Fixation and distribution of bacterial activity in sand to induce carbonate precipitation for ground reinforcement

The mechanical properties of soil (cohesion, friction, stiffness and permeability) are important parameters for engineering constructions and ecosystems in sedimentary environments. BioGrout is an in situ soil strengthening technique involving microbial-induced carbonate precipitation (MICP). This process involves hydrolysis of urea by bacteria containing the enzyme urease in the presence of dissolved calcium ions, resulting in calcium carbonate precipitation. In order to control the BioGrout process for engineering applications, it is necessary to improve understanding of the relevant phenomena and develop efficiencies to enable up-scaling of the technology to suit commercial applications. Control of a homogeneous distribution of bacterial activity in a sand bed is considered crucial in order to prevent clogging during injection and provide homogeneous reinforcement results. This paper describes a methodology to distribute and fix bacteria (with their enzyme activity) relatively homogeneously in a sand bed, before supplying cementation reagents. The methodology is based on a two-phase injection procedure: a bacterial suspension is injected into the sand body, immediately followed by a fixation fluid (i.e. a solution with high salt content). It is proposed that bacteria are retarded by adsorption and filtration processes and are permanently adsorbed to the sand grains when overtaken by the fixation fluid. The presented experimental approach for optimizing bacterial fixation in porous media can be used as a tool to design the treatment protocol for engineering applications in practice.