Effects of Biological Stabilization on the Water
Retention Properties of Unsaturated Soils

Downloaded from ascelibrary.org by Rahim Saffari on 05/14/19. Copyright ASCE. For personal use only; all rights reserved.

R. Saffari 1; E. Nikooee, Ph.D. 2; G. Habibagahi, Ph.D. 3;
and Martinus Th. van Genuchten, Ph.D. 4

Abstract: The soil water retention curve (SWRC) is one of the most fundamental characteristics of unsaturated soils. Because unsaturated
soils are subjected to a range of natural processes and engineered treatments, a thorough understanding is needed of how their retention
properties change when exposed to each separate treatment. In recent years, several biological treatment methods, such as microbial-induced
calcite precipitation (MICP), have been introduced as environmentally friendly techniques. This study investigates the effect of biologicaltreatment by means of Bacillus sphaericus on the SWRC of both coarse-grained and fine-grained soil samples. Pore fluid pH, X-ray
diffraction, and scanning electron microscopy tests are used to better delineate the mechanisms affecting biologically induced alterationsof SWRCs. Results showed that higher bacterial concentrations produced higher air-entry values in coarse-grained soils. However, thefine-grained samples showed an initial increase followed by a decrease in the air-entry value when the bacterial concentration increased.
Changes in the soil porous structure and double-layer thickness are thought to be main reasons causing the different behavior of fine-grainedsoils. DOI: 10.1061/(ASCE)GT.1943-5606.0002053. © 2019 American Society of Civil Engineers.
Author keywords: Microbially induced calcite precipitation; Microbial-induced calcite precipitation (MICP); Soil water retention curve;
Bioclogging; Double layer; Air-entry value.

Introduction
Much of the Earth’s surface is covered by soils that are partiallysaturated with water. The surface layers in arid and semiarid regions, which cover more than one-third of the Earth’s surface, certainly fall into this category (Fredlund and Rahardjo 1993; Dijkema
et al. 2017). Unsaturated soils are subject to far more complexhydromechanical processes than saturated soils. Because of theneed to understand the behavior of unsaturated soils in engineering
projects, the mechanics of unsaturated soils has received much attention in recent years (Fredlund and Rahardjo 1993; Lu and Likos
2004; among many others).
The mechanical behavior of unsaturated soil samples is known
to be very much affected by the degree of fluid saturation. In fact,
the basic hydraulic and mechanical properties of unsaturated soils
are governed largely by the soil water retention curve (SWRC),
which specifies the degree of fluid saturation as a function of
the imposed suction (or capillary pressure). The soil water retention
curve is a fundamental relationship in unsaturated soils for which
different functions have been proposed over the years (e.g., vanGenuchten 1980; van Genuchten and Nielsen 1985; Fredlund and1Graduate Student, Dept. of Civil and Environmental Engineering,
School of Engineering, Shiraz Univ., 71348-51156 Shiraz, Iran (corresponding author). Email: Rhm.saffari@gmail.com
2
Assistant Professor, Dept. of Civil and Environmental Engineering,
School of Engineering, Shiraz Univ., 71348-51156 Shiraz, Iran.
3
Professor, Dept. of Civil and Environmental Engineering, School ofEngineering, Shiraz Univ., 71348-51156 Shiraz, Iran.
4
Visiting Professor, Dept. of Earth Sciences, Utrecht Univ., 3584 CB
Utrecht, Netherlands; Associate Scientist, Center for Environmental
Studies, São Paulo State Univ., 13506-900 Rio Claro, São Paulo, Brazil.
Note. This manuscript was submitted on May 1, 2018; approved on
November 30, 2018; published online on May 14, 2019. Discussion period
open until October 14, 2019; separate discussions must be submitted forindividual papers. This paper is part of the Journal of Geotechnicaland Geoenvironmental Engineering, © ASCE, ISSN 1090-0241.

