A performance-based method for the design of drainage trenches used to stabilize slopes

Drainage trenches are widely used as a measure for slope stabilization. Their effect is generally quantified in terms of a safety factor against a possible collapse mechanism of the slope. A more rational and economical design approach consists in admitting the occurrence in some time intervals of limit conditions in the slope, and evaluating the performance of the slope in terms of accumulated permanent displacements of the unstable soil mass. The resulting displacements should avoid the occurrence of significant damage and preserve the serviceability of existing structures. In this context, a new method is proposed in the present paper for the design of drainage trenches used to control the mobility of translational landslides periodically activated by rainfall. This method is based on some analytical equations that relate rain to the groundwater regime as modified by the trenches, and a simple sliding block model to evaluate the landslide displacements. The proposed method also accounts for the delay of the hydraulic response at different depths. The above-mentioned equations are validated by several comparisons with numerical results obtained using a finite element code or published in the literature. Finally, the method is applied to some case studies to show the effectiveness of the drainage trenches to control the landslide mobility owing to rainfall.