Evaluation of In Situ Moisture Content at Long-Term Pavement Performance Seasonal Monitoring Program Sites

Time domain reflectometry (TDR) has become one of the most reliable nondestructive methods for measuring in situ soil moisture content. TDR sensors developed by the Federal Highway Administration are being used in the Long-Term Pavement Performance (LTPP) Seasonal Monitoring Program (SMP) to monitor the in situ moisture content at 64 LTPP sites. The main goal of this study is to develop procedures to produce good estimates of in situ gravimetric moisture content. All the TDR traces in the LTPP information management system database that were recorded at LTPP SMP test sections were processed using the approach described in this paper. To estimate the in situ gravimetric moisture content, methods were selected to interpret TDR traces. An algorithm and a computer program, Moister, were developed to implement these TDR interpretation methods. Then the apparent length of the TDR trace and the dielectric constant of the unbound material were computed. Models were developed to relate dielectric constant with in situ volumetric moisture content. Finally, gravimetric moisture content was computed using the volumetric moisture content value and dry density of the soil. A diagnostic study of the computed gravimetric moisture content was also conducted to evaluate the reasonableness of the computed moisture content.