Effect of soil type and compaction methods on the confined stiffness of soil-geogrid composites

When subjected to vehicular loading, pavements undergo relatively small deflections consisting of elastic and plastic components. Under repeated loading, the plastic components of these deflections accumulate and result in permanent deflections that typically cause pavement distress such as surface rutting and asphalt cracking. To improve pavement performance and extend service life, geosynthetics have been used as stiffening elements in the unbound aggregate layers, such as base layers, providing lateral restraint against particle displacement. The lateral restraint and the resulting improvement in layer stiffness rely on the tensile properties of the geosynthetic and the shear behavior of the soil–geosynthetic interface. These concepts are encapsulated in the soil– geosynthetic composite (SGC) model and the resulting parameter, the stiffness of the soil-geosynthetic composite (K_{SGC}), reflects the composite behavior under small strains. In this study, soil geosynthetic interaction (SGI) tests were conducted to investigate the effect of aggregate type, density, and compaction method on the K_{SGC} of geogrids. Accordingly, specimens were tested in the SGI setup at multiple density levels using aggregates of different angularity and grain sizes. In addition, the effect of the compaction method on the results was investigated with specimens prepared using the existing impact-based compaction procedure and the vibratory compaction procedure, which was developed as part of this study to eliminate particle crushing during specimen preparation. Evaluations with different aggregate types show that particle size highly influences the K_{SGC} parameter of soil-geogrid composites, as both angular and rounded aggregates of the same particle size distribution yield comparable results . However, the K_{SGC} parameters obtained with angular aggregates are consistently higher than those obtained with rounded aggregates. In addition, the results show a positive correlation between aggregate density and K_{SGC}, which can be attributed to the increased interaction (interlocking and friction) at the interface. Lastly, the results obtained from specimens prepared using impact-based and vibratory compaction methods were found to be in good agreement, provided that the same soil density is achieved. This validates the use of vibratory compaction developed in this study, which is recommended as the preferred method of specimen preparation, particularly for aggregates that are prone to particle crushing.