Most of the current geofoam design methods adopt the working stress concept. To design pavement structures with geofoam as subgrade according to the AASHTO and other pavement design guides, the resilient modulus, California Bearing Ratio (CBR), and modulus of subgrade reaction of geofoam need to be determined. The standard test methods are not suitable for geofoam because they usually prescribe higher stress than can be applied on geofoam. This paper presents modified tests methods designed for geofoam to determine the three parameters. Modified tests showed geofoam has resilient modulus unacceptable for subgrade by normal standards. CBR values interpreted by a modified method yielded higher values than conventional methods but still less than acceptable soils. Modified plate load tests indicated low values of modulus of subgrade reaction for geofoam. The concrete slab-geofoam composite subgrade was evaluated by numerical modeling. By considering the composite action of the concrete slab and geofoam, higher resilient modulus and modulus of subgrade reaction values were obtained. Based on the modified tests and modeling, an approach for selecting appropriate geofoam design parameters is proposed.