Evaluation of Compacted Silt Characteristics by Ultrasonic Pulse Velocity Testing

Unintrusive measurement of field modulus and moisture content of roadway subgrade and fill materials greatly enhances field control during construction. Establishing a measurement method in a laboratory setting provides an initial assessment of a tools possible usefulness in the field. An ultrasonic test system was used in this research in order to determine the wave velocities of cylindrical soil specimens in a laboratory setting in support of a Briaud Compactive Device (BCD) research project. The BCD is a non-destructive test method that can quickly determine soil modulus both in the laboratory and the field. Cylindrical soil samples of silt were compacted in two different molds: a standard proctor mold 152.4-mm (6-inch) diameter and a smaller 71.1 mm (2.8-inch) diameter mold. The p-wave and the s-wave velocities of samples manufactured in each of the molds were compared against sample moisture content and the dry unit weight. Trends in wave velocity versus moisture content were investigated using the two types of molds and compacting using two compactive efforts. It was found that specimens prepared in the two different molds generate similar trends of wave velocity versus moisture content. In general, the smaller compaction mold could be used for specimen preparation for ease in preparation, but care and experience are needed in order to match targeted dry unit weight values on a given compaction curve. Regardless of compaction mold sizes and compactive efforts, the wave velocities show the strongest correlation with moisture content. This ultrasonic test system is useful for calibration of the (BCD) in the laboratory setting, and could be applied to testing of samples extracted from the field.