The You-Ye-Lin landslide has caused long-term intermittent large ground movements during torrential rainfall since Typhoon Morakot occurred in 2009. To solve the landslide problem, the Soil and Water Conservation Bureau designed and constructed a subsurface drainage system consisting of two drainage wells (vertical shaft with multilevel horizontal drains) to draw down the groundwater table and stabilize the slopes made unstable from the landslide.
This study investigated the field-monitoring and in situ conditions of this system, with an emphasis on the design
and construction of subsurface drainages, and evaluated the systems efficiency at stabilizing the slopes using 3D numerical analyses. The numerical results demonstrated that the subsurface drainages were functional and capable of
accelerating the drainage of rainwater caused by high-rising groundwater levels during the torrential rainfalls of typhoon season. The large quantity of groundwater drawdown achieved by the subsurface drainages protects the slopes from fast deterioration and maintains the slopes stability to satisfactory design and safety standard.
Key Words: subsurface drainage, drainage well, horizontal drains, groundwater drawdown, slope stability