Biomediated soil improvement in the mitigation of liquefiable sandy soil

Abstract

Soil liquefaction is one of the catastrophic effects that result from earthquakes. It is a phenomenon that occurs when loose, saturated, cohesionless soil loses its strength and stiffness as a result of rapid loading. Several techniques have been employed to mitigate the effects of soil liquefaction. However, these techniques either require high energy for its execution, or the chemical admixtures used may have adverse effects on the environment. Consequently, biocementation via microbial induced carbonate precipitation (MICP) and enzyme induced carbonate precipitation (EICP) was explored as a technique to mitigate soil liquefaction. The bacterial strain used in the MICP process was Bacillus megaterium. Meanwhile, a plant-derived urease enzyme was used in EICP. In this study, experimental based research was conducted to examine the feasibility of biocementation in the mitigation of liquefaction in sandy soil. The research is divided into three main phases. The first phase examines the effect of environmental factors (pH, temperature and salt content) on the growth of B. megaterium. Test tube tests were conducted to determine the amount of calcium carbonate (CaCO3) precipitates at different cementation reagent concentrations. Based on the test tube test’s results, the EICP method of treatment was adopted to continue with the second and third study phases, due to the amount of calcite produced in the process. The second phase evaluates the effectiveness of EICP treatment on sandy soil through a series of unconfined compressive strength (UCS) tests. The effects of factors, such as curing temperature (4, 10, 20, 30, 40 and 50ºC), the concentration of cementation reagent (0.25, 0.5, 0.75, 1.0 and 1.25 M), number of treatment cycles (1, 2 and 3 cycles) and relative density (loose, medium and dense), palm oil fuel ash (POFA) content were examined on the biocemented soil. The third phase evaluates the effect of biocementation on the cyclic resistance of sandy soil, in terms of confining pressure, Cyclic Stress Ratio (CSR) and relative density, through a series of cyclic triaxial tests. The liquefaction potential of treated soils was investigated with respect to the development of excess pore pressure. The optimum environmental growth conditions, in terms of pH, temperature and salt content, were pH 7, 30°C and 1% (w/v) NaCl, respectively. Findings from the test tube tests showed the mass calcium carbonate precipitate increased when the concentration of cementation reagent (CCR) was increased from 0.5-1.0 M; irrespective of the curing period for both MICP and EICP. Findings from the UCS tests showed a linear relationship between UCS values at various cementation reagent concentrations and average calcium carbonate content. Furthermore, the strength of biocemented sandy soil was attributed to not only the calcite content formed within the soil but also the extent of soil density. The increase in cycles of treatment via surface percolation led to higher strength and CaCO3 content, irrespective of CCR. Image analysis, using Image J software, confirms the reduction in the area of pore spaces within the SEM images, with an increase in the number of cycles of treatment. The addition of POFA to the biocemented soil helped in reducing the ammonium content released. Results from the cyclic triaxial test showed that the EICP treatment improved the sand’s resistance against the generation of pore water pressure, as indicated by the greater number of cycles required to induce liquefaction. It can be concluded that biocementation via EICP can be an effective method of mitigating liquefaction in sandy soil.