Using 3-D numerical analysis and empirical methods, this study performed a series of steady-state drainage analyses under a constant groundwater recharge rate to estimate the groundwater drawdown attributable to subsurface drainage of horizontal drains for large landslides. The levels of groundwater drawdown calculated through the numerical method and through the empirical method, as proposed by Crenshaw and Santi (2004), coincided. The normalized groundwater recharge rate is the most crucial parameter influencing the drawdown pattern of groundwater levels and is alternately related to the spacing of horizontal drains and the groundwater constant head boundary at the upper slope. The horizontal drains will promote drawdown and will maintain the groundwater level at a safe depth under a larger drainage intensity if they are configured with a smaller spacing and are installed in a permeable soil stratum.
Key Words: Horizontal drain, Subsurface drainage, Normalized recharge rate, Groundwater drawdown surface.