Subgrade Damage Approach for the Design of Airport Flexible Pavements

Abstract

Current mechanistic-based design methods for the design of airport flexible pavements use vertical strain criteria to consider pavement rutting. Variations in subgrade strength, and aircraft traffic are considered based on cumulative damage according to Miner's hypothesis. Cumulative damage is calculated as the ratio of the number of load applications to the number of allowable load applications to failure. The total cumulative damage factor can not exceed one. New design procedures based on Subgrade Stress Ratio (SSR= ratio of deviator stress to unconfined compressive strength) have emerged for better considering subgrade rutting in airport flexible pavements. To consider the effect of traffic mix loading and sequence (stress history) and variations of subgrade strength on the rutting performance of subgrades, analyses of laboratory data and computed subgrade stress distributions were conducted. The permanent deformation laboratory data indicate that if a soil is subjected to an increased or decreased repeated stress sequence, the highest applied stress produces most of the permanent deformation with lower repeated stress level having little contribution on permanent deformation. Analyses of stress distributions based on aircraft type, aircraft wander, pavement thickness, and subgrade strength show that the distribution of high stress levels (above 85% of the allowable SSR) have a significant influence on the rutting performance of airport flexible pavements. This paper presents a subgrade damage approach that supports design procedures that use subgrade stress ratio or subgrade stresses for limiting subgrade rutting. The approach is based on an extensive literature review of current damage concepts included in current mechanistic-based design procedures, soil permanent deformation laboratory data, and airport traffic conditions. The research included in this paper was conducted at the FAA's Center of Excellence for Airport Pavement Research located at the University of Illinois at Urbana-Champaign.