Study On Dielectric Constant And Water Content Measurement Of Highly Conductive Geomaterials By TDR Technique

Water content of geomaterials is an important property in geotechnical engineering. The electrical conductivities of common geomaterials are low. With the development of geoenvironmental engineering, highly conductive geomaterials such as contaminated soil and municipal solid waste (MSW) are of great concern by many researchers. Water content measuring and monitoring of geomaterials are very important for determination of geotechnical parameters, stability analysis, monitoring transport of pollutants, water balance analysis of landfill, leachate collection and recirculation, and acceleration of MSW degradation.Time domain reflectometry (TDR) is a technique based on electromagnetic properties of geomaterial-air-water mixtures, which allows for water content measurement by measuring the dielectric constant Ka and electrical conductivity?of geomaterials and has the advantages of fast measurement speed, high accuracy and automated monitoring. However, it was observed that traditional travel time analysis method is challenging if not impossible when testing geomaterials with high electrical conductivities due to significant energy attenuation. Moreover, due to the restriction of sensors, the surface reflection analysis method which is suitable for highly conductive geomaterials is only used in laboratory for water content measurement of highly conductive soils and has not been applied to in situ water content measurement of highly conductive geomaterials.Based on the principle of surface reflection method, systematic analysis was carried out to quantify the effects of sensor type, material and size on the accuracy of the measurement through finite element analysis and numerical modeling of TDR system. A new TDR sensor suitable for measuring dielectric constant of geomaterials with different electrical conductivities had been designed. The waveform acquired by the new sensor can be analysed use both travel time method and surface reflection method.The dielectric constant and water content of different type of soils, especially highly conductive soils, were measured by both laboratory test sensor and field test sensor. In the case of low electrical conductivity, the results of dielectric constant measured by surface reflection method gave similar accuracy as the travel time method and the relative error of measured value of (?) was within±10%. However, the high electrical conductivity results in a higher estimation of dielectric constant by travel time analysis. In comparison with oven dry method, the absolute error of measured value of water content by surface reflection method and travel time method was within±3%. Moreover, surface reflection method provides higher accuracy for soils with high electrical conductivities while travel time method fails. The field test sensor gave similar measuring accuracy as laboratory test sensor and was easy to use.The results of laboratory test and field test of MSW indicate that the water content of highly conductive MSW can be determined by surface reflection method. Although physical properties of different components in MSW are quite different, the dielectric constants of them are similar. Therefore, the dielectric constant of MSW has almostly nothing to do with the proportion of its components, but is determined by water content and porosity. The relationship of Ka and volumetric water content?can be expressed by linear equation and is inseparable with porosity. Therefore, independent calibration equation should be established for MSW with different porosity in practical application. The (?)-?relationship, in which the influence of porosity was taken into account, was proposed based on large number of experimental data. The absolute error of water content calculated by the equation and measured by oven dry method was within±5%.?=(0.0323n±0.101)(?)+(0.1429n-0.2646)According to the results of both field test and model test, the new TDR sensor could monitor the variation of water content of MSW during degradation. The change of water content measured by TDR agree well with that calculated by leachate production. In leachate recirculation test, the sensor is sensitive to the change of water content, which can monitor the change of water content caused by the variation of leachate in MSW.