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Last but not the least, future experiments might be
performed on samples having a different degree of saturation and initial
density to check the efficacy and accuracy of the proposed study. Furthermore,
other different tests like triaxial test etc. may be employed to check the
behavior of treated soil under different mechanical and hydraulic conditions.
Stabilization using MICP is very cost effective compared to other conventional
techniques and a very green, sustainable and eco-friendly technique which
promises a great future.
Proper field testing to confirm lab results
Slower microbial process
Limit of aerobic bacteria is not effective in deep soils
Excessive ammonia production beyond human safety
While this study might be extensive, it certainly is bound
by its scope but it would serve as a benchmark for future studies of MICP on
fine-grained soils. Problems with MICP will still be present for future
research to be carried out upon such as:
This study would be the first of its nature to study the
distribution of bacterial solution through the sample and an attempt at
achieving homogenous treated sample. Fine-grained soils will be investigated
and how their properties respond to microbial treatment, which in turn would
decrease its various problems, would be presented. Bacterial and cementation
reagent concentration would be altered as well to find the average trend and
put forward an optimum content of both. All of these observations are either not
looked into or literature regarding them is very little and hence this study
serves as a foundation for future work in this regard.
For phase-2, the bacterial solution would be injected into
the sample, which should probably a column or a large tank, and analyze the
distribution of the bacterial solution along the sample. According to Whiffin
(2007), balancing the rate of urea hydrolysis in the column with the delivery
of reactants via the flow rate is essential to precipitate calcium carbonate at
locations where required. In order to produce a more homogeneous result, the
balance between supply and conversion needs be shifted. For example, faster
flow rates will move the cementation reactants further into the column allowing
less time for reaction along the path, and similarly lower conversion rates
will leave more reactants in the fluid, also resulting in further infiltration
distances. The precipitation amount will be measured by the increase in PH and
calcium carbonate content throughout the sample. The whole sample will be
divided into smaller (equal) samples for lab testing to check the homogeneity.
Treated samples and untreated samples will be exposed to
Direct Shear and 1-D Consolidation tests. Graphical and analytical comparisons
would be drawn between both types of samples for their differences in shear
strength and settlement behaviors. The coefficient of compression (Cc), the
coefficient of compressibility (mv) and Coefficient of Recompression (Cr)
values would be calculated from oedometer tests to predict the primary
consolidation and compressibility potential of both samples. To determine the
shrink-swell potential, the coefficient of linear extensibility (COLE), liquid
limit (LL), Plasticity Index (PI) and Free Swell Index would be determined for
both treated and untreated samples and collectively a comparison will be done.
This study will be divided into two phases: the first phase
will check the effect of MICP on the properties of soil while the second phase
would aim to achieve a homogeneous distribution of calcium carbonate throughout
the soil. For the purpose of the phase-1, soil samples will be collected and
divided into two groups; treated and untreated samples. Bacteria will be
cultured in the lab so as to promote the growth of its colonies while the
cementation reagent would consist of Urea, Calcium Chloride and Nutrient broth.
Furthermore, the bacterial concentration would be kept constant and content of
cementation reagent would be altered. This would be the first set of treated
samples, while the other set would have constant cementation reagent and
bacterial content would be changed. The effect of both the bacterial and
cementation reagent content on the treatment would be analyzed and an optimum
amount of both would be found.
Since microbial ground improvement is a recent and new method
of altering soil properties, therefore it still has a lot of room for
improvement. The center of research in this field has always been sandy soils,
to bring about cohesion in them and reduce their permeability, and fine soils
have not been researched enough for MICP. The major obstacle for Geotechnical
engineers over the past years has been the large-scale application of MICP. In
all the studies conducted, homogenous distribution of the bacterial content was
not achieved in the samples. There is a primary restriction on the transport of
microbes in soil matrix because of the size of pore throats makes bio-grout
treatment not deeper in soils (Kumari 2017). Also, urease-producing bacteria in
bio-grout development are aerobic which means it cannot precipitate carbonates
deep in the ground due to the absence of oxygen and a weak precipitate is
formed. There is also limited research carried out on fine-grained soils, which
properly documents the comparison of treated and untreated samples and comments
on whether MICP could be as effective in clayey soils as it is in sandy soils.
