Development of New Subgrade Failure Model for Flexible Pavements in FAARFIELD
In November 2016, the United States Federal Aviation Administration (FAA) released an updated version of its standard computer program for airport pavement thickness design and evaluation, FAA Rigid and Flexible Iterative Elastic Layered Design (FAARFIELD). The new version, FAARFIELD 1.41, contains a number of changes with respect to the modeling and design of flexible pavements. This report documents improvement to the method of assigning modulus values to aggregate base and subbase layer materials in FAARFIELD. It also documents the development of a new flexible failure model relating computed subgrade vertical strain to the number of coverages of the aircraft gear corresponding to expected structural failure of the flexible pavement. In this revised failure model, the logarithm of coverages to failure is defined mathematically as a Bleasdale function of vertical strain when failure coverages are equal to or greater than 1000. For coverages less than 1000, the failure model follows a linear tangent line to the Bleasdale curve. The parameters of the Bleasdale failure model were derived analytically by making use of the alpha factor curves for four- and six-wheel aircraft gears incorporated in the FAA’s COMFAA computer program, and the resulting design model was compared to full-scale traffic tests from the FAA National Airport Pavement Test Facility (NAPTF). Data are presented to show that the new model is generally less conservative than the previous (FAARFIELD 1.3) model and provides a better fit to observed failure data, in particular at higher traffic coverage levels.
Author: Izydor Kawa