This study investigated the efficiency of subsurface drainage systems—which comprise drainage wells and galleries—to analyze the slope stability of landslides in Lishan in central Taiwan during Typhoon Toraji in 2001
(with subsurface drainage remediation) and Typhoon Amber in 1997 (without remediation) by using the numerical
analysis method. The drainage effects of drainage wells and galleries were simulated by assigning a series of line-type
and point-type drainage boundary conditions along drainage boreholes in the numerical model. The numerical results
and monitoring data indicated that the subsurface drainage systems for handling landslides in Lishan are functional and capable of accelerating drainage of infiltrated rainwater caused by prolonged heavy rainfall and reinforcing slope stability against rapid deterioration during landslides.
Keywords: large landslide, subsurface drainage, drainage well, drainage gallery, potential sliding surface.