"The behavior of flexible pavements under traffic and environmental loading can be significantly affected by subsurface conditions. Inadequate support conditions under the surface can lead to excessive pavement deformations, often leading to structural and functional failure. This research effort focused on assessing the effects of base/subbase and subgrade layer conditions on flexible pavement behavior. The results of this study are presented in the form of two journal manuscripts. The first manuscript focuses on utilizing pavement structural and functional evaluation data in making pavement rehabilitation decisions. Visual distress surveys and Falling Weight Deflectometer (FWD) testing are often carried out by agencies as a part of their pavement preservation programs. Although back-calculation of individual layer moduli from FWD data is a common approach to assess the pavement's structural condition, the accuracy of this approach is largely dependent on exact estimates of individual layer thicknesses. Considering the lack of pavement layer thickness information for all locations, this study used Deflection Basin Parameters (DBPs) calculated from FWD test data to make inferences regarding the structural condition of individual pavement layers in conventional flexible pavements. The adequacy of DBPs to assess the structural condition of individual pavement layers was assessed through Finite-Element (FE) Modeling. Subsequently, four selected pavement sections in the state of Idaho were analyzed based on this method to recommend suitable rehabilitation strategies. The second manuscript focused on studying how improvements to subsurface layers can affect the flexible pavement behavior over expansive soil deposits. A recently completed research study at Boise State University investigated a particular section of US-95 near the Idaho-Oregon border that has experienced significant differential heave due to expansive soils. Laboratory characterization of soil samples indicated the presence of highly expansive soils up to depths of 7.6 m (26 ft.) from the pavement surface. Through subsequent numerical modeling efforts, a hybrid geosynthetic system comprising geocells and geogrids was recommended for implementation during pavement reconstruction. This research effort focused on evaluating the suitability of polyurethane grout injection as a potential remedial measure for this pavement section. Laboratory testing of unbound materials treated with a High-Density Polyurethane (HDP) demonstrated that resilient modulus and shear strength properties could be improved significantly. Finite Element modeling of the problematic US-95 pavement section indicated that depending on the treated layer thickness, the differential heave magnitude can be reduced significantly, presenting polyurethane injection as a potential nondestructive remedial measure. ."--Boise State University ScholarWorks.