Alternative techniques and materials are constantly being investigated to develop innovative solutions for the construction of highway embankments and other types of earth structures. Improvement of the subgrade soils for highway embankments can be critical for implementing sustainable and economical applications, especially for cases where recycled or other alternative materials are used. In this part of the research, an extensive laboratory investigation has been performed to investigate the use of fly ash and homopolymerpolypropylene (HPP) to stabilize clayey soils. Various combinations of the soil with fly ash and HPP mixtures were prepared in the laboratory and experiments were performed to evaluate the engineering properties of these mixtures. Standard Proctor compaction tests were conducted in the specially designed Harvard miniature compaction equipment. The compaction characteristics and the unconfined compressive strength of samples under different treatment conditions were investigated. This paper presents the findings from the laboratory study with particular emphasis on the use of alternative materials that provide economic benefits as well as reduced environmental impact. © 2012 American Society of Civil Engineers.