Work performed in this research project was in support of the effort by the North Carolina Department of Transportation to develop criteria to discern the need for undercutting soft soils and the adequacy of the subgrade after stabilization measures are implemented. The work was specifically focused on performing field testing and collecting data at a site located in the North Carolina Piedmont geologic area. Three test pads were constructed on comparable subgrade conditions with subgrade stabilization measures that included the use of select fill material and geosynthetic reinforcement coupled with a relatively thin layer of Aggregate Base Coarse (ABC). Field loading was applied using 1000 passes of a loaded construction truck. Several parameters were monitored during loading including stresses, deformation, and moisture and suction levels. A fourth pad was also constructed to test lime stabilization; however loading of this pad was not performed since the lime used for stabilization did not meet NCDOT’s specifications. A surface profiler (Lidar-based surveying) was used at periodic intervals to monitor plastic deformation and wheel path rutting with loading cycles. The field testing also included collection of data from Light Weight Deflectometer (LWD) and Dynamic Cone Penetrometer (DCP) to assess subgrade properties. Results indicated that the measured vertical stress near the interface of the subgrade for the geosynthetics-reinforced sections initially increased, but then decreased with increasing number of truck passes This decrease was attributed to progressive densification of the aggregate base course and the mobilization of the reinforced layer tensile strength. The reinforced sections experienced surface deformation on the order of 0.5-0.7 inches and the select fill-stabilized section showed a 1.2 inches of deformation after 1000 truck passes. In addition to the field data, two models were developed for the estimation of the resilient modulus (Mr) on the basis of the Light Weight Deflectometer (LWD) and Dynamic Cone Penetrometer (DCP) data. Mr values from these models can be used along with the subgrade’s shear strength to discern the need for undercutting. Using the monitored data, the proposed undercut criteria by Borden et al 2010 were deemed to be valid at the site with the Piedmont residual subgrade soil. Sections with stabilized measures met the criteria for adequate subgrade. Guidelines regarding thickness of the ABC layer with various subgrade strength values were proposed based on the results of dynamic 3-D numerical analyses. A chart was proposed to enable the selection of an appropriate thickness of ABC layer based on the measured DCPI of the subgrade soil.
