互層砂土對自升式平台船 基腳支承力之影響

Abstract

本研究以模型實驗方法探討互層砂土對風機安裝船支撐基腳承載力之影響。本研究使用渥太華砂模擬海床砂土，進行直徑D = 100 mm之spudcan 1g模型試驗。本研究在交通大學基礎模型試驗室自行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、支撐基腳及資料擷取系統。為備置三種相對密度(Dr = 26%、Dr = 51%及Dr = 85%)砂土試體，本研究使用空中霣降法備製均勻乾砂土壤試體，將乾試體於試驗土槽內浸水，並以真空幫浦吸除土壤孔隙內殘餘之空氣，以模擬海床飽和土壤行為。本研究首先進行砂土飽和度試驗，實驗結果顯示渥太華砂經浸水，並以真空幫浦抽除土壤孔隙內殘餘之空氣後，飽和度可達99.97±0.14%。本研究探討四種互層砂土分別為(1)上層85%與下層26%; (2)上層51%與下層 26%; (3)上層26%與下層 85%; 及(4)上層26%與下層 51%。依據實驗結果，本研究得到以下各項結論。Spudcan貫入飽和砂土試體，所得之承載力qu實驗值略高於依SNAME (2008)公式求出之理論值。此結果與Lu (2007)以離心機模型試驗求出之實驗值高於以SMAME設計手冊求出之理論值結果一致。Meyerhof and Hanna (1978)理論假設淺基礎下側破壞面完全發生在下層。但Lu (2007)的離心機實驗顯示，當spudcan貫入通過互層土壤的界面時，造成之土壤破壞面同時切過上層及下層土壤。Meyerhof and Hann理論假設在spudcan貫入砂土過程，上下層土壤之體積、密度、及剪力強度皆保持為一常數。實際上當spudcan向下貫入即向上提腿時，鬆砂及緊砂受剪可能發生體積收縮(contraction)或膨脹(dilation)。上述土壤行為是導致腳掌支承力曲線理論值與實驗值不同之主要原因。Spudcan貫入不同相對密度之砂土，腳掌底下砂土破壞面所影響之深度大約介於1.0D至1.2D。

In this study, the effects of layered sand on the bearing capacity of the spudcan were investigated. Ottawa sand was used as soil specimen to conduct 1g physical model tests for the model spudcan with a diameter of 100 mm. Testing facilities used at the National Chiao Tung University consisted of the vertical loading system, soil bin, spudcan, and data acquisition system. To simulate the seabed soils with three densities (Dr = 26%, Dr = 51% and Dr = 85%), the air pluviation method was used to prepare the 900 mm-long, 900 mm-wide and 900 mm-high dry soil specimens. After the specimen was submerged in the soil bin, the air trapped in the specimen was removed by a suction pump. Tests were conducted to investigate the degree of saturation of the specimen. Test results indicated that, after submergence and suction, the degree of saturation of the specimen reached 99.97±0.14%. Four different types of layered-sand specimens were prepared: (1) upper layer with relative density Dr = 85% and lower layer with Dr = 26%; (2) upper layer with Dr = 51% and lower layer with Dr = 26%; (3) upper layer with Dr = 26% and lower layer with Dr = 85%; (4) upper layer with Dr = 26% and lower layer with Dr = 51%. Test results indicated that the experimental bearing capacities were about 0 to 20% higher than those calculated with the equation suggested by SNAME(2008). The experimental bearing capacities obtained with centrifuge model tests by Lu(2007) were also higher than the values estimated with SNAME. In the equation of SNAME, it was assumed that the volume, relative density and shear strength of soils in the upper and lower layers were kept a constant. However, during the penetration and lifting of spudcan in the soils, volume contraction in loose sand and dilation in dense sand would occur when sheared. Change of volume, density, and shear strength due to shearing was probably the main reason that caused the different bearing capacity curves between theoretical and experimental studies. For a model spudcan penetrating sandy soils with different densities, the depth of failure surface in soil was about 1.0D to 1.2D