Simulation and verification of overhanging soil layers stability of gully heads in Yuanmou Dry-hot Valley based on moment method

Abstract: Gully erosion is one of the major contributors to severe land degradation in the Yuanmou Dry-hot Valley Region of Southwest China, due to the frequent erosive precipitation events in the rainy season and special lithological conditions. Most active gullies in the study area are characterized by steep gully headcut, and headward erosion has been recognized as the main process of gully expansion. In particular, scour holes are often developed in the open side of gully headcut, which cause the frequent collapse of overhanging layers of gully heads in the process of headward erosion, and thus accelerate gully headward development. It is, therefore, of great significance to carry out the study on the collapse of overhanging layers of gully heads during the gully headward processes. In this study, based on the field investigations, a generalized physical model of active gully head was established. Then, the stability of overhanging layer of active gully head was studied by the momental analysis method, considering the main factors i.e. headcut height, the shape of scour hole and the development of tension crack at gully head. The results showed that the stability of overhanging layer of gully head was affected by various factors, such as the soil water content, the height of scour hole, the location and depth of tension crack. The results also indicated the difficulty in studying the mechanism of the gully head collapse process. When other parameters were assigned with default values, the stability of overhanging layer of gully head was decreased with soil water content and depth of tension crack (p<0.001) as an exponential function, and decreased with height of scour hole (p<0.001) as a linear function. As the distance between the tension crack and the headcut wall increased, the stability of overhanging layer increased significantly (p<0.001). Furthermore, to test the reliability of the evaluation of stability of overhanging layers, 4 gully heads molded by man with scour holes and the flow scouring experiments were employed in the field. When the collapse of overhanging layers of gully heads occurred, soil water contents of the whole collapse soil and the collapse surface soil were determined by oven-dry method with 6 repetitions. The results showed that the measured soil water contents of the whole collapse soil and the collapse surface soil were 69.8%-87.6% and 82.0%-95.5% of the model estimated values, separately. The small difference in soil water contents between the measured values and estimated values also indicated that this evaluation model was very valuable for the prediction of the collapse of overhanging layers of gully heads in the Yuanmou Dry-hot Valley Region. Moreover, the soil water content of the collapse surface soil was much closer to the model estimated value, which demonstrated that the development of tension crack on the overhanging layers of gully heads did affect their stability. These results are helpful to supplement meaningful theories to the erosion mechanism study and conservation practices implementation of active gully heads in the Yuanmou Dry-hot Valley Region. However, due to the complexity and uncertainty of the collapse of overhanging layers, further studies are needed to focus on the process of collapse of active gully heads, which can further improve the evaluation reliability of the stability model.