Abstract: Expansive soil is prone to trigger destruction of overlying structures due to characteristics of water absorption expansion and water loss shrinkage. We examined the physical and expansive properties of polyaluminum chloride (PAC) modified expansive soil via particle analysis test, limit moisture content test, free expansion rate test, expansion rate test, shrinkage test and SEM test. Results demonstrated that the clay particle content, hydrophilicity, liquid limit and plasticity index of PAC modified expansive soil decreased; and also the expansion and shrinkage indicators, inclusive of free expansion rate, non-loaded expansion rate, loaded expansion rate, linear shrinkage rate and volumetric shrinkage rate, of PAC modified expansive soil all declined within a certain amount of PAC admixture. A large amount of flocculants improved the structure of soil by filling the pores of soil and cementing soil particles. The mechanism of PAC modifying expansive soil lies in a large number of high valence [Al]^m+ and [Al_m(OH)_n]^3m-n flocculants ionized in water medium, which could reduce surface charge by ion exchange, electric neutralization, cementing adsorption and flocculation enmeshment, so that clay particles gather together to lower the expansion and contraction of expansive soil.

Key words: expansive soil, polyaluminum chloride, sodium silicate, swell-shrink characteristic, microstructure