It is well known that the strength of sand improved by chemical grouting is influenced by many factors including the type of base resin, silica concentration, soil grain and density. Although the strength is several to several tens of times larger than that of the silica hydrogel, the mechanism for increasing the strength has not been clarified. In this paper, a new proposal was made on the strength mechanism in which the volumetric shrinkage of silica hydrogel in sand offers confining pressure to the sand skeletons. In order to prove the validity of the mechanism, a series of unconfined compression strength tests, splitting tensile strength tests and elastic wave tests were carried out. Furthermore, the strength development of the improved soil was estimated by equations proposed on the basis of the mechanism.
