| Summary: | 碩士 === 國立交通大學 === 土木工程系所 === 97 === The slope stability and transportation of underground pollution in unsaturated soils much depend on the soil moisture content, thus, there is a demand for monitoring spatial and temporal variation of the soil moisture content. The water content characteristics of unsaturated soils has a close relation with soil electrical resistivity. However, electrical resistivity depends not only on soil moisture content, but also on the groundwater characteristics and geological factors. Therefore, it is difficult to monitor soil moisture distribution by ERT alone. Earlier study proposed to integrate electrical resistivity tomography (ERT) with Time Domain Reflectometry (TDR), which can monitor soil moisture content and resistivity simultaneously for on-site calibration. However, the sandbox physical model tests revealed that, during simulated rainfall infiltration and drying process, there exists a hysteresis between soil moisture content and resistivity, in which a resistivity value corresponds to different soil moisture content depending on whether it is in drying or wetting process..
In order to enhance the accuracy of soil moisture imaging based on ERT, this research was aimed to investigate the hysteresis phenomenon. First, experiments were conducted to show whether the hysteresis is due to non-uniform spatial sampling of TDR probes and configuration of the sandbox physical model. The results showed that the vertical installation of TDR probes can reduce the hysteresis loop slightly, but the hysteresis phenomenon is still significant. This research was then turned to more fundamental investigations, including temperature effect and experiments using devices that can produce more uniform samples and better control the testing soil moisture. Significant temperature effect on soil resistivity was observed. This study proposed a temperature compensation model and further measurements were temperature corrected. Experimental results from different wetting and drying speed showed that the hysteresis seems related to the wetting and drying speed. Similar hysteresis phenomenon was observed in quick wetting and drying process while there is no significant hysteresis in slow wetting and drying process. The hysteresis is attributed to different air and water distributions in soils during wetting and drying process.
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