Disintegration behavior of strongly weathered purple mudstone in drawdown area of three gorges reservoir, China

Abstract

Disintegration behavior of purple mudstone in the drawdown area of the Three Gorges Reservoir (TGR) in China, as a result of exposure to cyclic wetting and drying (WD) conditions, promotes bank slope instability during reservoir operation. To better understand the disintegration behavior of the mudstone in the drawdown area during reservoir operation, 63 strongly weathered purple mudstone (SWPM) samples from the Triassic Badong group (T2b) in Badong County of the TGR area were collected for laboratory-based cyclic WD experiments. After each WD cycle, measurements of the grain size distribution curve, the clay minerals contents and the ion (K+, Na+, Ca2+ and Mg2+) content in the soak solution, tests of water absorption, and observations of microstructure were conducted on the SWPM samples. Grain size distribution curves show that the SWPM samples disintegrate during WD cycles. The ion contents in the soak solutions and clay mineral contents in the SWPM samples indicate that the clay minerals in the surficial zone of the samples run into the soak solution by hydration between deionised water and clay minerals when each wetting procedure is performed, which weakens the cement between the grains and promotes the disintegration of the SWPM samples. The microstructural observations and water absorption variations exhibit increases in pores and cracks in the surficial zone of the SWPM mudstone sample, resulting from clay mineral content losses during the wetting procedure and shrinking in volume of the SWPM sample under the drying procedure in each WD cycle. The decrease in clay mineral content and increases in pores and cracks in the surficial zone of the SWPM sample are the main reasons for the disintegrations of the SWPM samples undergoing WD cycles, with the disintegration proceeding in the form of a peeling onion.