Abstract: To investigate the effect of the delayed compaction on the engineering properties of the improved soil, this study focuses on the subgrade soil from an expressway in Hunan Province. Various samples with different cement contents and delayed compaction time were prepared, and multiple series of compaction tests and the unconfined compressive strength (UCS) tests were conducted to assess the influence of the delayed compaction on key engineering characteristics. Subsequently, a regression model was developed to quantify the degree of influence. The findings indicate that the maximum dry density (MDD) of the cement-modified soil decreases with the increasing cement contents and the delayed compaction time. The delayed compaction has an adverse effect on the UCS for the cementimproved soil; therefore, it is crucial to ensure a proper connection between the mixing and the compaction processes. As the duration of the delayed compaction increases, the stiffness of the cement-modified soil decreases while its toughness increases. Regression models for the MDD and the UCS considering both the delayed compaction and the cement contents were established, demonstrating a close agreement between measured values and predicted values. It is important to reasonably consider the impact of the delayed compaction when dealing with the subgrade filling for cement-improved soils.

Key words: Subgrade, Soil Improvement, Cement, Delayed Compaction, Maximum Dry Density, Unconfined Compressive Strength