Moisture effects on the mechanical behavior of flexible pavement subgrades

All pavement materials contain moisture in various forms and amounts. The effects of moisture on the mechanical behavior of these materials vary from small for some bound materials to very significant for some fine grained soils. Moisture effects become critical particularly during the spring thaw, but also where prolonged exposure to water sources occurs. The pavement design engineer must consider the effect of moisture on the mechanical properties of the constituent materials, and how different pavement layers interact under wet conditions. This doctoral dissertation focuses on the effect that moisture in the subgrade has on the mechanical response and performance of flexible pavement structures. The research work is based on experiments on a series of full-scale pavement test sections that were built inside a testing facility where the temperature and soil moisture were artificially kept constant at predetermined values. The test sections were subjected to accelerated traffic by means of a heavy vehicle simulator. The test results were analyzed and relationships between subgrade soil type, moisture content, load intensity and number of traffic repetitions to failure were established. The effects of subgrade soil moisture content on subsurface strain and stress distributions were explored and relationships between resilient deformation and the progression of permanent deformation were examined. The existing failure criteria based on limiting the resilient strain at the top of the subgrade were revised in view of the new experimental data. The data indicate that criteria must be specific for each soil type, and the subgrade soil moisture condition must be established in order to predict permanent deformation. The virtual sensor experimental method is presented and used to build profile and contour cross sections of strain and stress. Comparing the stress and-strain figures in the vertical direction to those in the longitudinal and transverse directions it is apparent that the loading regime significantly differs from that used in laboratory triaxial tests. Based on the experimental data, subgrade permanent strain models were developed for each soil type as functions of subgrade moisture content, load intensity, and number of traffic repetitions.