The application of microorganisms to improve the mechanical properties of soil is a new developing research area. A new native bacteria extracted from soil was introduced for the biological improvement of soil geotechnical parameters. The isolate was identified as Acinetobacter calcoaceticus S1. Sporosarcina pasteurii was used as a positive control. Direct shear tests were performed on the nontreated soil and soils treated with bacteria to determine the shear strength, adhesion and angle of internal friction. The treatment period was 40 days. The shear wave velocity was measured.The results showed that the untreated sample had relatively constant shear strength, but the shear strength of the treated soils increased significantly. The soil treated with A. calcoaceticus had greater shear strength. The angle of internal friction increased for the treated soils with A. calcoaceticus (39.3%) and S. pasteurii (28.6%). The greatest cohesion was found for soil treated with A. calcoaceticus, reaching 0.66 and 0.56 kg/cm2 for S. pasteurii. The shear wave velocity in the treated soils increased significantly. The results confirmed the ability of native A. calcoaceticus to improve soil geotechnical parameters. Calcium carbonate precipitation fills the voids between soil particles and forms a gel, which makes effective connections between soil particles and makes them coalesce and grow larger.
