precipitation (MICP) for soil stabilization

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State-of-the-art review of biocementation by microbially induced calcite
precipitation (MICP) for soil stabilization
Article in Geomicrobiology · August 2016
DOI: 10.1080/01490451.2016.1225866
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State-of-the-Art Review of Biocementation by Microbially Induced Calcite
Precipitation (MICP) for Soil Stabilization
Donovan Mujah , Mohamed A. Shahin, and Liang Cheng
Department of Civil Engineering, Curtin University, Perth, WA, Australia
ARTICLE HISTORY
Received December 2015
Accepted August 2016
ABSTRACT
Biocementation is a recently developed new branch in geotechnical engineering that deals with the
application of microbiological activity to improve the engineering properties of soils. One of the most
commonly adopted processes to achieve soil biocementation is through microbially induced calcite
precipitation (MICP). This technique utilizes the metabolic pathways of bacteria to form calcite (CaCO3)
that binds the soil particles together, leading to increased soil strength and stiffness. This paper presents a
review of the use of MICP for soil improvement and discusses the treatment process including the primary
components involved and major affecting factors. Envisioned applications, potential advantages and
limitations of MICP for soil improvement are also presented and discussed. Finally, the primary challenges
that lay ahead for the future research (i.e. treatment optimization, upscaling for in situ implementation
and self-healing of biotreated soils) are briefly discussed.
KEYWORDS
Bacteria; calcite; microbially
induced calcite precipitation
(MICP); soil biocementation;
soil improvement
Introduction
The multidisciplinary research between geotechnical engineers
and microbiologists has paved a way into a new frontier of
knowledge called geobiology. This realization came to provide
engineers opportunities to consider soil as a living ecosystem
rather than an inert construction material. The use of microorganisms as a potential catalyst in soil biocementation was first
suggested by Whiffin (2004) and Mitchell and Santamarina
(2005). Since then, countless research has advanced considerably high in this field.
Although DeJong et al. (2013) provided a comprehensive
review regarding the emerging biogeochemical processes and
their potential geotechnical applications, the current paper is
focused on the history and state-of-the-art of soil improvement
using biocementation through microbially induced calcite precipitation (MICP). It specifically discusses the factors affecting
the effectiveness of MICP in soil improvement and compares
the projected cost incurred by MICP compared to other conventional techniques for field applications. Moreover, envisioned applications, potential advantages and limitations of
MICP in soil improvement are presented and addressed. The
current challenges and future work on biocementation by
MICP are also outlined.
Current soil improvement practice
The current practice in soil improvement includes the use of
the following techniques: (1) addition of natural and synthetic
materials such as recycled glass fibers, tires, fruit brunches,
jutes, polypropylene, polyester and geosynthetics (Ahmad et al.
2012; Mujah 2016; Mujah et al. 2013, 2015); (2) injection of
chemical grouting or deep mixing using cement (Bahmani
et al. 2014; Kamei et al. 2013) and/or lime (Ciancio et al. 2014;
Di Sante et al. 2015); and (3) application of sand columns
(Dash and Bora 2013; Deb et al. 2011) or stone columns (Castro and Sagaseta 2011; Zhang et al. 2013). However, the majority of these techniques are dependent on mechanical or
man-made materials, which requires substantial energy for
their production or installation. Out of these techniques, chemical grouting is the most commonly used soil improvement
method. However, this technique is often costly and requires
many injection wells for treating large volumes due to the high
viscosity or fast hardening rate of the injected fluids. Furthermore, chemical grouting significantly reduces the permeability
of treated soils, which hinders groundwater flow and limits
long distance injection, making the large-scale treatment unfeasible. Moreover, chemical grouting increases the pH of groundwater to highly alkaline levels and thus can cause serious
environmental problems and contribute to ecosystem
disturbance.
The abovementioned limitations in the current practice of
soil improvement necessitate exploration of new alternative
technologies that should be environmentally-friendly and sustainable, and able to fulfill the increasing demands for ground
improvement especially for civil engineering infrastructure
developments. Hence, new exciting opportunities for utilization
of biological processes have been recently proposed, which have
been made possible through interdisciplinary research at the
confluence of microbiology, geochemistry and geotechnical
engineering. This field of study is relatively new and many years
of exciting research lie ahead to fully optimize the biological
CONTACT Donovan Mujah [email protected] Department of Civil Engineering, Curtin University, Perth, WA 6102, Australia.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ugmb.
© 2016 Taylor & Francis Group, LLC
GEOMICROBIOLOGY JOURNAL
http://dx.doi.org/10.1080/01490451.2016.1225866
technology through laboratory scale experiments, full in situ
implementation and reliable monitoring of performance in
real-life situations as well as commercialization of the product
to meet society needs (Parmar and Singh 2014). In this paper,
one of the most emerging and promising biological soil
improvement techniques, i.e. microbially induced calcite precipitation (MICP) or biocementation, is reviewed and discussed
in some detail.
Microbially induced calcite precipitation (MICP)
MICP is a biologically driven calcium carbonate (calcite or
CaCO3) precipitation technology, which includes the following
two mechanisms of biologically controlled and biologically
induced CaCO3 precipitation. In the biologically controlled
mechanism, the organism controls the nucleation and growth
of the mineral particles, and independently synthesizes minerals in a form that is unique to the species regardless of the environmental conditions. Examples of biologically controlled
mineralization are presented by Barabesi et al. (2007), which
showed that CaCO3 mineralization was achieved molecularly
using Bacillus subtilis. On the other hand, in the biologically
induced mechanism, production of CaCO3 is somewhat dependent on the environmental conditions (Barabesi et al. 2007; De
Muynck et al. 2010).
Microorganisms used in MICP process
Bacillus pasteurii or Sporosarcina pasteurii are the most preferred bacteria reported in the literature for the MICP process
because of their ability to produce high amount of precipitates
within a short period of time due to their high urease activity
(Bang et al. 2001; Dhami et al. 2013; Wei et al. 2015). S. pasteurii is a type of aerobic bacteria that is ubiquitous in soil and
able to hydrolyze urea to produce carbonate by the generation
of adenosine triphosphate (ATP) through the secretion of urease enzyme (Ivanov and Chu 2008). The urease activity of pure
ureolytic bacteria culture proposed in the literature for ground
improvement ranges from 4 to 50 mM urea/min (Al-Thawadi
2011; Burbank et al. 2012; Whiffin et al. 2007).
So far, the demand to obtain a reliable and constantly high
urease activity of urease as reported in the literature is only
achieved by cultivating pure ureolytic bacteria strain under
sterile conditions to prevent any contamination and growth of
urease-negative bacteria in the culture (Al-Thawadi 2008).
Therefore, the production of highly active ureolytic bacteria
represents a major cost factor of MICP application (Cheng and
Cord-Ruwisch 2013). Consequently, prior to any full scale
MICP application, such as soil liquefaction or embankment stabilization, an economically viable production of ureolytic bacteria is necessary.
