Characterization of elastoplastic behavior and retrieval of active zone depth for expansive soil slopes in the middle-route channel head of the South-to-North Water Diversion Project, China, using InSAR time series

Expansive soil is very destructive for light infrastructures, while the South-to-North Water Diversion Project (SNWDP), one of the most critical projects in China, has a considerable section constructed by it. The significant displacements by the expansive soil slope (ESS) lead to severe damage to the slope prevention in the SNWDP, possibly resulting in slope failure with no timely and appropriate preventive measures. In this study, taking the channel head of the middle route of the SNWDP (Henan, China) as a case, we employed synthetic aperture radar (SAR) data of 134 Sentinel-1 scenes spanning 2017–2021 to characterize the ground displacement on the ESS and, after that, retrieve the active zone depth (AZD) that is a significant parameter to determine the treatment depth. An elastoplastic deformation model under wetting and heating effects (EDWHE) is proposed to accurately interpret ESS's nonlinear deformation, considering the soil lags responding to wetting and heating. The results show that the excavated canal generally has positive LOS displacements, maximum to 18 mm/year, while the filled canal with negative, minimum to −25 mm/year. The InSAR results of our model show a significant improvement compared to traditional models and have a good agreement with GNSS observations. Besides, we find the movement pattern of the ESS sliding toward the slope toe, manifesting the potential risk of progressive landslide. Then, we derive the soil's elastic deformation by the EDWHE and observe that the elastic swelling and shrinkage become frequent and drastic in the monsoon season, which is the principal reason for the expansive soil geohazard. Moreover, we employ a time lag approach to retrieve the AZDs with high spatial resolution and clear distribution patterns. Our works provide a methodology to study ESS's behavior and give the possibility for detecting extensive expansive-soil geohazard and helping subsequent preventive measures.