Undrained cyclic response of silty sands improved by microbial induced calcium carbonate precipitation

Loose saturated silty sand can be prone to liquefaction but improving their soil properties is challenging due to their lower permeability compared to clean sands. Microbial induced calcium carbonate precipitation (MICP) is a new, natural and sustainable soil improvement method, which can increase the shear strength and stiffness of soils. In this study, MICP treatment is applied to silty sands with fines content in the range of 0-35% and a constant preshear void ratio 0.7 +/- 0.05 to observe improvement in their liquefaction resistance. The specimens are treated until reaching a shear wave velocity about 400 m/s. Cyclic direct simple shear testing is used to evaluate changes in the liquefaction resistance of untreated and MICP treated silty sands. The results show that by applying MICP the liquefaction resistance increases significantly for all fines contents tested, and the treatment efficiency depends on the level of fines content, which dictate the relative density, and the fabric governing the structure. As examples, the 35% fines content specimen has shown the highest improvement, which is related to a higher relative density at this fines content. The silty sand specimens with 5% fines content has a metastable structure which makes the soil more sensitive to cyclic loading. Although the cyclic resistance has increased by applying MICP at this level of fines content, its sensitivity to cyclic loading has remained unchanged. The results of this study demonstrate that MICP improves the cyclic strength of silty sand and can provide an alternative soil improvement method for silty sand.