Cheng and Cord-Ruwisch (2013) have successfully demonstrated a production of highly urease active bacteria by nonsterile chemostat culture cultivated at specific conditions (high
pH of 10 and high concentration of urea of 0.17 M) that only
favor the growth of urease active bacteria. This allows reproducibility of continuously enriched bacteria on site. Burbank
et al. (2011) has achieved a successful soil cementation through
in situ enriched indigenous urease active microorganisms using
enrichment medium containing molasses, urea, sodium acetate
trihydrate, ammonium chloride and yeast extract. All these
methods would be useful for producing a large amount of ureolytic bacteria for cost-effective, large-scale applications of MICP.
Calcite precipitation by urea hydrolysis
The crucial reason as to why urea hydrolysis is the most desired
CaCO3 precipitation method adopted by researchers is that the
process is straightforward and easily controlled (Dhami et al.
2013), and can generate up to 90% chemical conversion efficiency of precipitated CaCO3 amount in a short period of time
of less than 24 h (Al-Thawadi 2011). The mechanism of
CaCO3 precipitation by urea hydrolysis can be categorized into
two stages: (1) urea hydrolysis and (2) CaCO3 precipitation
(Burbank 2010; Cheng 2012; Hammes 2003; Hammes et al.
2003; Hillgartner et al. 2001; Martinez 2012; Montoya 2012;
Waller 2011). The chemical reactions of urea hydrolysis are
given as follows (Cheng et al. 2013):
CO NH ð Þ 2 2 C H2O ! CO2 3 ¡ C 2NH4C (1)
Ca2 C C CO2 3 ¡ ! CaCO3 (2)
During the urea hydrolysis stage, 1 mol of urea (CO(NH2)2)
is hydrolyzed to produce 1 mol of carbonate (CO32¡) and
2 mol of ammonium (NH4C) ions. During the CaCO3 precipitation stage, the introduced calcium ions (Ca2C) derived from
calcium chloride (CaCl2) in the cementation solution reacts
with the carbonate ions (CO32¡) to form 1 mol of calcium carbonate (CaCO3) crystals.
Ferris et al. (2004) revealed that the ureolytic bacteria precipitate CaCO3 crystals by generating carbonate from urea
hydrolysis in which the crystal was formed in three stages: (1)
the development of supersaturated solution, (2) nucleation at
the point of critical saturation (i.e. the supersaturation at which
CaCO3 actually initiates), and (3) spontaneous crystal growth
on the stable nuclei. These stages are crucial for soil stabilization because CaCO3 crystals morphology (type, shape and size)
may differ, depending on the attainment of the supersaturation
condition. Depending on the polymorph of CaCO3 crystals (i.e.
calcite, vaterite or aragonite), the strength of biocemented soil
can be affected (Al-Thawadi 2013; Dhami et al. 2013). Mitchell
and Ferris (2006) showed that ureolytic bacteria can produce
larger CaCO3 crystals by increasing the size of crystals propagated during the nucleation stage as opposed to bacteria-free
artificial groundwater medium. Their finding implied a crucial
direction toward obtaining larger CaCO3 crystals through the
supplement of greater ureolytic bacterial concentration.
Biocementation by MICP for soil stabilization
Biocementation has been at work in nature since time immemorial through biodeposition of sand over a long period of
time. Examples of such natural occurrence can be found in
Pinnacles and Lake Thetis in Western Australia and East Cliff
in England. Observations from nature have paved the way into
exploring a new branch in Geotechnical Engineering called Biogeotechnology, which focuses on natural sand metamorphosis
2 D. MUJAH ET AL.
into biosandstone by improving its engineering properties
using microorganisms such as bacteria (Achal et al. 2015). The
biomineralization of the precipitated CaCO3 crystals links the
soil particles together through an effective bridging that is predominantly concentrated at the point of contacts (pore throats)
forming a menisci shape due to the capillary force (Tuller et al.
1999). The conceptual illustration of the CaCO3 precipitation
distribution in the soil matrix is presented in Figure 1, and an
example of a scanning electron microscopy (SEM) image that
verifies the effective bridging phenomenon is shown in Figure 2.
It can be seen from Figures 1 and 2 that the precipitated CaCO3
did not fully fill the pores between the soil particles, allowing
drainage of fluid movement. It was reported by many researchers (Cheng et al. 2014b; Chou et al. 2011; Keykha et al. 2014a,
b) that the biocemented sand can retain sufficient permeability
values ranging from 1.0 £ 10¡6 m/s to 5.0 £ 10¡3 m/s. Recent
research conducted by the authors of the current paper showed
a successful transformation of natural sand into biosandstone
in the laboratory, as presented in Figure 3.
Soil treatment process by MICP
In order to ensure a successful ground improvement by MICP,
the introduction and retention of ureolytic bacteria or urease
enzyme inside the soil matrix is crucial. The retained bacteria
can induce CaCO3 precipitation through the supply of injecting
a cementation solution. Improper bacteria retention could lead
to the bacteria being flushed away or detached by a subsequent
injection of the cementation solution, leading to uneven distribution of bacteria and resulting in non-uniform CaCO3 precipitation and strength within the biocemented soil.
The introduction of bacteria to soil can be achieved generally through two main methods including: (1) injection method
and (2) premixing method. The injection method warrants the
flushing of bacteria solution top down and a retention period
to be observed for bacteria to be attached to the sand grains
before injection of the cementation solution. In the premixing
method, the bacteria are mechanically mixed with the soil prior
to introduction of the cementation solution. Each method is
discussed further below.
Injection method
In order to attach the bacteria into the soil grains, Harkes et al.
(2010) added 50 mM calcium chloride (CaCl2) fixation solution
after the initial injection of the bacteria culture. Microbial
attachment toward the sand grains was promoted via an
increased ionic strength of Ca2C ions, which encourages bacterial adsorption onto the surface of sand grains (Torkzaban
et al. 2008). Whiffin et al. (2007) introduced the injection technique to treat a 5 m long sandy soil column in which both the
bacteria culture and cementation solution were injected alternately from top to bottom of which the vertical flow of the reactants was regulated by a peristaltic pump. It was found that
CaCO3 was precipitated into the entire 5 m of treated sand column; however, the CaCO3 distribution was not uniform along
the column length. It was also reported that a minimum of
about 60 kg/m3 of CaCO3 content was precipitated to achieve a
compressive strength of at least 300 kPa. van Paassen et al.
(2010a) applied the horizontal injection method for treatment
of a large-scale experiment of soil volume of 100 m3 and discovered considerably high variation in the peak strength of the
UCS values suggesting non-uniformity along the biotreated
sand volume. Qian et al. (2010) injected the cementation solution into MICP treated column and found that the highest UCS
Figure 1. Schematic diagram showing the effective bridge formation (modified after Cheng et al. 2013).
GEOMICROBIOLOGY JOURNAL 3
strength value was close to 2 MPa but non-uniformity distribution of CaCO3 was detected.
