A case study of a rainfall-induced landslide involving weak interlayer and its treatment using the siphon drainage method

Local collapse or even overall movement may occur due to excavation at the slope toe, especially when there is a weak interlayer in the stratum. This process frequently produces many tension cracks on the slope surface, increases the sensitivity of groundwater to rainfall and thus promotes the development of landslide. The Houpucun landslide, which is located in Tonglu County, Zhejiang Province of China, is a typical case in which the weak interlayer and the tension crack play an important role in triggering the slope instability. In this study, the deformation process and the hydraulic characteristics of this landslide were analyzed based on monitoring of the slope displacement, rainfall and hydraulic head. The results show that the slope deformation has a strong correlation with rainfall and groundwater table. The large number of cracks on the slope surface provide favorable conditions for the infiltration of rainfall and that the hydraulic head rise rapidly under the action of rainfall. Because the mechanical strength of the weak interlayer greatly decreases when saturated, the slope is prone to failure under rainfall. Numerical simulations were carried out to compare the stability of the slope before and after rainfall. The results show that the factor of safety of the slope is close to one once the weak interlayer is immersed by groundwater. A new siphon drainage technique was employed to control the groundwater table and improve the stability of the slope, which has proven to be a practicable and efficient countermeasure for the case in hand.