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We present results of P-wave velocities and acoustic emission (AE) activities of a calcareous sandstone, collected from the Three Gorges Reservoir (TGR) area, under uniaxial loading to investigate the stress-induced damage evolution under both dry and deionized water saturated conditions. Chemical analyses of the sandstone-soaking water and microstructural observations of the sample were also conducted to study the chemical effects of water on the microstructure of sandstone. The results show that decline of P-wave velocity and accelerated AE activities in all samples are initiated after adequate volume dilatation has developed, yet the presence of water obviously postpones both processes to a higher stress ratio. Accumulated AE number is much less in saturated samples, a combined result of water-induced wave attenuation and water-enhanced microcracking with less fracturing energy. It is also shown that dissolution of carbonate cement in water takes place at room temperature. Both stress-induced inter and intra-grain cracking are significant in dry samples, while cracks in saturated samples mostly follow grain boundaries. Based on these results, three possible mechanisms responsible for the water-weakening effects of TGR sandstone, i.e., the Rehbinder effects, stress corrosion and pressure solution, are discussed in detail. A conceptual model involving pressure solution of carbonate cement is presented to explain the micro-structure evolution and damage process of TGR sandstone in the presence of water.

An experimental work is presented that aims to improve our understanding of water–rock reaction, under both neutral and acidic conditions, and its effects on the mechanical deterioration of sandstone in Chongqing, southwest China, where a lot of heritage rock statues are carved in the same strata. SCB tests were conducted to evaluate the weakening of fracture toughness of Chongqing sandstone after water and acid erosions, and a high-speed camera was employed to capture its cracking behaviors under different hydrochemical conditions. Microstructural observation was also carried out to investigate the cracking path at a micro-scale, and the chemical analyses of sandstone soaking water in a duration of up to 120 days allows to reveal the evolving process of water–rock reaction. The results show that compared to acid solutions of pH = 3 and 5, the strength weakening effect of distilled water on Chongqing sandstone is more severe. Crack propagation rates captured by the high-speed camera vary between 20 and 60 mm/s in the SCB tests. There is no significant difference in the data for dry and water-saturated samples, and the same for the samples in the two acid fluids. Yet the crack growth rates of the latter two groups are apparently greater than those of the former two. The chemical reaction between water / acid solutions and Chongqing sandstone is mainly pertinent to the cementation and grain boundaries, which weakens grain contacts and promote crack propagation. This mechanism results in a low KIC, long crack length, i.e., more tortuosity along grain boundaries, and high crack propagation rate.

A particle-based discrete element model combined with digital image processing to take into account the grain geometry and size distribution is used to investigate the mechanical and microcracking behavior of Beishan granite (BG), a typical crystalline rock potential for nuclear waste disposal in China. First, the procedure to construct a digital image-based grain-based model is introduced. The unsupervised K-means clustering algorithm is used to determine mineral boundaries, and four micro-parameters are assigned according to the Weibull distribution to reflect micro-heterogeneity of rock. Then, the model is calibrated to the macro-properties of BG obtained from laboratory tests, and consistency between model simulation and laboratory test results of both microcracking processes and failure modes is found. It is shown that intragrain cracks are initiated successively in quartz, feldspar, and mica; and their numbers in quartz and feldspar are much more than that in mica. Crack cluster, forking cracks and cataclastic microcracking are typical in quartz, while the “voids” is found in mica. Grain property heterogeneity has a significant impact on micro-cracking behavior and macro-properties of the model, with greater heterogeneity resulting in a lower uniaxial compression strength and elastic modulus, and a more scattered distribution of microcrack inclination. The modeling procedures presented here provide an effective method for investigating mechanical and damage behavior of granites, with specific consideration of heterogeneity involving the distribution of mineral grains and their mechanical properties.