改良式時域反射量測技術在橋墩沖刷監測之應用

Improved Time Domain Reflectometry Measurement for Monitoring of Bridge Scour

王凱(Wang, Kai)

指導教授 ： 林志平

國立交通大學/工學院/土木工程系所/博士(2019年)

摘要

在台灣坡陡急流的險峻環境下，橋墩安全的主要威脅來自於沖刷現象。一般而言，橋墩基礎最危險的時刻為洪峰期間，此時的沖刷監測不僅能提供橋梁安全的預警訊息，更能拓展建立沖蝕模式與瞭解動態沖刷過程，然而，堅固耐用且兼具經濟性的即時沖刷過程監測系統，尚未有成熟的發展與應用。
TDR導波式的監測技術具相當大的潛力來發展耐用型的橋墩沖刷監測，本研究首先概念式提出鋼桿式與鋼纜式的導波元件，並考量其可量測的範圍、耐用性、與現地安裝的便易性。沖刷鋼桿為一個中空鋼棒，並鑲嵌金屬棒在帶有絕緣的槽溝中，來形成TDR導波;而沖刷鋼纜，則由兩條鋼纜線(其中一條外層披覆絕緣材)，來讓電磁波傳達至鋼纜周圍，進行量測。此兩種型式的感測器，係透過室內靈敏度量測試驗，來評估達到最佳的導體配置。另外，研究中提出一個適用在TDR由上而下量測的，全走時沖刷深度演算法。此法無需識別難以量測的水/泥沙的介面反射，且其利用簡易的雙步驟回歸濾定介質速度，來用於沖刷深度估算。透過定量的不確定性分析的測試結果，試驗驗證了所提出的校準程序和沖刷演算法。在80 cm的測量下，預測與實測的沖刷深度之間的誤差小於3.3 cm，且誤差百分比大部分在5 ％以內。隨後，研究亦在台灣台中的東勢大橋進行了現場安裝測試，來進行橋墩沖刷監測。
基於在現地-東勢大橋案例所遭遇的問題，包括感測波導的固定問題、結垢影響和雨潑效應。本研究進一步改良TDR感測鋼纜線，提出一種創新的單一束制式且自下而上量測的感測鋼纜，來提高波導器的耐用性並避免結垢與雨潑的不利影響，實現更具穩固性的波導器和測量性能。而透過將圍繞同軸電纜的兩組鋼纜線（作為波導的兩個相對電極）扭絞成的感測鋼纜，更可提高波導器的耐用性。在沖刷量測的方向，則係透過感測鋼纜內置的同軸電纜來改變成由下而上，以消除在空氣/水界面間的多重反射。利用研究所提出的波形前後差異的分析方式，甚至可以消除在阻抗不匹配系統中所測量的複雜信號。而實驗結果證實，在80 cm的土厚測量下，沖刷測量的最大誤差和誤差百分比僅分別小於3 cm和5％。
考量到TDR系統在現地監測環境下，可能會出現信號干擾與雜訊和短路現象。現行感測鋼纜的平衡式配置，容易造成量測雜訊，且降低了量測效能。研究另針對平衡與非平衡式的波導器，進行訊號品質的進一步探討與改進。根據試驗結果，研究進一步改良提出新式束制式感測鋼纜，該感測鋼纜使用三根HDPE線(用以分隔相對電極)，來避免系統短路;另外由兩組鋼絞線形成波導（其中一組絕緣）;以及一條用於改變電磁波測量方向的50歐姆CNT-400同軸電纜製成。借助此特殊的配置設計，新式TDR感應電纜可以提高信噪比（SNR），減少水砂石磨損造成的訊號影響並延長其感應距離。此外，透過絕緣材磨損與回淤試驗，研究中所提出的更新參考波形的差異分析法，可以有效克服感測鋼纜上的絕緣材磨損影響。在6.0 m大型砂槽沖刷試驗下，量測與實際沉積物厚度之間的最大絕對誤差僅小於0.14 m。
由於現地的沖刷深度伴隨著水位的變化而有所不同，在多變的環境條件組合下，TDR信號響應可能非常複雜。因此，本研究透過使用數值模擬和實驗室沖刷實驗來評估沖刷和回淤現象，對TDR沖刷測量信號之影響。根據實驗和數值模擬結果，研究提出了一個全面的象限分析法，來輔助判釋TDR的沖刷信號，與其所對應的現地狀況。此外，研究亦針對四種商用的同軸電纜信號品質，和系統的應用匹配性，進行了評估和比較。由於同軸電纜的電阻效應，在現地長距離量測下特別顯著。本研究建議在感測鋼纜連接接頭處，與TDR系統測站之間的監測距離，應小於50 m，以避免信號衰減。最後，研究選在高雄市的新旗尾橋，落實了TDR沖刷監測系統，同時並安裝雷達波水位計來進行橋墩沖刷監測。而研究中所提出的TDR橋墩沖刷監測系統，亦成功地捕獲了與暴雨期間的沖刷現象，伴隨著水位和流量的上升。唯未來研究建議持續透過現場安裝經驗，來進行修正與改良TDR沖刷監測系統，以提供更穩定與可靠的量測資料。

