The Design Of TDR Probe And Monitoring Technology Of Water Content And Dry Density

Soil water content and dry density are important parameters for quality control in the process of construction of Embankment, slope, dam and foundation treatment in geotechnical engineering. Soil water content and dry density are needed to be measured by the traditonal method and intruments and couldn't be monitored in a long time. Since TDR(Time domain reflectometry) could measure the soil water content and dry density quickly, easily and accurtately, it had been greatly applied in engineerings. The standard of ASTM provided the method of measurement and the probe. However, the probe of the standard could only measure the surface soil water content and dry density and couldn't be embedded in deep soil to monitor the soil water content and dry density in a long time. The traditional probe couldn't meet the demand of measurement of water content and dry density in different scales. Therefore a new probe needed to be designed.Soil would show the electrochemical property in an external electromagnetic field. This property had a great relationship with the soil volume ration of each component, the soil type and water. The dielectric constant reflected the physical parameter of the degree of polarization in an alternating field and the degree of energy stored in the material. The dielectric constant of soil water content was affected by water content. The conductivity reflected the physical quantity of the charge of activities in the electric field. The conductivity of soil were mainly affected by the conductivity of solid particles, soil porosity, saturation, and the conductivity of the pore fluid. The technology of Time domain reflectometry (TDR) could be used to measure the dielectric constant and the conductivity.According to the compositon of the TDR system, the form of the probe, the spacing of the probe and the diameter of the probe were analyzed in system. We used the finite element software HFSS and Matlab program to calculate and propse reasonable method of the probe design. When the probe was longer than 20cm, the error due to the instrument were less than 1%. The ratio of probe spacings to probe diameter should not be greater than about 10, which would reduce the energy concentration around the probe. Finally, a model test probe and field test probe were developed.Several relationships between the dielectric constant and water content were introduced. We could obtain the water content by the dielectric constant measured by TDR using the relationship between the dielectric constant and water content. There are two meathods of the joint measurement of water content and dry density:two step method and one step method. Two step method and one step method requied the calibration of parameters before the start of measurement. This paper did the calibration on 4 different types of soil and established the database of paramters.The model test probe was used to monitor the process of foudation eastablishment and saturation. The results of the water content and the dry density at the process of foudation establishment in different depths showed that:the abosolute error of water content was less than 3% and the relative error of dry density was less than 3%. The results had a great accuracy. When the foudation establishment was completed, water was impoured into soil from the bottom of the model tank with pressure. The TDR probes were embeded to monitor the process of saturation. The results of experiment showed that:the soil didn't reach the full saturation at the end of experiment and the degree of saturation is 94%. The field test probe was used to obtain water content and dry density at different depths in field. The error of water content and dry density was less than 5%, which- would meet the requirements of engineerings.