Addressing the challenges of homogeneity, quality control and waste handling in soil bio-cementation: A large-scale experiment

In an attempt to go beyond the conventional laboratory experiments widely reported in literature around the emerging technique of soil bio-cementation, this work addresses key challenges related to its large-scale application. Precisely, a state-of-the-art installation with a draining bottom boundary is introduced and a novel treatment strategy, based on ex-situ hydrolysis within a 1000 L bioreactor, is described. Hydrolyzed solutions are injected in a tank filled with 0–4 mm sand, via a system of eight injection tubes to treat a total surface of 40 m² across a depth of 2 m. A multilevel, spatial and temporal quality control system is used to monitor the injection processes across several cycles via chemical and hydraulic means. In total, 20.8 m³ of reactant solutions are supplied to the targeted zone, equal to one pore volume and over 120 chemical analyses are carried-out. Reaction efficiencies overall exceeded 80%, while by increasing the number of treatment cycles, and thus calcification levels, a gradual increase in the recorded pressure at the injection inlet was captured, that reached up to 75 kPa. Zones where the injection pressure increased the most are found to yield better resistance in the vicinity of the corresponding injection tube. A dynamic penetrometer campaign reveals that increase in the tip resistance, is found to exceed 5 MPa and yields more homogenous response across the bottom 0.5 m of the tank, which is believed to reflect the effect of initial confinement on the deposition of calcite. For the zones with the highest cementation, correlated φ’ values yield a 5° increase, while the oedometric modulus is found to double. The results suggest that ex-situ bio-cementation, where hydrolysis occurs in bioreactors instead of inside the soil mass, is capable of yielding similar precipitation efficiencies and mechanical improvement compared to traditional bio-cementation, where bacteria are injected directly into the soil. Finally, the monitoring of MICP at the scale of typical geotechnical works is discussed along with the problematic of residual ammonium, which in this study is found to reach absorded quantities of 4 mol/L.