Selection and application of bacteria for bioclogging of sandy soil

Abstract

Sand is the fundamental building material for any construction work. Being the most abundant material in the desert, this project explored the possibility of constructing a fully man-made reservoir by microbial geo-techniques. The most probable current technique to construct would be to use chemical grouting methods to bind the loose sand particles together, but these grouts are usually toxic in nature and could not be used to construct a drinking water reservoir. Thus microbial geo-techniques – bioclogging and biocementation, which are environmentally friendly, are explored for this application. Initially, bacteria that are suitable for these two techniques, nitrifying bacteria for bioclogging and urease-producing bacteria for biocementation, are selected and cultivated. Experiments were conducted to determine the suitable conditions for the growth and activities of the bacteria. The bacteria were then applied to sand columns with the appropriate conditions and the treated sand columns were tested for permeability and compressive strength.

The use of excess urea to calcium chloride in the molar ratio of 3:1 for biocementation experiments was shown to maintain a high pH of above 8.5 with at least 80% of calcite formation. The biocemented sand samples showed a decrease in permeability from 1.10 x 10-4 m/s to 2.16 x 10-5 m/s within 6 days. The air-dried biocemented sand samples, which underwent 6 treatments, showed an unconsolidated undrained compressive strength of 0.139MPa compared to 8.42kPa for the bioclogged sand sample. The bioclogged sand sample showed a reduction in hydraulic conductivity from 1.10 x 10-4 m/s to 2.90 x 10-5 m/s, which was comparable to the biocemented samples, within 30 days.

In conclusion, biocementation is a recommended method due to its higher compressive strength and its shorter time frame to reduce the hydraulic conductivity compared to that of the bioclogged sand samples. Biocementation is also a more cost-effective method compared to the chemical grouting methods available today.