© ASCE

Xing 1994; Aubertin et al. 2003; Tarantino 2009), including theirrelationship with the mechanical and other properties of unsaturated media (Mualem 1976, Fredlund et al. 1996, Vanapalli et al.
1996; Khalili et al. 2008, Alonso et al. 2010).
Soil water retention is highly dependent on soil texture (notably
particle-size distribution), soil structure, and various environmental
conditions, including soil mineralogy, soil solution strength andcomposition, external loads, and temperature. Thus, for each engineering application, it is important to consider the individual effectsof various environmental conditions, including changes in soil
structure and chemical composition, that may alter the retention
properties of soils.
Soil improvement projects often involve engineering applications for which the retention properties of soils are of considerable importance. In recent decades, new environmentally friendly
techniques for soil improvement have been introduced. These techniques employ various biogeochemical processes, such as bioclogging and biocementation, to change the macroscale mechanical andhydraulic properties of a soil. Bioclogging, for example, has beenused for sealing purposes in geotechnical engineering (Seki et al.
1998; Dennis and Turner 1998).
Various places on or near the Earth’s surface contain naturally
cemented sands, such as stromatolites in shallow high-salinity watersof Western Australia. Formations of this type have inspired the engineering use of microbial-induced calcite precipitation (MICP).
They also point to the durability of this technique; as indicatedby DeJong et al. (2013), MICP-treated sand is expected to be stable
for more than 50 years as long as favorable conditions regarding pH
and alkalinity persist. A 50-year durability is consistent with the lifetime of many earth structures. Because of lower energy consumption
and lower CO2 emissions, MICP is now increasingly used as an environmentally sustainable alternative to chemical and other soil
improvement techniques.
MICP and resulting biocementation processes have been used
to increase shear strength (DeJong et al. 2006; Al Qabany 2011;

04019028-1

J. Geotech. Geoenviron. Eng., 2019, 145(7): 04019028

J. Geotech. Geoenviron. Eng.

Downloaded from ascelibrary.org by Rahim Saffari on 05/14/19. Copyright ASCE. For personal use only; all rights reserved.

Burbank et al. 2013), reduce the liquefaction potential of soils(Montoya et al. 2013; He et al. 2013), and/or improve the mechanical properties of problematic soils such as swelling, collapsible, anddispersive soils (Sadjadi et al. 2014; Sisakht et al. 2015; Moravej
et al. 2015, 2018; Saffari et al. 2017; among others). A comprehensive review of recent advancements in biological soil improvement
techniques has been provided by DeJong et al. (2013).
The mechanical behavior of biologically improved soils (especially but not limited to their surface layers) that are exposed tovarious and often time-dependent hydrological conditions verymuch depends on how changes in prevalent biogeochemical processes lead to changes in the soil water retention properties. Biogeochemical processes can potentially have a broad range of effects,
from changes in the soil microfabric (such as interlayer cations andthe diffuse double-layer thickness), to changes in the soil macrofabric (such as the overall pore-size distribution and the presenceof soil macropores), which all can alter the retention properties of
unsaturated soils.
Several recent studies have addressed the effect of chemical soilstabilizing materials, such as lime and fly ash, on the retention
properties of artificially cemented soils (e.g., Russo 2005; Puppalaet al. 2006; Thudi 2007; Aldaood et al. 2014; Wang et al. 2015;
Suazo et al. 2016; Zhang et al. 2017). But only a very few works
have focused on the effect of bacteria on the soil retention properties. Those studies did not investigate the effect of bacteria involved
in MICP processes (e.g., Dello Sterpaio 2012), and the bacteria involved had different metabolic paths, metabolic products, and biochemical effects. Furthermore, most of the studies concerned theeffects of microorganisms on the retention properties of coarsegrained soils. Therefore, to the best of the authors’ knowledge, nocomprehensive studies have been carried out specifically on the impact of biogeochemical effects induced by MICP processes on thewater retention properties of unsaturated soils. Given the complex
interplay of soil structure and biochemical reactions, especially forfine-grained soils, a need exists for well-designed experiments toaddress these shortcomings in current literature.
Following the aforementioned reasons, this study explores
these different effects using a series of experimental tests on
biologically stabilized soils using MICP technique at various
bacterial densities. For this purpose, the effects of microbial
calcite precipitation on basic soil properties (Atterberg limits)
as well as soil water retention behavior were investigated.
The soil water retention data were obtained by means of filter
paper and pressure-plate techniques. Pore fluid chemistry andmicrofabric of fine-grained soils play an important role in their
hydraulic behavior. Therefore, pore fluid pH measurements aswell as X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests were performed with the aim of examining theunderlying mechanisms affecting biologically induced alterations of SWRCs.
The experiments were carried out using a low-plasticity clay.
After a first set of experiments with the clay, another set of tests
was performed on a coarse portion of the sample to distinguishthe effect of biogeochemical processes on relatively fine-grained
versus coarse-grained media, and at different (microscale andmacroscale) levels.
The structure of the paper is as follows. First, the sample preparation and biological treatment techniques are described, and a brief
explanation is provided of the adopted experimental procedures.
Next, the results from different tests are presented and discussed.
The paper ends with concluding remarks and suggestions forfurther studies. A brief discussion about the durability of MICP
effects, as well as challenges in field applications of MICP, ispresented in the Appendix.
© ASCE