Another issue with urease-driven MICP is excessive production of ammonia beyond
safety threshold (Yu et al. 2015). To avoid this problem, asparaginase-based
MICP process produces less ammonia, as reported by Li et al. (2015), however
further research regarding this is needed.
Gap in Literature:
Some studies conducted on fine-grained soils include Saffari
et al (2017) in which fine swelling soil was treated with different
concentrations of MICP. The results revealed that the biological treatment
increased the soil peak internal friction angle by 14%, while the soil cohesion
was almost tripled for the sample treated by the bacterial solution with
optical density (OD) of 2.3 (Saffari et al 2017). Compressibility and shear
strength of a fine-grained organic soil was tested before and after microbial
treatment, which showed a 20% increase in the carbonate content of the organic
soil (Hanifi, Waleed, Ibrahim 2015). Shear Strength was also reported to have
increased as well as the compressibility decreased.
In order to evaluate the potential MICP, a 5-m sand column
was treated with bacteria and cementing reagents under conditions that
simulated field conditions (Whiffin, van Paassen, and Harkes 2007). To evaluate
the potential of bio-grout for field applications, it was scaled up for a
larger sandbox of size 100 m3 where sand was treated and homogenous treatment
was attempted. The results concluded with turning sand into bio-sandstone by
bio-grout with UCS as high as 12 MPa (van Paassen et al. 2009). As high as UCS
value of 34 MPa was reported in soil while using bio-grout at different MICP
treatments (Whiffin 2004). Unconfined compressive strength and permeability
tests were performed on sand samples and MICP treatment increased the strength
of the treated samples (Al Qabany and Soga 2013). The magnitude of this
increase depended on the concentration used in the treatment. The use of a
high-urea calcium chloride concentration solution resulted in a rapid drop in
permeability, whereas the use of a low-chemical-concentration solution was
found to result in a more gradual and uniform decrease in permeability.
Bacillus pasteurii, now reclassified as Sporosarcina
pasteuriis, a highly urease active bacteria, plays an important role in CaCO3
precipitation (Bang et al 2001, Dejong et al 2006, Dejong et al 2010). The
bacteria along with CaCl2, Urea and nutrient broth are injected into the soil
as a solution which produces crystals of calcium carbonate in the soil matrix
by urea hydrolysis and cements the soil.
Previous studies regarding this method have been majorly conducted on
These drawbacks of conventional techniques provide enough
reasons to look for an alternative method which guarantees cost effectiveness
and environmental safety along with a greater depth of improvement and not
interfering with natural groundwater flow. Microbial calcite precipitation has
recently been a center of research for geotechnical engineers as it is not
environmentally damaging and is cost-effective in achieving desired qualities
of soil. Also, Whiffin (2007) demonstrated improvement of the load-bearing
capacity of the soil without making the soil impermeable to fluids.
Current Methods of ground improvement include densification,
solidification, dewatering, replacement, mechanical compaction and
cement/chemical grouting. All of these methods of improvement are either costly
or environmentally unfriendly. The cost of chemical-based soil grouting is
estimated between $2 and $72 per cubic meter of soil, while the costs of the
raw materials for the bio-grouting could be in the range from $0.5 to $9.0 per
m3 of soil in cases when the waste materials are used as carbon source for
microbial growth (Ivanov and Chu 2008). Bio-grout is also calculated as cheaper
material over conventional grout in another study (Suer et al. 2009). These traditional measures are unsuitable for
the treatment of large amount of soils due to their limitation of the zone of
influence or due to the high viscosity or short hardening time of the injected
grouts. Furthermore, these techniques significantly lower the permeability of
the soil that prevents groundwater flow and limits the injection distance,
making large-scale treatment unfeasible (van Paassen et al. 2009).
