About calcium carbonate precipitation on sand biocementation

• Published in: Engineering geology Vol. 271; p. 105612
• Main Authors: Cardoso, Rafaela, Pedreira, Rita, Duarte, Sofia O.D., Monteiro, Gabriel A.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20.06.2020
• Subjects: Control tests; Grading size distribution; Ground Improvement; MICP; Porosimetry; Sands; Sands; Ground Improvement; Control tests; Grading size distribution; Porosimetry; MICP
• ISSN: 0013-7952; 1872-6917
• DOI: 10.1016/j.enggeo.2020.105612

The contribution of indigenous bacteria and of some possible precipitation due to chemical reactions occurring during the treatment of sandy soils through microbially induced calcite precipitation (MICP) is investigated in this paper considering two different grading size distributions. This is a novelty, because the presence of the feeding solution alone in the soil (usually used as control test, and named here as CICP) is usually not quantified as it is assumed that the total amount of calcium carbonate is formed only due to ureolytic activity of the bacteria added to soils. However precipitation can occur because bacteria naturally exist in soils and also, even if less likely, due to CO2 sequestration in a process called chilled ammonia process. The justification is because the ions necessary for forming calcium carbonate, both by the indigenous bacteria or by CO2 sequestration are supplied in the feeding solution (solution with urea, ammonium and a calcium source). Experimental tests were performed on a set of sand samples subjected to biological treatment using bacteria and another set exposed to chemical treatment using only the feeding solution, after checking that natural bacteria present in the soil would not interfere with the process. The samples were prepared with similar dry density. They were analysed through mercury intrusion porosimetry tests, chemical analysis and unconfined compression strength tests performed after partially drying the samples. Although the hydrolysis of urea made by the added bacteria speeds up the precipitation of calcium carbonate, the results showed that CICP appears to be independent from sand grading size distribution and introduces significant strength increment. For this reason, the treatment will have effect even if the bacteria added die. Nevertheless, it is important to promote adequate conditions for these bacteria to survive because higher strength values were measured in the samples where MICP treatment was done, and therefore the effectiveness of the technique increases in significant manner. •CaCO3 precipitation on sand treatment due to the addition of feeding solution without adding bacteria is investigated.•CICP appears to be independent from sand grading size distribution•CICP introduces significant strength increment but higher values were found adding bacteria.•Because of CICP, the treatment is expected to have effect even if added bacteria die.•Treatment effectiveness increases in significant manner if bacterial activity is promoted.