Abstarct: Abstract Due to the ever-increasing growth of the population followed by the development of construction in the past decades, obtaining the necessary knowledge and information to improve existing weak and problematic soils has become more important. Soil improvement is the conscious strengthening of the geotechnical properties of weak soils to obtain their optimal performance. Nowadays, in order to further develop and research chemical soil improvement methods, various chemicals are used to change soil resistance parameters. The importance of environmental issues has led to the use of environmentally friendly additives instead of traditional materials such as cement and lime.One of these chemicals is biopolymers. Different researchers proved that the addition of biopolymers strengthens soil resistance parameters and improves other engineering properties of soil. In this thesis, the ability of sodium alginate biopolymer as an environmentally friendly additive in the sand improvement of bandar anzali has been evaluated. In this research, using a triaxial device, a set of undrained consolidated triaxial tests on carbonated sandy soil samples of Bandar Anzali in the unimproved state and improved with biopolymer concentration of 0.1, 0.2, 0.3 and 0.4 percent by weight of the soil, under confining pressures of 100, 200 and 300 kPa and during the processing time of 1, 7, 14 and 28 days and with a relative density of 60 percentage has been done and the effect of adding this biopolymer on the shear behavior of the soil has been investigated. The results of these experiments showed that the shear behavior of soil improves by adding calcium alginate hydrogel. It was also observed that with the increase in biopolymer concentration, confining pressures and processing time of the samples, values of shear stress, maximum deviatoric stress and cohesion of improved samples increased and a slight decrease in the friction angle of improved samples was also observed. By examining the changes in the stress-strain curves and the Mohr-Coulomb failure envelope of the samples improved with hydrogel, it was observed that at a concentration of 0.4% and a processing time of 7 days, about 26% in maximum deviatoric stress and about 60% in shear stress has increased . As a result, the effectiveness of this additive in 0.4% of biopolymer and the duration of 7 days of processing has been more visible.