The use of hydraulic barriers to control the water leakage and flow into underground structures such as tunnels has become a common method in geotechnical engineering practice. There are different types of materials used for hydraulic conductivity control purposes such as soil-bentonite and cementbentonite mixtures, and chemical admixtures including asphalt, sodium silicate and so on. Underground hydraulic barrier materials are required to well-resist against the in-situ earth pressure and hydrostatic water pressure. In this study, a gel–type polysaccharide biopolymer gellan gum–has been used to reduce the hydraulic conductivity of sand. It results from the fact that gellan gum biopolymer forms high viscous hydrogel once thermo-gelated, which is expected to have a high structural resistance even at high external pressure conditions. Therefore, the hydraulic conductivity of gellan gum-treated sand is assessed with vertical confinement and water pressure increase. It is interesting to observe that the hydraulic conductivity of sand exponentially decreases with gellan gum content.