Full-Scale Pavement Testing

Subgrade Strains Measured in Full-Scale Traffic Tests With Four- and Six-Wheel Landing Gears

Vertical strains were measured in the subgrade of a conventional flexible airport pavement test item during full-scale traffic tests at the National Airport Pavement Test Facility. The tests were run to complete failure over approximately 12,000 repetitions. Load was applied in separate wheel paths by landing gear configurations representative of typical four- and six-wheel jet transport aircraft landing gears. Wheel load was 45,000 pounds on both gears over the complete test. The wander path was 2 meters (6.5 ft) wide with 66 repetitions in a complete wander cycle. Multidepth deflectometers (MDDs) were located at a single lateral position relative to the center of the wander pattern in each wheel path. Average strains in the top 150 mm (6 in) of the subgrade were computed from subgrade deflections measured with the MDDs. Representative time histories of the strains are shown to illustrate the response behavior at different lateral positions of the gear in a wander cycle and at different times in the test from the start of the test to complete failure. The relationship between recovered and unrecovered strains at different positions in the wander cycle is also illustrated by the time histories. Over a complete wander cycle, unrecovered strains vary from large compressive to large expansive relative to the mean value. Net unrecovered strain over a complete wander cycle is very small both in absolute terms and relative to the unrecovered strains at individual wander positions. The net unrecovered strain over a single wander cycle represents the permanent deformation in the subgrade accumulated over that wander cycle. When accumulated over the complete test to failure, these small increments of permanent deformation represent the contribution of the subgrade to total rut accumulation. Both the recovered strains and the unrecovered strains increased in magnitude as testing progressed to failure. Also, the ratio of unrecovered strain to recovered strain increased as testing progressed. Summaries are given of the recovered and unrecovered vertical strains for six complete wander cycles at different times as the test progressed from start to end. Summaries are also given of the recovered and unrecovered strains at one wander position over many wander cycles to show the relationship between maximum strain and load repetitions. The strain measurements demonstrate that the response of the pavement is extremely complex and that realistic theoretical modeling of the pavement structure requires a model which includes permanent deformation, moving loads with wander, repeated loading, and representation of structural parameters as functions of temperature and accumulation of damage.

Authors: Gordon F. Hayhoe, Navneet Garg