AbstractSeveral monotonic triaxial consolidated drained (CD) and consolidated undrained (CU) tests were conducted to investigate the effect of biogenic gas formation through microbial-induced denitrification on the mechanical response of poorly graded sand at different degrees of saturation (Sr) and varying relative density. The results demonstrated that the effect of desaturation on the mechanical response varied significantly depending on the relative density. For loose sands, the buildup of pore pressure during soil contraction is prevented. Consequently, the undrained strength increases with decreasing degrees of saturation, whereas dense sands indicate the opposite effect in which negative pore pressures resulting from soil dilation are suppressed, reducing the undrained shear strength. Medium dense sands displayed the highest increase in undrained strength, but the increasing effect displayed a peak value at a specific degree of saturation and decreased with a further reduction in saturation. Irrespective of the relative density, a critical degree of saturation exists at which a further decrease in saturation has no additional effect on the undrained response. Furthermore, it was indicated that the critical state line in the e-logp′ plane shifted as a result of biological desaturation, but the shift appeared to be independent of the degree of saturation within the tested range of Sr values from 0.57 to 0.94.