Improvement of Electrical Properties Measurement by Time domain Reflectometry Applied in Compacted Soil Quality Control
|關鍵字:||時域反射儀;碾壓土壤品管技術;高導電度土壤;介電頻譜量測;乾密度量測;含水量量測;TDR;Compaction soil Quality control;highly conductive soils;dielectric spectroscopy measurement;dry density measurement;water content measurement|
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.