Comparison of experimental techniques for biocementation of sands considering homogeneous volume distribution of precipitated calcium carbonate

¹ IST, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisbon, Portugal
² INESC-MN, R Alves Redol, 9, 1000-029 Lisboa Portugal
³ IBB, Av. Rovisco Pais, 1 1049-001 Lisbon, Portugal
⁴ CERIS, Av. Rovisco Pais, 1 1049-001 Lisbon, Portugal

^* Corresponding author: rafaela@civil.ist.utl.pt

Abstract

Microbially induced carbonate precipitation (MICP), or biocementation, consists in using microorganisms living in the soil to produce calcium carbonate (biocement). This mineral bonds the grains and therefore improves the soil hydro-mechanical properties. When using this technique, one of the challenges is to ensure homogeneous treatment in the entire volume. In this study, an experimental device was developed to apply this treatment in cylindrical soil samples with 7.2 cm diameter and 12 cm height. Two distinct sample preparation techniques were tested: (i) pre-mixing the soil with bacteria, and then inject the feeding solution; (ii) inject bacteria followed by injecting the feeding solution. In both, the injection conditions varied in two distinct ways: (i) infiltration column, from the top and (ii) injecting through a perforated central tube. The homogeneity of the biocement in the volume was evaluated using X-ray and SEM images from small samples taken from different locations in the specimens and analysing different parameters. Mercury intrusion porosimetry (MIP) and CaCO₃ dissolution tests revealed uneven distribution of CaCO₃ content between the top and bottom sections, as well as along radial direction. The most homogeneous samples were found when bacteria were premixed with the soil before injecting the feeding solution. Unconfined compression tests (UCS) were also performed in samples with and without treatment. The treatment increased stiffness and strength significantly and soil rupture occurred mostly near the bottom, where the lowest CaCO₃ contents were detected.