683038 event 1751987952 1751987982 School of Civil and Environmental Engineering

Ph.D. Thesis Defense Announcement


Influence of Micro-scale Mechanisms on Macro-scale Properties of Bio-cemented Soils

By Shaivan Shivaprakash

Advisor:

Dr. Susan E. Burns

Committee Members: Dr. Sheng Dai (CEE), Dr. Xing Xie (CEE),
Dr. David Frost (CEE), Dr. Yuanzhi Tang (EAS)

Date and Time: July, 21, 2025. 11:00 – 2:00 PM EST

Location: SEB 122

Due to its versatile applications, microbially induced carbonate precipitation (MICP) via
urea hydrolysis has gained considerable interest in diverse fields, ranging from soil sciences to
applied microbiology, to geomicrobiology to civil and environmental engineering. The objective
of this thesis is to elucidate the role of micro-scale mechanisms on the macro-scale behavior of
bio-cemented soils, and the overarching goal of this research is to address the growing need for
sustainable ground improvement techniques in geotechnical engineering.
A series of bench scale column experiments were performed to investigate the impacts of micro-scale factors on the implementation of MICP under a range of experimental conditions. The
scope of the MICP technique was expanded beyond traditionally tested silica sands to include the
carbonate-rich soils to improve the versatility and robustness of the technique, which showed that
the pre-existing carbonate soil particles served as preferential sites for calcite crystal growth due
to their lower energy barrier compared to nucleation on silica particles. These findings were further
validated through MICP experiments on artificially graded carbonate-sand mixtures, where similar
calcite morphologies were observed on Iceland Spar (pure calcium carbonate) particles. A novel
experimental setup was introduced in this research to capture the effect of non-uniform
cementation on the evolution of shear-wave velocity of the bio-cemented column. The testing
setup successfully captured the spatial variation in Vs with treatment depth, as well as the average
stiffness of the entire bio-cemented column, overcoming the limitations of previous studies and
provided quantifiable improvements in geotechnical engineering properties with treatment depth
as a result of bio-cementation.
Additionally, micro-scale visualization performed on biologically preserved bio-cemented
specimens showed bacterial cells entombed within calcite crystals and serving as nucleation sites,
growth of individual bacterial cells and colonies predominantly near the precipitated calcite, and
formation of bacterial chains connecting different calcite crystals together resulting in the
formation of cementation bonds between soil particles. These findings provided direct evidence
for key hypothesized mechanisms in the MICP process and offer a more comprehensive, holistic
understanding of the physicochemical and biological interactions that drive soil bio-cementation. Micro-scale observations of precipitated calcite morphologies and cementation bonds
across all tested soils revealed three primary mechanisms responsible for increase in shear-wave
velocity: contact cementation, particle-coating, and pore filling and matrix cementation. These
mechanisms were shown to be influenced by the physical properties of the soil, including particle
size, shape, mineralogy, and specific surface area (SSA). Importantly, the traditional linear
relationship between Vs and calcite content, widely used in the literature, failed to account for the
observed variability across these different soil types and morphology. To address this limitation,
Vs was related to normalized calcite content, defined as calcite content divided by the specific
surface area of the soil. The model demonstrated improved performance in capturing the variation
in Vs across both the experimental dataset of this study and a broader literature dataset. This model
was found to be generalizable across a wide range of soils, including those with varying particle
sizes, mineralogy, and morphological characteristics, thereby providing a more robust framework
for predicting the stiffness of bio-cemented soils during the MICP process.

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