Improvement of soft clay layers is highly significant for the efficient utilization of the national land. Thus, the development of advanced methods and materials is currently necessary. Quicklime, as a injection material for the soil improvement, is possibly applied as the useful method for the improvement of soft clay layers. Based upon the experimental study, the following conclusions were obtained. When quicklime was employed, a substantial strength increase was observed from the initial stage of injection. In overall, the present experiments showed that the improvement effects of soft clay layers using the quicklime are appeared to be substantially better than those of lime or sand. Therefore, the possibility of quicklime usage is significantly high in domestic country with abundant lime reserves.