斜移斷層潛移錯動對高架橋樁基礎性能之影響評估

Abstract

臺灣地震工程領域在集集地震之後致力於探討斷層尖端的強地動力對於上部結構與基礎設計的影響(Huang et al., 2017)，然而，極少數將地震引致地表破裂以及斷層潛移錯動納入考量；諸多文章討論地震產生之動態效應對結構互制行為的影響，斷層長期潛移錯動對結構的影響評估在文獻中仍然相對少見。臺灣中央地質調查所於2021年公告36處活動斷層，其中有8條為斜移斷層。南北狹長的自然地質條件，使得線型結構物，如：高速公路、鐵路與高速鐵路等，難以避免地跨越既存的活動斷層影響範圍。在這些線型高架橋樑結構中，樁基礎為最主要的結構形式。 本研究著重於探討群樁結構受斷層潛移之互制行為，研究區域位於高雄燕巢區，臺灣高鐵於此跨越車瓜林斷層。根據GPS測量報告顯示（趙家賢，2019），位於斷層影響範圍內之橋面板自完工以來 ，上下盤已有可觀測之相對偏移量，顯示車瓜林斷層具有潛移特性。綜合現地調查與監測資料，我們確定車瓜林斷層西南段在燕巢區域為斜移斷層，同時兼具右移與逆衝分量，橫移抬升比估計為7:1。 研究採用縮小尺度(1:100)的物理砂箱試驗，以石英砂作為上覆土層之材料，並放入2×2群樁基礎。在給予不同斜向滑移之邊界條件下，利用數值分析方法校核砂箱成果，討論分析關鍵因素，包括雷氏剪切角度、斷層擴展距離、三角剪切帶範圍以及樁帽三方向之位移旋轉等變化。結果顯示，樁帽水平向之位移與樁帽偏擺旋轉量(Yaw)與斜移斷層帶在砂箱中的不對稱分布有相關性，當斷層帶偏向下盤側明顯，樁帽水平向之位移量漸增而樁帽偏擺旋轉量(Yaw)則漸減。樁帽翻滾旋轉量(Roll)則在群樁基礎同時受抬升與水平錯動時，有較明顯之趨勢。根據模擬結果，群樁基礎三維位移、旋轉與其受潛移錯動之斷層型態有關，本研究基於砂箱試驗成果，校核數值模擬的材料參數、模型邊界等條件，進一步進行全尺度多跨高架橋群樁基礎之變形行為性能評估。 利用現場橋樑結構監測數據，校核全尺寸離散元素法之模型並用於初步變形性能評估。比較數值模擬量值與現場橋墩水準點位監測值，顯示約有80%的擬合度。本研究發現樁帽的最大剛體旋轉發生在斷層尖端投影位置附近，上部結構變位趨勢對應於不同的樁組旋轉和位移，這些成果對後續安全補強及加強監測的對策方案，提供了參考比對的依據。

Earthquake engineering has devoted much effort to considering seismic ground motion in the design of infrastructures, but rarely brings the effect of earthquake fault ruptures into the analysis. Although more studies have focused on the topic of fault-soil-structure interaction due to sudden fault offset in recent years, the assessment of structural performance under long-term fault creeping is still rare in literature. In Taiwan, the Central Geology Survey has announced 36 active faults so far. Due to geological settings and natural terrain conditions, linear structures such as Highways and High-Speed Rail are difficult to avoid crossing the fault zone. Of these infrastructures, pile groups are often used as the foundation to support the superstructure. This study aims to evaluate the performance of the pile group foundation subjected to fault creeping. We focus on the area where the High-Speed Rail of Taiwan crosses an active fault Chegualin Fault. Reports indicate that the ridge deck has been offset by observable value due to faulting since the construction was completed. Based on field investigations and monitoring data, we confirmed that the Chegualin Fault in its west-southern segment is an oblique fault, which has a ratio between right-lateral and uplifts up to 7:1. The study used lab-scale sandbox experiment with a 2×2 pile foundation. Under different oblique-slip ratios, key factors were compared, including fault propagation, triangular shear zone, and 3D deformation of the pile cap. The results showed a correlation between the horizontal displacement of the pile cap and the yaw rotation of the pile cap with respect to the oblique-slip fault. According to the simulation results, the three-dimensional deformation of the group pile foundation and fault patterns are related to its susceptibility to lateral movement. Based on the findings of the sandbox experiments, this study will further conduct a performance assessment of full-scale elevated bridge group pile foundations. The fault location and the deformation zone were also identified at the site scale. With the aid of in-situ structural monitoring data, full-scale discrete element modeling can be calibrated and used for preliminary safety evaluation. The comparison between simulation and in-situ bridge pier displacement shows approximately 80% agreement. We found that the superstructure distress is corresponding to differential pile group rotations and displacements. The maximum rigid-body rotation of the pile cap occurs close to the projected fault tip.