Performance of Biocemented Sydney Sand Using Ex Situ Mixing Technique-Article

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The study evaluates ex situ mixing as a method for using microbially induced calcite precipitation (MICP) to strengthen loose Sydney sand. Sand was mixed with urease-producing <em>Bacillus megaterium</em>, urea, and calcium chloride, allowing bacteria-driven calcium carbonate to bond sand grains. The researchers assessed specimens using unconfined compression and drained triaxial tests, bender-element measurements, and 1-g footing models containing a 38-mm-diameter biocemented column.

Biocementation substantially increased soil strength and stiffness. In triaxial tests, biocemented specimens were three to five times stronger than uncemented sand, with strength generally increasing as calcite content rose. Calcite was distributed relatively uniformly through the specimens. Shear-wave velocity also increased during curing and was related to calcite content, suggesting it may help monitor cementation non-destructively.

In the footing tests, cemented columns improved foundation stiffness and reduced settlement. A column made with 15% urea reduced settlement by about 50% in dry sand and 32% in wet sand. Gypsum-cemented columns generally achieved greater maximum strength than biocemented columns, particularly in wet conditions.

Overall, the tests demonstrate that ex situ mixing can produce relatively uniform biocemented soil and improve foundation performance. However, the results are preliminary: the model tests were conducted at low, 1-g stress levels, and further research is needed to establish field-scale performance and assess factors such as mixing conditions, curing time, and temperature.

Keywords

microbially induced calcite precipitation

MICP

biocementation

Sydney sand

Bacillus megaterium

urea

calcium chloride

unconfined compression

triaxial tests

shear-wave velocity