近接施工引致潛盾隧道三維變形特性之研究

Abstract

臺北捷運大部分路網為地下潛盾隧道段，隨著捷運路網之普及與土地資源之短缺，愈來愈多新興結構物之基礎開挖將緊鄰捷運隧道，對列車營運及隧道結構安全產生威脅。因此，近接施工是近年來大地工程實務上極為重視的新課題。 欲提供捷運隧道之安全預警，須仰賴合適的監測系統。目前針對近接施工，捷運隧道採用的監測方法多是沿襲真圓度檢測之概念，透過量測隧道環片橫斷面(in-plane)上數個測點的二維相對變形值，如內空變位、軌道沉陷等，與規範容許變形值相比，判定是否該採取行動。惟近接施工屬於複雜的三向度問題，故此法能提供之預警效果有限，實有必要深入探討隧道三維變形特性，方能發展更完善的監測系統，助於隧道安全評估。 本研究利用三維有限差分程式FLAC3D分別建立潛盾隧道及深開挖數值模型，並依據解析解與案例監測資料與分析結果相互比對，以檢核施工程序之適用性，最後建立近接施工數值模型。藉由數值途徑可模擬潛盾隧道受近接施工擾動之力學行為，並依序由隧道全段及特定剖面之觀點，以in-plane及out-of-plane之分析角度探討隧道之變位與應力增量狀況。 研究結果顯示：近接施工對潛盾隧道的三維力學行為影響甚大。環片變形方面，隧道環片中心之偏移量普遍高於隧道徑向變形，縱向變位之存在更凸顯現有監測方法之不適。藉由臨界破壞線之發展，可明白近接施工引致的隧道破壞潛勢為一三維分佈情況。隧道應力增量方面，藉由out-of-plane分析可知隧道環片於縱向上之軸力將隨著接近開挖工區而產生受拉或受壓之變化，勢對隧道整體結構造成不利影響。未來宜發展更完善之監測方法，進一步考量隧道縱向之受力情況，以三維概念進行評估，如此一來隧道安全方有保障。

Most underground network of Taipei Mass Rapid Transit (MRT) System was composed of shield tunnel. As MRT lines increase and lands decrease in Taipei megalopolis, more and more constructions such as foundation-excavation approach existing shield tunnel closely, and the kind of situations is called neighboring construction. Neighboring construction might threaten the safety of MRT operation and tunnel structure, and therefore has become an issue which is emphasized gradually during these decades. Conventional tunnel monitoring items are applied popularly in shield tunnel during neighboring construction, such as convergence deformation or railroad settlement, which neglects longitudinal mechanical properties of tunnel. These current methods are established in two-dimensional plane (in-plane). However, neighboring construction is complicated and belongs in three-dimensional phenomenon, thus current methods of tunnel monitoring are inadequate and should be improved. First of all, this study builds up numerical models of shield tunnel and deep excavation separately by FLAC3D, and the models have been verified by comparing its calculated results with existing analytical solutions and actual case data. The second, this study combines two models into one model for simulating the responses of a shield tunnel encountering neighboring construction. Based on the neighboring construction model, the three-dimensional deformation and stress increment of whole tunnel and specific profile can be discussed in the views of in-plane analysis as well as out-of-plane analysis. Results of numerical simulation indicate that neighboring construction has a significant influence on three-dimensional deformation and stress variation of shield tunnel. In the part of segment deformation, the deformation of segment center is more than convergence deformation generally, and there is longitudinal deformation between segments. With the distribution of critical failure line, the potential failure site of shield tunnel which is caused by neighboring construction could be a three-dimensional distribution. In the part of stress increment, as a result of out-of-plane analysis, the longitudinal force of shield tunnel would be compressed or tensile along the axial direction, and the appearance of variation may cause damage to tunnel structure which is fragile. Accordingly, this study recommends that more appropriate tunnel monitoring method should be developed in the future, and it might be a better way to evaluate shield tunnel with three-dimensional concept so that provide early warning for tunnel safety.