It should be noted that the injection method is still the most
commonly preferred MICP treatment method as it lies in the
injection conditions (i.e. flow, pressure and hydraulic gradient)
that can be easily controlled during testing, leaving a room for
treatment adjustment which allows injection from both vertical
and horizontal directions. Also, this method allows fully saturated, partially saturated and unsaturated conditions of soils to
be prepared since the flow of the reactants is controllable.
Despite such appraisals, one of the major disadvantages of the
injection method lies in the uneven distribution of bacteria and
Figure 2. Scanning electron microscopy (SEM) image showing the effective bridge formation.
Figure 3. Sand metamorphosis: (a) natural sand; and (b) biocemented sand (biosandstone)
4 D. MUJAH ET AL.
inhomogeneity of CaCO3 distribution, which result in a nonuniform treatment of biocemented soil strength throughout the
length of tested columns. When bacteria were injected through
the pore space of sand, they are likely to be filtered through the
sand with a long-linear reduction of microbe concentration
along the injection path (Ginn et al. 2001). Also, the reaction of
cementation solution with bacteria during the penetration leads
to less reagent moving to deeper areas and resulting in a localized cementation around the injection points and repeated
injection of cementation solution eventually leads to pore plugging at the region near the injection source, leading to uneven
distribution of CaCO3 precipitation (Cheng and Cord-Ruwisch
2014). To improve this drawback, a slower injection rate of bacterial suspension was suggested by Harkes et al. (2010) to allow
for a sufficient bacteria delivery to more distant locations along
the treated soil column to counter the issue of CaCO3 distribution uniformity. It was also proposed that a waiting period
should be applied between the injection of bacteria suspension
and cementation solution to allow for bacteria to be transported along the sand column and firmly attached before the
application of the cementation solution (Al Qabany et al.
2012). Whiffin et al. (2007) recommended that the flow rate of
cementation solution should be increased to allow more
reagents to reach deeper locations into the treated soil column.
Surface percolation method
By simply spraying or trickling bacterial suspension and
cementation solution alternately onto the soil surface followed
by the solution penetration into the soil driven by gravity, soil
was successfully cemented up to 2 m deep (Cheng and CordRuwisch 2014). The main advantage of this method is that the
solution injection does not require heavy machinery due to the
free-draining of water movement. However, this method may
lead to limitation of treatment for fine grained soils (e.g. silt or
clay) due to soil low infiltration rate and permeability. It was
found that the cementation reaction was limited to 1 m deep
for fine sand of size along an entire 2 m long of coarse sand column with UCS values varied between 850 kPa and 2067 kPa. The surface percolation was also tested in a large container, and showed a
reasonable homogeneous distribution of CaCO3 and strength
in 3D scale. This is probably due to the phenomenon of selfadjustable preferential flow path during the treatment (Cheng
and Cord-Ruwisch 2014). In reality, fine grained soils are usually encountered deep into soil deposits. Therefore, possible
applications of the surface percolation method may include
dust suppression, track basement stabilization and embankment construction.
Premixing method
In this method, the bacteria are premixed mechanically with
soil until a desired homogeneity is achieved. By applying this
method, Yasuhara et al. (2012) obtained biocemented sand
samples of UCS values ranging from 400 kPa to 1.6 MPa.
Although this method did not produce as high UCS values as
the other treatment methods, it placed the homogeneity issue
at rest. Zhao et al. (2014a) claimed that almost 83% of the
CaCO3 precipitated in biocemented samples using the premixing method was homogeneously distributed throughout the
treated sand columns.
Despite the fact that the premixing method solved the
homogeneity problem, it remains the least favorable MICP
method because it causes disturbance to the local soil. This is
critical because soil disturbance may lead to a pseudo stress
development in the soil sample as a result of the vigorous mixing between the soil and the cementing agent. Also, the unmeasured stresses applied during mixing of soil samples complicate
the soil stress history and make it difficult to discern during
mechanical testing.
Recently, Zhao et al. (2014a) fully submerged the premixed
bacteria-sand matrix in a mechanically operated tank reactor
containing cementation solution. In contrast to the traditional
injection method, the cementation solution was allowed to
freely diffuse under the concentration gradient into geotextile
wrapped bacteria–sand samples by the action of magnetic stirrer. It was reported that about 6.6–8% (by weight) of produced
CaCO3 achieved UCS strength between 1.76 MPa and
2.04 MPa for a laboratory scale 100 mm length soil column.
Although Zhao et al. (2014b) found that the produced CaCO3
was fairly distributed in the soil samples tested, it is impractical
to apply the diffusion method in the field as it requires an
installment of a geotextile wrapping to be used as a protective
membrane which can accelerate the rate of diffusion between
the chemical substances into the treated samples. The preinstallment of geotextile wrapping may disturb the integrity of
Figure 4. Different CaCO3 distribution within the pore spaces: (a) uniform; (b) preferential distributions (DeJong et al. 2010); and (c) actual distribution.
GEOMICROBIOLOGY JOURNAL 5
treated soil and can also induce a pseudo-stress history that
may contribute to an inaccurate strength value.
Geotechnical engineering properties of biocemented
soils
Permeability
Permeability measures the ability of porous materials to allow
the passage of fluid. In MICP, permeability is of utmost importance because the technique is preferred for soils that are pervious or semi pervious in nature such as coarse grained soils (e.g.
sand or gravel). Porous materials with high permeability can
prevent the development of excess pore water pressure during
loading. In general, MICP can be utilized to increase soil
strength while keeping sufficient permeability (in case of soil
biocementation) or completely block the soil pores (in case of
soil bioclogging). It should be noted that soil drainage condition is related to its packing density in which the macro scale
behavior of soil mass results from the interaction between the
soil inter-particle levels (Cho et al. 2006). According to Chu
et al. (2013a), a good drainage passage having a hydraulic conductivity value of at least 1 £ 10¡4 m/s must be maintained in
order for the bacteria and cementation solution to penetrate
into the desired sand depth, to ensure homogeneous CaCO3
precipitation throughout the treated soil depth.
In soil biocementation, MICP facilitates permeability retention for biocemented soil samples better than the other cementitious materials such as ordinary Portland cement (OPC). For
example, Cheng et al. (2013) suggested that a loss of permeability in soil samples treated with Portland cement is due to the
occupation of pore spaces by water insoluble hydrates formed
as a result of the cement hydration reaction with pore water.
