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Microbially induced calcite precipitation (MICP) is a bioinspired improvement method of granular soils, as it can significantly affect their engineering characteristics. In fact, calcite nanoparticles produced through MICP increase the angle of internal friction and cohesion intercept of granular soils. As a consequence, the shear strength values of granular soils are improved following the increase in these two parameters. However, MICP is not very efficient in strengthening fine-grained soils. In this study, using Sporosarcina pasteurii for MICP treatment, the effects of biocementation and plastic and non-plastic fines (clay and silt) on the shear strength parameters of sandy soils were investigated after 7, 10 and 15 d of treatment to analyse the viability of MICP in different sand mixtures in the field. Direct-shear tests, X-ray diffraction and scanning electron microscopy were used as investigative tools. The results obtained in this study showed that there was a significant improvement in the shear strength of the samples, while noticeable modification occurred within the first 7 d of treatment and fines reduced the MICP potential for strengthening of sandy soils. Although MICP boosted the cohesion intercept and the angle of internal friction of the samples, a great amount of the gained strength was due to cohesion growth.

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