關鍵字

橋墩沖刷 ； 時域反射法 ； 導波器設計 ； 沖刷監測 ； 結構物健康監測 ； 雷達波水位計

並列摘要

Scour is a major threat to bridge safety, especially in Taiwan’s harsh fluvial environments. Scour monitoring is crucial for providing early warning of bridge safety and extending the knowledge of scour process. However, a robust and economic real-time scour monitoring device is still not fully developed.
Time domain reflectometry (TDR) is an emerging waveguide-based technique holding great promise to develop more durable scour monitoring device. This study presented new types of TDR sensing waveguides in forms of either sensing rod or sensing wire, taking into account of the measurement range, durability, and ease of field installation. The sensing rod is composed of a hollow grooved steel rod paired up with a metal strip on the insulating groove, while the sensing wire consists of two steel strands with one of them coated with an insulating jacket. Factors affecting the measurement sensitivity were identified and experimentally evaluated for better arranging the waveguide conductors. A data reduction method called total travel time method with TDR top-down measurement for scour-depth estimation without the need for identifying the water/sediment reflection and a two-steps calibration procedure for rating propagation velocities were proposed. The test results validated the proposed calibration procedure and the data reduction method. The errors between predicted and measured scour depth were less than 3.3 cm and the error percentage was mostly within 5% in a 80 cm model experiment. Besides, a trial field installation was implemented at Dong-Shi bridge in Taichung, Taiwan for measuring the bridge scour process.
Because the practical problems encountered in the Dong-Shi bridge including weak waveguide package design, fouling effect, and rainfalls effect, this study focused on further improving the concept of TDR sensing wire for better sensor package and measurement performance. An innovative bundled bottom-up sensing cable was proposed to improve sensor durability and avoid the adverse effect of sensor fouling. The sensor durability is enhanced by twisting two sets of steel strands (as two opposing electrodes for the waveguide) around a coaxial cable into a composite sensing cable. The sensing direction is changed by the inner coaxial cable for getting rid of the multiple reflections from the air/water interface. Taking advantage of a new data reduction method by using time-lapse differential waveform, even the complicated signals measured in the mismatched impedance system can be eliminated. The experimental results indicated that, the maximum error and error percentage of scour measurements were less than 3 cm and 5%, respectively, in a 80 cm model experiment.
Considering the signal noise and that the abrasion of the coating may result in shorted circuit in the field, this study further introduced a new bundled TDR-based sensing cable constructed by three HDPE wires for avoiding shorted circuit, two sets of steel strands for forming the waveguide (one of which insulated), and one central 50 ohm CNT-400 coaxial cable for changing sensing direction of EM wave. With special configuration design, the new TDR sensing cable can enhance signal to noise ratio, reduce the abrasion effect, and extend the sensing distance. In addition, a base waveform update method for improving scour depth calculation when insulation abrasion occur was proposed and validated. The maximum absolute error between measured and actual sediment thickness was less than 0.14 in a 6.0 m full-scale scour experiment.
Since scour is accompanied by changing water level in the field, this study further used both numerical simulations and laboratory scour experiments to evaluate signal patterns as influence by changing field conditions. A comprehensive quadrant analysis method was proposed to interpret TDR scour signals. In addition, the cable resistance effect was investigated. The monitoring distance between sensor head and TDR station was recommended to be less than 50 m to avoid excessive signal attenuation. Furthermore, a TDR scour monitoring system together with a radar water level sensor were installed at the Hsin-Chi-Wei bridge Kaohsiung, Taiwan. The proposed system successfully captured the scour phenomenon which was associated with the water level and discharge rise during a storm event.

並列關鍵字

Bridge scour ； Time domain reflectometry (TDR) ； Waveguide design ； Scour monitoring ； Structural health monitoring (SHM) ； Frequency modulated continuous wave (FMCW)

參考文獻

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