An experimental study of coral sand enhanced through microbially-induced precipitation of calcium carbonate

By injecting Pasteur bacillus suspension and CaCl₂-urea solution, the coral sand was solidified through the microbially-induced precipitation of calcium carbonate. Then a series of experiments including the permeability, strength and microstructure tests was conducted on the solidified soil. It is shown that the bacteria activity of Pasteur bacillus decreases slowly with time elapsing, which satisfies the requirement of the improving coral sand. Due to the bio-mediated improvement, the permeability of coral sand pillars decreases by 1 to 2 orders in magnitude. The stress-strain curve of improved coral sand pillars can be divided into 3 stages: stress increasing slowly with strain, stress increasing fast with strain and stress suddenly falling with strain. The unconfined compressive strength of specimen is up to 14 MPa before specimens being compressed into brittle failure. The compressive strength increases with the increase of dry density and the decrease of permeability. Unlike the silica sand improved by microbe, the coral sand improved by the current technology has a structure that the calcium carbonate precipitates wholly around coral sand particles. Hence the bonding between two particles by calcium carbonate is relatively weak, which properly explains the insignificant decrease of permeability.