The paper reports the results of a FEM analysis performed to examine the use of trench drains to protect a slope unstable only after heavy rainfalls. For an initially unsaturated slope, the efficiency of the drainage system is defined by the mean piezometric height values, h_0(t) and h(t), calculated for the same time t, in the absence and in the presence of drains, respectively, and therefore is a non-constant value that gradually increases during the rainfall event, tending to constant values for longer precipitations. In particular, the efficiency of trench drains is defined based on geometric characteristics during the transient stage when the trenches interfere with soil saturation, slowing development of positive pore pressures and allowing the slope to remain stable for much longer than without trenches.
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