砂土密度對風機安裝船支撐基腳承載力之影響

Abstract

本研究以室內實驗方法探討砂土密度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為實驗用砂，進行1g spudcan模型試驗實驗。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。為備置不同相對密度砂土試體，本研究使用空中霣降法形成均勻土壤試體，將乾試體於試驗土槽浸水，並以抽吸幫浦抽除土壤孔隙殘餘之空氣，模擬海床浸水土壤行為。本研究於交通大學進行直接剪力試驗，實驗結果顯示，渥太華砂之尖峰內摩擦角隨著砂土之相對密度之增加而加大，本研究建立下列相對密度Dr與砂土尖峰內摩擦角peak之經驗公式: φpeak = 0.0867(Dr) + 28.3。渥太華砂之殘餘內摩擦角不隨砂土之相對密度而改變，殘餘內摩擦角維持31.7度。本研究探討砂土密度對浸水及抽氣造成之密度變化，實驗結果顯示，鬆砂單粒晶構造顆粒之排列鬆散，浸水及抽氣皆造成砂土顆粒重新排列成為較緊密之排列。緊砂顆粒原本即已緊密排列，浸水及除氣皆無法對相互鎖緊之顆粒結構造成明顯的改變。支撐腳掌貫入不同密度砂土試體實驗求得之承載力，與SNAME設計手冊求得之理論值相互符合。造成此結果原因是由於，在Spudcan向下貫入緊密原始砂土層過程中，不斷向下切割新的破壞面，故必須採用原始緊密土層之尖峰強度來估算spudcan貫入緊密砂土之承載力。當試體相對密度越大，則貫入造成試體表面最大隆起量越大，是由於緊砂受剪體積發生收縮然後膨脹，造成試體表面隆起。最大隆起位置介於中心線左右約1.3~1.7倍基腳直徑之間，砂土密度對最大隆起位置影響不大。Spudcan 向下貫入砂土時，回流區之海床傾斜角接近砂土殘餘摩擦角31.7度，原因是基腳貫入海床土壤後，兩側砂土回流填補基腳通過造成之孔洞，故回流區表面形成安息角。

In this study, the effects of soil density on the bearing capacity of the spudcan penetrating submerged sandy soil were investigated. Ottawa sand was used as soil specimen and 1g physical model tests were carried out. The testing facilities constructed 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 different densities, the air pluviation method was used to prepare uniform soil specimens.Direct shear test results indicated that the peak internal friction angle of Ottawa sand increased with increasing soil density. An empirical relationship between and the peak friction angle and the relative density of soil was proposed. The residual internal friction angle of sand did not change with increasing soil density, and remained 31.7 degree. The dry sand in the soil bin was submerged, and then the air trapped in the submerged soil was removed by a suction of 0.5 atm for 9 hours. Submergence and suction significantly increased the density of the loose sand. However, little density change due to submergence and suction was observed in dense sand. The experimental bearing capacity of the spudcan penetrating soils with different densities were in fairly good agreement with those calculated with the equation suggested by SNAME (2008), using the peak internal friction angle of soil. This finding may be explained by the fact that, during the penetration of spudcan into the seabed, new failure surface were cut continuously in the undisturbed virgin soils, there fore it became necessary to use the peak shear strength of soil to determine the bearing capacity of the spudcan. The amount of ground heaving of the specimen increased with increasing soil density. The ground heaving is probably due to the contraction followed by dilation of dense sand under shearing. The position of maximum ground heaving was located between about 1.3~1.7 times diameter of spudcan from centerline.In the back-flow zone,the measured seabed inclination angle was almost identical to the residual friction angle 31.7 degree of the sand.This is because the soils on both sides of the penetratim zone back flowed to fill the cavaity due to spudcan penetrating . An angle of repose was formed on the surface of the back-flow zone.