Time Domain Reflectometry for Compaction Quality Control

Abstract

Time Domain Reflectometry (TDR) is a promising new technology for control of compaction now that a procedure has been developed to include density measurement in addition to water content measurement. The TDR method employs the transmission of electromagnetic waves in a "cable" of known length where soil is the dielectric medium between the leads of the cable. Because the length of the "cable" is known, it is possible to determine the dielectric constant of the soil. The dielectric constant is strongly affected by the water content and density of soil but is relatives unaffected by soil texture and salt content. A simple equation relates dielectric constant to density and water content. By conducting two tests, one insitu, and one on the same soil compacted in a mold, both water content and density can be determined as long as the water content of the two specimens are the same. The testing procedures are straight forward and simple. Specially designed equipment makes the test practical and robust. Examples show that accuracy is quite good. Ongoing work is examining applicability of this method for soils with large particle sizes and for soils with high activity, additives, and stabilizers. This paper also previews some ongoing developments of theoretical modeling that allow for a more complete understanding of electromagnetic wave propagation in soil.