Strength and failure characteristics of Jurassic Red-Bed sandstone under cyclic wetting–drying conditions

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Abstract
Due to cyclic fluctuations of reservoir water levels, bank slopes in drawdown areas are subjected to wetting–drying cycles. In order to reasonably evaluate the stabilities of sandstone slopes in the drawdown area of the Three Gorges Reservoir, it is a primary premise to obtain the strength and failure characteristics of the sandstones undergoing wetting–drying cycles. In this paper, the conventional triaxial compression tests, ultrasonic velocity and porosity measurements and microstructural observations were conducted on Jurassic Red-Bed sandstone (JRS) specimens undergoing wetting–drying cycles. The results from the triaxial experiments indicate that the peak strengths of the JRS are dramatically reduced after the first wetting–drying cycle, and then remain approximately constant with increasing number of wetting–drying cycles. The failure modes of the JRS samples undergoing different wetting–drying cycles are all brittle failures under low confining pressures ( ≤ 15 MPa). The decrease in P-wave velocity and increase in porosity with increasing number of wetting–drying cycles reveals the raise of damage level of the sandstone specimens, which is the main reason for the decline of peak strength. Detailed microstructural analysis has shown obvious argillization phenomena after undergoing wetting–drying cycles, which weakens the cements between grains in the sandstone and increases the damage of the sandstone.

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