Effect of Horizontal Drain for Slope Stability During Rainfall Using Transient Seepage Analysis

This research aims to use transient seepage analysis to determine the effect of using horizontal drains in reducing groundwater levels and increasing slope stability during the rainy season. The drain used was a perforated pipe drilled and inclined horizontally into the slope to release the pore water pressure in areas prone to landslides, thereby lowering the groundwater level. Numerical modeling research was conducted to determine the effect of rainwater seepage and slope stability on soil using a coupled program based on the Finite element method (FEM) and Limit equilibrium method (LEM) with SEEP/W and SLOPE/W, respectively. Five scenarios were carried out by varying the rain conditions and soil seepage parameters without and with horizontal drain. The results showed that horizontal drain can increase the safety factor by up to 1.7 and 1.1 times during uniform rain modelling for 4 days in good and poor drainage soil conditions, respectively. Furthermore, it raised the safety factor with a ratio of 1.03 times in the real time rainfall intensity of 90 days. The rise in safety factor occurred after 56 days of rain with an average duration of 6 hours/day. These results indicate that the effectiveness of horizontal drain is very sensitive to various parameters (rainfall parameters and soil parameters), hence it must be conducted carefully to mitigate landslides.