Evaluating the Efficiency of Subsurface Drainages for Li-Shan Landslide in Taiwan

Abstract. This study investigates the efficiency of subsurface drainage systems includes drainage wells (vertical shaft with drainage boreholes or horizontal drains) and drainage galleries (longitudinal tunnel with sub-vertical drainage boreholes) for the slope stabilization of Li-Shan landslide in central Taiwan. The efficiency of the subsurface drainages is verified through a series of two-dimensional (2-D) rainfall induced seepage and slope stability analyses without and with subsurface drainages remediation during two typhoon events. Numerical results and monitoring data both show that the groundwater level at B5 monitoring station with subsurface drainages remediation during Toraji Typhoon (2001) is about 40 m lower than that without remediation during Amber Typhoon (1997), and the factor of safety Fs of the first potential sliding surface (1^st-PSS, the most critical potential sliding surface) is promoted simultaneously from 1.096 to 1.228 due to the function of subsurface drainage systems. In addition, the Fs values of the three potential sliding surfaces (1^st- PSS, 2^nd-PSS, and 3^rd-PSS) stabilized by subsurface drainage systems are constantly maintained greater than unity (F_S>1.0 or F_S≥1.217) during rainfalls with return periods increases from 25 to 50 and 100 years. This demonstrates the subsurface drainage systems in Li-Shan landslide are functional and capable of accelerating the drainage of infiltration rainwater induced from high intensity and long duration rainfall and protect the slope of landslide from further deterioration.