自升式平台船基礎於砂土(上層)-黏土(下層)之承載力評估

Abstract

(1)目的: 近年台灣政府大力推動離岸風力發電(offshore wind power)發展，而風機的安裝與維護則是依靠自升式平台船(jack-up barges)的運作。雖然它廣泛地被運用於離岸石油與天然氣產業(offshore oil and gas fields)，但是其尺寸與樣式與應用在離岸風電的有差距。為了確保自升式平台船所座落之土層的安全性，討論自升式平台船基礎承載力(bearing capacity)為本研究主要目的，同時也是為了降低工程可能帶來的人員傷亡及財物損失。 (2)過程及方法：以往若單純討論單一土層的承載力計算，往往不符合實際工程現地所會遇到的多層及互層土壤，這讓所計算出的承載力與實際情況有相當大的落差。本報告探討基礎座落於砂土(上層)-黏土(下層)的互層土壤，藉由固定砂土層厚度(H)與改變基礎寬度(B)來探討厚寬比(H/B)對極限承載力的影響。研究利用Matlab與FLAC來進行數值分析以及土壤受力後的塑性流降伏區，最後藉由這些計算基礎工程上最重要的結果---安全係數。 (3)結果：由本報告的分析結果得知基礎極限承載力的大小，主要與砂土層厚度與基礎寬度之間的比值有相當大的關聯。然而現地情 況較書中所介紹之理論模式複雜許多，例如黏土的不排水剪力強度隨深度增加，而其影響因素也待之後做更進一步之研究。

Taiwan government promotes the offshore wind power development in recent years, and the installations and maintenances of wind turbines depend on the operations of jack-up barges. Although they are widely used in offshore oil and gas fields, the sizes and types are quite different with those employed in offshore wind farms. The main purpose of this study is to investigate the bearing capacity of jack-up barge footings, in order to guarantee the safety of them on proposed soil strata during construction, and to reduce the possible damage to the people and the properties on the barges. The bearing capacity calculation based on foundations in single soil layer does not reflect the actual in-situ condition, stratified deposits, so there exists a big gap between calculation results from single soil layer and actual bearing capacity. The research work focus on the effects of the thickness-width ratio (H/B) to the bearing capacity assessment of the foundation on the sand overlying clay, by changing footing width (B) but maintaining the same sand layer thickness (H). Software MATLAB and FLAC are utilized to conduct the numerical simulation and analyze the yielding zone of the plastic soil flow, respectively, and finally obtain the most important result “Factor of Safety” on foundation engineering from these calculations. According to the analysis results in this report, the bearing capacity of the foundation is highly relative to the ratio of the sand layer thickness to the footing width. However, the actual field conditions are much more complicated than those assumed in the theoretical models in textbooks, such as the undrained soil strength increasing with the depth, and the further research are required to explore the effects of these factors.