Excavated soils and rocks generated from construction works in Japan sometimes contain heavy metals derived from nature. Without proper treatment, these soils will generate hazardous leachate and cause environmental contamination. Sorption layer method is a newly developed technology which has a potential to be an economical countermeasure. It utilizes a permeable soil layer with sorption capacity lying beneath piled soil and rock in embankments to retain the chemicals of concern dissolved in the leachate, thus making the effluent meeting the environmental standard. In this study, the performance of sorption layer by mixing decomposed granite soil with Ca/Mg composite immobilizing agent is discussed. The mechanical, hydraulic and sorption performances of soil-immobilizing agent mixture affected by different mixing ratio and curing period is evaluated. Experimental results showed that curing increase the unconfined compressive strength of the mixture. Higher immobilizing agnet content exhibited higher sorption capacity.