Meanwhile, the loss of permeability in biocemented soil samples is because of the occupation of CaCO3 crystallization in
the soil pore spaces. The CaCO3 crystals cause a slight volume
change in the pore spaces as opposed to the hydrates, hence
ensuring good drainage that allows a liquid passage through
the biocemented soil matrix.
van Paassen (2009) reported 60% reduction in the initial
permeability of biotreated soils at approximately 100 kg/m3
CaCO3 precipitation, whereas Ivanov et al. (2010) recorded a
permeability reduction of 50–99% using 1 M cementation solution. Al Qabany and Soga (2013) used 0. 5 M cementation solution and found a reduction of 20% in the initial permeability
value at 2% CaCO3 precipitation. Larger CaCO3 crystals were
produced and clogged the pores when a high concentration
solution was used. Therefore, for samples treated with solution
of high concentrations (0.5–1 M), the reduction in permeability
is usually greater as opposed to those treated with solution of
low concentrations (0.1–0. 5 M). However, inhomogeneity
along the sand column samples can still be attributed to the
localized clogging. It is thereby recommended that a low concentration solution should be used if less permeability reduction is desired, to ensure a uniform consistency of CaCO3
precipitation. A solution with low concentration may produce
more uniform precipitation pattern and stronger samples for a
given amount of CaCO3 precipitation.
Although the MICP technique could retain sufficient soil
permeability after treatment, it can be used for soil clogging, a
process to significantly reduce the hydraulic conductivity or
permeability of porous soil media. Ivanov and Chu (2008)
introduced the concept of bioclogging, by filling the pores with
the cementing agent (CaCO3) derived from bacteria. Their
results showed a significant permeability reduction (5 £ 10¡5
m/s to 1.4 £ 10¡7 m/s) in loose clean sand samples after treatment with bacteria signaling the potential use as a sealant to
wastewater or agriculture treatment ponds and landfill sites.
Similarly, Chu et al. (2013b) used a strain of Bacillus sp. bacteria
to promote bioclogging in sand. It was observed that permeability of biotreated sand varied with the content of precipitated
calcium. It was also suggested that for bioclogging of sand to
occur, precipitated CaCO3 of 9.3% w/w or higher is required.
Porosity
Porosity is the amount of voids in a material. Qian et al. (2010)
characterized the effectiveness of cementation in terms of the
porosity of cemented sand samples and its reduction, and
found that the porosity was reduced to 25% after MICP treatment. Tagliaferri et al. (2011) used X-ray imaging and quantitative a 3D digital image analysis to analyze the crushed
biocemented bonds and found that the overall porosity of biocemented soil was reduced to 30%. Although the porosity value
was reduced, the CaCO3 precipitates were found to fill the soil
pores of sand grains. It should be noted that the porosity governs the effectiveness of MICP treated samples by means of
controlling the replacement of the pore content of sand grains
by CaCO3 (Rong et al. 2012). As the degree of cementation
increases, the amount of precipitated CaCO3 increase and
higher amount of CaCO3 crystals replaces the pore content of
the inner structure of the soil matrix, leading to higher strength.
Stiffness
Soil stiffness, commonly known as soil elastic modulus (E), is
the ratio of stress over strain. Soil stiffness is closely related to
the bonding strength between loose soil grains. Cheng et al.
(2013) compared the elastic modulus of biocemented sand with
other types of geomaterials such as concrete, gravel and soft
rock, and found that the biotreated sand is the most flexible
among the materials tested. In earthquake prone areas, less stiff
soil can provide an extra time for evacuation due to its ability
to maintain significant residual strength even after failure. Lee
et al. (2013) performed MICP on residual soil and found that
the stiffness behavior of biocemented residual soil is similar to
that of biocemented natural sand.
Previously, researchers studied the effects of cementation on
strength and stiffness of granular soils using a variety of different cementing agents namely the Portland cement, gypsum
and sodium silicate (Amini and Hamidi 2014; Fernandez and
Santamarina 2001; Haeri et al. 2006; Sharma et al. 2011). It was
found that the strength and stiffness of cemented materials
increase with the increase of the amount of cementing material
in the soil matrix; although the amount of cementing material
required to produce a certain cementing effect may vary. Based
6 D. MUJAH ET AL.
on this fact, Montoya and DeJong (2015) studied the effect of
biocementation on stress–strain behavior of biotreated sand
and found that the stiffness was dramatically improved with
the increase of MICP cementation (i.e. CaCO3 content).
It is worth noting that the effective stress path as well as the
drainage condition influence the MICP treated soils in a way
that it can reduce the rate of stiffness due to the degradation of
cementation prior to failure. The stress paths of a given soil
depend on the initial, in situ and final state of soil sample.
A study carried out by Ruistuen et al. (1999) suggested that the
stress path dependent behavior of weakly cemented soil is due
to the shear-enhanced compaction at which increasing the
cementation was shown experimentally to reduce the stress
sensitivity.
Shear strength
Shear strength is the magnitude of shear stress that a soil can
sustain and depends strictly on the shear strength parameters
of soil including the cohesion (c) and friction angle (f).
Duraisamy and Airey (2012) correlated the shear strength of
liquefiable sandy soil treated with MICP to the degree of
cementation using the shear wave velocity technique. It was
found that the shear strength of biocemented soil was strictly
affected by the increase in soil cohesion resulting from the
increase in the cement content, while the friction angle was not
greatly affected by the cementation process. In contrast, Chou
et al. (2011) reported a large increase in soil friction angle but a
small increase in soil cohesion was detected for almost all
treated samples using MICP which was catalyzed by three conditions of Sporosarcina pasteurii (growing, resting and dead
cells). It was also found that the peak shear strength of biocemented soil was higher compared to untreated specimens, and
generally higher in the growing cell treatment than that of other
treatment methods.
Ng et al. (2012) applied MICP using Bacillus megaterium to
treat a residual soil and found that the shear strength ratio of
treated to untreated soils was increased at values ranging from
1.40 to 2.64. Montoya and DeJong (2015) observed that the
shear strength of MICP treated sand was dramatically
improved with the increase in MICP cementation. With
increasing cementation level, the peak shear strength increased
leading to a transition in the stress–strain behavior from strain
hardening to strain softening. Cheng et al. (2013) also discussed
the cohesion and friction angle of biocemented soil samples
treated under different degrees of saturation and showed that
at lower saturation degree, the precipitated CaCO3 crystals contributed more to improving the soil cohesion than the friction
angle. On the other hand, regardless of the saturation degree,
both the cohesion and friction angle increased at higher CaCO3
content due to the filling effect of the calcite crystals in the soil
pore spaces.
Unconfined compressive strength (UCS)
The unconfined compressive strength (UCS) is the most commonly used test to describe the strength of biocemented soils,
as reported by many researchers (Cheng et al. 2013; Harkes
et al. 2010; Ivanov et al. 2015; Whiffin et al. 2007; Zhao et al.
2014a). Available results in the literature reported that the lowest recorded UCS value was 150 kPa, whereas the highest value
was 34 MPa, at different MICP treatments (Whiffin 2004). van
Paassen et al. (2010b) revealed an exponential relationship
between the CaCO3 content and UCS values of biocemented
soils. This indicates that despite having the same amount of
CaCO3 precipitated crystals, the mechanical response of MICP
treated soil can vary significantly depending on the effective
CaCO3 precipitation mechanism. Despite the fact that the UCS
testing is commonly used to characterize the strength property
of cemented soils because it allows large number of samples to
be tested at the same time (Al Qabany and Soga 2013), triaxial
testing is also recommended for further investigation of the
response of biocemented soils toward the monotonic and cyclic
loadings as it simulates the natural behavior of soil in the field.
