Geotechnical properties of biocement treated sand and clay

The application of microbial technologies to improve the mechanical properties of soils is a new and fast developing research area in geotechnical engineering. Microbially induced calcium carbonate precipitation (MICP) is the most commonly employed biocementation method. Application of MICP for sand has been studied by many researchers. However, few studies had been carried out for fine-grained soils such as clay or other types of low permeability materials. In this research project, optimization of biocementation in sand and feasibility of using biocement to improve the mechanical properties of clayey soil were studied. Element tests using small cylindrical samples as well as model tests using soil of up to one cubic meter in volume were carried out. Different chemical compositions and types of bacteria were tested for both sand and clay. The properties of the MICP treated soil were assessed by unconfined compression tests, triaxial tests, direct simple shear tests, and flexible wall permeability tests using a triaxial cell. The results show that biocementation using urease producing bacteria (UPB) with low activity is effective for both small samples and relatively large samples in the model tests. Results of consolidated drained (CD) triaxial tests on sand with different degrees of biocementation treatment indicate that the strength gained through bio-treatment is related mainly to the increase in cohesion provided by the biocementation effect. In addition to increasing the shear strength, the permeability of sand is also reduced by bioclogging. One method is to form a calcite crust of 2 to 3 mm thick on the top surface of sand. This method can reduce the coefficient of permeability of sand from 10 -4 m/s to 10 -8 m/s. The possible applications of MICP to fine grained soils including kaolin, marine clay and bentonite were explored. The experiments show that a higher shear strength was observed for clayey soil mixed with UPB and cementation reagents compared to pure soil under the same water content. However, excess calcium cation used for the MICP process may impede the strength improvement. Another possible method is to make clay balls through premixing with bacteria and cementation reagents.