Abstract The inclusion of randomly distributed short virgin polypropylene fibers (C3H6) in clay has proven to significantly improve the geotechnical properties of clay such as shear, compressive, and tensile strengths, ductility, volume change, and so on. Those improvements have triggered great attention on the possibility of mixing fibers with clay to form a desirable composite. Because the percentage of fibers used is arbitrarily chosen by the users, the purpose of this study is to determine if there is an optimum fiber content and if it is a function of the type of fiber used. Compaction properties of clay-fiber composite using commercially available synthetic polypropylene, synthetic polypropylene fibers (monofilament and fibrillated) and kaolinite were determined. It was found that for a clay-fiber mix, there is an optimum fiber content and it is different for different types of fiber.