Synergistic effect of rainfall and pipe leakage erosion on the formation and development of loess caves in Weinan city, Loess Plateau, Northwest China

Rapid urban expansion and climate change often bring about loess cave disasters on the Chinese Loess Plateau. It is believed that human activity and rainfall can synergistically alter the hydrogeological conditions and further accelerate the formation of loess caves. However, thus far, few cases or lines of evidence have been reported to support this conclusion. The study provided an example of the formation of loess caves in Weinan city, Loess Plateau, Northwest China, which was related to rainfall and erosion due to pipe breakage and leakage. The precipitation data were collected to examine the effect of rainfall erosion. Moreover, through a field investigation and multi-channel analysis of surface waves (MASW) survey, ten loess caves that were adjacent to aging underground pipes were systematically investigated. The physical and mechanical properties that were affected and unaffected by the loess caves were compared to explore the influence of the pipe breakage and leakage, i.e., the water content, void ratio, and self-weight collapsibility coefficient. The results showed that the spatial variation in these properties could reflect the alteration of the loess caves under the combined action of rainfall and pipe-breakage and leakage erosion. We shed light on this synergistic effect on loess cave formation and proposed a four-stage formation model: formation of the upper negative relief, initiation of the inner cave, upper to lower coalescence, and volume enlarging. New insights were provided on the development process of the loess caves and the synergistic role between human activities and a natural force. The findings can help us to more comprehensively understand the formation mechanisms and development trends of the loess caves to predict and prevent loess cave disasters on the Chinese Loess Plateau.