Microstructure
The particle-to-particle contact mechanism helps to strengthen
the biocemented soil particles together. SEM allows researchers
to have a direct and closer look at the CaCO3 bonds developed
at the inter-particle soil grains and provides insights into
explaining the improvement mechanism of biocemented soils
(Sham et al. 2013). Cheng et al. (2013) found that not all
CaCO3 crystals precipitated in the sand pores necessarily contribute to the shear strength of biocemented soil but rather the
crystals forming the effective bridges that link the sand grains
together at the inter-particle level, as previously shown in
Figure 2. DeJong et al. (2010) presented a clear explanation
in relation to the distribution alternatives of CaCO3 precipitation within the pore spaces. This includes the “uniform” distribution which means an equal thickness of CaCO3 precipitation
around the soil particles, producing a relatively small bonding
between the sand grains, and “preferential” distribution which
indicates the particle to particle contacts of CaCO3 precipitation at which the CaCO3 crystals contribute to the soil
improvement. Figure 4a shows a schematic diagram of the
abovementioned two different CaCO3 distributions within the
soil pore spaces. However, based on the SEM image, the
CaCO3 precipitation follows an “actual” distribution in which a
significant amount of CaCO3 is precipitated in the vicinity of
the particle to particle contacts, as shown in Figure 4b.
According to DeJong et al. (2010), there are two parameters
that govern the spatial distribution of CaCO3 (i.e. the biological
behavior and filtering process). The reduced shear stresses and
availability of nutrients at the grain contacts are the main reason that microbes prefer to be in the smaller surface features,
such as near the particle to particle contacts. The increased
CaCO3 precipitation in the region of particle to particle contacts is the result of the greater concentration of microbes aligning themselves to that particular area. The filtering process
occurs in the soil pore spaces when the precipitated CaCO3 is
formed in the pore fluid and subsequently released and suspended within the pore fluid space. This process forces the precipitated CaCO3 to re-attach near the region of the particle to
particle contacts as the pore fluid flows through the pore throat
(i.e. the smallest pore space connecting two larger pore cavities). Since the filtering process is dependent on the pore space
and the relative size of the suspended CaCO3, its effect becomes
GEOMICROBIOLOGY JOURNAL 7
more significant as the pore throat space decreases due to the
continuous loading. With the belief in the effective bridge
mechanism, many researchers have come up with the same
conclusion (Akiyama and Kawasaki 2012; Park et al. 2014;
Rong and Qian 2014; Rong et al. 2012, 2013; Sel et al. 2014;
Tobler et al. 2012) and provided similar SEM images to those
of Cheng et al. (2013), showing the particle to particle contact
points developed in the micro scale level and indicating the formation of CaCO3 which improves the shear strength of biocemented soil.
Shear wave velocity
The non-destructive technique using the bender elements (BE)
has been employed to determine the progressive strength development of soil (Sharma et al. 2011). Martinez et al. (2013) performed a real time monitoring of one-dimensional flow for a
half-meter-scale column improved with MICP using S-wave
velocity measurements. It was concluded that the S-wave velocity acts as a proxy to increase the small-strain shear stiffness as
cementation occurs at the particle to particle contacts. The
study also pointed out that the S-wave velocity measurements
are effective monitoring indicators of MICP soil improvement
for both temporally and spatially based conditions. Recently,
Montoya and DeJong (2015) captured the change in the smallstrain stiffness of biocemented soil during shearing using the
S-wave velocity method. It was concluded that an indication of
cementation degradation as a function of the strain level can
possibly be deduced from the change in the small-strain stiffness. The advantages of this technique include the non-destructive examination of biocemented soil samples and capability to
measure the soil strength as a function of time in a real time
domain. Hence, it can be applied to determine the changes of
ground improvement in the field over a long period of time
(Piriyakul and Iamchaturapatr 2013).
Factors affecting formation of CaCO3 crystals in MICP
treatment
The CaCO3 crystallographic patterns (i.e. size, shape and distribution) play a significant role in determining the engineering
response of MICP treated soils. This is because different size,
shape and distribution patterns of CaCO3 precipitation can
produce different strength responses of biocemented soils (Al
Qabany and Soga 2013). In this section, the critical factors
affecting MICP treatment such as temperature, urease activity/
availability of nucleation sites, pH level, degree of saturation
and concentration of cementation solution are discussed based
on published work in the literature.
Temperature
The effect of temperature on MICP is complex as it affects the
urease activity of microorganisms, growth and nucleation rate
of CaCO3 crystals and solubility of CaCO3. Nemati and
Voordouw (2003) demonstrated that an increase in temperature from 20C to 50C enhanced the production rate of
CaCO3 from the enzymatic reaction and hence affected the size
and shape of the formed CaCO3 crystals. Rebata-Landa (2007)
showed that at a temperature higher than 60C, the CaCO3
production ceased to occur due to the death of microorganisms.
Hence, it is vital to know the most optimum temperature for
formation of CaCO3 crystals, as it contributes to the highest
strength.
Recently, Cheng et al. (2014b) conducted a preliminary
study to determine the effect of room (25C) and higher (50C)
temperatures on the strength of MICP treated sand. It was
found that although about three times more CaCO3 crystals
were precipitated at 50C, the strength of biotreated soil samples was 60% less than that of the samples treated at 25C.
Upon investigation through SEM, it was concluded that the
CaCO3 crystals produced at higher temperature were relatively
small (2–5 mm in diameter) and fully covering the sand grain
surfaces. Meanwhile, CaCO3 crystals formed at room temperature were larger in size (15–20 mm in diameter) and deposited
mainly at the soil pore throats. The precipitated larger size
CaCO3 crystals would certainly have more contact points linking the soil grains together, hence contributing to higher
strength.
Urease activity/bacterial concentration
The urease activity is an indication of the hydrolysis rate of urea
by the ureolytic bacteria (Whiffin 2004). Nemati and
Voordouw (2003) suggested that an increase in the urease
activity enhances the extent of CaCO3 precipitation because
bacterial cells act as nucleation sites in MICP process (DeJong
et al. 2010). Hammes and Verstraete (2002) mentioned that the
availability of the nucleation sites has been found to be crucial
in governing the urease activity that dictates the amount of produced CaCO3. Basically, once the injected bacteria are attached
in the soil, they would act as nucleation sites for the CaCO3
crystals precipitation to occur by catalyzing the reaction
between Ca2C ions and the CO32¡ ions to form CaCO3 that
eventually link two or more soil particles together as they grow
in size (DeJong et al. 2010). As the availability of the nucleation
sites depends greatly on the amount of the attached bacteria in
the soil, the amount of supplied bacterial cells (i.e. the concentration of the urease activity injected into the sand column) is
imperative.
