Laboratory Testing of Soil Using the Superpave Gyratory Compactor

Earthwork construction methods have changed substantially over the past 50 years, although the Proctor laboratory test for evaluating compacted densities has remained relatively unchanged. One shortcoming of the Proctor test is that it uses impact loads to compact soil in a stiff non-yielding mold, whereas modern field compaction methods rely on a combination of kneading, vibration, and increased normal pressures to achieve high densities. A suite of laboratory tests were conducted on soils using the Superpave gyratory compactor to analyze the primary variables that influence soil compaction. Gyratory compaction tests were conducted on four soil types that represent a broad range of materials that may be encountered during construction. Gyratory dry densities were compared to standard and modified Proctor maximum dry densities to explore potential differences between gyratory and impact laboratory compaction methods. The results indicate that increasing the confining pressure is the most effective method of increasing the dry density of fine-grained soils, while increasing the number of gyrations is the most effective method of increasing the dry density of non-cohesive, granular soils. Although the gyratory compaction process has been demonstrated to simulate field compaction of hot mix asphalt, results from this study demonstrate that gyratory compacted maximum dry densities are not substantially different than dry density values obtained using traditional laboratory Proctor tests, even though the physical process of achieving compaction are quite different. Furthermore, gyratory compaction encounters many of the same disadvantages as the Proctor test when compacting cohesionless soils at relatively high water contents.