- 學位論文
以填補法降低海床砂土之基腳足印互制作用
Mitigation of Spudcan-footprint Interaction in Sand with Infilling Method
摘要
本研究以室內1g模型實驗方法探討自升式平台船支撐基腳（spudcan）遭遇足印 （footprint）之對策。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，以砂斗開口大小及砂斗落距控制備置乾砂試體之相對密度為15%；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之氣體，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%。本研究採用直徑D = 200 mm之模型基腳貫入長1,200 mm、寬900 mm、高900 mm之試體，第一腳貫入目的為製造足印，第二腳貫入海床情況分為三種：(1) 貫入足印；(2) 貫入填平足印；及 (3) 貫入填砂平台。第二腳貫入點之偏移距離有四種：(1) 原點貫入 β = 0；(2) 偏移距離 β = 0.25 D；(3) 偏移距離 β = 0.5 D；及(4) 偏移距離 β = 1.0 D。依據實驗結果本研究獲得以下結論。 研究發現，Spudcan貫入足印中心點 ( β = 0) 時，因為第一腳造成足印凹陷，初始貫入呈現空貫，測得垂直力 Vz = 0。通常自升式平台船有4支或6支支撐腳，若其中1支支撐腳踩空，可能發生如同貫穿式 (punch-thorough) 的破壞。足印下方砂土因第一腳貫入時造成嚴重擠壓，鬆砂試體承受巨大垂直應力導致體積收縮，土壤孔隙比e下降，單位重增加，其強度與勁度也增大。隨著貫入深度增加，當spudcan下方圓錐開始接觸砂面後，spudcan承受之垂直力 Vz 迅速增加。若採用足印填平法，於任何spudcan貫入偏移距離，皆可有效提升基腳貫入造成之最大垂直力 Vz, max，試驗測得之基腳貫入支承力增加約5% ~ 20%。Spudcan貫入足印造成之最大水平力，發生於貫入偏移距離為 β = 0.25 D。當貫入深度z = 0.2 D時，無論偏移距離為何，使用填補兩法皆能減小spudcan再次貫入之最大水平力，其中以貫入偏移距離 β = 0.25 D及0.5 D效益最佳，最大水平力減少量皆為73% ~ 76%。於偏移距離為 β = 0.25 D，spudcan貫入足印造成最大之彎矩。在不同偏移距離中，以於 β = 0.25 D貫入填平足印可獲得效益最佳，彎矩減少量可達約63% 。
並列摘要
In this study, the mitigation of spudcan-footprint interaction in sand with infilling method was investigated by 1g physical model tests. Ottawa sand was used as soil specimen, and the diameter of the model spudcan was 200 mm. To simulate a uniform seabed material, the air-pluviation method was used to control the distribution of the soil density in the soil bin. After reaching the desired specimen thickness of 0.9 m, water was introduced from the base of dry sand specimen. Then, the remaining air in the flooded sand sample was removed by a constant suction pressure of 50 kPa for 8 hours. In this study, the purpose of the first penetration of spudcan into the soil specimen was to create an existing footprint. Then, the spudcan was repenetrated into three types of seabed conditions: (1) into the footprint; (2) into the infilled footprint; and (3) into the infilled footprint covered with a thin sand blanket. To investigate the effects due to re-penetration offset, four categories of offset distance were conducted: 0, 0.25 D, 0.5 D, and 1.0 D conducted in this study. Experimental results indicated, due to the existence of the footprint, re-penetration at the center of the footprint would cause no vertical force at the beginning stage of re-penetration. During the first penetration, soil beneath the footprint was compressed and a volumetric contraction occurred. The void ratio of soil decreased and its unit weight increased. Therefore, for the re-penetration, as the spudcan touched the soil surface, the vertical force increased rapidly. After adopting the footprint infilling method, for the offset distances used, infilling method would increase the maximum vertical force about 5% to 20%. For the spudcan to re-penetrate a footprint, maximum horizontal force and bending moment would occur at the offset distance of 0.25 D. As the penetration depth reached 0.2 D, two infilling method used in this study could effectively reduce the maximum horizontal force. At the optimum offset distance 0.25 D and 0.5 D, maximum horizontal force could be reduced for 73% to 76%. At the offset distance of 0.25 D, the maximum bending moment that acting on the spudcan could be reduced for about 63%.