模型試驗探討乾濕顆粒流之行爲

Abstract

本研究利用不同粒徑的乾顆粒(礫石、砂、玻璃珠)與選取豐丘與神木現地兩種不同類型土石流(礫石型、一般型)之土石級配縮小尺寸來當試驗材料，於砂箱模型中模擬其流動及行為之探討。試驗採用可控制開關的鋁製漏斗型降土設備，落距為50 cm，使試體落至15∘坡面後流動。試驗的過程中，以攝影機觀察並擷取試體顆粒之運動影像，再應用質點影像測速儀(Particle Image Velocimetry, PIV)擷取影像及進行分析。此外於流動段坡趾處架設衝擊力量測裝置，記錄流體之衝擊力大小。 乾顆粒流考慮不同粒徑與量體，試驗結果發現，礫石撞擊坡面後跳動距離大，初始速度較快，但流動過程中，顆粒間因相互碰撞、摩擦等使流速以20~35%比率急遽下降，並快速堆積於坡面上，故堆積影響範圍小，1/4吋礫石堆積的長度與寬度比值為2.06，3/8吋礫石則為2.33；當量體增加時，此堆積現象會更為明顯，此時1/4吋與3/8吋礫石堆積長寬比為2.16與2.27。而砂流動性較佳，流速雖以4~6%比率逐漸下滑，仍能以較快速度進入堆積區，影響範圍較大，長寬比為1.79。此外，礫石在撞擊量測裝置時，因流速已經變很緩慢，所得衝擊力會低於砂顆粒之衝擊力。另考慮球形且表面光滑之玻璃珠，在流動過程中因與坡面摩擦甚小，會因重力加速度而以5~13%比率增加其流速，而玻璃珠為大顆粒，在這些條件下，玻璃珠衝擊力會遠大於其他乾顆粒試驗所得者。 土石流試驗考慮不同類型與量體，由本試驗得知，此兩種因素對流動速度影響不大，但衝擊力方面有部分相同與不同之處，皆出現兩波尖峰值，且集中於流體剛撞擊時。當量體增加一倍時，其衝擊歷時大致相同，並不會延長，反而是撞擊後雍高，衝擊面積變大，使測得力量上升。然若將試驗所量到衝擊尖峰值除上其作用面積，可發現單位面積所受的力大致相同；但含大顆粒較多的礫石型單位面積之衝擊力大於一般型土石流約為1.167倍。

This study focused on the behavior of both dry and wet granular materials flowing down from upslope by performing model tests in a sand box. The dry test materials of different particle sizes were gravels, sands, and glass beads. The wet test materials were selected according to the similitude laws and the particle size distribution of the materials at two debris flow sites, Feng-chiu and Shen-mu, respectively. The material at Feng-chiu is composed mainly of gravels and is called gravel type, while that at Shen-mu has more fines and is called general type. The test materials were stored in an aluminum hopper before test, and were subsequently released from a distance of 50-cm high onto a 15˚ slope by a control switch of the equipment. During the test the movement of the particles was recorded by three video cameras, and the analysis of the images of particles was performed by a particle image velocimetry (PIV). Moreover, a device used as the obstacle was set up at the toe of the slope for measuring the impact force of the material. To test the dry granular flows, different particle sizes and quantities were considered. The test result showed that the gravels traveled a larger distance after impacting the slope than the sands; it consequently resulted in faster initial velocity. However, during the flowing process, the velocity of the gravels decreased quickly because of the collision between particles as well as the friction from the slope surface. As a result, most of the gravels deposited on the slope, having no obvious run-out zone. As the amount of gravels was increased, this phenomenon became even more obvious. On the contrary, the sands have better fluidity and moved with faster speed than the gravels. Therefore, the sands had a clear run-out distance and accumulated in the toe area of the slope. In addition, the impact force induced by the gravels was less than that of the sands due to their different velocity characteristics. The spherical glass beads have smooth surfaces, and their velocities increased all the way down to the toe of the slope. Consequently, the impact force induced by the glass beads was the highest among the three granular-material flows. In testing wet flows, two types of flow with different quantities were taken into consideration. The test result showed that the composition of flow had little effect on its velocity, and that both flows displayed two obvious peaks in the time history of impact force. It was found that the duration of impact force remained nearly unchanged, even though the quantity of flow was increased. After impacting the obstacle, the flow ran up, impacted a larger portion of the obstacle, and induced higher impact force. Nevertheless, the impact force per unit area was about the same irrespective of the flow type. Further, the gravel-type flow induced higher impact force per unit area than the general-type flow.