Mechanical performance and upscaling of bio-improved soils

Ecological awareness, pressing climate considerations and rising need for soil stabilization push forward the quest for alternative solutions in geotechnical engineering such as Microbially Induced Carbonate Precipitation (MICP). Precipitated crystals serve as a natural cement, densifying and holding together previously unbonded grains, thus improving a soil's hydro-mechanical properties. However, despite abundant research in the last decade, and numerous foreseen applications, a solid breakthrough of the technology from the lab-bench towards geotechnical practice remains to be seen. The dissertation focuses on the major aspects currently governing the future of biocementation for soil improvement, namely, the mechanics of treated materials and the upscaling of the technique. Relative to these optics, a large portion of this research involves experimental campaigns completed at different scales and complementary frameworks. They serve to consolidate findings, contribute to finer quantifications, and expand the spectrum of investigated mechanical responses to novel considerations, relative to more representative stress paths and boundary conditions as those encountered when addressing shallow geotechnical problems. While the central attention of previous literature predominantly focused on the peak response, the work herein, in addition to the peak, covers novel or previously understudied aspects such as the response at lower strain ranges, laterally constrained loading, treatment permanence, creep, large-strain range, critical state, and residual improvement. As such, by means of experimental mechanical tests, microstructural investigations, and constitutive considerations, the physical roots of biocementation, namely densification and bonding, are investigated on the elasticity, peak, softening and critical state. Mirroring bench-scale practices on field sites cannot meet all the upscaling considerations, namely, treatment homogeneity, monitoring control, waste elimination, treatment permanence, and economic competitiveness. Hence, in this work, a state-of-the-art upscaling setup is constructed, and a novel injection approach based on ex situ hydrolysis in a bioreactor is put forward, specifically tailored to address the bio-chemical quality control and the bottleneck of residual ammonia. Overall, this thesis offers new insights and consolidates understandings relative to the mechanical response and upscaling of bio-cemented sands.