於深開挖工程案例中，可能因為開挖基地大、地界線距開挖基地尚有一段距離、需要較寬廣之施工空間以及扶壁需作為永久結構物等原因，在開挖設計上會設置外扶壁，但對於外扶壁之研究卻相當缺少。因此，本研究首先針對二個座落於台北盆地且設置外扶壁之深開挖案例，進行三向度有限元素法之數值模擬分析及比較驗證，以瞭解設置外扶壁對開挖引致變位之影響，並探討設置外扶壁對於減少最大壁體變位與地表沉陷之成效。為了要更加掌握外扶壁抑制開挖引致變形之特性，本研究建立假設案例進行一系列參數研究，以瞭解不同影響因子對於外扶壁減少開挖引致變形之效率，並探討外扶壁變形控制之主要機制。
根據研究結果顯示，外扶壁變形控制之主要機制為連續壁與扶壁結合的組合勁度，故設置外扶壁以提高組合勁度除了可抑制開挖引致之變形外，亦可提高壁體勁度，使壁體變形趨向剛性變形。因此，本研究挑選出影響外扶壁抑制壁體變位與地表沉陷較明顯之參數因子，進行藉由設置外扶壁達剛性擋土系統之參數研究，提出外扶壁平面配置之關係式作為外扶壁配置之設計方法，並以兩個支撐開挖工法案例及三個無支撐開挖工法案例驗證其適用性，供實務上使用。

In deep excavation projects, the use of outer buttress walls is often necessary due to factors such as the large size of the excavation site, significant distance from property boundaries, the need for a wide construction space, and the requirement for permanent structures. However, research on outer buttress walls is currently lacking. Therefore, to clarify the effect of outer buttress walls on the movements of excavations and investigate the effectiveness of outer buttress walls in reducing maximum wall deflections and ground surface settlements, two deep excavation cases with outer buttress walls in the Taipei Basin are used to simulate and compare by using three-dimensional finite element method.
To better understand the characteristics of outer buttress walls in reducing excavation-induced deformations, this study establishes hypothetical cases for a series of parameter studies. The objective is to examine the efficiency of various influencing factors in reducing deformations induced by excavation and to investigate the main deformation control mechanism for outer buttress walls.
The research findings indicate that the main deformation control mechanism for outer buttress walls is the combined stiffness of wall and buttress walls. Therefore, the installation of outer buttress walls not only helps reduce deformations caused by excavation but also enhances the stiffness of the wall, leading to a more rigid deformation. Based on these results, this study identifies the key parameters that significantly affect the ability of outer buttress walls in reducing wall deflections and ground surface settlements. A design method for the configuration of outer buttress walls to achieve a rigid retaining system is proposed. A relational equation for the layout of outer buttress walls is developed and validated through two supported excavation cases and three strut-free excavation cases, aiming to provide practical application.

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