Mechanical Properties of Piles Formed by Microbially Induced Carbonate Precipitation: Experimental Investigation and Numerical Simulation

Abstract

Microbially induced carbonate precipitation (MICP) utilizing a urease active bioslurry is an ecofriendly method that can improve soil strength. However, the micromechanisms, such as ion diffusion, production rate of CaCO3, porosity, and permeability of pile reinforced by bioslurry, require further investigation. In this study, both biopile model tests and a coupled fluid-flow, solute transport and biochemical reactive model were conducted to analyze the mechanical property and biocementation mechanism of pile formed by urease active bioslurry. Results showed that the simulated CaCO3 content along the biopile length after 120 h grouting was close to test results. The UCS of the biopile decreased from 3.44 MPa to 0.88 MPa and the CaCO3 content decreased from 13.5% to 9.1% with increasing depth. The largest reduction in CaCO3 content was observed in the middle part of the biopile as the CaCO3 crystals in the upper part hindered the downward transport of the cementation solution. The morphology of CaCO3 crystals was influenced by cementation solution concentration, as evidenced by the predominance of spherical vaterite crystals in the upper part of the biopile and rhomboidal calcite crystals in the middle and lower parts. During the grouting process, the concentration of calcium ions and urea decreased, while the ammonium ion levels increased with depth due to the utilization of calcium ions and urea for CaCO3 precipitation and ammonium ion production. The production rate of CaCO3 first increased rapidly to reach a peak value and then decreased. The porosity and permeability demonstrated both linear and nonlinear decreasing trends as the CaCO3 concentration increased. The largest reduction in porosity and permeability, reaching 20% and 58% in the biopile top.