Laboratory Performance Comparison of Stabilized Undercut Subgrade Under Cyclic Loading

This study evaluated the performance of undercut subgrade stabilization measures during construction traffic loading prior to final paving. Twenty-two simulated undercut sections with different stabilization configurations over a typically undercut Coastal Plain clay subgrade were built in a large-scale test pit. The subgrade was placed at a California Bearing Ratio of ∼2–3 % and stabilized with granular layers, granular layers reinforced with geosynthetics, and lime. Granular layers consisted of either aggregate base course (ABC), sandy select fill, or a multi-layer system with both soil types. The four geosynthetics tested were a woven reinforcement geotextile, a woven separation geotextile, and two biaxial polypropylene geogrids. The soft nature of the subgrade and its consequences on the ability to compact the ABC layer show the importance of carefully analyzing the results when viewed on a comparative basis. Cyclic plate loading simulating construction traffic showed that thicker granular layers produced less surface displacement, barring subgrade strength differences from remolding effects. Tests with lime stabilized subgrade showed the least magnitude of deformation over initial and post-rut repair cycles. ABC tests with geotextile showed improvement over unreinforced sections but only when placed at depths approximately equal to the loading plate diameter and after initial displacements mobilized the geosynthetic strength.