Soil moisture and density monitoring methodology using TDR measurements

Soil moisture content (MC) and density of pavement materials exert a great influence on pavement performance. Cyclic traffic loading with increasing MC in materials can accelerate changes in density significantly. Time domain reflectometry (TDR) is a non-destructive method for measuring MC and density of soil. In this study, TDR measurements were taken in the laboratory for a typical road material at two different conditions; at constant MC with different density and at constant dry density with different MC. Based on TDR measurements, a theoretical formulation between voltage drop occurring for the passage of an electromagnetic wave through the soil and the electrical conductivity (EC) is developed. This relationship is further correlated to develop density function providing the opportunity to obtain MC information. For the validation of the calibration functions, sensors are installed in roads to obtain field data. Laboratory results point out that the calibration models are independent of moisture and density, and the field instrumentation shows the satisfactory accuracy of the method. The newly established calibration models allow for observing the routine performance providing a better understanding of the material behaviour for example rutting of roads.