梳子壩受土石流衝擊時動態行為之研究

Abstract

由於台灣地質較為脆弱，河川多短小流急，加以位處亞熱帶地區，每逢颱風豪雨，常有山崩、土石流等坡地災害發生，其中尤以土石流發生迅速，破壞力大，影響區域較廣，最值得注意。近年來在坡地過度開發下，土石流災害更加嚴重，而921集集地震後，原本破碎之地質狀況亦愈形脆弱，提高土石流災害發生之可能性。 本研究之室內水槽試驗以等重量替代法對現場土石材料之粒徑分佈曲線加以修正。試驗中，於混凝土梳子壩上設置土壓計與加速度計，並連接自動資料擷取系統，量測壩體受土石流衝擊時之動態行為與反應，進行衝擊力及相關攔阻功效研究。本研究將梳子壩之下方固定，假設其為一不均勻斷面懸臂樑，在自由端量測之振動加速度，於時間域下積分，求其速度-時間與位移-時間之關係，並以擬靜態側向彈性勁度之假設，分析壩體所受之衝擊力，再計算出土石流作用於壩體之合力與位置。 研究結果顯示:當壩體角度較大時，所受之衝擊力相對增加，其貯砂率與分離能力均提高，而壩體中央處其所受之衝擊力較兩側為大；在壩體角度較小時，土石流對壩體衝擊力有明顯地減小。由試驗結果可得合理之衝擊力作用位置為距壩底高0.112m(3L/8)∼0.15m(L/2)處，而以林弘群(1994)流體動壓之觀點，推估所得衝擊力作用位置為距壩底高約（L/3）0.11m處。

Due to the weakness of geologic condition in Taiwan, with the short and rigid flow in the streams, the typhoon and heavy storm often happen because of Taiwan locating at tropic region. It mainly causes the debris flow and other slope destruction. With the rigid debris flow, it contains a large amount of devastative power, which would affect a large spread of areas. Recently, under the over excavation of the slopes, the damage caused by debris flow has became essentially especially after the 921 Chi-Chi earthquake. The occurring possibility of debris flow is arisen out of the weakening geologic situation. With the use of Equal-Weight Replacement Method, the grain size distribution curve obtained by the in situ grain size can be correlated. The impact experiment is performed in the water flume. The soil pressure meter and acceleration meter are attached to the sambo dam model stricken on the water flume. Through the Automatic Data Log System, the dynamic behavior and reaction on the sambo dam can be recorded when the impact produced by the debris flow. And the research is aim to study the impulsive force and corresponding blocking effect on sambo dam. It is assumed that the cross section of sambo dam is in the form of inhomogeneous cantilever beam. By the measurement of vibrating acceleration at the free end, respect to the integration with time, it estimates the relationships between velocity & time and displacement & time. Consequently, the assumption of statically lateral-elastic stiffness is assist to assess the impulsive force and then, the resultant force and its location acting on the sambo dam under the impact of debris flow can be calculated. The research result reveals that the greater the impulsive force, the higher the inclination of sambo dam. It also increases the sand-retaining ratio and separation power of the dam. The central part of sambo dam receives the most of impulsive force that is greater than its lateral sides. Decreasing the inclining angle of sambo dam, it is obviously lower the impact acted by the debris flow. The location of resultant-impulsive force can sound reasonably because it ranges from 0.112m (3L/8) to 0.15m (L/2) far from the base of sambo dam. And that location is also the same as the result (0.11m [L/3] from the base) derived by Lin (1994) with perspective to the dynamic pressure of fluid.