To investigate the optimal configuration and performance of subsurface drainage systems (comprising nine engineering units, seven drainage wells, and two drainage galleries) in relation to the Li-Shan landslide, a 3-D numerical model of the central area of the landslide was generated based on a substantial amount of information (boring logs, contour topography, geological surveys, soil/rock laboratory tests, and groundwater monitoring data). Steadystate subsurface drainage analysis and slope stability analysis using a strength reduction method were performed systematically.
The validity of the numerical model was verified by comparing the coincidences of numerical results with
monitoring data in terms of groundwater variation and location/depth of the potential sliding zone. An evaluation of the interactive effects and drainage efficiency, in relation to the sequential operation of the nine engineering units, was performed using the contribution percentage of groundwater drawdown (PCGW %), increasing percentage (FS %) and enhancement contribution percentage (PCFS %) of the factor of safety (FS), and the location of potential sliding zone.Upon completion of the subsurface drainage system, the main potential sliding zone on the northern side of the landslide area was stabilized due to effective groundwater drawdown, and it was eventually transferred to the adjacent area of the western ridge. In addition, as the subsurface drainage construction was progressing and becoming gradually operational,the stability of the potential sliding zone was enhanced with an increase in the factor of safety (FS=1.131 1.300).
Key Words: Deep subsurface drainage system, drainage well, drainage gallery, potential sliding zone