Overlay Procedure by Corps of Engineers Reconsidered

The 1982 Consultant Board review of the U.S. Army Corps of Engineers (USACE) overlay procedure is re-examined in the light of more recent analytical developments that provide pathways to implementing the recommendations formulated at that time. The objective of the 1982 Review was to identify shortcomings in the legacy overlay design procedure, determine methods of improving overlay design, and recommend research required to develop and validate an airport overlay design procedure that would yield equivalent performance for flexible and rigid overlays on a rigid base pavement. The history of the existing USACE equations is traced, and new equations proposed in the aftermath of the 1982 Review are re-evaluated. These equations were not recommended at the time as substitutes to the prevailing USACE approach since these could not be verified by the limited field data available. This paper re-examines the USACE procedures using the method of transformed sections, which is found to corroborate previous findings. It is concluded that expecting agreement between theoretical results and the overlay thicknesses actually employed in the field is outrageously optimistic, given the stochastic nature of fatigue, which necessitates a database of forbidding size. To accomplish a comparison between theoretical predictions and field observations, one must first eliminate the use of Coverages from the USACE procedure, given that field estimates of this parameter can easily fluctuate within an order of magnitude. It is shown that selection of a structurally equivalent homogeneous slab thickness, h_o, is tantamount to fixing a threshold stress level for the calculation of the required overlay slab thickness, h₁. Once the problem becomes reduced to an exercise of matching stresses, excellent agreement may be restored between the assumed values of h_o and the backcalculated h₁-values, using mechanistic analytical tools. The methodology employing the method of transformed sections is deemed to be a suitable replacement for the USACE overlay procedure, being a mechanistic-based interpretation of it.