Uniformity evaluation and improvement technology of sandy clayey purple soil enhanced through microbially-induced calcite precipitation

Abstract

In order to improve the uniformity of calcite precipitation and engineering practicability, a series of tests using bacillus megaterium (BNCC 336739) were conducted to enhance sandy clayey purple soil, with different concentration bacterial solution and cementation reagent flowing to the samples perforated in the center with different grouting speed. Based on the mineral component (XRD) and soil microstructure (SEM), cementation mechanism was analyzed. Based on measurement of CaCO₃ production and unconfined compressive strength tests, the influence law of grouting factors on CaCO₃ production amount (C), CaCO₃ uniformity (s), CaCO₃ deposition rate (P), unconfined compressive strength (UCS) and stiffness (elastic secant modulus E₅₀) were analyzed and the correlation between C, s and UCS, E₅₀ were analyzed. The results show that the uniformity can be improved by perforation grouting, and the UCS and E₅₀ of samples treated by MICP increased by 105.58% and 464.14%. The CaCO₃ induced by bacillus megaterium are 1–5 µm calcite crystal, which cemented and wrapped soil particles. The higher the concentration of bacteria solution and cementation reagent and the slower the grouting speed are, the bigger the C and the s. The C has a lower threshold of 2.5% and an upper threshold of 5%, the UCS of samples treated by MICP significantly increases with the increase of C in the interval, and the UCS growth becomes slow or even negative outside the interval. The smaller the s is, the bigger the UCS and E₅₀ are, and this effect is small when C< 4% and is significant when C> 4%. With the effect of s, the UCS and E₅₀ of sample treated by MICP increase with different speed and then reduced as the increase of C. It provides scientific reference for the application of MICP technology in purple soil area.