以TORSA彈塑性基礎梁法模擬開挖土堤的方法探討

Abstract

在工區範圍較大，形狀規則，且周遭環境較不複雜等前提下，土堤可做為開挖時的輔助工法，並且可以隨著永久工程的進行而在短時間內分區與分階移除，或在永久工程已經可以提供足夠支撐的階段下進行移除，因此在某些情況下，它們提供了在某些條件下降低成本與時間之方案。深開挖可使用土堤作為臨時支撐，其目的在幫助穩定擋土牆，並減少位移量，但目前針對設置土堤之設計仍以經驗工法為主。 目前工程中常使用輔助軟體TORSA進行開挖設計，但TORSA現有條件未提供模擬土堤之功能，故無法直接預測具有土堤狀況之擋土壁變位。為探討模擬土堤的合理力學機制，本研究以明挖斜坡土堤開挖為背景進行研究，由於土堤的規劃將直接影響開挖安全性與引致變位的大小，因此土堤留設形狀/體積大小與其所能提供之支撐性變成是相對重要的課題，可以看出土堤在黏土中開挖所能提供的效益較大，這可能是因為相對堅硬的土壤(如礫石層)本身自立性佳，開挖引致的側向變位並不大，較難看出土堤所能提供的效果。相對地，軟弱黏土開挖所產生的變位較大，土堤所達到的成效就益發明顯。 本研究首先利用有限元素法軟體PLAXIS進行反算分析，再運用TORSA現有功能針對不同條件下進行模型驗證，最後建立了三種簡化的分析方法來考慮土堤支撐擋土壁的情況，分別為：側向土壓力法、折減覆土載重與土壤參數法以及等值開挖深度法等方法。研究利用現地案例驗證結果與參數分析結果發現以折減覆土載重與土壤參數法假設土堤，可得較為合理的結果，使用此方法所得之最大誤差值遠小於其他方法。而使用側向土壓力法則對壁體變位結果很敏感，如果無法準確預估側向土壓力的值，容易有高估或低估變位的結果，且側向土壓力法與等值開挖深度法對於土堤有效面積之折減因子RA非常敏感，亦會有高估或低估壁體變位的情況，於黏土層中影響更顯著。

Under the condition that the area is large, the shape is regular and the surroundings are less complex, earth berm can be used as an auxiliary method in excavation, can be partitioned and removed in a short period simultaneously with the permanent work in progress. Alternatively, it can be removed once the permanent structure can provide sufficient support. Thus, in some cases, the option of using earth berm can possibly reduce costs and time under certain conditions. Earth berm in deep excavation is recognized as temporary support, which helps to stabilize retaining walls and reduce displacement, but the current design details for the assumption of earth berm are still mainly based on empirical methods. The earth berm could be possibly selected as additional temporary support measure to stabilize retaining walls and reduce related displacement. The commercial beam-spring model software TORSA is a widely used tool to simulate deep excavations. However, it could not include the function of berm in the analysis so the lateral wall displacement of retaining structure could not be predicted successfully herein. To explore the reasonable mechanism of earth berm, Cases of open cut with additional earth berm are selected for further studies. Details of the berm directly affect the safety of excavation and the magnitude of the deformation, retention of the shape/volume of soil berm and its performance become relatively important issues. From this study, it confirms that the use of berm could reduce the displacement caused by the excavation, especially for the clay layer. In contrast, the berm might not contribute significant impacts for excavation in gravel. As the gravel is relatively hard, the displacement caused by excavation is not as large as the excavation in soft soil and this might connect to the statement above. In this study, the back-analysis using finite element method software PLAXIS was first undertaken and the validations using TORSA under various conditions were carried out. Three simplified analytical methods were established and examined for evaluating the performance of earth berm. Results show the method using reduction of overburden load and soil parameters method could possibly give better results. The lateral earth pressure method is sensitive to the wall displacements. Thus, if the lateral earth pressure cannot be accurately estimated, the displacement may be overestimated or underestimated by using this method. In addition, the lateral earth pressure method and the raised excavation surface method are sensitive to the reduction factor of the effective area of the earth berm RA, the wall displacement may also be overestimated or underestimated, especially in clay layers.