Length: 1800 m; Width: 1600 m; Average Depth: 70 m; Area: 200 ha; Volume:
1.4x108m3.
Damages
Active secondary sliding at the toe and flanks of the landslide caused
damage to the cultivated fields and roadway almost every year.
Geology, Mechanism of Failure and Type of Movement
This large scale landslide is located in the southern portion of a
hilly region within a highly-concentrated landslide zone. The landslide
zone is underlain by 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 the deformation from folding
and faulting, the strength of the bedrock is low. The mountain is comprised
of an andesitic intrusive body situated upslope of the slide, and functions
as the source area for ground water. It is apparent that these factors
contributed to sliding. Based on boring data, three slide planes have been
identified: the Slide Plane I is about 70 m deep; the Slide Plane II is
about 100 m deep; and Slide Plane III is about 135 m deep. Measurements
taken over the past several years has revealed no movement along all three
slide plane. However, the slope stability analysis indicates that Slide
Plane I has the lowest factor of safety. Therefore, the potential for slope
instability still exists. Furthermore, ground water moving near Slide Plane
I is the contributing factor for secondary sliding at the toe area. Because
of the above reasons, the mitigation measures have oriented towards controlling
Slide Plane I.
Mitigation Measures
Serious effort for mitigation work commenced in 1988. The primary concerns
for this large scale landslide are: 1) the slide planes are very deep;
and 2) ground water supplied from the andesite body upslope of the slide
increases the possibility of movement of the slide. Therefore, subsurface
drainage control works (including three drainage tunnels) and installation
of drainage wells at the head portion of the slide is the main thrust of
mitigation work. Drainage Tunnel No. 1 was completed in 1994, and pile
works have been executed to control the active secondary landslide.
