Laboratory assessment of permeability of sand and biopolymer mixtures. This research presents a method of creating seepage barriers in a sandy soil using biopolymer additives (biosubstance), which consist of polysaccharides and water. Polysaccharides strongly interact with water to produce a viscous suspension. The paper aims to investigate the influence of a biosubstance employed in a highly permeable sandy soil. Amount of the biopolymer used in a sample were 0.5, 1.0 and 1.5%, by dry weight. The test results indicate that the hydraulic conductivity significantly decrease with the amount of biosubstance added but only slightly increase when curing time gets longer. It is thought that such application, which is a relatively new soil improvement technique, could be used as a seepage barrier installation required to protect some geotechnical works including foundation, underground structures and waste disposals.