Parametric study of horizontal drains for slope stability measure: A case study in Putrajaya, Malaysia

Horizontal drains are commonly used in stabilizing slopes. This paper presents a case study of slope failure in Precinct 9, Putrajaya, Malaysia which buried 23 cars and caused the evacuation of 1000 people from their apartments. Recorded data showed that it had been raining heavily with a total cumulative rainfall of 210 mm two days prior to the slope failure occurrence. Post failure investigations suggested that the slope failure is caused by the rise in groundwater level that increased the pore water pressure thus reducing the shear strength. Therefore, horizontal drains were proposed as a remedial measure to enable the lowering of groundwater level to a safe level. Parametric study was carried out to determine the suitable length and ideal location of horizontal drains installation. Geophysical electrical resistivity survey was carried out to determine the ideal location for installing the horizontal drains. The Finite Element seepage analysis and Limit Equilibrium method were used to examine the effect of horizontal drains on slope stability during rainfall. The minimum length required for the horizontal drains is 22.5 m and the best location for installation is at the toe below the lowest berm of the slope. Field monitoring results from piezometers and measurements of discharge rate verified that the installation of horizontal drains are able to lower the groundwater level effectively.