Materials and Methods
Basic Soil Properties
First, the effects of biological treatment on the water retention properties of a fine-grained (swelling) soil sample were studied. The soil
sample used was a synthetic mixture of three different media: a silty
sand, a kaolinite, and a bentonite, with weight percentages of 70,
15, and 15, respectively. The same type of soil was used by Sadjadi
et al. (2014) to investigate the effectiveness of biological stabilization of swelling soils.
Based on the results of a sieve analysis [ASTM 2487 (ASTM
2011)] and following the Unified Soil Classification System, theprepared synthetic soil is classified as a clay with low plasticity.
The results of a standard Proctor compaction test [ASTM D698
(ASTM 2012)] revealed that the optimum moisture content of
the soil was 0.138, and the soil sample had a maximum dry unit
weight of 18.2 kN=m3 .
Sample Preparation and Experimental Procedure
To investigate the effects of biological treatment on the water
retention properties, soil samples were treated with solutions ofBacillus sphaericus bacteria. Lyophilized ampules of Bacillus
sphaericus under the strain name of PTCC 1487 were obtained
from the Iranian Organization of Scientific Research (IROST),
Tehran, Iran. Ingredients of the media used to cultivate the bacteria
are described in Table 1. Except for urea, the various ingredients ofthe culture media were poured into distilled water, after which theresulting solution was autoclaved for 20 min at 120 C. Urea wasthen added by means of filter paper having a pore size of 0.22 μm.
A major focus of this study is the concentration of the bacterial
solution and its effects on changes in the soil water retention properties. The concentration of a bacterial solution is often expressedin terms of its optical density (OD) as measured using a photospectrometer. The following procedure was used to determine the OD.
The system was calibrated with the culture media without bacteria
to obtain a reference line, followed by optical density measurements of the bacterial solution (determined based on the reference
value). To determine the required time for the growth of bacteria, a
growth rate curve was obtained first. The curve specifies the timerequired to reach a certain optical density. The growth rate curve
obtained for the Bacillus sphaericus bacteria is presented in Fig. 1.
For the water retention measurements, soil samples having
moisture contents of 0.12, 0.14, 0.16, 0.18, 0.20, 0.22, and 0.24
by weight were prepared. The samples were allowed to equilibratefor 24 h in closed-lid containers, after which they were staticallycompacted. The compaction rate was set at 2 mm per minute. The
compacted and treated samples were then kept in zipped plastic
containers for 4 days, after which filter papers were placed on
the samples for the retention measurements. Following ASTM
D5298 (ASTM 2016), three filters (Whatman No. 42) were placed
beneath the sample to measure the matric suction. The weightsof the filters were determined after 10 days using a balance with

Table 1. Culture medium composition
Component of the liquid culture media
Sodium bicarbonate
Nutrient brothAmmonium chloride
Yeast extract
Urea

04019028-2

J. Geotech. Geoenviron. Eng., 2019, 145(7): 04019028

Amount (g=L)
2.12
3.00
10.00
20.00
10.00
J. Geotech. Geoenviron. Eng.

0.3

2.5

OD=0, experimental
OD=1.7, experimental
OD=2, experimental
OD=2.3, experimental
OD=0, VG fit
OD=1.7, VG fit
OD=2, VG fit
OD=2.3, VG fit

Treatment solution =12%
0.25

Water content

Optical Density

2
1.5
1

0.2

0.15

0.5
0
1

2

3

4
Time (Day)

5

6

7

Downloaded from ascelibrary.org by Rahim Saffari on 05/14/19. Copyright ASCE. For personal use only; all rights reserved.

102

103

104

Matric suction (kPa)

Fig. 1. Measured growth rate diagram of Bacillus sphaericus.

