Message from Arno Hefer

Fri, 13 Nov 2026 01:02:41 GMT

Background

The Federal Aviation Administration (FAA) subgrade failure criterion for flexible airport pavements relates the maximum vertical compressive strain at the top of the subgrade to the number of aircraft gear-load coverages to structural failure. The published model was developed from FAA aircraft loading relationships and evaluated against full-scale pavement testing (Kawa, 2017).

Interpretation and application

The two expressions form one continuous failure relationship. Their designations (C < 1 000 and C ≥ 1 000) refer to calculated coverages to failure (structural capacity), not applied traffic demand. The transition at 1 000 coverages to failure corresponds to a maximum vertical compressive strain at the top of the subgrade of approximately 1 765 µε (0.001765).

Applied coverages at the pavement location being assessed must account for aircraft wander and gear configuration. Aircraft passes are converted to coverages using an appropriate pass-to-coverage (P/C) ratio before applying the criterion; this traffic processing is undertaken externally. Further information on the pass-to-coverage (P/C) concept and the treatment of aircraft wander is provided in FAA AC 150/5320-6G, Section 3.12.8.

Failure definition

The criterion addresses structural failure associated with subgrade deformation (rutting), rather than a specified allowable surface rut depth. In the FAA full-scale tests used to assess the model, failure was defined by at least 25.4 mm (1 in.) of surface upheaval outside the trafficked area, indicating general shear failure of the supporting layers (Kawa, 2017).

References

Kawa, I. (2017). Development of New Subgrade Failure Model for Flexible Pavements in FAARFIELD. FAA Technical Report DOT/FAA/TC-17/28.
Federal Aviation Administration (FAA). (2021). Airport Pavement Design and Evaluation. Advisory Circular 150/5320-6G.

edited by on 11/13/2026
edited by on 11/13/2026

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Message from Arno Hefer