Subgrade soil evaluation for the design of airport flexible pavements

This dissertation documents procedures and methodologies for characterizing subgrade cohesive soils (moduli, stress-strain, strength, permanent deformation) for airport flexible pavement mechanistic-based design procedures. The dissertation also documents the development of subgrade design criteria (S ubgrade Stress Ratio - SSR = subgrade deviator stress / subgrade compressive strength) for considering subgrade rutting.;The study indicates current state of the art technology in soil resilient behavior is adequate to support subgrade modulus inputs to a priori mechanistic-based flexible airport pavement design procedures.;Soil stress-strain and strength characteristics are important inputs to structural models incorporating Mohr-Coulomb failure criteria (such as Finite Element Programs), and subgrade design criteria based on subgrade stress or stress to strength ratio. The proposed SSR criteria uses soil unconfined stress-strain characteristics to consider rutting potential of cohesive soils.;Subgrade vertical strain criteria (permissible subgrade compressive strain - number of load applications) are not adequate for considering subgrade rutting performance. SSR design criteria are proposed for considering airport subgrade rutting. Permanent deformation (for a number of load applications)-SSR relationships exhibit a limiting stress, the threshold stress level, below which a fairly consistent permanent deformation accumulation rate is obtained with increased stress applications. Subgrade rutting is considered by limiting SSR to acceptable levels, usually in the range of 0.50 to 0.60 (around the threshold stress level). Lower SSR levels may be appropriate for certain high rutting susceptible soils. Such approach ensures that the pavement system exhibits a stable subgrade rutting performance. Total anticipated surface rutting can be limited to acceptable only if AC and granular base rutting are limited to tolerable values.;A subgrade rutting prediction procedure is also presented. The procedure can support the development of limiting surface/subgrade criteria. The approach is an extension of the concepts introduced for the SSR criteria. The procedure uses the power model (Log permanent strain - Log number of load repetitions) and simple unconfined compressive stress-strain tests to estimate permanent deformation. The power model parameters show a logical and consistent link with available engineering properties and factors affecting permanent deformation.;The concepts and procedures developed in this study are based on a comprehensive laboratory testing conducted with the National Airport Pavement Test Facilities (NAPTF) cohesive subgrade soils. NAPTF full-scale pavement data will be used to validate the concepts and procedures.