Gandhi et al. (1995) mentioned that the nucleation of new
crystals would compete with the process of crystal growth if
nucleation of new crystals prevails over the growth of the existing crystals. It is fair to assume that the levels of urease activity
(i.e. the total amounts of bacterial cells) significantly affect the
CaCO3 precipitation pattern. It has been reported by DeJong
et al. (2010) that the bacterial cells can act as nucleation sites
for new crystals precipitation. When more bacterial cells are
present in the soil matrix, the produced CO32¡ ions are consumed mainly by nucleation of new CaCO3 crystals due to the
abundant nucleation sites, instead of growing the existing
CaCO3 crystals, resulting in the precipitation of more new
small CaCO3 crystals. In case of small amount of bacterial cells,
nucleation of new CaCO3 crystals is inhabited by the low number of bacterial cells present in the soil matrix. This phenomenon may facilitate the growth of individual crystals instead of
formation of new crystals. Therefore, smaller growth of CaCO3
crystals in the lower urease activity condition would be
8 D. MUJAH ET AL.
expected. This phenomenon has been well studied in the pure
chemical CaCO3 production where high amount of nucleation
sites results in small crystals and vice versa (Al-Thawadi and
Cord-Ruwisch 2012). However, similar work has not been carried out for MICP and this can only be validated through SEM
observation. To date, no study was found in the literature
directly linking the effect of different urease activities to the
availability of nucleation sites toward the CaCO3 crystals precipitation patterns and the final strength of biocemented
samples.
pH level
The change in pH level, which is due to the formation of the
hydroxyl ions (OH¡) generated from the production of ammonium ions (NH4C), helps to create an alkaline environment
suitable for CaCO3 precipitation (DeJong et al. 2010). The presence of OH¡ ions raises the pH around the cells (Rebata-Landa
2007), and this is validated by Ferris et al. (2004) who showed
that MICP favored alkaline environment (6.5 soil biocementation, variability of the pH values can influence
the bacterial transport and adhesion, which is an important factor affecting achieving homogenously distribution of CaCO3
crystals precipitation. As discussed earlier, uniformly distributed CaCO3 crystals across biocemented soils is desirable in
MICP treatment because it produces uniformly well-cemented
samples that possess greater strength. A preliminary study carried out by Cheng et al. (2014b) pointed out that the relationship between the initial soil pH and formation of CaCO3
crystals is a function of the CaCO3 solubility variation generated as a result of the different initial pH values. Until a proper
SEM image is made to examine the CaCO3 crystals precipitation patterns under the effect of supersaturation condition (i.e.
the change in pH value), debates regarding this issue will
continue.
Degree of saturation
Cheng and Cord-Ruwisch (2012) was the first to suggest that
the distribution of CaCO3 crystals can be controlled and
restricted to the interparticle contact points by controlling
the degree of saturation of biocemented soils during MICP
treatment. This claim was made upon an observation of
sand columns treated at lower degree of saturation which
exhibited higher strength at similar CaCO3 level. Following
this, Cheng et al. (2013) investigated the effectiveness of
MICP at various degrees of saturation of 20, 40, 60 and
100%. The study found that MICP works best at lower
degree of saturation of 20% and gives higher strength at
even lower CaCO3 precipitation within the soil matrix.
At lower degree of saturation, CaCO3 crystals were formed
at the effective locations of the interparticle connections.
However, at a full degree of saturation of 100%, the CaCO3
crystals were formed at unnecessary locations which filled in
the pore voids, resulting in an ineffective soil improvement.
These findings contradict previous belief that the highest
strength is achieved at fully saturated condition (Al-Thawadi
2013; van Paassen 2009; Whiffin et al. 2007). With this information at hand, it was concluded that lower degree of
saturation (e.g. 20%) gives the most effective CaCO3 crystals
precipitation (as shown by the SEM image) and also better
UCS strength values as opposed to the biocemented soils
treated fully saturated.
Concentration of cementation solution
The efficiency of CaCO3 crystals formation is believed to be
affected by various cementation solution concentrations. This
is attributed to the fact that more homogeneous CaCO3 crystals
distribution along the sand matrix is usually observed at lower
cementation solution concentration. On the other hand, the
precipitated CaCO3 crystals at higher cementation solution
concentration are usually randomly formed in the soil voids
due to the faster precipitation induced by the higher cementation solution concentration (Okwadha and Li 2010). The
CaCO3 formation was also found to be more effective at lower
cementation solution concentration. For example, Al Qabany
and Soga (2013) conducted an experiment using sand samples
treated under different cementation reagents of 0.1, 0.25, 0.5
and 1 M urea-calcium chloride solution and found that the
treated sand with lower reagent concentration gave higher
strength compared to that treated with higher reagent concentration. The lower concentration led to more homogeneous
CaCO3 crystals formation at the particle contact points which
contributed to the strength improvement with minimum soil
disturbance and permeability reduction. This is consistent with
the findings of Ng et al. (2014) who claimed that biocementation was found to be more effective on the residual soil treated
with 0.5 M cementation reagent compared to that treated with
1 M cementation reagent. CheNg et al. (2014a) even produced
biocemented sand columns using low concentration of Ca2C
source (i.e. 10 mM) from seawater; however, a greater number
of injections were required to achieve the same amount of
CaCO3 crystals precipitation when lower cementation reagent
concentration was used.
Field application of MICP
In their attempts to verify the effectiveness of MICP for in situ
implementation of biocementation, some researchers have performed field trials and upscaled experiments. For example, van
Paassen (2009) carried out an experiment to treat 1–100 m3 of
sand in the laboratory and found that the strength of biocemented sand was significantly increased upon MICP treatment;
however, distinct spatial heterogeneity was recorded. Possible
reasons for these observations include (van Paassen 2009): (1)
non-homogeneity of CaCO3 distribution as a result of the
unspecified location of available amount of urea to react with
bacteria to form CaCO3; (2) amount of supplied reagents and
the way of supplying it into the soil (i.e. injection or surface
percolation); and (3) flow of reagents which may follow the
preferential flow along the phreatic surface. Along the preferential flow paths, where the flow resistance is lower than other
areas, soils continuously receive reagents, leading to reasonably
higher content of CaCO3 than in other areas. The locally produced CaCO3 crystals precipitated in the soil pores reduce soil
porosity and permeability, causing an increase in the flow
GEOMICROBIOLOGY JOURNAL 9
resistance and leading to development of new preferential liquid flow paths (Cheng and Cord-Ruwisch 2014).
Martinez (2012) conducted an upscaling experiment to simulate field implementation of MICP using 0.5 £ 0.5 £ 0.15 m
target treatment zone. The study found that at the micro-scale
level, the distribution of CaCO3 around the particle to particle
contact points was the main contribution to the improved
strength of biocemented sand while at the macro-scale level,
non-homogeneity of CaCO3 distribution along the soil matrix
was observed. The results of the macro-scale experiment were
in agreement with those obtained by van Paassen (2009).
