Research on the rainfall-induced regional slope failures along the Yangtze River of Anhui, China

During the highway construction along the Yangtze River in Anhui, China from 2015 to 2018, regional slope failures occurred frequently near the routes and constituted significant hazards to infrastructures. Especially from June to September in 2016 and 2017, the high-temperature weather and intensive rainfall hit this region, triggering a lot of slope failures. These slope failures have two puzzling features: (1) low height (2.5-5 m) or gentle dip angles (8-25 degrees). Such height and dips are unlikely to fail in theory; (2) slope failure emerged immediately during rainfall, while the slope materials consist of clay soil with extremely low permeability. Field investigations, laboratory tests, and a large-scale slope model test were conducted to investigate the failure modes and mechanism of the slope failures. The results show (1) low steep slopes generally show failure modes of surface erosion, or repeated local failures around the slope shoulder, while the gentle slopes often display failure modes of overall failure or even landslides; (2) the slope material mainly contains clay mineral of illite and displays strong shrinkage ability, which is prone to forming desiccation cracks during drying evaporation. Desiccation cracks can significantly improve the infiltration capacity of soils with three or four orders of magnitude. Shear strength of the soil is sensitive to water and decreases sharply with the increased water content; (3) the large-scale slope model test confirms that desiccation cracks can induce slope failure by providing preferential flow pathways for rainwater to rapidly infiltrate into deep soils. Based on the above results, the difference of failure modes and scales between the steep slope and gentle slope is interpreted. It is inferred that desiccation cracks are difficult to develop stably and constantly on the inclined surface of steep slopes due to the intense surface runoff. Thus, surface erosion and shallow flow-slip dominate the failure modes of the low steep slopes. Conversely, a gentle slope surface is favorable for the development of desiccation cracks. Hence, overall slope instability or a landslide is more likely to occur in a gentle slope after long periods of drying-wetting cycles.