Volume 36, Issue 1 (January 2013)
Using a Complex-Impedance Measuring Instrument to Determine In Situ Soil Unit Weight and Moisture Content
In situ measurements of soil unit weight and moisture content play a critical role in conventional compaction quality assurance and quality control procedures. Recently, there have been a number of attempts to develop alternative electrically-based test devices that can be used to measure the in situ unit weight and/or moisture content of a compacted soil; these devices are intended to serve as alternatives to more traditional tests such as sand cone, rubber balloon, drive cylinder, or nuclear density gauge tests. The study described in this paper focuses on the use of a relatively new electrically-based in situ soil test device that uses measurements of soil complex impedance, soil capacitance, and soil resistance to infer in situ soil unit weight and moisture content; this device is typically referred to as a complex-impedance measuring instrument (CIMI). This paper provides a detailed explanation of current CIMI operating principles and also describes the utilization of a CIMI for field- and laboratory-based testing. The CIMI used in this study was calibrated and assessed in two field compaction projects in which different silty sands were used for construction. A mold-based calibration approach was developed for building an electrically-based soil model using the CIMI; this approach provides an alternative to field calibration of the device. In order to perform a more complete assessment of the CIMI in a controlled environment, a series of CIMI tests were conducted in a large “field box,” and the resulting in situ measurements of soil unit weight and moisture content made using the CIMI are compared with the results from nuclear density gauge, sand cone, and drive cylinder tests. The advantages and disadvantages of field versus mold calibration of the CIMI are discussed, and side-by-side assessment of the CIMI relative to other conventionally used compaction control tests allows the reader to assess the accuracy of this device.