DeJong et al. (2014) developed a scaled repeated five-spot
treatment model for examining the feasibility of MICP in field
applications. The proposed model allowed experimentally uniform treatment to be achieved even under highly active microbial conditions. This was achieved by installing: (1) production
wells (to show spatial trend); and (2) sampling wells (to show
temporal trend) in microbial content indicating breakthrough,
growth and decay. Monitoring the microbial activity and tracking the urea variability in the macro-scale experiment was
made by the installation of bender elements that proved handy
for capturing both the spatial and temporal changes in the
mechanical properties of biocemented sand during treatment.
Gomez et al. (2015) performed the first ever field-scale biocementation tests. The study focused on the surficial application of MICP to provide surface stabilization and improve the
erosion resistance of loose sand deposits for future revegetation and dust control. Using the dynamic cone penetration testing, an improvement of approximately 28 cm near the
targeted depth of 30 cm was observed. The results showed that
upon further technique and solution optimization, MICP could
be used to treat larger-scale applications. It was also indicated
that the low-concentration treatment solution achieved the
greatest improvement, whereas the high and medium
concentrations were not optimal. This was attributed to the
modest spatial variability that occurred across the column
depth.
Envisioned applications of soil biocementation
Once the MICP process has been fully optimized experimentally, further field applications can be realized. Although field
applications of soil biocementation are still in their early stage
of conception, more research is being tailored to examine the
upscaled effect of MICP process in longer soil columns and
larger improved area. Thus, the envisioned applications
(Table 1) of soil biocementation are important so as to open up
more alternatives to the present research dealing with MICP.
Advantages of MICP for soil biocementation
Cost effectiveness
As opposed to other soil improvement techniques involving the
use of cementation agents, MICP is currently relatively costly to
be implemented in the field. The difference in the cost of various cementing agents toward soil improvement applications is
compiled in Table 2. Although the initial cost of MICP installation is relatively more expensive than other cementing agents,
Whiffin et al. (2007) stated that MICP is cost-saving because
the bacterial enzyme can be reused in subsequent (two to three)
applications of treatment using the same cementation solution.
This means that MICP offers cheaper treatment in the long
run. Similar observations were reported by Ismail et al. (2002)
and Al-Thawadi (2013).
Ivanov and Chu (2008) compared the raw material costs
between microbial and chemical grouting, and found that the
cost of chemical grouting ($2–$7 per m3 of soil) is cheaper than
microbial grouting ($0.5–$9 per m3 of soil). Although it seemed
that microbial grouting is more expensive than chemical grouting, it was pointed out that the microbial grouts may be nontoxic while chemical grouting may pose detrimental effects
toward the environment. To reduce the cost of MICP for field
applications, some alternatives were suggested in the literature.
This includes the production of non-sterile chemostat culture
containing highly urease active bacteria, cultivated at high pH
of 10 and high concentration of urea (0.17 M), which allows
bacteria enrichment to be reproduced continuously on site
(2013) and the use of Ca2C ions dissolved in seawater as
Table 1. Envisioned applications for soil bio-cementation.
Envisioned applications Possible mechanism References
Self-healing of soils A portion of bio-cementation bonds degrade when loaded beyond its yield
strength. The degraded MICP bonds can be healed by re-initiating the biogeochemical process, returning the cemented sand properties to pre-shearing
levels
Harbottle et al. (2014); Montoya and Dejong
(2013)
Slope stabilization The bio-cemented bonds help to strengthen the failure plane surface to provide
additional stability needed to prevent slope failures
DeJong et al. (2010, 2013)
Settlement reduction The bearing capacity of bio-cemented soils is increased; hence, settlement of
foundation is reduced
DeJong et al. (2010, 2011); van Paassen
et al. (2010)
Erosion control MICP increases the bio-cemented soil resistance to the erosive forces of water
flow along the sea shores and river banks
CheNg et al. (2014); DeJong et al. (2006)
Liquefaction prevention Similar to the concept of self-healing, the post-shearing loads could re-initiate the
MICP process; hence, preventing further liquefaction damages
Montoya and DeJong (2015); Montoya et al.
(2013)
Table 2. Application costs of different cementing agents for soil improvement.
Cementing
agents
Yield strength
(MPa)
Cost per m3
treatment ($) References
MICP 0.5–2.5 20–60 Cheng (2012)
Portland cement 0.5–3.8 NC Ismail et al. (2002)
Gypsum 0.2–1.8 NC Ismail et al. (2002)
Chemical grouting NM 2–72 Ivanov and Chu (2008)
NM, not measured.
NC, not calculated
10 D. MUJAH ET AL.
potential substitute for the commercial CaCl2 source (Cheng
et al. 2014a).
System reliability
The reliability of MICP treatment contributes to the current
preferential of MICP process over other techniques. This lies in
the ability of MICP treatment to be easily adjusted, both
mechanically and biologically, to give an optimal treatment
according to the type and condition of the local soil. The duration and homogeneity of MICP treatment can also be adapted
to a specified timeframe and the presence of biological processes that have the potential to enhance spatial uniformity. In
addition, the implementation of MICP treatment in soil
improvement is flexible because it is easy to control treatment
process using bacteria, which encourages the potential use of
this technique in the retrofitting of construction sites.
Promoting sustainability
Soil biocementation using MICP promotes the concept of sustainability through the use of natural materials such as microorganisms as the primary source of cementing bond. Achal and
Mukherjee (2015) reported the myriad use of MICP in the construction industries indicating its strategic role as possible green
technology for the future. Although part of the end product of
hydrolysis of urea is ammonia, which may be deemed to be detrimental to the groundwater, the ammonia could be fed back
into the surroundings as fertilizer if proper plans and precautions are taken. Moreover, the CaCO3 bonds between the soil
particles would not permanently alter the subsurface conditions
of biocemented soils (DeJong et al. 2011).
Limitations of MICP for soil biocementation
Soil compatibility
One of the main disadvantages of MICP is that it can only be
utilized for specific soil sizes. As reviewed by Fragaszy et al.
(2011), the technique is currently only suitable for treating
sands of particle sizes equal to 0.5–3 mm. Thus, it would be a
great challenge ahead to further expand the usage of MICP
toward improving and enhancing fine-grained soils such as silt
and clay.
Treatment homogeneity/uniformity
Treatment homogeneity (i.e. uniform distribution of CaCO3 along
the treated soil matrix from top to bottom) remains the most critical component to date that requires more attention. Certain techniques have been proposed in the literature to achieve homogeneous
biotreated soil columns in the laboratory. For example, Harkes
et al. (2010) argued that the fixation and distribution of bacteria
through an effective injection method is really crucial in achieving
homogeneous CaCO3 precipitates within a sand column while preventing clogging at a certain point inside the treated column.
