Quality Control of Earth Fills Using Time Domain Reflectometry (TDR)

Abstract

Measurement of soil density and water content in compacted fills is the principal means of quality control to assure adequate performance. Current testing methods have various limitations, including the use of hazardous materials, limitations in accuracy, the need for extensive calibration, or the test duration. A new technique using time domain reflectometry (TDR) to measure the water content and density of soil is introduced. The purpose of this paper is to present a historical and theoretical background of this new approach. Prototype equipment was developed for routine use in the quality control testing of compacted soils. The method was evaluated with theoretical study and laboratory experiments. The results of the TDR method are compared with results from conventional methods on actual construction sites. The advantages and limitations of this new method are also discussed. A TDR device is used to transmit an electromagnetic wave into the soil and receive a reflected waveform. The dielectric constant of the soil can be calculated from the travel time of the waveform. Water content and density are the basis used to assess the quality of a compacted earth fill. These are the same factors that affect the dielectric properties of soil; so a relationship between them can be established. A procedure to measure soil density in-situ using TDR was developed. This procedure obtains results comparable in accuracy to existing methods such as the nuclear density gage and the sand-cone test. The test duration is approximately 15 minutes. A new approach is under development to interpret the reflected waveform and extract additional information regarding the electromagnetic soil properties. This approach appears to be very promising and has the potential to provide even more rapid and accurate test results for assessing the water content and density of compacted soils.