Prediction of deflection response of flexible pavements under dynamic loads

Analysis is made of deflection-response data collected under moving vehicles and a falling weight deflectometer (FWD). The vehicles included log trucks, B-727 and A-4 aircraft, F-15 load cart, and other light vehicles. The tests were conducted in three separate studies. All pavements had asphalt concrete (AC) surfaces, and subgrade materials included sand, lean clay, and heavy clay. The test sites were instrumented with velocity transducers and/or multi-depth deflectometers (MDD) to measure deflections at various depths within the pavement structures.;The response signals from the various vehicles and test sites were examined for frequency of response as a function of speed, load, and position within the pavement. The deflection measurements were related to vehicle speed.;Moduli of the pavement layers and subgrade were computed from deflection basins produced by the FWD and the vehicles through a backcalculation technique using layer-elastic theory (WESDEF program). Comparison of computed deflections at offsets and depths were made to measured values. Results using moduli determined from FWD basins showed computed deflections for the FWD loads to match well with measured values. However, these moduli were not successful in similar comparisons of vehicle deflections.;Frequency of the vehicle deflection response, determined from response-time signals, were on the order of 0.2 to 2.5 Hz (corresponding to 5 to 25 mph), and were higher near the pavement surface. The FWD signals had frequencies ranging from 10 to 21 Hz, and were fairly constant with depth.;A set of moduli were determined using the layered-elastic program WESLEA, that gave computed vehicle deflections at depths directly beneath the wheel loads, which were in agreement with measured values. In all cases, this required a soft layer (low modulus) at the top 12-in. of the subgrade with a higher modulus subgrade beneath. Multiple regression equations were used to relate the vehicle moduli to the FWD moduli, and a procedure was enumerated that makes appropriate adjustments to the FWD moduli.