Today regarding population growth and better use of land and resources need to strong base is more than any other period. For this reason, a lot of improvement tecniques in the world is running and it has its own advantages and disadvantages of each them. chemical, physical, mechanical, biological and electrical are the most important methods in soil improvement. Some of these methods to bring environmental problems that can not be used in some places or all places. Biological Improvement of a innovative, novel and environmentally friendly use of microorganisms and biochemical processes that led to deposition of calcium carbonate, which makes soil particles to each other and improve soil physical and strength properties. In this paper, mixed culture performance against single culture of bacteria on physical and strength properties of sandy soil was studied. Bacillus pasteurii and orosarcina urea bacteria was used as single culture and Bacillus pasteurii+Bacillus subtilis and Sporosarcina urea+Bacillus subtilis were used as mixed culture. Unconfied compression test results as an indicator of the strength properties showed that unconfied strength of the improved samples by Bacillus pasteurii+Bacillus subtilis and Sporosarcina urea+Bacillus subtilis were obtained 1690.3 kPa and 771.8 kPa, respectively, that these amount show 22% and 38% increasment of unconfied strength to single culture. Triaxial test (UU) to compare improvement samples with sandy soil was performed on 50, 100 and 150 kPa confined pressure. Falling head test results as an indicator of the pHysical properties showed that the MICP method decreases the permeability factor of samples. Permeability factor of the improved samples by Bacillus pasteurii+Bacillus subtilis and Sporosarcina urea+Bacillus subtilis compared to single culture has decreased 34% and 17%, respectively. Direct shear test was performed to evaluate time of curing 7, 14 and 21 day and rainy conditions on field. Results showed that with increasement time of curing, angle of internal friction and cohesion will increase in both of conditions (dry and spraied). Finally precipitated calcium carbonate has been shown by SEM and XRD tests. Keywords Soil improvement, Dune Sand, Mixed culture, Single culture, Unconfined Compressive strength test, Direct Shear test, Falling head test
