Bio2Cementation: A novel treatment coupling clay aggregation and bio-cementation in sand-bentonite porous media

Abstract

The geo-technical quest to couple technical, environmental, and economic innovation, has increased recent attention towards bio-inspired soil strengthening techniques. This thesis presents a proof-of-concept for a coupled clay inhibition and bio-cementation treatment tested in sand-bentonite, referred to as Bio2Cementation. Fine particles are first aggregated using a nitrogen based compound. By binding the electrical double layer of clay minerals, the aggregates become chemically and physically stable. Thereafter, bio-cementation treatments hydrolyze urea to precipitate calcium carbonate crystals within the pore space. The crystals bind the mineral particles, increasing the strength and stiffness of the soil.
State of the art considerations regarding enzyme induced calcite precipitation and guanidinium hydrochloride research are used as the theoretical foundation for the treatment’s design. A diverse sand-bentonite matrix is tested, comprised of 10% and 30% bentonite, to evaluate the applicability limits of the technique. The implementation is tested in flow-cells, whereby soils are injected with Bio2Cementation treatments. Experiments show the dominant role of guanidine for stabilizing clay particles – the matrix aggregates, hydraulic conductivity improves by two orders of magnitude, surface charge interactions are minimized, and swelling is halted irreversibly. Hydraulic conductivity calcultations, unconfined compressive strength tests, image analysis of Micro-CT scans, and scanning electron microscope imaging evidence to notably improved bio-cementation following guanidine injections. Consistently, the optimized enzyme induced calcite precipitation was found to crystallize vigorously in 10% bentonite samples, but less successfully in optimally compacted 30% bentonite soils. The concept of Bio2Cementation is proven to work within certain limitations.
Future research should explore the role of different clay minerals such as kaolinite, in view of better defining the treatment’s engineering applicability in-situ. Additional strength testing, soil-structure interaction analyses and environmental impact studies are also recommended