應用時域反射法於碾壓土壤檢測之電學性質量測技術改良

Abstract

碾壓土壤結構物與民生安全息息相關，夯實品質檢測之重要性不得輕忽，為可快速且準確進行品管控制之目的，近幾年國際上積極發展快速、準確之非核子方法量測儀器，此些新興方法雖可增加施作效率但目前之量測準確度皆尚無法取代核子密度儀。本研究之主要目的便是針對較具潛力的TDR量測技術進行多面向評估工作，根據評估結果提出既有TDR量測技術可能之改善方法以及未來非核子式量測技術的研發方向，並根據此結果進行TDR量測分析技術改善。 TDR量測技術(ASTM D6780-05)包含有雙步驟法及單步驟法，多面向評析結果顯示，單步驟法之理論具有缺陷，不建議使用，而雙步驟法則有進一步改善的空間，主要改善工作在於其採用走時分析法所遭遇到在高導電度土壤量測、土壤種類差異影響及量測有效頻率變動之問題，另提出改良式介電頻譜分析法做為未來發展無須取土單次直接量測的非核子式技術(寬頻介電頻譜分析法或多物理量融合技術)的基本分析工具。 在改善TDR雙步驟法之走時分析問題上，提出頻率域相位波速法，獲得視介電度值之有效頻率，研究結果顯示，此法可有效量測頻率250-900MHz之視介電頻譜，降低走時分析之不確定性；而針對高導電度土壤應用之問題，提出TDR能量指數法，以反射係數之大小作為分析物理量，測試結果顯示，此法可在高導電度土壤中進行有效量測，且有較不受土壤孔隙水導電度影響的優點，惟其感應範圍較集中於表面，在應用上須針對此特性進行應用方法的調整；相對於前述僅採用特徵值之兩種分析法，改良式寬頻介電頻譜分析法直接量測土壤介電頻譜，可同時達到前述兩種方法所欲解決的問題，測試結果顯示，此法至少可有效量測10-300MHz之介電頻譜，搭配頻率域相位波速法可達到10-900MHz之寬頻介電頻譜量測，且其分析較既有介電頻譜分析方法更為簡便，對於未來發展TDR現地應用技術有極大助益。 本研究所進行之TDR應用於碾壓土壤電學量測分析技術改良之結果皆有室內試驗資料獲得驗證，在現有TDR雙步驟法中已可立即應用，而其亦為後續無須取土單次直接量測的非核子式技術研發提供適用之分析工具，將可有助於在碾壓土壤介電頻譜行為基礎理論上之研究。建議後續可在本研究之基礎上，針對多種物理量(介電頻譜、彈性波速、導電度、熱導性質)進行碾壓土壤行為的研究，加以提出恰當之理論模型，研發TDR寬頻介電頻譜檢測技術或多物理量融合之量測技術，以達到發展無須取土單次直接量測的非核子式技術的目標。

The moisture content and dry density are crucial parameters for quality control of compacted soils. Conventional measurement methods are time consuming while the nuclear method although efficient is potentially hazardous. Fast non-nuclear measuring systems are being developed but none of them are accurate enough yet to replace the nuclear gauge. The main purpose of this study was to evaluate the performance of the time domain reflectometry (TDR) technique, which was considered having the most potential to replace nuclear gauge. Based on the results of evaluation, several improvement methods were then proposed to overcome difficulties associated with current TDR technique and future R&D direction for non-nuclear method were suggested. Current TDR compaction soil quality control technique (ASTM D6780-05) includes two methods: one-step method and two-step method. The one-step method was shown to be theoretically flawed and not generally accurate, and was suggested to be suspended. The two-step method is theoretically sound, but the accuracy may not be accurate enough in current practice. To upgrade the performance of the two-step method, improvement methods were proposed to overcome various problems found, including the difficulty in measuring highly conductive soils, effect of soil type, and lacking effective frequency in the apparent dielectric constant measured by current travel time approach. A simplified dielectric spectroscopy method was also proposed as the fundamental measurement for further development of non-nuclear one-step method. To reduce the uncertainty caused by the travel time approach and provide the effective frequency to the apparent dielectric constant, a frequency domain phase velocity approach was proposed which can effectively measure apparent dielectric constants in the frequency range from 250 MH to 900MHz. The TDR amplitude index method was introduced to overcome the difficulty in measuring highly conductivity soils. However, it should be noted that the sampling volume of this method is limited to the surface vicinity. Probe and measurement should be adapted to account for this behavior. Taking on a different direction, measurements of complex dielectric permittivity circumvent aforementioned problems of conventional TDR method and provide much more information for simultaneous inversion of water content and density. A simplified approach was proposed to obtain wide-band complex dielectric spectrum using the full waveform in a much more practical way. Experiments showed that it is feasible even in a complicated field setup and provides reliable complex dielectric measurement from 10 to 300MHz. When integrated with the frequency domain phase velocity analysis, the frequency range can be extended to 900 MHz. This wide band dielectric spectrum provides the most important physical information for development of next-generation one-step method. Fusion of dielectric information with other multiple physical parameters (such as elastic wave velocity, thermal conductivity, etc.) may provide more reliable estimation of water content and dry density. Thus, further researches are recommended to investigate and model the behavior of the multiple physical parameters.