Mechanical behavior of bio-cemented soils

The thesis presents a study of the mechanical properties of sands treated by MICP (Microbial-induced calcium carbonate precipitation), a promising bio-mediated methods with little carbon footprint. Previous studies were mainly based on poorly graded sands with very fine grains (< 1 mm) and were often qualitative rather than quantitative. Many monotonic and cyclic triaxial tests were done on five untreated and lightly treated sand mixtures with grain diameters up to 5 mm. Test results show that MICP treatment greatly enhances the resistance of the soils to both monotonic and cyclic solicitations, even when samples are not totally cemented, the relative effect of density and cementation depending on the percentage of fine grains (FC). In both monotonic and cyclic tests, the relation between the enhancement of the mechanical properties and the deposited calcium carbonate content is complex, characterized by a minimum CaCO3 content, which increases when FC decreases, below which its effect is not visible. Above the "minimum value", the resistance increases linearly with the CaCO3 content. Microscope observations confirm the complex interplay of several mechanisms that can explain the effect of MICP treatment through the bridging of particles, surface roughness (coating) and density (pore-filling) increases. Additional bio-physico-chemical tests helped interpret the results.