Optimization of microbial-induced calcite precipitate–zeolite stabilisation of silt-sand soil for geotechnical application using response surface methodology

Abstract

Microbially-Induced Calcite Precipitation (MICP) is an innovative soil improvement technique in geotechnical engineering, which deals with microbiological activity to enhance soil properties. Studies conducted using the techniques have shown its potentials in geotechnical engineering applications. However, the ammonia generated as by-product during the MICP process limits its broader application. This study aims to optimize the MICP input parameters to minimize the ammonia generation, by incorporating zeolite, using Response Surface Methodology (RSM) for design of experiment. Urease positive bacteria (Lysinibacillus fusiformis) was isolated, characterized and identified to specie level using biochemical and molecular DNA test, which was then used for the MICP treatment. Series of laboratory experiments were conducted, varying pH, zeolite content, bacterial suspension density, cementation reagent concentration, and compactive effort. These input variables were optimized to improve shear strength, reduce permeability, and lower the ammonia production. The results reveals that the preference optimized MICP treatment condition for silt-sand were 3.2E+09cell/mil concentration of Lysinibacillus fusiformis, 0.65mol cementation reagent, 6.4 pH level, 3.9g zeolite and WAS compactive effort and the predicted responses at these optimized conditions were 6.3E-06cm/sec permeability, 22.43kN/m² shear strength and 1.53E-06mol ammonia content and the experimental response values at the optimized condition were 7.2E-06cm/sec permeability, 19.86kN/m² shear strength and 1.68E-06mol ammonia content respectively. This study demonstrated effectiveness of using Lysinibacillus fusiformis as a urease producing bacteria for MICP and the potential of incorporating zeolite into the process, which significantly improves soil strength, decrease permeability, and reduce the production of excess ammonia. Thus, the optimized MICP process with zeolite presents an effective sustainable solution for reducing ammonia, while enhancing the geotechnical properties of silt-sand soil.