Irrigation-induced Loess Landslides Risk Evaluation And Control

To solve the agricultural water supply problems in the Loess Plateau, river water has been elevated to irrigation areas. Irrigation water goes deep down into loess layers until it reaches underlying relatively impermeable clayey layers. Then long-term accumulation of water in the loess layers begins triggering massive loess landslides. Irrigation-induced loess landslides are common in the Loess Plateau, especially on the edges of main irrigation areas in Shaanxi, Gansu, Ningxia and Qinghai provinces. Most of the landslides are large and medium ones with an obvious mass developing feature, the extreme of which can be a long strip of landslides spreading dozens of kilometers. Under current circumstances, the key issues are to evaluate risks of irrigation-induced loess landslides in a reasonable way and to develop corresponding risk mitigation measures. On the one hand, irrigation is the basis for people's lives living in the irrigation areas; on the other hand, irrigation has resulted in severe landslide disasters. Therefore, trade-off between the two contradicted parties is the ultimate goal for landslide risk evaluation and control.Taking the 9th Jiaojia landslide in Heifangtai area as a typical example, this thesis carried out risk management method of "irrigation water penetration-evolution of groundwater flow field-changes of shear strength parameters-safety factor of the slope-reliability-risk evaluation-risk control". Based on massive material collection of irrigation-induced loess landslides, the cause of this type of landslides was analyzed. Considering the changes of infiltration recharge conditions induced by massive irrigation in Heifangtai, a numerical model of groundwater was built with MODFLOW software to predict the groundwater flow field, and the law of groundwater table change was summarized. Based on massive tests of physical and mechanical properties of loess in the area, analysis of variation characteristics of parameters, together with FEM-based deterministic stability analysis and Monte-Carlo random reliability analysis of the slope were performed and slope failure probabilities under different irrigation conditions were obtained. Based on landslide risk management theory and systematic analysis of the value loss of elements at risk, risk of single life and property risks under different irrigation conditions were evaluated. After an overall consideration of allowable risk level and agricultural irrigation demands, comprehensive risk control measures which incorporate irrigation control, vertical drainage, lateral drainage and simple engineering treatment were put forward.