0.0001 g precision, thus providing measurements of their moisture
contents. The matric suction was obtained based on the middle
filter beneath the sample. The calibration graph of ASTM D5298(ASTM 2016) was then employed to find the suction value corresponding to each moisture content, leading to plots of the soil water
retention curve.
To better understand the effects of biological stabilization on the
microfabric and macrofabric properties of the soil, additional tests
were carried out on the coarse portion of the samples. For this purpose, the portion passing through a No. 40 sieve but retained by a
No. 200 sieve were used. The coarse-grained portion of the sample
drained quickly at relatively low values of the matric suction andhence did not need high-suction experiments. A pressure-plate apparatus was utilized to obtain the water retention data of the control
and treated coarse-grained soil samples. The pressure-plate experiments used coarse-grained samples having unit dry weights of
17 kN=m3 , which were prepared using similar procedures as thoseused for the fine-grained samples.
Finally, to investigate expected mineralogical alterations inducedby the Bacillus sphaericus bacteria, a series of complementary experiments were carried out, including SEM and XRD measurements,on the treated and untreated samples. For the XRD tests, a D8Advance (Bruker AXS Corporation, Karlsruhe, Germany) was used,
and the SEM tests used a Cambridge S360 (Leica Cambridge,
Cambridge, UK). The pore fluid chemistry (notably pH) andAtterberg limits were monitored before and after treatment.

Results and Discussion
Results of the various experiments are now provided. First, the results of the soil water retention tests are presented, followed by how
Bacillus sphaericus bacteria affected the soil physical and chemical
properties. Then, the effects of bacteria on the soil microfabric andrelated changes in the water retention properties of the soil aredescribed.
Results of the Soil Water Retention Experiments
Results of the effect of Bacillus sphaericus on the treated soil samples are presented in two parts. At first, measurements of the matricsuction of the samples treated with a 12% biological solution(6% bacterial solution and 6% calcium chloride) are presented, followed by those for experiments using a 14% biological solution(7% bacterial solution and 7% calcium chloride). The concentrations of bacterial solution and calcium chloride are expressed interms of weight percentages of the dry weight of the soil. The soil
water retention measurements are presented in terms of gravimetricwater contents.
© ASCE0.1
101

8

Fig. 2. Soil water retention curves of fine-grained samples mixed with
12% treatment solution, as obtained using the filter paper technique, atvarious bacterial densities. VG stands for the van Genuchten water
retention equation.

0.3
Treatment solution=14%
0.25

Water content

0

OD=0, experimental
OD=1.7, experimental
OD=2, experimental
OD=2.3, experimental
OD=0, VG fit
OD=1.7, VG fit
OD=2, VG fit
OD=2.3, VG fit

0.2

0.15

0.1
101

102

103

104

Matric suction (kPa)

Fig. 3. Soil water retention curve of the fine-grained samples mixed
with 14% treatment solution, as obtained using the filter paper technique, at various bacterial densities.

Gravimetric SWRCs of the samples treated with the 12% and
14% biological solutions are depicted in Figs. 2 and 3, respectively.
In addition to the amount of treatment solution, the OD of the solution (as a measure of the bacterial concentration) was considered
as an additional variable. Soil water retention curves were obtained
at OD values of 0, 1.7, 2, and 2.3.
Observed water contents (w) as a function of the suction head(h) were analyzed in terms of the standard van Genuchten (1980)
equation given bywðhÞ ¼ wr þ

ws − wr½1 þ ðαhÞn 1−1=n

h≥0

ð1Þ

where wr and ws = residual and saturated gravimetric water contents, respectively; and α and n (with n > 1) = quasi-empiricalshape factors. The value of α is approximately the inverse of
the air-entry value of a soil. The water retention curves as fitted tothe data using the RETention Curve (RETC) code of van Genuchten
et al. (1991) are shown in Figs. 2 and 3. Values of the van Genuchten(VG) parameters obtained for the samples mixed with the 12% treatment solution are presented in Table 2, and those for the 14%
treatment solution are listed in Table 3.
As shown in Figs. 2 and 3, by treating the soil with the culture
media and bacterial solutions, the suction value required to drain
the soil to a certain water content increases first as the SWRC
moves upward, but then the curves move back and downward with

04019028-3

J. Geotech. Geoenviron. Eng., 2019, 145(7): 04019028

J. Geotech. Geoenviron. Eng.

0.02

Table 2. van Genuchten hydraulic parameters obtained for biologicallytreated fine-grained soil sample mixed with 12% treatment solution

VG parameterwswrnα (kPa−1 )

0

1.7

2

2.3

0.217
0
1.17
0.00722

0.249
0
1.43
0.00101

0.236
0
1.36
0.00162

0.230
0
1.30
0.00264-parameter (1/kPa)

Optical density

Treatment solution 12%
Treatment solution 14%
0.015

0.01

0.005

0

Downloaded from ascelibrary.org by Rahim Saffari on 05/14/19. Copyright ASCE. For personal use only; all rights reserved.