Harkes et al. (2010) also indicated that two phases of injection procedure where the fixation solution is injected immediately after the
injection of bacterial suspension are better than the parallel
injection of the two substances at once since they lead to immediate
bacterial flocculation and CaCO3 precipitation near the inlet or
injection point, causing local clogging at that point. Similar findings
were reported by Tobler et al. (2012) who mentioned that more
homogeneous CaCO3 distribution was found using the staged or
two phases injection compared to the parallel injection where the
bacteria were found to be accumulated near the inlet point. It was
found that the fixation solution (mainly consisting of high salinity
solution, i.e. calcium chloride, CaCl2) retarded the movement of
bacteria and helped the adsorption of bacteria into the sand grains.
It was also found that flushing low salinity solution after bacterial
culture may remobilize the bacteria from attaching into the sand
grains and return them to the liquid phase. The ability of bacteria
to be spread out evenly inside the entire column was found to be
the key point to achieve homogeneous CaCO3 precipitation along
the treated length of the soil column. While this technique was
compatible with the small scaled column, no further evidence was
provided as to whether the proposed technique would work in the
upscaled column experiment (e.g. >1 m length). It should be noted
that more research is needed on this particular limitation in order
to optimize MICP as a potential in situ soil improvement
technique.
Ammonia as by-product
One of the disadvantages of MICP urea hydrolysis is the end
product, i.e. ammonia, a substance which is believed to have a
repugnant odor and detrimental impact to health. According to
the Australian Standards (2015), if more than 0.5 mg/L is to be
consumed at a time as direct drinking source, it would certainly
lead to fatal disease to infants like in the case of “blue baby syndrome.” Hence, it is vital to develop a strategy to avoid ammonia leaching to groundwater level at which the intake maybe a
water source for drinking purpose. The authors hereby proposed two pronged strategies: (1) treating the biocementation
ammonia-rich effluent before discharge; and (2) back-feeding
the ammonia as a fertilizer to the surrounding plants.
Future research
The field of biocementation involves a multidisciplinary
research at the confluence of geotechnical engineering, microbiology, ecology and chemical engineering. Despite the fact
that several researches from the above fields have developed
main sets of data and interpretations, currently no study has
yet attempted to determine the optimum MICP process in
terms of the cost and factors involved, for potential commercial
implementation. Once the factors affecting the MICP process
have been optimized in the laboratory at the micro level (i.e. at
the particle to particle contact points) and macro level (i.e. soil
columns set up), further research in terms of upscaling to field
applications at the mega level can then be executed and predicted in the complex true natural environment. The complexity of the coupling effects among the flow, mixing and reaction
contributes to the limited progress in MICP upscaling. Specific
challenges ahead with respect to MICP upscaling include controlling the flow and transport through heterogeneous media,
durability of treatment, permanence of the mixing technique
and mapping of the subsurface stratigraphy at the particle level.
GEOMICROBIOLOGY JOURNAL 11
Future research should also enlarge the scope of MICP applications, not just in terms of strengthening and improving soils
but also harnessing the soil ability to self-heal using the premixed microorganisms in the soil matrix. Bacteria can be reactivated upon loading and undergo the same microbial reaction inside the soil, provided that ample cementation solution
is supplied. By doing so, MICP treatment would be able to heal
the degraded CaCO3 bonds post-shearing (Harbottle et al.
2014). Montoya and DeJong (2013) was the first to observe the
improved behavior of MICP treatment after the healing process. The healing ability of MICP can be used to prevent additional settlement and damage to structures or soils during
earthquakes and aftershocks, for example.
Also, the plausibility of using seawater which already has
calcium ions that provides calcium chloride source naturally
deposited in the solution as a potential substitute for the prepurchased manufactured calcium chloride should not be forgotten. A preliminary study carried out by Cheng et al. (2014a)
has shown the potential use of seawater as a replacement for
the calcium source in the CaCO3 precipitation during MICP
process. It was found that the UCS of biocemented sand samples achieved two times higher strength (with the same amount
of crystals produced) than that of MICP treatment by highly
concentrated calcium and urea solution retaining up to 30%
permeability which signifies a good drainage potential.
Conclusions
This paper provided state-of-the-art review of soil improvement using MICP. Commonly adopted soil improvement techniques have been briefly discussed and limitations of
employing such techniques were discussed. The possible
CaCO3 precipitation mechanisms and potential use of MICP
were carefully reviewed. Soil biocementation using CaCO3 precipitation by urea hydrolysis was explained at great length comprising the biological, chemical and various MICP treatment
processes involved as reported in the literature. The improved
engineering properties of biocemented soils and factors affecting the effectiveness of CaCO3 precipitation were examined.
Lastly, envisioned applications, advantages, limitations and
some thoughts for future research for soil biocementation using
MICP were proposed. Based on the review carried out in this
paper, the following conclusions can be drawn:
Alternative technique that employs the use of natural,
readily available and sustainable materials like bacteria
for soil stabilization is discussed. Although the technique
promised a sustainable approach in utilizing bacteria as
the main reaction catalyst, the issue of the urea hydrolysis
by-product (i.e. ammonia) and the energy intensive production of purified calcium chloride still serve as the
main concern of MICP for soil stabilization.
MICP treatment has high potential for improving the
engineering, mechanical and physical properties of biocemented soils. The envisioned applications of MICP
encompass self-healing of soil, slope stabilization, settlement reduction and erosion control as well as liquefaction
prevention.
Urea hydrolysis is the most preferred CaCO3 precipitation mechanism because it can be easily controlled and
possesses about 90% of CaCO3 production efficiency in a
short period of time. The most favorable bacteria for soil
biocementation come from, but not limited to, highly urease active bacteria (e.g. Sporosarcina pasteurii).
The injection method is the most commonly used technique for MICP treatment in the laboratory in which the
bacteria and cementation solution are injected into the
soil pores.
It is vital to know the effectiveness mechanism of CaCO3
precipitation because it translates to lower cost of MICP
treatment. In return, this can pave the way into possible
commercialization of the technique in the near future.
The advantages of MICP for soil improvement include
lower cost as opposed to chemical grouting method and
other man-made materials, treatment reliability and overall concept which promote sustainability in tandem with
future needs.
Among the limitations of soil biocementation by MICP is
its restricted use in fine-grained soils, upscaling and treatment homogeneity. These parameters entail further future
research in this field.
More research should be focused on optimizing the MICP
process at both the micro and macro levels before its
direct application in the field. Specific challenges need to
be investigated including the flow and transport of
medium through the heterogeneous media, durability of
treatment, permanence of the mixing technique and mapping of the subsurface stratigraphy at the particle level.
Furthermore, self-healing in terms of pre-shearing and
post-shearing of biotreated soils after major earthquakes
and the corresponding aftershocks together with the use
of seawater as substitute for salt are other potential directions in MICP technology.
There are numerous possibilities for this new and exciting branch of biogeotechnology since it offers a sustainable solution for a better tomorrow. Regardless of the
current challenges, MICP has an untapped potential for
relieving some of the present day concerns in soil
improvement.
ORCID
Donovan Mujah http://orcid.org/0000-0002-0829-8548
Liang Cheng http://orcid.org/0000-0002-1767-2108
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14 D. MUJAH ET AL.
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