DEVELOPMENT OF MECHANISTIC FLEXIBLE PAVEMENT DESIGN CONCEPTS FOR THE HEAVYWEIGHT F-15 AIRCRAFT (AIRFIELD, COMPUTER DESIGN, CBR, ALGORITHMS)

A new configuration of the F-15 aircraft has been developed and is in use by the U.S. Air Force. This heavyweight F-15 has a 30-kip/355-psi wheel loading. A heavier-weight F-15 with 36-kip/395-psi wheel loading has been proposed for use. The F-15 will become the controlling aircraft for design of airfield Light-Load Pavements. A review is presented of the concepts and development of the present Department of Defense (DOD) method for flexible airfield pavement design.;The structural model used in this study to calculate pavement structural responses (stresses, strains, deflections) is the finite element program ILLI-PAVE. This program considers the pavement as an axisymmetric solid, and accommodates stress-dependent materials and soils and stress corrections according to Mohr-Coulomb failure criteria. Multiple regression analyses are performed on an extensive ILLI-PAVE data base (400 runs) to develop prediction equations (algorithms) for pavement structural responses of interest. These equations have high statistical precision when compared against the ILLI-PAVE data base. Therefore, they may be used in lieu of running ILLI-PAVE, which generally requires a main-frame computer. Equations developed from a much smaller data base, consisting of three levels of each of the variables (i.e., 3('4) factorial = 81 runs), were found to provide acceptable accuracy. Using 3('4) factorial data bases, algorithms are developed for the heavyweight F-15 load, heavier-weight F-15 load (36-kip/395-psi), 24-kip/355-psi load, and 36-kip/355-psi load.;Pavement test section data obtained from the literature are analyzed using ILLI-PAVE. Transfer functions are derived relating calculated pavement responses to coverage till failure.;The components of a mechanistic design procedure are discussed. A mechanistic design example is presented and compared to the DOD design for the same conditions. Critical pavement responses recommended for design are maximum radial tensile strain at the bottom of the asphalt concrete and maximum subgrade stress ratio (deviator stress/compressive strength). It was found that four inches of asphalt concrete may not be sufficient to prevent premature fatigue cracking of pavement subjected to long term use by the heavyweight F-15 aircraft. Also, the DOD designers for the heavyweight F-15 aircraft may be overly conservative for subgrade rutting.