PREDICTING PAVEMENT RESPONSE DURING THAW WEAKENING PERIODS USING THE FALLING WEIGHT DEFLECTOMETER . THIRD INTERNATIONAL CONFERENCE ON BEARING CAPACITY OF ROADS AND AIRFIELDS. PROCEEDINGS, NORWEGIAN INSTITUTE OF TECHNOLOGY, TRONDHEIM, NORWAY, JULY 3-5 1990. VOLUMES 1-2

This paper presents the results of a programme of tests to study the performance and structural capacity of pavement structures during thaw weakening periods. Four pavement test sections were constructed in the Frost Effects Research Facility (FERF). Both full depth and conventional pavement structures were studied. The subgrade was inorganic clay of high plasticity (CH) and either gravelly sand or poorly graded sand with few fines was used as sub-base material. Results of the first 2 freez-thaw cycles, with maximum frost depth of 122 cm and 152 cm respectively are presented here. Falling weight deflectometer (FWD) measurements were taken both before freezing and during the thaw periods. Several relationships were evaluated to characterize pavement response during the thawing periods: the impulse stiffness modulus (ISM), first sensor deflection ratios; fourth sensor deflection ratios; and deflection basin area. The ratio of the 4th sensor deflection during thaw to the same sensor deflection prior to freezing was evaluated as an indicator of the relative subgrade strength. This ratio, called the subgrade strength index (SSI) was used to characterize the subgrade strength during thaw. It was found that the clay subgrade was weakened by a factor between 2.0 and 2.2 compared to the prefrozen strength. This implies a 50% to 60% reduction in the modulus of CH subgrade. A method using the FWD deflection measurements to determine thaw depth is also presented. Good correlations between deflection basin areas and thaw depth were obtained using this method, and it was also found that SSI could be used to estimate thaw depth to within 2 cm for thaw depths up to 90 cm. (TRRL)