Volume Measurement of Expansive Soils and Its Application during the Design of a Retaining Wall in Texas

Predicted values for the shrinkage and swelling potentials of expansive soils are principal parameters used in the design of ground-supported structures because unsteady soil movement affects long-term structural reliability. To characterize the shrink and swell indexes of a soil, ${\gamma }_h$, the change in volume of the soil must be measured after exposure to varied controlled conditions. Several volume measurement methods have been commonly employed in geotechnical engineering; however, all of these methods have practical limitations so engineers have long sought an alternative test method. A method developed by geologists for rock samples was adapted to determine the volumes of engineering soil samples, and it was implemented during an investigation of highly expansive soil samples from a construction site located on Interstate 35 in San Antonio. Twenty samples from two geotechnical borings were analyzed following a pressure plate apparatus test protocol, which subjected the soils to varied levels of pressure and suction (moisture) and then determined how their volumes changed. Volumes were determined at various stages of testing to an uncertainty of $\pm 0.3\%$ or better in a manner that proved repeatable and nondestructive. Shrinkage index values derived from these analyses ranged from 0.0068 to 0.0317 (mean of 0.0161) and the swelling index values ranged from 0.0077 to 0.0304 (mean of 0.0161), which are consistent with values found by other studies of expansive soils. Thus, the effective measurement of soil sample volumes enabled the determination of the shrinkage and swelling indexes of expansive soils, which made it possible for geotechnical engineers to design a more cost-effective highway retaining wall.