INVESTIGATION OF MOISTURE-INDUCED VARIATION IN SUBGRADE MODULUS BY CROSS-CORRELATION METHOD

The change in moisture content of subgrade soils and its resultant effects on structural pavement performance are of interest to many highway engineers. One of the primary sources of moisture in thee subgrade is water from precipitation. The purpose of this research was to examine the feasibility of statistically relating the variation in subgrade modulus with the corresponding time history of precipitation. Two locations on KS-4 in northeast Kansas were monitored from March through November. Dynamic pavement surface deflections were obtained each week by Dynaflect, and time histories of subgrade moduli were constructed from normalized Dynaflect deflection parameters. A corresponding time history of precipitation at the locations was obtained from the local U.S. Weather Bureau station. A cross-correlation study by computer of the two times series revealed a statistically significant correlation between precipitation history and variation in subgrade modulus. the time required for the subgrade to reach its weakest state after a rainfall was found to be rather long--up to 3 weeks might be required. The interrelationship of other factors, such as runoff characteristics and temperature, that might influence this time delay, was not considered in this study. (Author)