Final Report: Subgrade Stability

Subgrade stability is a term which relates to the strength and deformation properties of the soil. Both properties significantly influence 1) the response of a subgrade to the heavy repeated loading of construction traffic and operations, 2) the ability to place and compact overlying material layers, and 3) the long-term performance of the pavement subgrade. Ideally the subgrade should be strong enough to prevent excessive rutting and shoving, and sufficiently stiff to minimize resilient deflection. In this report a) concepts for characterizing field subgrade stability conditions are developed; b) subgrade stability requirements are established for construction (sinkage and compaction considerations); c) remedial actions (undercut and backfill, moisture-density control, lime treatment of subgrades) for inadequate subgrade stability conditions are described and evaluated; and d) assessment guidelines for considering which remedial procedure(s) should be used for a particular situation are presented. Moisture-density-CBR (California bearing ratio) data for five typical Illinois soils are presented in Appendix A. Appendix B presents a documented hypothesis describing the mechanism of subgrade softening under conditions of high moisture content and heavy repeated loading.