Establishment of subgrade undercut criteria and performance of alternative stabilization measures.

729574864Final report; Aug. 2007-Dec. 2009.PDFTech ReportFHWA/NC/2008-07Soil stabilizationBackfillingNorth CarolinaNorth Carolina. Dept. of TransportationBorden, Roy H.North Carolina State University. Dept. of Civil, Construction, and Environmental EngineeringNTL-HIGHWAY/ROAD TRANSPORTATION-Soils and GeologyUS Transportation CollectionThe main objectives of the research work were to develop a systematic approach for determining whether or not undercut isnecessary, and to investigate the adequacy of stabilization measures as implemented in conjunction with the undercut approach.The research work encompassed small and large scale laboratory testing, limited field testing using DCP, and numerical analysesand modeling. The following milestones were achieved based on the work performed in this project:i. Establishment of undercut design criteria based on the magnitude of strength and modulus of the subgrade soils. Theundercut criteria were based on meeting a deformation limit state of 1 inch for both pumping and rutting, but with the additionalrequirement that bearing resistance of subgrade is at least twice the applied tire pressure to minimize the potential for rutting.ii. Development of a procedure for the use of DCP to provide input data for undercut criteria on the basis of modulus andstrength. The proposed procedures are based on the analysis of the DCP data using wave mechanics and offer the advantage ofusing the DCP data to discern the need for undercut with depth (by applying the proposed undercut criteria incrementally) at thevarious phases of design and construction.iii. Development of guidelines for specifying a stabilization measure to achieve adequate subgrade support. Five stabilizationmeasures were investigated through the performance of 22 large scale laboratory tests. These included the use of select fill,aggregate base course (ABC), geogrids with ABC, geotextiles with ABC, and lime stabilization. The performance of eachstabilization measure was investigated in the laboratory and through numerical analyses.iv. Demonstrating the applicability of the proposed measures in several field configurations. Four field cases were idealizedfrom actual project sections. The field cases were numerically modeled with the stabilization measures, and observations weremade regarding subgrade response under static, proof roll, and cyclic loading.v. Performance of a comparative cost analysis to illustrate the relative cost of each stabilization measure in relation tomeasured performance (surface deformation). The cost analyses were performed with results presented in a normalized form inan attempt to account for variables such as strength of the stabilization measure and subgrade deformation