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Seasonal Variation in the Subgrade and Base Layers of U.S. Highway 20, Iowa

Seasonal variations in ground temperature and moisture content influence the load carrying capacity of the pavement subgrade layers. To improve pavement performance, pavement design guidelines require knowledge of environmental factors and subgrade stiffness relationships. As part of this study, field instrumentation were installed along U.S. Highway 20 near Fort Dodge, Iowa, to monitor the seasonal variations in temperature, frost depth, groundwater levels, and moisture regime in the pavement subgrade after construction. Dynamic cone penetrometer, nuclear gauge, and Clegg hammer tests were performed at 64 test points in a 6 ft. by 6 ft. grid pattern to characterize the subgrade stiffness properties (i.e., resilient modulus) during construction operations. The purposes of this paper are to present the field instrumentation results and the observed changes in subgrade stiffness properties due to freeze-thaw effects based on in situ test measurements.