ENGINEERING BEGAVIOUR OF MICROBIALLY CEMENTED SOILS USING SPOROSARCINA PASTEURII

Abstract

Microbial induced calcite precipitation (MICP) is a novel biomediated ground improvement method that can bind the sand grains together and improve the engineering properties of soil in a sustainable and environmentally-friendly method. The present study investigated the feasibility of using the MICP method in soil treatment of the Yamuna river basin soil to increase soil shear strength and stiffness while reducing permeability by the induced calcium carbonate precipitates as calcites. Sporosarcina pasteurii, a common alkalophilic soil bacterium with a high urease activity, was used to facilitate the biochemical reaction that induced CaCO3 precipitation. The soil treatment was carried out using a bacterial solution (BS) containing bacterial cells and a cementation reagent solution (CRS) containing urea and CaCl2 as nitrogen and calcium sources. Treatment was carried out with different molar concentrations of 0.25 M, 0.5 M, 0.66 M, and 0.75 M CRS. The optimum concentration for effective soil treatment was 0.66 M CRS for cost-effectiveness and to avoid wastage of chemical reagents. The improvement in soil strength of the biocemented sand was determined by unconfined compressive strength and direct shear tests and the effect of clogging by xix permeability tests. In addition, a significant increase in the grain sizes was observed in the biocemented sand. Higher CaCO3 content with more mineral precipitation and higher strength was observed in the various biocemented sand. The improvements in UCS of the various biocemented sand varied from 0.55–2.2 MPa with approximately 4–11% CaCO3 content. The CaCO3 content in the MICP-treated specimens is comparable to 5–15% CaCO3 content in the natural cave soils, stones, and rock masses. The 14 days MICP treated biocemented sand have achieved more calcite content and higher strength than those treated for prolonged durations due to washing away of detached calcites and decementation. The strength improvement in the biocemented sand treated with sterile and non-sterile treatment solutions was also determined. The UCS achieved was 835 kPa in biocemented sand treated with non-sterile treatment solution prepared with tap water has indicated its use for typical field conditions by skipping the sterilization process. The cohesion and angle of internal friction of the various biocemented specimens varied from 28–34 kPa and 9–10°, respectively, confirming the strength development in the treated biocemented specimens. The effectiveness of fine content in the biocemented sand was confirmed by the higher calcite content of 14-15% and UCS of ~ 850 kPa than those without fine contents. The permeability tests confirmed the bioclogging of sand pores by the calcite crystals, which were reduced to three order-of-magnitude from the initial day at 0.6x10-4 m/s to 1.1 x 10-7 m/s on the 7th day of treatment which is acceptable to control leakage for aquaculture ponds. In addition, the formation of thin water impermeable biocemented crust layers of all the biocemented sands may benefit in mitigation of surface erosion and seepage from water bodies. xx The scanning electron microscope (SEM) images confirmed the presence of microbial beds in the biocemented sands from the imprints left by the bacteria as hollow spaces on the calcite crystals surfaces. In addition, the presence of biocementation and bioclogging between the sand grains by the calcite crystals and the variation in calcite distribution on the bacteria and sand surfaces were viewed in the images. Lastly, the permeability retention in the biocemented sand, unlike the cement grouting, is advantageous for applying more treatment solutions to control further strength enhancement.