Determination of mechanical and hydraulic properties of polyacrylamide-added bentonite-sand mixtures

Bentonite-sand mixtures are typically used in engineering applications such as cutoff walls for waste containment facilities or barriers in landfills and freshwater reservoirs. However, low or reduced mechanical and hydraulic performance of a bentonite-sand mixture might result in the failure of the engineering structure. In order to enhance the mechanical and hydraulic properties of a bentonite-sand mixture, an anionic and a cationic polyacrylamides with a content of 1, 2, 5, 10, and 15% respectively were added to the bentonite-sand mixtures. The polyacrylamides used in this study were water-soluble synthetic polymers with high molecular weights. They had enhanced flocculation capability when coming into contact with soil and water. Standard Proctor compaction, consolidated-drained direct shear, unconfined compression, falling-head permeability, and free swell tests were performed on the bentonite-sand mixtures that had different bentonite contents. Test results indicated that cohesion, unconfined compressive strength, maximum dry unit weight, and swell index increased while internal frictional angle, permeability, and optimum water content decreased as a result of the polyacrylamide addition. The rate of change was much higher when the polyacrylamide content was increased up to 2%. The performance of the anionic polyacrylamide was found to be slightly better than that of the cationic polyacrylamide in terms of both strength and permeability. In conclusion, 2% anionic polyacrylamide addition resulted in a sufficient increase in the shear and compressive strength, a slight increase in the swell index, and a sufficient decrease in the permeability of the bentonite-sand mixtures.