Table 3. van Genuchten hydraulic parameters obtained for biologicallytreated fine-grained soil sample mixed with 14% treatment solution
Optical density
VG parameterwswrnα (kPa−1 )

0

1.7

2

2.3

0.217
0
1.17
0.00722

0.251
0
1.52
0.00051

0.226
0
1.18
0.00242

0.227
0
1.10
0.0181further increases in the bacterial concentration. The authors hypothesize that bacterial soil treatment affects the water retention properties of unsaturated soil through different biogeochemical processes.
By increasing the bacterial concentration (from OD ¼ 0 to 1.7),
processes such as bioclogging and biocementation lead to smaller
pore radii. Therefore, the number of pores with smaller radii that
contribute to the same water content increases; consequently, thematric suction required to drain the soil to that water content rises.
Furthermore, it is expected that the air-entry value (which is closely
related to the pore-size distribution) will be altered.
Bioclogging and biocementation are accompanied by otherbiochemical processes, most notably by the effect of bacterial treatment on soil pH. As bacterial cell concentrations further increase
and as the soil pH changes, another mechanism comes into play:the effect of biological treatment on the double diffuse layer ofthe soil. As the cell concentration increases, the ammonium andcalcite production rates are affected. For example, Stocks-Fischer
et al. (1999) demonstrated that at higher cell numbers of calciteprecipitating bacteria, the ammonium production rate decreases
considerably, which means that at higher cell concentrations, thefinal pH will be less than that at lower cell concentrations. The
lower the pH, the lower the diffuse double-layer thickness (Mitchell
1993; van Olphen 1991; Gratchev and Towhata 2013). The lower
the double-layer thickness, the lower the water retention tendency(François and Ettahiri 2012).
However, care should be taken because the results of StocksFischer et al. (1999) described the metabolism of Bacillus pasteurii,
whereas the present study used Bacillus sphaericus (anothercalcite-precipitating bacteria). Nonetheless, one can at least speculate that the changes in the water retention curves, first upward andthen downward, can be attributed to the twofold influence of thecell concentration on soil pH and precipitation-induced poreclogging, including their competition. Additional evidence for thechange in soil pH of the treated soil are presented separately insubsequent sections where the biochemical reactions involved arealso summarized.
The presence of bacteria and their active involvement in the biogeochemical processes operative in a complex clay-water environment involves the two processes causing the forward and thenbackward (nonmonotonic) movement of the SWRCs. Variationsin the α-parameter for the biologically treated clay mixed with© ASCE

0

0.5

1

1.5

2

2.5

OD

Fig. 4. Effect of different treatment solutions and bacterial concentration on the α-parameter of fine-grained soil (as obtained using the van
Genuchten retention model).

12% and 14% treatment solutions are depicted in Fig. 4. The
α-parameter, which is closely related to the inverse of the soil airentry value, is of importance when considering the hydromechanicalbehavior of unsaturated soils. The two treatments (12% and 14%)show different trends in the value of α with increasing OD values(bacterial concentrations), which are attributed to the competition
between different biochemical processes and their effect on soil fabricand chemistry.
For fine-grained soils, it is important to investigate how both
capillary water and adsorbed water are influenced by microbial
calcite precipitation. Although van Genuchten’s classical model
provides a general understanding of how retention properties areaffected, another model is needed to account for adsorbed water.
For this purpose, the model proposed by Lu (2016) was used toinvestigate the effect of Bacillus sphaericus on the adsorbed watercontent of fine-grained soil samples. The model of Lu (2016) isexpressed as follows:

 
 
h − hmax mwðhÞ ¼ wac 1 − exphpﬃﬃﬃ h − hc
þ 0.5 1 − erf
2
hc

 
 

h − hmax m
1½1 þ ðαhÞn n−1
× ws − wac 1 − exphð2Þ
where ws = saturated water content, as before; 1=α = air-entry
suction; n = pore-size distribution coefficient; hc = cavitation suction; hmax = highest suction; wac = adsorption capacity; and m =adsorption strength. Whereas the original formulation of Lu (2016)is expressed based on volumetric water content, here, the same
formula in terms of gravimetric water content will be used. When
all parameters of Lu’s (2016) equation are obtained, one can determine the adsorbed water content using

 
 
h − hmax m
wa ðhÞ ¼ wac 1 − expð3ÞhValues of the Lu (2016) parameters obtained for the samples
mixed with the 12% treatment solution are presented in Table 4,
and those for the 14% treatment solution are listed in Table 5. Fig. 5
presents the variation in the α-parameter as obtained using Lu’s
(2016) method versus optical density. The α-parameter values obtained with the Lu (2016) method followed the same pattern as

