Use of Time Domain Reflectometry for Determination of Water Content an Density of Soil

Abstract

A new method is developed for measuring in-place density and moisture content of soil using the technique of Time Domain Reflectometry (TDR). The method is applicable for construction control of earthworks. The method is extended for measuring moisture content of soil in the laboratory and in the field for various other geotechnical purposes. Laboratory uses include measuring moisture content of soil in compaction molds; field uses include measuring moisture content of soil in Shelby tubes and soil retrieved during drilling and sampling operations. In the course of achieving these goals, this study develops the principles of TDR probe design, test methodology, and relationships of TDR measured dielectric constant with density and moisture content of soil. TDR was originally developed to locate faults in cables and transmission lines. For the last 20 years, it has also been used (mostly in soil science) to measure the volumetric moisture content of soil by using its capability of measuring the dielectric constant of soil. Some use has been made for measuring moisture content for engineering applications but soil density had to be known before volumetric moisture content could be converted to gravimetric moisture content which is used in geotechnical engineering. This study proposes a method of using TDR technique for measuring density of soil and extends the use of TDR for measuring moisture content of soil for various geotechnical purposes. In order to achieve these goals, this work develops various transmission lines/probes and devices. It develops coaxial transmission line (CTL) probes, composed of coaxial apparatus (CA) and cylindrical cells (CC). It develops multiple rod probes (MRP) which extend the capabilities of existing MRP. Analytical and experimental investigations are made to design different parameters of the probes. Investigations are carried out to study the effect of probe rod installation on measurements and experimental methods. Experiments are conducted to develop calibration equations to relate dielectric constant with moisture content and density of soil. The work describes how the TDR method can be used to measure in-situ density and moisture content of soil. The procedure makes use of the designed probes and devices. Significant testing of the procedures on a variety of soil types, along with comparisons with other measures of density and oven dry moisture content indicates that this new method is quick, safe and sufficiently accurate for measuring in-place density and moisture content of soil.