The Transportation and Deposition of Catastrophic Landslides in Taiwan: Insight from Granular Discrete Element Simulation
Date Issued
2010
Date
2010
Author(s)
Tang, Chao-Lung
Abstract
The catastrophic landslides are not rare in the world, but often cause a great loss of lives and properties. Just from 1992 to 2001, the landslides caused more than nine thousand people to die in the world. Taiwan is located in the junction of Eurasian plate and Philippine Sea plate, the most active orogenic belt in the world with young and unstable geological condition. Frequent earthquakes and typhoon season can also contribute to induce large catastrophic landslides. In recent years, the most impressive catastrophic landslide events in Taiwan are Tsaoling repeated landslides, Jiufengershan landslide induced by the 1999 Chi-Chi earthquake and 2009 Hsiaolin buried event caused by the Morakot Typhoon.
Most of the landslide investigations emphasize the landslide triggering mechanism, for the landslide transportation and the deposit-impact areas are less discussed. In this study granular discrete element method will be adopted to study and discuss catastrophic landslide behavior in Taiwan, including the Tsaoling landslide, Jiufengershan landslide and Hsiaolin village events. The study focuses on the Tsaoling repeated landslides to predict the location of the next collapse, the failure mechanism of the Tsaoling landslide, the velocity of transportation, the impact area, and topographic migration of the landslide area. The discrete element method is a time-explicit calculation, and the simulation process can express the crack development and large scale displacement of features, so the landslide movement simulation can be very appropriate for the granular discrete element method.
After the seismic simulation triggered, the parts of the block collided with others during the vibration simulation. The upper part of the block slid a short distance and then stayed on the slope, and the lower part of the block was collided down by the upper part block. When the collision pushed down a certain distance to the lower part of the block, the shear surface just lost the strength of the sliding plane and induced the landslide during the 1941 earthquake. The cracks developed from shear surface to ground surface during the vibration simulation. The distribution of the crack concentrated in some certain zones, just fitted to the future landslide detachment surface (1942 and 1999 events).
Based on a 2-D discrete element simulation of the Tsaoling landslide induced by the Chi-Chi earthquake, the reduction of friction at the sliding surface is predicted. A self-lubrication mechanism is suggested in our study to explain the low residual friction of about 0.15. Low friction favored the propagation of a 125 million cubic meters of rock debris which crossed over and collided against the Chingshui River valley. The maximum velocity of sliding block can reach about 45 m/sec. A quasi-rigid behavior of the sliding mass during the sliding process may explain the 7 survivors after sliding 2250 m. However, in the 3-D simulation, the friction coefficient must be reduced to 0.03, and the maximum velocity and runout distance shall be 78 m/sec and 3495 m to obtain an appropriate result. They have a large difference. The application direction of force in 2-D model is only on one plane, the grain and force behaviors would not be lateral dissipation, so the following grains can only collide back and forward. However, in the 3-D model most of the energy will dissipate because of lateral spreading. If the friction consumes too much kinetic energy, the accumulation appearance must be obviously different from actual deposit area. However, the 3-D model is more likely to reflect real circumstances. Thus, the maximum sliding velocity of the Tsaoling landslide is higher in the two-dimensional model. Either 2- or 3-D models show that if the landslide block is broken before sliding, the upper layer of the landslide block would be disturbed and buried, so the residents living on the block cannot survived after sliding more than 2 kilometers. Hence, the Tsaoling 1999 landslide block induced by the Chi-Chi landslide is cohesive.
There are arguments that the collapse mechanism of the 1999 Jiufengershan landslide is shear out or buckling. The 2-D simulation can only discuss the results of a profile. However, the 3-D simulation profiles show the different failure mechanisms in the landslide block at a friction coefficient of 0.05. For the toe particles, the southern section of the collapse mechanism is buckling with short runout distance, and the northern section of the mechanism is shear out with long runout distance. Thus, the mechanisms of large landslide are not the unitary. The single evidence cannot be explanation to a large landslide.
For the Hsiaolin Village buried by the huge landslide mass induced by the 2009 Morakot Typhoon, the 3-D granular discrete element simulation revealed that the 590 Highland played a key role. If the debris did not cut the 590 Highland, most of the debris would flow into the northern gully of the Hsiaolin Village, and form a higher natural dam. The Hsiaolin Village might not be destroyed in instant. The residents of Hsiaolin might escape before the dam collapsed
The revised model revealed that most of the debris crashed and buried Hsiaolin Village after the modification of 590 Highland. For landslide model construction, the factors of consideration are not only before and after topography, but the terrain changes during sliding. It can make a proper assessment to predict the path and the deposit area.
Subjects
Landslide
Granular discrete element method
Runout distance
Buckling
Shear out
SDGs
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