Liquefaction Mitigation of Silty Sands with Microbial Induced Calcium Carbonate Precipitatio

Liquefaction is a phenomenon that happens in loose, saturated cohesionless soils such as sands and silty sands resulting in devastating consequences such as large settlements, collapse of buildings, and differential settlement. Liquefiable soils require remediation to reduce the consequences of liquefaction. There are many methods available for improving sands, however, many of these methods can be restricted when being applied to silty sands. These restrictions include lower permeability, environmental impacts, and higher costs. Therefore investigating the application of a new and more natural soil improvement method which can overcome these restrictions is necessary. Microbial induced calcium carbonate precipitation (MICP) is a new emerging soil improvement technique that utilizes biochemical activities for cementing the soil. Ureolytic bacteria are applied to hydrolyze urea and increase the alkalinity of the medium. In the presence of calcium, calcium carbonate precipitation takes place. Research has shown that MICP can increase the undrained shear strength and stiffness of soils while decreasing their permeability.;In this study, MICP is applied to investigate the changes in the shear response of silty sands at varying levels of fines content. For that purpose, undrained monotonic and cyclic direct simple shear tests were performed on untreated and MICP treated silty sand specimens at different levels of silt content. The level of treatment was detected using shear wave velocity measurements. All the specimens were treated up to a shear wave velocity of about 400 m/sec. The results show that different parameters affect the changes in shear response as a result of biocementation such as the level of fines content, relative density, and the skeleton and interfines void ratios. The level of improvement also depends on the fabric governing the shear response of silty sands which can be sand, silt or both depending on the fines content and void ratio of the soil.;Cyclic direct simple shear tests (DSSc) were performed on untreated and MICP treated soil specimens with varying levels of silt content and at different levels of cyclic stress ratio. The results show that the cyclic resistance improves with MICP. The level of improvement depends on the fabric and structure of the soil. The improvement in cyclic resistance is compared to natural sand and aged mine tailing material which shows that MICP can mimic natural processes which produce cementation.;The lower permeability of silty sand is a restriction for applying conventional soil improvement methods. Also, the permeability of soil decreases with applying MICP; therefore, investigating the extent of this reduction and its effect on the in situ application is of interest. Constant head permeability tests were performed on fine Nevada sand and silty sand specimens treated up to different levels of shear wave velocity. The permeability of the soil decreases by applying MICP on both sand and silty sand but the level of reduction is similar which a promising result is.;Numerical analysis was performed using finite element Seep/W and Sigma/W programs to model the injection process of MICP treatment in situ. The modeling and calculations were performed for the untreated state and after the soil had reached moderate levels of cementation (e.g. shear wave velocity of about 400 m/sec). The results show that with reduction in permeability, higher levels of excess pore water pressure are generated. In order to increase the level of allowable injection rates, the improved strength and stiffness must be sufficient to counteract with the higher levels of generated excess pore water pressure so that failure does not occur in the soil.