Kay County Shale Subgrade Stabilization Revisited

This paper presents a re-examination of a research project on US-77 in Kay County, Oklahoma concerning chemical stabilization of an 8-inch compacted shale subbase layer completed in 1983. The primary focus of this paper was to examine the changes in the pavement surface deflection and backcalculated modulus of the stabilized subbase layer within the project test sections over time. The original research project test sections were identified and tested with a trailer mounted Falling Weight Deflectometer (FWD) unit in August 2010. The FWD deflection survey performed in July 2000 and the original Benkelman beam deflection survey performed in June 1985 following the original pavement construction were used in this pavement analysis. The pavement had been overlain with two thin asphalt overlays. The FWD deflection data was normalized by ratio of the new thickness to the original pavement thickness so that the original Benkelman beam deflection data could be used. The pavement analysis uses the two-layer Hogg model and three-layer model in the Modulus 6.0 software to estimate the subgrade modulus and 8-inch stabilized subbase layer subgrade modulus respectively. The conclusions are the following: a) the pavement surface deflections indicate that the stabilized subbase layer sections have relatively stiff subgrade support, b) the three-layer model predicts a higher subbase layer modulus, and c) chemically stabilized subbase layers do not significantly lose strength or degrade with time and can be depended on as part of the structural section.