考慮扶壁效應於開挖行為之簡化分析

Abstract

扶壁與地中壁工法是目前常用的深開挖工程輔助措施，連續壁受到扶壁或地中壁的束制而減少側向變形量，其變形行為亦稱為三向度效應。然而，目前工程界常採用單向度彈塑性基礎梁法或二向度平面應變的方法設計連續壁，其分析程式無法考量扶壁與地中壁的三向度變形行為。因此，本研究使用兩種分析方法預測連續壁變形，其一為三維有限元素法PLAXIS 3D，可合理反應案例真實的變形行為；其二為單向度分析程式RIDO，將三向度效應考量為修正因子進行簡化分析，其簡化分析方法為：將扶壁兩側之摩擦力視為地盤改良的效應，以提升被動土壓力，使開挖側的土壤勁度增加。 本研究首先針對兩個深開挖案例進行RIDO與PLAXIS 3D分析，由現地監測值與分析結果顯示，扶壁長度過長時，簡化分析方法高估而導致牆變位過小，因此簡化分析方法必須適當修正。再根據開挖尺寸、開挖深度、扶壁配置數量、扶壁間距與扶壁長度等影響因子進行參數研究，並修正簡化分析方法。 本文提及的修正方法是藉由修正扶壁長度 (Bb) 來改善簡化分析方法，根據參數研究的結果，歸納出以下結論：一、當開挖長度大於40公尺時，建議使用圖4.11中的Bb最小邊界線，以避免低估連續壁牆變位。二、對於大開挖基地，增加扶壁長度比增加扶壁數量更能有效地抵抗連續壁牆變位。最後，RIDO程式搭配修正後的簡化分析方法，能夠避免高估連續壁牆變位的情形發生，並可合理模擬含有扶壁工法的開挖行為。

The diaphragm wall deflection can be reduced by the restraining effect from cross walls and buttress walls, the so called three-dimensional (3D) effect. Practically, the prediction of wall deflection has been frequently conducted by one-dimensional (1D) and two-dimensional (2D) analyses under plane strain condition. However, 1D and 2D analyses are inappropriate, when the 3D effect becomes significantly profound in excavation. Therefore, 1D analysis program RIDO along with a simplified approach is performed herein to capture 3D effect, considering the side-wall frictions as the effect of soil improvement to enhance the lateral passive earth pressure. In addition, PLAXIS 3D is used to validate this approach. Two case histories were studied to verify the simplified approach in this thesis. When the Bb is getting greater, the approach underestimates the maximum wall deflection. Therefore, a parametric study was conducted to improve the simplified approach by discussing the influence factors such as excavation size and depth, buttress wall length (Bb), the number of buttress walls. From the results of the parametric study, the following conclusions can be drawn. Firstly, the corrected Bb proposed in this study was used to improve simplified approach. Secondly, when the excavation length is greater than 40 m, the minimum boundary of corrected Bb in Fig. 4.11 is suggested to use. Thirdly, increasing Bb is more effective than increasing the number of buttress wall to reduce the diaphragm wall deflection, especially for large excavation size. Lastly, the corrected approach is capable of capturing the 3D effect in excavation with buttress wall.