Chemical stabilization of subgrades is one of traditional technologies to provide a pavement construction platform. Laboratory test results of a typical mix design including soil strength and stiffness measurements are usually well documented in the short term. However, the long-term performance data of stabilized pavement are lack and desired for further development of this technology.

In order to address those problems, nine test sections were selected to assess engineering properties of old stabilized subgrades in Texas, Oklahoma, and Kansas. Six subgrades were stabilized with lime and three subgrades stabilized with fly ash. Ages of these stabilized subgrades ranged from 5 to 28 years. Both laboratory and in-situ tests were performed. Laboratory tests include moisture content, sieve analysis, pH test, scanning electron microscope, and unconsolidated-undrained test. In-situ tests include dynamic cone penetrometer, falling weight deflectometer, light weight deflectometer, plate load test, and soil sampling. Using engineering research international (ERI) data analysis software, the subgrade layer moduli were backcalculated based upon FWD tests results.

Soil types, pH values, mineralogical and microstructure analysis, and the improvement ratios between stabilized and un-stabilized subgrades were presented in this study. At some test sites, the field observation found that lime was not uniformly mixed with subgrades. SEM analysis shows some cementing products formed and existed in lime stabilized subgrade samples. Based on the laboratory and in-situ test results, the improved soil strength and stiffness remained after many years of construction.