In this study, the effects of bacterial calcite
precipitation on the settlement, bearing capacity and swelling potential of a
fine-grained expansive soil will be studied. The degree of homogeneity of
calcite precipitation throughout the sample would also be studied as it is a
major concern for the application of this method in the actual field. In
previous studies, it has been attempted to check the depth of improvement by
this method (Whiffin 2007) but not achieved yet. Shear strength of a
fine-grained soil has also been successfully increased with MICP (Safari et al.
2017) and organic soil had its compressibility coefficient, as well as the
magnitude of its primary consolidation, decreased (Canakci, Sidik and Kilic
2015). Research done on finer soils is still very limited and a proper
comparison of the changes, the bacterial precipitation brings about in the
treated soil with respect to the untreated soil will be presented in this
study. The aim of this research is to look into a cost-effective and
eco-friendly method of improving the shear strength, decreasing settlement and
swelling of a fine swelling soil by MICP. Percentage of Bacterial contents
added to the soil will be varied in order to find the optimum microbial
content. Furthermore, an attempt will be made to develop a method of
application of MICP on a larger scale, analyze the depth of improvement and
attempt at achieving a homogenous distribution of microbial treatment.
Aims and Objectives:
These problems are majorly associated with clayey soils and
therefore microbial calcite precipitation would be used to remediate these
problems. Another problem discussed in this study would be the uneven
distribution of calcium carbonate which is precipitated by bacterial action.
This results in an uneven strength achievement in the sample which is not
feasible in field application.
Swelling of soil is defined as an increase in the volume of
soil when it is exposed to moisture and a soil shrinks when it dries out.
Shrink-swell nature of soil can cause differential settlement, ground heave and
foundation cracks. These type of soils are known as Expansive soils such as
montmorillonite and bentonite or soils which have a greater fraction of these
will exhibit such behavior. Estimates of the total cost of damage due to
swelling soils, one of the least publicized geologic hazards, were estimated at
$2 to $7 billion in the U.S. in 1987 (Jones and Jones, 1987) and can be
considered to be as much as twice today.
Under loads, all soils will settle, causing settlement of
structures founded on or within them. Soil Settlement is defined as the
decrease in the volume of a soil mass, when a load is applied on it, due to the
expulsion of air and water from the voids. If the settlement is not kept to a
tolerable limit, the desired use of the structure may be impaired and the
design life of the structure may be reduced. Structures may settle uniformly or
non-uniformly. The latter condition is called differential settlement and is
often the crucial design consideration (Budhu, M. 2006).
Statement of Problem:
Treating sandy soil using a microbially induced calcite
precipitation (MICP) process has been studied substantially in the past years.
However, it is still a challenge to apply this approach to treat fine-grained.
This research would explore the application of microbially induced calcite
precipitation (MICP) method, which is used to produce calcium carbonate
crystals in the soil matrix by urea hydrolysis using a urease-producing
bacteria, to mitigate problems associated with fine-grained such as excessive
settlements, low bearing capacity and high shrink-swell potential. In past
studies, MICP has been used to improve the properties of different soils with
successful results but background literature of the effects of MICP on
fine-grained soils is still very limited and thus the objective of this
research is to provide research results related to that based on laboratory
Current Geotechnical measures to improve soil properties
through mechanical or chemical means provide satisfactory results but these are
very costly and are not environment-friendly. Therefore, soil stabilization by
the use of bacteria/microbes has recently emerged as a new method for altering
soil properties. Biomineralization has been proven to be environmentally
friendly and cost-effective method of improving the properties of soil
according to the need of the project. Bio-mineralization is the process where
micro-organisms produce minerals mainly carbonate products that provide a basis
to develop Bio-grout (Kumari 2017). Changes made by this precipitation to the
soil could improve the mechanical properties (strength, stiffness, cohesion,
friction), decrease the permeability and modify the strength properties of soil
(Whiffin, van Paassen, and Harkes 2007; DeJong et al. 2010).
Ground Improvement deals with the modification of soil
properties so that it meets the performance and strength requirements of the
structure that is to be built upon it. The rapid shift of population towards
urban areas demand a great deal of construction and civil infrastructure to
accommodate the general public. This makes it compulsory to have improved soil
conditions in order to avoid any financial or loss of life due to excessive
deformations in the soil.