Field Performance Evaluations of Illinois Aggregates for Subgrade Replacement and Subbase—Phase II

The project objective was to validate the results from ICT Project R27-1, which characterized in the laboratory the strength, stiffness, and deformation behaviors of three different aggregate types commonly used in Illinois for subgrade replacement and subbase applications, through accelerated loading of full-scale pavement working platform test sections. Six different test “cells” were constructed at different combinations of aggregate material quality and subgrade strength, and were tested to failure using the University of Illinois Accelerated Transportation Loading Assembly (ATLAS). Each cell was tested along two different wheel paths representing two different aggregate layer moisture contents (Cells 1-5), or geotextile reinforcement conditions (Cell 6). Performances under loading were monitored through surface profile measurements as well as transverse scanning with ground-penetrating radar (GPR). Field and laboratory test results highlighted the importance of considering aggregate quality in the thickness design of aggregate layers for construction platforms. Thick layers of uncrushed gravel placed over a weak subgrade mainly underwent internal shear failure due to excessive movement of the aggregate particles. Crushed aggregate layers constructed with high relative compaction, on the other hand, showed significantly higher resistance to internal shear deformation and permanent deformation accumulations. Prolonged exposure to moisture and freeze-thaw effects was found to be beneficial for a crushed dolomite material with high amounts of nonplastic fines probably due to carbonate cementation within the fine fraction. Failure of test sections under flooded conditions was primarily caused by excessive deformation in the subgrade layer. Recommendations were made based on the study findings for improved material selection and thickness designs of aggregate working platforms.