Mechanism of wave deterioration of mica quartz schist under dry-wet cycle

Dry-wet cycle is a critical factor contributing to the frequent landslide and collapse geological disasters in the schist area of northwest Hubei; however, limited research has been conducted on the deterioration mechanism of physical and mechanical properties of schist under the dry-wet cycle. In this study, the degradation of mica quartz schist widely distributed in northwest Hubei Province was investigated by the acoustic wave testing under the action of dry and wet cycling. The deterioration mechanism of mica quartz schist was analyzed from both macro and micro perspectives, combined with the apparent characteristics of the samples and the scanning results of electron microscopy. The results show that the porosity of mica quartz schist increases gradually with repeated dry-wet cycles, particularly during the first five cycles, where porosity increased by 43.74%. Afterward, the rate of increase slowed considerably. The wave velocity of mica quartz schist decreases continuously, showing rapid decline in the early stage and gradual flattening in the later stage. The wave velocity deterioration is the largest along the direction of large angle intersection between the radial and schistosity plane, and the smallest along the direction of parallel schistosity plane. In the dry conditions, the anisotropy index of wave velocity increased significantly, by 15%, whereas under the saturated conditions, the anisotropy increased slightly, with an increase of 1.6%. The maximum damage degree index is 0.28% in the direction of large angle intersection of the schistosity plane. The parallel schistological direction is the smallest (0.169%). In the mica quartz schist sample, the reddish-brown material precipitates continuously, and the schistosity gradually develops and spreads, and cracks. Micropores and microcracks develop and expand continuously, and the porosity increases gradually, leading to the decrease of wave velocity. The uneven distribution of pores and fissures results in obvious wave velocity anisotropy. This study provides a basis for exploring the mechanism of rock damage and deterioration under dry-wet conditions.