Expected stiffness changes during compaction in laboratory and field

The use of stiffness indexes to assess soil compaction has the potential to revolutionise the earthworks industry. However, conventional Proctor based compaction methodology fails to adequately explain the relationship between soil modulus and density. To address this issue, this paper presents recent laboratory and field trial results and analysis demonstrating the importance of soil degree of saturation when evaluating the effectiveness of using stiffness indexes to monitor and assess compaction. During a recent compaction study of silty sand samples, it has been found that the small-strain shear modulus (G0) and dry density (rho d) can be simply related provided changes in degree of saturation (Sr) are monitored. Multi-staged, one-dimensional compaction tests have been conducted at varying moisture contents, and first arrival shear wave velocity measured via Bender Elements (BE). Results presented in this paper show that G0 increases with increasing rho d at a constant Sr rather than constant water content (w). Further analysis indicates that additional compaction past a critical Sr reduces the stiffness response G0 at a constant w, and that when compacting across the typical range of Sr of 40-70%, G0 appears to be largely independent of rho d and is instead indexed by moisture content. At a construction phase field compaction trial in Australia, field dry density was measured against Intelligent Compaction (IC) unit Compaction Meter Value (CMV). Two outcomes of the field trial are discussed. As in the laboratory G0 trial, CMV, a stiffness related parameter, and field dry density were found to be practically independent. Drawing upon the common results of these two trials, a practical framework is proposed involving a method-based specification with respect to the compacted state and stiffness indexes, one in which adequate compaction and strength could be achieved while reducing the requirement for regular nuclear densometer gauge testing.

Automated summary

The importance of soil degree of saturation in evaluating the effectiveness of using stiffness indexes to monitor and assess compaction is demonstrated through laboratory and field trial results and analysis. The results show a simple relationship between the small-strain shear modulus and dry density, and additional compaction beyond a critical saturation reduces the stiffness response.