Evaluation of the Mechanical Responses in Clay Subgrade under Aircraft Loading

The Federal Aviation Administration has conducted a full-scale accelerated pavement testing experiment, Construction Cycle 9 (CC9), to evaluate the performance of various flexible pavement structures. The purpose of this study, specifically, was to evaluate the behavior of clay subgrades under aircraft loads. In CC9, a specific area (referred to as LFC5S) utilized a thin P-401 Hot Mix Asphalt layer along with a low-strength clay subgrade. The clay subgrade was instrumented with pressure cells and coil sensors that monitored clay responses during trafficking. The LFC5S was trafficked using a dual-wheel gear with a 160 kN (36,000 lb) wheel load and 1,379 kPa (200 pounds per square inch) tire pressure. In addition, there was an unanticipated 5-month pause in the trafficking sequence where LFC5S was unloaded. Based on the instrumentation data, the vertical deflection at the top of the subgrade increased over the experiment, with a temporary return to the original deflection magnitude after the pause in trafficking. Vertical stress did not substantially change over the experiment. No relationship was observed between the volumetric moisture content and the increase in the vertical deflection. However, a strong relationship was found between the change in volumetric moisture and the deflection at the top of the subgrade. Overall, an increase in the vertical deflections resulted in an overall decrease in the modulus of 72% and 80% before and after the pause in trafficking, respectively. Finally, the modulus of the subgrade recovered to approximately 80% of the initial modulus after the 5-month pause in trafficking.