The large scale landslide of our interest is located in the southern portion of a hilly region (see Figure 1) within a highly-concentrated landslide zone. It is underlain by the Upper Tertiary Miocene marine mudstone. The region is situated within a NNE to SSW-trending structure exhibiting a fold axis (anticline and syncline axis) and faults. Due to deformation from folding and faulting, the strength of the bedrock is low. The mountain is composed of an andesitic intrusive body situated upslope of the slide, and functions as the source area for ground water. The other contributing cause of landslide is infiltration of prolonged rain water prior to the addition of snowfall and snowmelt which contribute to increase the groundwater. The past history of slide activity is unknown; however, there has been some erosion during every snowmelt season. It is apparent that these factors contribute to sliding. Based on the recent data the slope movement is about 4 cm per year and this continuous movement explains the fissures during the construction of the tunnel as shown in Photo 1. Furthermore, ground water moving near the slip surface is suspected to be the contributing factor for secondary sliding at the toe area. Due to the above reasons, mitigation measures are being implemented. Serious effort for mitigation work commenced in 1988. The primary concern for this large scale landslide is mainly ground water supplied from the andesitic body upslope of the slide which increases the possibility of movement of the slope. Hence subsurface drainage control works (see Figure 2) by tunnels is found to be the main thrust of mitigation work.

Figure 1. Location of the drainage project


Table 1. Summary of tests results
Normal stress (kPa) Time to reach the residual state in
Total settlement of the upper platen in the container
h (mm)
Residual stress
in (kPa)
Flush PFT Flush PFT Flush PFT
50 33.2 22.7 0.221 0.110 21 17
100 33.2 13.7 0.296 0.196 37 27
200 36.2 16.7 0.352 0.149 62.5 56.7
3001 37.7 16.7 0.425 0.242 101.9 80
4002 16.7 9.2 0.310 0.483 125 117.5
Total 157 79 fr =  15.33 15.48

1 Value discarded in the calculation of fr (PFT procedure)
2 Values discarded in Calculation of fr (Flush procedure)

Table 2. Factor of safety (Fs) by different methods
Water level condition Simplified Method of Slices Simplified Bishop Janbu
  Flush PFT Flush PFT Flush PFT
Full water level 1.06 1.03 1.11 1.09 0.93 0.91
Low water level 2.40 2.38 2.50 2.5 2.21 2.19