Biogrout, ground improvement by microbial induced carbonate precipitation

Abstract

Biogrout is a new ground improvement method based on microbially induced precipitation of calcium carbonate (MICP). When supplied with suitable substrates, micro-organisms can catalyze biochemical conversions in the subsurface resulting in precipitation of inorganic minerals, which change the mechanical soil properties. This study focuses on one of these biochemical conversions: microbially catalyzed hydrolysis of urea inducing calcium carbonate precipitation in sand. This Biogrout process comprises the following steps: Sporosarcina pasteurii, a bacterial species containing a large amount of the enzyme urease are cultivated, injected in the ground and supplied with a solution containing urea and calcium chloride. Urease catalyzes the conversion of urea into ammonium and carbonate and the produced carbonate precipitates with calcium as calcium carbonate crystals. These crystals form sticking wedges between the sand grains increasing the strength and stiffness of the sand. The remaining ammonium chloride is extracted and disposed. The thesis comprises the necessary steps to develop this process from a laboratory experiment to a practical application, culminating in an unprecedented 100 m3 field scale experiment in which 40 m3 of sand was biologically cemented within 12 days stretching over a distance of 5 m. Engineering tools are established such as empirical correlations between the CaCO3 content and strength or stiffness, which enable to design treatment procedures for several emphasized applications, such as increasing the stiffness of railroad embankment or improving the stability of limestone room and pillar mines. Some of the remaining issues of this Biogrout process include the required removal of ammonium chloride and the use of axenically cultivated aerobic organisms with consequent decaying urease activity in time due to a lack of oxygen in the subsurface. To avoid both these issues the suitability of other possible MICP processes for ground improvement is evaluated and the potential of the most promising alternative, denitrification, is shown in laboratory experiments.