Discussion: Predicting compaction properties of soils at different compaction efforts

Di Matteo and Spagnoli (2021) introduced a modelling framework (Equations 9–12 in their paper) for converting the optimum moisture content (OMC) and maximum dry unit weight (MDUW) between standard Proctor (SP) and modified Proctor (MP) compaction energy levels (CELs) for fine-grained soils. Their framework employs measured OMC and MDUW values for one of the CELs, together with the liquid limit (LL), to predict the corresponding values for the other CEL. Model development/calibration was performed using a database, comprising both SP and MP compaction test results for 49 fine-grained soils (LL = 17–98%), compiled entirely from various sources in the research literature. A second database of 14 SP:MP compaction data pairs, also compiled from different literature sources, was employed for validation of the model. The paper under discussion (Di Matteo and Spagnoli, 2021) offers a reasonably practical and more reliable alternative to the relatively poor prediction performance of traditional empirical correlations that rely solely on soil index properties (typically Atterberg limits and/or gradation information) for OMC and MDUW estimations. However, as demonstrated in this discussion contribution, it is the discussers’ viewpoint that, when attempting to convert OMC and MDUW parameters between two CELs, the LL makes no or little contribution, but rather it restricts the SP⇄MP conversions to those soils falling specifically within the LL range used for model calibration. In other words, it is the discussers’ viewpoint that reliable SP⇄MP conversions can be achieved without employing LL measurements.