ANALYSIS OF IN SITU GRANULAR-LAYER MODULUS FROM DYNAMIC ROAD-RATER DEFLECTIONS

The major objective of this study was to investigate the ability of elastic-layer theory coupled with nonlinear dynamic modulus tests to predict pavement deflections in a way comparable to dynamic road-rater deflection measurements on three highway sections in Maryland. It was found that theoretically predicted deflections were two to four times the measured road-rater deflections for all three pavement sections studied at all four road-rater sensor locations and for all times throughout the year in which road-rater deflections were made. To obtain equality between predicted and measured deflections, the granular-layer modulus was adjusted by using a K sub 1-factor. A linear log-log relationship was evident when the K sub 1-adjustment factor was plotted versus measured surface deflections. It was concluded that the current laboratory method of modulus characterization underestimates the modulus of a granular material and that an increase in surface deflection results in a decrease of the granular-layer modulus. An investigation was undertaken to determine whether the effect of shear strain was responsible for the decrease in granular-layer modulus. It was found that the granular- layer modulus decreases with increasing shear strain and that shear strain is proportional to surface deflection. The former finding substantiates other research dealing with the dynamic response of granular soils. The study of the K sub 1-factor and modulus adjustment also led to the development of equations that relate measured surface deflection to in situ granular-layer modulus. For two of the pavements analyzed, predicted granular-layer modulus showed very good agreement with the modulus required for equal computed and measured deflections. (Author)