04019028-4

J. Geotech. Geoenviron. Eng., 2019, 145(7): 04019028

J. Geotech. Geoenviron. Eng.

Optical density
Lu parameters

1.7

2

2.3

2.011 × 105 2.016 × 105
1,422
192.8
0.15650.18670.038120.01797
1.223
5.408
0.004077
0.004782

2.013 × 105
186.3
0.174
0.01626
7.239
0.007089

Table 5. Values of the Lu (2016) model parameters obtained for biologically treated fine-grained soil sample mixed with 14% treatment solution

Treatment solution=14%

OD=0
OD=1.7
OD=2
OD=2.3

0.8
0.6
0.4
0.2
0
0
10

1

10

2

3

4

10
10
10
Matric suction (kPa)

5

10

6

10

Fig. 7. Adsorbed retention curve for the fine-grained soil samples
mixed with 14% treatment solution.

Optical density
Lu parameters

02.464 × 105
645.9
0.154
0.017211.544
0.01017hmax (kPa)hc (kPa)wacmn
α (kPa−1 )

1.7

2

2.3

2.589 × 105 2.009 × 105 2.358 × 105
1,592
532.4
4,312
0.18680.191
0.17860.029130.0302
0.026
1.125
4.937
2.262
0.005094
0.0074850.01483

0.016
Treatment solution 12%
Treatment solution 14%-parameter (1/kPa)

0.014
0.012
0.01
0.008
0.006
0.004
0

0.5

1

1.5

2

2.5

OD

Fig. 5. Effect of different treatment solutions and bacterial concentration on the α-parameter of fine-grained soil [as obtained using the Lu(2016) retention model].

1
OD=0
OD=1.7
OD=2
OD=2.3Treatment solution=12%0.8

acNormalized adsorption
water content (w/w )Downloaded from ascelibrary.org by Rahim Saffari on 05/14/19. Copyright ASCE. For personal use only; all rights reserved.

02.469 × 105
645.7
0.154
0.017181.544
0.01017hmax (kPa)hc (kPa)wacmn
α (kPa−1 )1Normalized adsorption
water content (w/wac )Table 4. Values of the Lu (2016) model parameters obtained for biologically treated fine-grained soil sample mixed with 12% treatment solution

0.6

0.4

0.2

0
100

101

102

103

104

105

106

Matric suction (kPa)

Fig. 6. Adsorbed retention curve for the fine-grained soil samples
mixed with 12% treatment solution.
© ASCE

those determined using van Genuchten’s equation. Figs. 6 and 7
illustrate the adsorbed water content curves. As indicated in thesefigures, although there is an increase in adsorbed water until an
optical density of 1.7, the values tend to decrease by increasing
the bacterial concentration. As mentioned previously, such a pattern
is attributed to the competition and effect of different mechanisms.
The involved mechanisms are listed in Table 6.
At lower bacterial concentrations, the effect of precipitated calcite crystals is dominant relative to the other mechanisms. The
effects of precipitated calcite crystals are twofold. Calcite crystals
precipitated on grains and in pores reduce the size of pores andincrease the suction required for the air phase to invade the partially clogged pores. The precipitated calcite on the grains can, in
fact, increase their surface water adsorption. A comparison between the adsorption energy of calcite and kaolinite presented
by Ohkrimenko et al. (2013) can shed light onto this effect(Fig. 8). Furthermore, as stated by Ercole et al. (2007) Bacillus
sphaericus produces extracellular polymeric substances (EPS) in
addition to calcite. Having a spongelike microstructure, EPS has
been shown to have a considerable water-holding capacity, as
much as 10 or more times of its dry weight (e.g., Zheng et al.
2018). The high affinity of precipitated calcite and the waterholding capacity of EPS can lead to the observed increase in
the adsorbed water content.
Precipitation of calcite and other biological byproducts (such as
EPS) on the surface of grains additionally can increase the grainsurface roughness, thereby increasing the specific surface area
as well as the water film thickness on the grains. There is muchevidence in the literature that an increase in surface roughness will
increase the water film thickness (Kibbey 2013; Zheng et al. 2015).
Given a relationship proposed by Tuller and Or (2005) connecting
the soil capacity for adsorbing water (θ) and the soil specific surface
area (i.e., θ ¼ h × SA × ρw , where h is water film thickness, SA
stands for specific surface area, and ρw is water density), one would
expect the adsorbed water content to increase. All of these mechanisms are in favor of increasing the adsorbed water content. As
stated previously, the pore-size reduction as a result of the production of EPS and calcite leads to an increase in capillary water retention, and hence the water retention capacity of a soil.
However, by increasing calcite precipitation (because of the increase in bacterial concentration), a lower pH is attained in the soilwater-bacteria system, which for the case of fine-grained soils affects
the double layer in two ways. The lower pH directly results in a lower
thickness of the double layer and consequently lower water retention.
In addition, the dynamics of calcite precipitation can be affectedby pH. Because pH is known to affect the precipitation-solution

