考量潛變效應之拱壩長期變位與動力行為分析研究

Abstract

翡翠水庫為行政院核定之國家一級關鍵基礎設施，其功能性與安全性維繫整個大臺北地區600萬民眾生命財產安全，自1987年至今已經營運33年。翡翠水庫除了在興建時結合各界之專業規劃、設計與完善良好的施工品質外，也在運作期間，建置完整大壩安全監測系統，並定期辦理水庫安全評估來確保大壩安全。 本研究目的係於翡翠水庫已臻完善的安全管理基礎上，針對其大壩變位預測及現有變位警戒值之安全評析，建立可以模擬預測大壩變位的有限元素模型。此模型主要考慮外在因素（氣溫歷時、水位變化）與內在因素（潛變效應），並利用此模型，改善其現有的變位警戒值，確認翡翠大壩之長期變位是否有安全上疑慮，作為未來翡翠水庫於安全管理上的參考。 本研究分析結果顯示，溫度效應係因壩體熱漲冷縮引致大壩主要向上游側變位；而水位變化效應係因大壩受水壓力作用影響，其變位方向向下游側變位；潛變效應則由於水位加載之因素也同樣往下游側變位。另透過累積變位的比重計算至2020年，觀察壩體監測儀器中變位幅度最大之處，於中央高程172.5m之分析結果顯示，整體趨勢係溫度隨時間比重下降，水位隨時間比重接近持平，潛變則隨時間比重上升，潛變造成變位所佔比例維持在24%；溫度造成變位所佔比例約50%；水位造成變位所佔比例約26%。 對於大壩長期變位預測，本研究採用臺灣大學土木工程系發表之臺灣本土化潛變模型Model B4 TW(2020)，搭配翡翠水庫興建時之實際潛變試驗資料，建立出屬於翡翠大壩之潛變預測公式，於中央高程172.5m之分析結果顯示，潛變變位從2020年15.73mm到2050年增加至18.346mm，30年增加的幅度約16%，2050年18.346mm 到2100年增加至21.04mm， 50年增加幅度約15%，潛變成長幅度已逐漸趨緩，且整體變位皆於警戒值範圍內，另經大壩長期應力與應變分析結果，潛變鬆弛效應造成的應變增量，並無造成大壩內部明顯應力重分配。整體而言，大壩內部應力與應變皆在安全範圍內。 過去對於大壩變位警戒值之訂定，著重於大壩受溫度及水位變化作用之影響，未完善考慮潛變效應對大壩變位之影響，故本研究優化過去大壩變位警戒值，分別考慮溫度、水位、潛變對大壩所造成之變位，並且將潛變所造成之變位於每十年進行更新，並針對現今警戒值查詢系統進行警戒值表格設定，對於翡翠大壩長期變位之預測，能考慮潛變效應之累積變位，更反映真實大壩變位行為。 大壩動力分析中，考慮壩體混凝土的動態彈性模數與基礎岩盤的動態有效變形模數，並利用Zangar 等效附加質量元素來模擬上游面的動態水壓力與壩體相互作用之影響，其模型能有效的反應真實結構行為。並以不同條件之靜力分析結果作為動力分析之初始應力，考慮潛變效應對大壩動力分析所造成的影響，在MDE地震力作用下，壩體變位狀況以及應力分布情形，提供在大壩安全管理下之參考。 關鍵字 : 混凝土、潛變、拱壩、有限元素分析、地震力。

The Feitsui Reservoir is a first-level national infrastructure approved by the Executive Yuan. The functionality of Feitsui Reservoir supports the safety of the lives and property of 6 million people in the Taipei metropolitan area. It has been in operation for 33 years since 1987. In addition to integrating professional planning and design in the construction of the Feitsui Reservoir along with high construction quality, a complete dam safety monitoring system has also been implemented, conducting regular reservoir safety assessments to ensure the safety of the dam. The objective of this study project is to predict the displacement of the dam based on the safety management of the Feitsui Reservoir by establishing a finite element model. This model shall mainly consider external factors (temperature and water levels) and internal factors (creep). The existing displacement warning value for safe management can be further improved. The analysis results of this study show that the temperature effect is due to the thermal expansion and contraction of the dam body, which causes the dam to be displaced mainly to the upstream side; and the water level change effect is that the dam is affected by the water pressure, and its displacement direction is displaced to the downstream side; The creep effect is also displaced to the downstream side due to the loading of the water level. In addition, through the calculation of the proportion of cumulative displacement until 2020, the largest displacement in the monitoring instrument of the dam body is observed. The analysis results at the central elevation of 172.5m show that the overall trend is that the proportion of temperature decreases with time, and the proportion of water level is close to the same with time. , the proportion of creep increases with time, and the proportion of displacement caused by creep remains at 24%; the proportion of displacement caused by temperature is about 50%; the proportion of displacement caused by water level is about 26%. For the prediction of long-term dam displacement up to 2100 AD, this research uses the localized creep model named Model B4 TW (2020) published by the Department of Civil Engineering of National Taiwan University. Model B4 TW (2020) combines the actual creep test data at the time of the construction of the Feitsui Reservoir and establishes a creep prediction formula belonging to the Feitsui Reservoir. The analysis result of the central elevation of 172.5 m shows that the creep displacement will grow from 15.73mm in 2020 to 18.346mm in 2050, an increase of about 15% in 30 years; the creep displacement will increase from 18.346mm in 2050 to 21.04mm in 2100, which is about 15% in the 50 years. The rate of growth of the creep displacement has gradually slowed down and the total displacement is within the range of warning values. Furthermore, the results of the dam long-term stress and strain analysis show that the strain increases due to the creeping relaxation effect, but it does not result in an obvious redistribution of the internal stress of the dam. In general, the internal stress and deformation of the dam are within a safe range. In the past, the setting of the warning value for dam displacement focused on the influence of changes in temperature and water level on the dam, and the influence of creep effect on dam displacement was not fully considered. Therefore, this research optimizes the warning values from the past considering the displacement caused by temperature, water level, and creep, respectively. Furthermore, this research also updates the changes caused by the creep every ten years and provides the warning value table for the current warning value query system. For the prediction of the long-term displacement of Feitsui Reservoir, the cumulative displacement of the creep effect can be well considered, which reflects the more real displacement behavior of the dam. In the dynamic analysis of the dam, the dynamic elastic modulus of the dam body concrete and the dynamic effective deformation modulus of the foundation rock are considered, and the Zangar equivalent additional mass element is used to simulate the influence of the dynamic water pressure on the upstream surface and the interaction of the dam body. The model can effectively reflect the real structural behavior. The static analysis results under different conditions are used as the initial stress of the dynamic analysis, and the influence of creep effect on the dynamic analysis of the dam is considered. Under the action of MDE seismic force, the dam body deformation and stress distribution provide a reference for dam safety management. Keywords: Concrete, Creep, Arch dam, Finite element analysis, Seismic force