Debris Flows and Landslides - River Dynamics and Integrated River Management - page 210

The model (Eq. 4.1) implies that an increase in loading, for instance due to rainfall and or supply of
water to the soil; an increase in the angle of the slip surface, for instance due to stream incision and
erosion of the toe of the sliding body; reduction in the yield strength, for instance during earthquake, and
reduction in the frictional coefficient, may result in landslides.
Seismic waves cause not only sudden reduction in the yield strength of the slip surface, but also cause
a sudden increase in pore pressure. Using real seismic waves, the possible seismic loadings on the sliding
surface due to the seismic excitation during earthquake have been studied. Through a newly developed
ring shear apparatus, the seismic loadings were applied successfully to the soils on the sliding surface to
mimic the seismic response of soil during an earthquake (Wang et al., 2007). Figure 4.23 shows the result
of a seismic simulation test, in which the normal stress is equal to W, or the weight of loading per area. It
reveals that due to the seismic loading, a certain amount of excess pore-water pressure was built up
within the saturated sliding surface, which led to the failure of the slope (Wang et al., 2007). As the pore
pressure increases to about 0.5 times of the normal stress, the shear resistance reduces sharply. As the
pore pressure increases to nearly equal to the normal stress, the shear resistance reduces to zero and
displacement increases quickly (sliding occurs). After failure, high excess pore-water pressure was
generated with an increase of shear displacement. This finally resulted in a great reduction in the shear
resistance and rapid movement.
Fig. 4.23 Result of seismic simulation test - Due to the seismic loading, a certain amount of excess pore-water pressure
was built up within the saturated sliding surface, which led to the failure of the slope (after Wang et al., 2007)
4.2.2 LandslideDams
4.2.2.1 Removal and Preservation of Landslide Dams
Large landslides often create landslide dams. Landslide dams may fail, be removed or be preserved. In
the past, humans could do nothing on the landslide dams, which failed or remained naturally depending
on the composition of landslide debris and on the flow discharge of the rivers. At Tianshan Mountain,
which is shared by China, Kazakhstan, Uzbekistan, Kyrgyzstan, and Tajikistan, the Sarez Earthquake
triggered the Usoi landslide with a volume of 2.2 billion m 3 . The landslide blocked the Murgab River
with the highest landslide dam in the world (500-600 m high), forming Sarez Lake, in 1911 (Gaziev,
1984). Almost one century has passed and Sarez Lake is still storing water and the water level is rising at
a rate of 18.5 cm/year (Schuster and Alford, 2004). The risk of dam failure is increasing following the
increasing water level and the safety of the dam has attracted world wide attention.
Some researchers have concluded that landslide dams will inevitably fail, probably due to channel
erosion during a flood. Many landslide dams fail within 1 year or less (Costa and Schuster, 1988; Schuster,
2000). Becker et al. (2007) have the same opinion and they indicated that no landslide dams formed in
large rivers on the West Coast in the central Southern Alps had survived to the present day. For example,