Abstract: The effect of sand and fly ash contents on the Guilin Red Clay mixtures was studied throughout the direct shear, California Bearing Ratio (CBR) and scanning electronic microscope tests. The following conclusions can be obtained: For the direct shear tests, the strength of the Guilin Red Claysand and fly ash mixtures has no significant difference at early curing age of the mixtures. With the curing age, the shear strength will increase and approach to a stable value. The optimum mixture contained 20% sand and 10% fly ash. In the CBR tests, the additives can effectively increase the CBR values and reduce the amount of the expansion. The microstructure study indicates that that additives change the microstructure of the Guilin Red Clay soils and improve the strength and stability.

Key words: Modified Guilin Red Clay, Shear Strength, California Bearing Ratio, Micro-Structures