Three-Dimensional Numerical Modeling of Transient Drawdown from Horizontal Drain Systems for Slope Stabilization

One of the major causes of slope failure is elevated groundwater; lowering groundwater levels and consequently, maintaining low pore water pressures is a key element toward landslide mitigation. Horizontal drains are a common method toward dewatering and are often installed in specified patterns, lengths, and locations to be most effective based on site conditions and operator experience. As the design of horizontal drains is often observation-based practice, we developed a three-dimensional solution for transient groundwater drawdown stemming from horizontal drain installation in an unconfined aquifer by using a custom-made finite difference solver. First, the solution was verified against experiment results. Then, a sensitivity analysis was performed to evaluate the effect of drain spacing and hydraulic conductivity on drawdown and factor of safety in a simplified hillslope. The results indicated overall effectiveness of horizontal drains and improvement in slope stability; however, changes in drawdown and consequently factor of safety varied with drain spacing, site hydrological conditions, and time after installation.