Effectiveness of trench and sub-horizontal pipe drains in clayey landslides: the influence of the slip zone permeability

The paper shows numerical results on the effects of two types of drain systems, trenches and sub-horizontal pipes, aimed to the stabilization of two deep clay landslides of the Italian Apennines (Costa della Gaveta and Varco d'Izzo, respectively). The results of three-dimensional numerical analyses carried out with a commercial FEM code show that the hydraulic conductivity of the slip zone is the most influencing parameter of drain systems. Long-term monitoring and experimentation indicate that the hydraulic conductivities of the slip zones of the two landslides are higher than those of their bodies and parent formation. This characteristic causes two effects: on the one hand, the permeable slip zone exerts a generalized drainage action that makes average pore water pressures in the slope decrease; on the other hand, the permeable zone can allow for fast and intense response to rainfall of pore water pressures just along it. Such latter effect causes a rapid response to rain of basal displacement rates. In the case of the CdG landslide, a hypothetical trench drain system can reduce significantly water pressures in the permeable slip zone only if they penetrate it. In the case of the VdI landslide, which is crossed by a tunnel with a system of 15 m long sub-horizontal pipe drains, very close to a slip surface, the numerical results show that the effects of the drains decrease with the slip zone permeability increasing.