04019028-5

J. Geotech. Geoenviron. Eng., 2019, 145(7): 04019028

J. Geotech. Geoenviron. Eng.

Table 6. Processes affecting the water retention properties of fine-grained soils in MICP technique
Process

Cause

EffectBioclogging due to precipitation of calcite and EPS on Increase in water retention
and among grains
Increase in grain roughness
Precipitation of calcite and EPS on the surface of grains Increase in water film thickness and adsorbed
water retention
Changing grain surface adsorption properties Precipitation of calcite and EPS (i.e., high water-holding Increase in adsorbed water retention
capacity of EPS and adsorption affinity of calcite)
Change in surface wettability
Precipitation of calcite and EPS on the surface of grains Depending on the amount of EPS and calcite,
different contact angles can be achieved
Change in double-layer thickness
Change in pH induced by bacterial metabolism andReduction in adsorbed water retention due to pH
calcite precipitation
decrease
Precipitation-solution dynamics of calcite in Change in pH induced by bacterial metabolism andReduction in adsorbed water retention due to pH
favor of the presence of free Ca2þ cations, and calcite precipitation
decrease
their substitution in clay structure

120

0.3

110

0.25

100
90
80
70

0.15
0.1

0
101

50

102

103

Matric suction (kPa)
0

5

10

15

20

25

30

35

Adsorption ( mole/m2)

Fig. 8. Comparison of the adsorption energies of calcite and kaolinite.
(Data from Ohkrimenko et al. 2013.)

dynamics of calcite [Eq. (4)], at lower pH values, the rate of calcium
precipitation is reduced (Krauskopf and Bird 1995)−
Ca2þ þ HCO−
3 þ OH → CaCO3 þ H2 O

ð4ÞFor these reasons, not all calcium cations would be consumed
and precipitated. Although the resulting free Ca2þ cations will not
influence a coarse-grained soil, for fine-grained soils, they will substitute cations present in the double layer and, consequently, decrease its thickness. The cation substitution mechanism has also
been proposed to influence the retention properties of lime-treatedand cement-treated soils (Thudi 2007).
Several other processes, generally having less influence, mayalso be operative. They include an increase in viscosity of adsorbed
water due to EPS (Roberson and Firestone 1992; Zheng et al. 2018)
and changes in the contact angle of the grain surface due to precipitation of EPS and calcite, which seem to generally lead tohigher contact angles [e.g., Ethington (1990) reported values of thecontact angle on pure calcite substrate]. Of course, the exact contact
angle would depend on the amount of precipitated EPS and calcite
on the grains, which requires additional experiments (e.g., bymeans of environmental scanning electron microscopy or X-ray
microtomography), possibly in future studies.
Still, the competition of the various processes discussed in preceding paragraphs appear to explain the patterns observed in theadsorbed water content curves as obtained with the Lu (2016)
model for the fine-grained soil samples.
© ASCE

0.2

0.05

60

40

OD=0, experimental
OD=1.7, experimental
OD=2, experimental
OD=2.3, experimental
OD=0, VG fit
OD=1.7, VG fit
OD=2, VG fit
OD=2.3, VG fit

Treatment solution=14%

Merck Calcite
Kaolinite

Water content

Isosteric heat of adsorption
Qst(kJ/mol)

Downloaded from ascelibrary.org by Rahim Saffari on 05/14/19. Copyright ASCE. For personal use only; all rights reserved.

Reduction in soil pore and throat radii

Fig. 9. Soil water retention curves of the coarse-grained samples mixed
with 14% treatment solution obtained with pressure-plate tests at various bacterial densities.

