Study on the cracking mechanism of strongly weathered purple mudstone under wetting and drying effect through experiments and molecular dynamics simulation

This study aims to investigate the cracking mechanism of strongly weathered purple mudstone (SWPM) samples in Three Gorges Reservoir (TGR) drawdown area, China under wetting and drying (WD) effect. The surface morphology and internal meso-structure changes of SWPM samples were observed by laboratory WD experiments and X-ray computed tomography (CT). Then, the Hyperspectral imaging (HSI) was conducted to explore the mineral components and mineral distribution on the crack sidewalls of the SWPM samples under WD effect, and the Atomic Force Microscopy (AFM) was applied to obtain the height profile of montmorillonite (MMT) on the surface of crack sidewalls based on HSI results. Subsequently, the molecular models contained clay mineral model and non-clay minerals model were established according to HSI analysis. The results showed that the cracks evidently appeared on SWPM surface after WD tests, the initial cracks in the mudstone sample extended continuously and connected pores increased rapidly under WD effect through CT images analysis. The hyperspectral images showed that the montmorillonite, albite and quartz were the main minerals of SWPM cracks area, the montmorillonite was the primary component of clay minerals, and the AFM images illustrated the existence of five layers of montmorillonite crystal layers on the montmorillonite region of the crack sidewalls. The MD simulation revealed that the interaction energy of SiO2 - MMT layers and MMT-MMT layers decreased with the increase of water content, and the electrostatic energy accounted for most of the energy. In wet condition, the distance of SiO2-MMT layers and MMT-MMT layers was raised with the swelling of montmorillonite due to the water intrusion. In dry condition, as the amount of water diminished, the MMT layers were close to each other and the spacing was shrinking, while the distance between MMT and SiO2 expanded continuously. The extension of the spacing SiO2-MMT layers in the dehydrated state was considerably more superior than that in the water absorption state, with the large apparent distance emerging between SiO2 and MMT, which illustrating the adjacent area of montmorillonite and detrital mineral in SWPM can easily appear microcracks under WD effect. This work directly explained the interaction behavior of SiO2-MMT layers and MMT-MMT layers at the molecular level, which provided a new perspective to comprehend the cracking process of strongly weathered purple mudstone in the TGR area.