以實驗方法探討海床傾斜角度對自升式平台船基腳貫入行為之影響

Abstract

本研究以室內模型實驗方法探討海床傾斜角度對自升式平台船基腳 (spudcan) 貫入行為之影響。本研究使用渥太華砂模擬海床砂土，進行直徑D = 150 mm之基腳1g模型貫入試驗。本研究在交通大學基礎模型試驗室自行設計及建造之基腳模型試驗系統包含：垂直荷重加載系統、試驗土槽、模型支撐基腳及資料擷取系統。為模擬台灣彰化西部沿海之疏鬆砂土，本研究使用空中霣降法備製均勻乾砂試體，以砂斗開口大小(10 mm)及砂土落距(300 mm)控制備置乾砂試體之相對密度為18%。將試體於試驗土槽內浸水，並使用真空幫浦吸除土壤孔隙內殘餘氣體，以模擬海床飽和土壤之行為，飽和疏鬆砂土試體之相對密度為26%。本研究探討海床傾斜角度為i = 0°、1°、2°、3°、4°、5°、6°、8°及10°。依據實驗結果，分別對基腳貫入承受之垂直力、水平力及彎矩進行討論。Spudcan貫入不同傾斜角度之飽和砂土試體，所得之垂直力Vz隨海床表面傾斜角增加而減小。基腳貫入水平砂土試體之垂直力實驗值，較依SNAME(2008)公式求出之理論值高出約23%。但是，海床傾斜角越大，基腳貫入量測到之水平力越大，且在本研究最大貫入深度zmax = 0.5D之前，作用於基腳之水平力是隨著貫入深度增加而增大。實驗結果顯示，當基腳邊緣點接觸到傾斜海床表面時，測得作用於基腳之彎矩為最大值，且維持此最大值至最大貫入深度zmax = 0.5D，基腳所受之彎矩大小不受海床傾斜角之影響。基腳拔出後造成的足印在縱剖面上坡足印斜面及下坡足印斜面與水平面之夾角為29.1度，足印中心沉陷量與海床傾斜角並無明顯之關係，且足印最大沉陷量約為基腳直徑之一半。

In this study, the effects of the penetration of spudcan into sloping seabed were investigated. Ottawa sand was used as soil specimen. 1g physical model tests were conducted for the circular model spudcan with a diameter of 150 mm. Testing facilities used at the National Chiao Tung University consisted of the vertical loading system, soil bin, spudcan, and data acquisition system. The air-pluviation method was used to control the uniformity of the soil sand. With the slot opening of 10 mm and a drop distance 300 mm of soil particles, the very loose relative density of 18% was achieved. After the specimen was submerged in the soil bin, the air trapped in the specimen was removed by a suction pump. The relative density of the saturated loose sand was 26%. In this study, the sloping angles of seabed selected for testing were 0°, 1°, 2°, 3°, 4°, 5°, 6°, 8°, and 10°. Test results indicates, during spudcan penetration, the vertical forces acting on the spudcan decreased with increasing sloping angle of seabed. The vertical force acting on the spudcan measured when penetrating a horizontal seabed was higher than the theoretical value estimated with the equation in SNAME (2008) by 23%. The horizontal forces acting on the spudcan increased with increasing sloping angle of the seabed. The measured horizontal force acted on the spudcan increased with increasing depth, until the maximum penetration depth 0.5D was reached. The bending moment acting on the spudcan reached a maximum value when the spudcan edge point touched the inclined seabed. On the longitudinal profile of the footprint, the inclination angle for both the uphill and downhill slope walls was 29.1 degree. Settlement at toe of the footprint was not affected by the inclination of the seabed. The maximum settlement in the footprint was about half the diameter of the spudcan.