under increasing uniaxial compressive load applied incrementally. The full-field 3D displacement fields are obtained in the specimen as a function of the load, from which both the volumetric and the deviatoric strain fields are calculated. Strain localization zones which lead to the eventual failure of the rock are identified. The results indicate that both shear and tension are contributing factors to the failure mechanism.

This study uses a three-dimensional crack model to theoretically derive the Hoek–Brown rock failure criterion based on the linear elastic fracture theory. Specifically, we argue that a failure characteristic factor needs to exceed a critical value when macro-failure occurs. This factor is a product of the micro-failure orientation angle (characterizing the density and orientation of damaged micro-cracks) and the changing rate of the angle with respect to the major principal stress (characterizing the microscopic stability of damaged cracks). We further demonstrate that the factor mathematically leads to the empirical Hoek–Brown rock failure criterion. Thus, the proposed factor is able to successfully relate the evolution of microscopic damaged crack characteristics to macro-failure. Based on this theoretical development, we also propose a quantitative relationship between the brittle–ductile transition point and confining pressure, which is consistent with experimental observations.

In this study, the meso-failure mechanism and fracture surface of Jinping marble were investigated by means of scanning electron microscope (SEM) with bending loading system and laser-scanner equipment. The Yantang and Baishan marbles specimens from Jinping II hydropower station were used. Test results show that the fracture toughness and mechanical behaviors of Yantang marble were basically higher than those of Baishan marble. This is mainly due to the fact that Baishan marble contains a large percentage of dolomite and minor mica. Crack propagation path and fracture morphology indicated that the direction of tensile stress has a significant effect on the mechanical behaviors and fracture toughness of Baishan marble. For Yantang and Baishan marbles, a large number of microcracks around the main crack tip were observed when the direction of tensile stress was parallel to the bedding plane. Conversely, few microcracks occurred when the direction of tensile stress was perpendicular to the bedding plane. The presence of a large number of microcracks at the main crack tip decreased the global fracture toughness of marble. The results of three-point bending tests showed that the average bearing capacity of intact marble is 3.4 times the notched marble, but the ductility property of the defective marble after peak load is better than that of the intact marble. Hence, large deformation may be generated before failure of intact marbles at Jinping II hydropower station. The fractal dimension of fracture surface was also calculated by the cube covering method. Observational result showed that the largest fractal dimension of Yantang marble is captured when the direction of tensile stress is parallel to the bedding plane. However, the fractal dimension of fracture surface of Yantang and Baishan marbles with tensile stress vertical to the bedding plane is relatively small. The fractal dimension can also be used to characterize the roughness of fracture surface of rock materials.

Uniaxial compression tests (UCTs) on 34 naturally fractured marble samples taken from the transportation tunnels of Jinping II hydropower station were carried out using the MTS815 Flex test GT rock testing system. Rockburst proneness index WET is determined for the marble samples with the UCTs. According to the number, size and spatial structure characteristics of the internal natural fractures of the marble samples, fractures are basically divided into 4 types, namely, single fracture, parallel fracture, intersectant fracture and mixed fracture. The mechanical properties of naturally fractured rocks (4 types) are analyzed and compared with those of intact rock samples (without natural fractures). Experimental results indicate that failure characteristics of fractured rocks are appreciably controlled by fracture distribution or fracture patterns. In comparison with intact rocks, the failure of fractured marbles is a locally progressive failure process and finally rocks fail abruptly. Statistically, the uniaxial compressive strengths (UCSs) of rocks with single, parallel, intersectant and mixed fractures are 0.72, 0.69, 0.59 and 0.46 times those of the intact rocks, respectively. However, the elastic modulus of the fractured Yantang marbles is generally not different from that of intact rocks. But the elastic moduli of Baishan marble with single, intersectant and mixed fractures are 0.61, 0.62 and 0.45 times those of intact rocks, respectively. Experimental results also indicate that WET of fractured marbles is generally smaller than that of intact marbles, which implies that rockburst intensity of fractured marble in field may be controlled to some extent. In addition, the bearing capacity of surrounding rocks is also reduced, thus the surrounding rocks should be supported or reinforced timely according to practical conditions.