地中壁於黏土層中深開挖對系統勁度及壁體變位之影響

Abstract

地中壁工法為常用於目前深開挖工程的輔助措施，以防止連續壁產生過多之側向變形量並減少對相鄰建築物的影響。Clough等人於1989年提出設計曲線，其考慮了土壤及擋土措施的效應，可初估開挖所造成的牆體位移。然而，由地中壁引起之三向度效應對於連續壁的抑制非常明顯。所謂的三向度效應為考慮輔助措施的存在以及開挖基地的大小。首先，針對三個已完成的開挖案例進行PLAXIS 3D分析，以驗證地中壁的存在確實對於抑制連續壁的側向位移有顯著的影響，以及發現其現地結果遠小於Clough曲線的預測值。 由案例探討可知Clough曲線已經不敷使用，若不進一步優化Clough曲線，將使得設計過於保守。因此，為了更準確地預測連續壁的側向位移，本研究在原始Clough的架構下結合地中壁的影響，透過系統勁度和基地底面隆起安全係數的調整來量化其地中壁效應；優化曲線後並延伸以涵蓋高系統勁度和高基地底面隆起安全係數，可用於估算地中壁影響下之牆體位移；另外，也蒐集其他案例來檢討其準確性及合理性。另外，進行三維數值分析進一步了解地中壁的影響，經過數值模擬發現地中壁間距為決定壁體位移的重要因素。根據數值結果，可得出15米的地中壁間距似乎能發揮其最佳之效益。 根據參數研究的結果，歸納出以下結論：一、修正後的Clough曲線可合理預估開挖所引致之壁體位移，並滿足設計所需。二、若要發揮地中壁之最大效益，15米的地中壁間距為最佳之設計值。

Cross walls are usually adopted to prevent excessive deformation of diaphragm wall and to minimize damage to the adjacent buildings. Three case histories were selected to demonstrate the presence of cross wall does have a significant effect on minimizing wall displacement. According to the validation of case histories, it is noted that the three-dimensional effect induced by cross wall is pronounced for the excavation in soft clay, which results in a pretty small field observation that is far below the results predicted by the design chart proposed by Clough et al. In order to have a better prediction on the wall displacement, this study incorporates the effect of cross wall within Clough’s chart by adjusting the system stiffness and factor of safety against basal heave. In addition, Clough’s original design curves are extrapolated to cover the uncharted area of both high system stiffness and high factor of safety against basal heave. The strengthening effect of cross wall leads to increase of system stiffness and factor of safety against basal heave that can be quantified by simplified approaches, which are incorporated within Clough’s scheme. With the revised scheme, the wall displacement under the influence of cross walls can be reasonably estimated, if the condition of soil, retaining wall and layout of project site had all been known. Other case histories are also studied to validate the revised scheme. Three-dimensional numerical analyses are also carried out to further calibrate the effect of cross walls. It is found that the spacing of cross walls is the most important factor that governs the magnitude of wall displacement. A typical spacing of 15 m between cross walls appears to be the optimal layout of cross walls if a low value of wall displacement is desired.