臺灣遽變式山崩傳送與堆積之顆粒流離散元素模擬

Abstract

山崩的分佈在全球非常普遍，且往往對民眾造成身命與財產極大的損失，僅就1992至2001年間，山崩在全球造成超過9千人死亡；台灣位於歐亞大陸板塊與菲律海板塊的交接處，為當今世界上最活躍的造山帶之一，地質極為年輕和不穩定，地震頻繁，加上每年夏秋二季的颱風侵襲，因此山坡地上極易發生大型遽變式的山崩事件。在台灣近年來最令人矚目的大型山崩事件有草嶺的連續崩塌事件 (包括1999年集集地震所誘發的山崩事件)、1999年集集地震所誘發的九份二山崩塌事件和2009年莫拉克颱風造成的小林村滅村事件。 近年來對於山崩的研究主要在於崩塌的誘發原因，對於山崩的運動過程及堆積區的衝擊範圍則是著墨較少，本研究採用顆粒體離散元素法 (Granular discrete element method) 來對台灣近年來幾個具代表性的遽變式山崩的運動行為討論之外，其中包括九份二山、草嶺、小林村的事件，並討論目前幾個大型山崩尚未解決的問題，包括是否能經由重複崩塌的草嶺來預估下次的崩塌地點、九份二山的崩塌機制，地形改變對山崩衝擊區的影響等。由於離散元素法在是以外顯時間的計算方式，且模擬過程中可以表現塊體的破裂行以及大尺度位移的特性，因此於山崩的運動過程可以做到非常恰當的模擬。 草嶺1941年的模型在輸入地震波模擬之後，我們發現塊體在受地震力作用時會有互相碰撞的現象，上方的塊體會向下碰撞，造成下方的塊體位移量增加，上方塊體的位移量減少，因此提高了山崩發生的機率，而在受地震作用的同時，塊體內部亦會產生許多裂隙，這些裂隙會有集中的現象，且集中的區域和未來山崩的位置恰好吻合。 除了1941年的崩塌外，我們同時動用二維及三維的程式來模擬草嶺1999年由集集地震所誘發的山崩，二維的模型顯示，草嶺山崩時的滑動面摩擦係數必需小於0.15時，最快滑動速度約每秒45公尺，土石才有可能越過清溪到達2000公尺外的倒交山；三維的結果更顯示滑動面的摩擦係數必需低到0.03以下，堆積區的形貎才有可能會符和真實的地形，而且最快速度可達每秒75公尺，最長的顆粒滑行距離更高達3495公尺，二者會有如此的差異是由於三維的模型要考慮到塊體側向擴張所增加的滑行距離，而耗損更多的動能，因此必需有低摩擦係數提高滑動的速度，才能有足夠的動能；而不管是二維或三維的模型都顯示，如果崩塌塊體在崩塌前就是破碎的，則崩塌體的上層在滑動時會受到嚴重的攪動而被掩埋，這樣就不可能有人在滑動超過2公里後而生存，因此草嶺在1999年因集集地所誘發的山崩塊體在崩塌前是一個有內聚力的塊體。 有關九份二山的崩塌機制是剪出或拱曲到目前仍有爭論，二維的模擬只能討論一個剖面的結果，但我們用三維模擬的結果發現，當集集地震誘發山崩後，滑動面的摩擦係數在0.05時，在模型的北、南二個剖面分別有不同的崩塌機制，在南邊的剖面其崩塌機以拱曲為主，故趾部的顆粒滑行的距離較短，而北邊的剖面的崩塌則是以剪出為主，趾部的顆粒滑行的距離非常遠，由以上的結果讓我們瞭解大型山崩的崩塌機有可能不是單一的，所以當我們在探討大型山崩事件時，如果只由局部的證據去推論整個山崩的機制是無法對整個山崩機制做完整的解釋。 有關2009年莫拉克颱風所誘發的小林村滅村事件之探討，經由三維顆粒離散元素法模擬得到小村村上方的590高地為小林村被掩埋的一個重要關鍵，如果崩塌的土石沒有把590高地削去大半，則大部份的土石會轉向流入小林村北側的溪溝，且形成一個更高的天然壩，但是小林村可能不會在一瞬間被毁減，在天然壩潰壩前村民也許會有足夠的時間逃脫。修正後模型的地形將590高地去除，則大部份的土石會直衝小林村，這也是導致滅村的主要原因。因此所以在山崩模型建造時，不但要考慮前後的地形，還要考慮到崩塌過程中的地形變化，才能做出正確評估或預測土石流動的路徑與堆積區域。

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.