剪力帶對於TDR錯動變形監測影響之探討

Abstract

時域反射法(time domain reflectometry, TDR)技術應用於大地工程監測已行之有年，其透過電磁波傳送於預埋地層內之同軸纜線，當地層滑動造成纜線之幾何形狀改變所傳回之反射訊號以分析滑動位置，且可透過網路進行遠端自動化、一機多功、低成本等優勢更符合工程上實際需求。然而，現地存在纜線電阻、纜線-灌漿材料-外填土壤三者材料互制與剪力弱帶寬度等影響因子而使得TDR錯動變形量化分析仍具有難度，其中纜線電阻之影響已能利用傳輸線理論合理考慮，而材料互制行為已由林文欽 (2007) 得到許多寶貴的結果，但對於剪力帶可能造成的影響仍然缺乏合理的試驗方法。 本研究將先從大型直剪儀試驗重複性不佳的缺點加以改善，在試驗具有重復性的前提下，著重於剪力弱帶之探討，於實驗室以剪力帶環片建立一套模擬現地剪動條件相似之物理模型：剪力帶環片，據以了解剪力弱帶對於TDR反射訊號與剪力位移量關係之影響，據以了解如何提昇感測靈敏度及量化分析之可行性。利用林文欽大型直剪儀及本研究的成果，修改林文欽所建議的TDR錯動變形監測安裝標準程序及量化分析，提供實務應用的參考；最後，參考文獻中嘗試以纜線加入節點束制物提升TDR反應靈敏度的概念，以試驗探討其可行性。 本研究為了盡量能模擬實際滑動狀況，剪力盒外圍部份視為不動體，因此，束制水泥試體與纜線，使其整體剪動模式更貼近現地，試驗結果雖然有較為可靠的數據（重複性佳），但使其在軟弱土層中之初始門檻值(δD)及靈敏度(S)皆下降。剪力帶的模擬試驗結果顯示，剪力帶寬度在試驗5公分範圍內對於TDR受剪反應之影響不大。重要的是在整理林文欽（2007）的試驗，以及更新其重複性試驗的數據後，發現不同勁度外填材料下靈敏度差異頗大，增加了現地量化變形量的難度，但可以考慮模擬試驗不同材料與現地材料的比較，推估可能的變形範圍。另外，TDR反射訊號經纜線加入束制節點後，能改善其靈敏度，但對於提早偵測變形幫助有限，如何利用束制節點獲得最佳的成效，仍需後續進一步研究。

Time domain reflectometry (TDR) is a relatively new technique based on transmitting an electromagnetic pulse into a coaxial cable grouted in rock or soil mass and watching for reflections of this transmission due to cable deformity induced by the ground deformation. It is advantageous in its automation, multiplex capability, distributed sensing, and low cost. However, quantitative interpretation of TDR monitoring remains difficult because the TDR response due to localized shear deformation is affected by cable resistance, soil-grout-cable interaction, and shear bandwidth. While the effect of cable resistance can be taken into account by the electromagnetic theory and the effect of soil-grout-cable interaction have well examined by Lin (2007), the influence of shear bandwidth has not been reasonably investigated. This study improved the repeatability of the large direct shear device developed by Lin (2007) and added the function of controlling the shear bandwidth. This device more realistically simulates the field condition with shear band. The effect of shear band on the TDR response is experimentally investigated with emphasis on how to enhance the sensitivity to shear displacement and quantify the shear displacement from the TDR response. Utilizing the results from Lin (2007) and this study, the standardized procedure of TDR installation and interpretation was revised for better practical use. In addition, the feasibility of improving the sensitivity and early detectability by adding amplifying blocks on the cable was investigated. To better simulate the field condition, the cable extending outside the shear box was fixed to avoid movement of cable relative to the grout. This revision significantly improves the repeatability of the TDR experiments. But it was found that the early detectability and sensitivity of TDR response to shear displacement in soft ground were overestimated in the previous study. The physical model reveals that the TDR response is not significantly influenced by the shear bandwidth within the testing range of 5cm. It should be noted that, after the improvement of the TDR shear device, the sensitivity of TDR response was found apparently dependent on the stiffness of ground materials, making quantitative interpretation of TDR measurement difficult. However, the range of deformation may be reasonably estimated by comparing the ground materials with several materials tested in the physical model. Furthermore, the amplifying blocks attached on the cable were found useful to improve the sensitivity to shear displacement but has a limited effect on enhancing the early detectability. Optimization of the amplifiers requires further study.