In order to further distinguish the effects of the aforementioned
mechanisms (i.e., bioclogging-induced changes in the soil pore
network and pH-induced changes in the double-layer thickness,among others), the SWRCs of the coarse portion of the soil samples(before and after treatment with the 14% biological solution) wereobtained. For the coarse-grained soil, the dominant retention
mechanism is capillarity, and the pH-induced changes in the soil
double layer should be absent, which would lead to a monotonic
change in the air-entry value and consequently the α-parameter.
The SWRCs of the coarse portion of the soil sample (passing a
No. 40 sieve but retained by a No. 200 sieve, which is designatedas sand) are shown in Fig. 9. The parameters of the van Genuchten
equation for this soil sample are presented in Table 7. The variationin α versus bacterial concentration is illustrated in Fig. 10. As
expected, in the absence of double-layer effects, the first hypothesized mechanism (bioclogging and biocementation) is the sole

Table 7. van Genuchten parameters obtained for coarse-grained soil
samples treated with 14% biological solution
Optical density
VG parameterwswrnα (kPa−1 )

0

1.7

2.0

2.3

0.223
0
6.32
0.0208

0.248
0
4.56
0.0144

0.249
0
5.31
0.00973

0.267
0
3.50
0.0088004019028-6

J. Geotech. Geoenviron. Eng., 2019, 145(7): 04019028

J. Geotech. Geoenviron. Eng.

0.025

8.8
Full bacterial solution (inclusive of CaCl2and culture media)
CaCl2+ culture media

8.6

0.02
8.4

pH

-parameter (1/kPa)-parameter versus OD

0.015

8.2
8

0.01

7.8

0.005
0

0.5

1

1.5

2

0

1

2

3

2.5

4

5

6

7

8

9

10

Time(Day)

Downloaded from ascelibrary.org by Rahim Saffari on 05/14/19. Copyright ASCE. For personal use only; all rights reserved.

OD

Fig. 10. Variation in the van Genuchten α-parameter as a function ofOD (sand sample and 14% treatment solution).

mechanism causing an increase in air-entry value (a decrease in
α-parameter), meaning a forward movement of the curves.
Effect of Bacteria on Soil Properties and MicrofabricAtterberg Limit Test
Table 8 presents the effects of biological treatment (with 14% biological solution, OD ¼ 2.3) on the plasticity properties of the soil interms of the Atterberg limits as obtained with ASTM D4318(ASTM 2017) standard tests. The results reveal that the liquid limit
reduced considerably as a result of biological stabilization, but thatthe plasticity limit was not noticeably affected. The change in theplasticity index is induced mainly by changes in the liquid limit.
This finding confirms the proposed mechanisms that biologicaltreatment of the fine-grained soil not only causes changes in the
pore-size distribution and soil porous structure, but also induces
biochemical processes affecting the clay mineral-water system.
These biogeochemical effects (pH changes, double-layer alterations, and pore fluid chemistry alterations) contribute to changes
in the water adsorption potential. They eventually alter both theAtterberg limits as well as the soil water retention curves, which
both are sensitive to changes in the soil microfabric. To further
elaborate the effects of biological treatment on the soil microfabricand to determine possible biogeochemical interactions, pH measurements and XRD and SEM analyses were performed. Results
are discussed in subsequent sections.
Soil Pore Fluid Chemistry: Measurement of Soil pH
In order to monitor soil pore fluid geochemical changes, variationsin the pH of two separate soil samples were investigated. The tests
involved one soil sample treated with the culture media and CaCl2
only, and another sample treated with the 14% biological solution(OD ¼ 2.3) in addition to the culture media and CaCl2 (precipitating agent). Results are presented in Fig. 11. The initial pH of thesolution containing culture media and CaCl2 was 8.0, and theuntreated soil sample had a pH of 8.3.

Table 8. Atterberg limits of fine-grained samples treated with 14%
biological solution (OD ¼ 2.3) and untreated samples
Soil property
Liquid limit (%)
Plastic limit (%)
Plasticity index (%)© ASCE

Untreated sample

Treated sample

41
17
24

30
16
14

Fig. 11. Variations in the pore fluid pH with time following treatment.

Consistent with previous studies (e.g., van Paassen 2009; Al
Qabany et al. 2012), the results in Fig. 11 indicate that the urease
enzyme is responsible for the initial increase in soil pH, whereas
calcite precipitation later tends to lower the pH back to neutral, withthe actual final pH of the soil sample depending upon the reaction
rates and substrate concentrations. As Fig. 11 indicates, for the mixture of the full bacterial solution and soil, a noticeable decrease in
pH occurred, which can be attributed to the formation of calcite
crystals. The decreasing pattern in pH with time is not that pronounced when the soil is treated with a solution containing only
the culture media and CaCl2 .
To better understand the effects of biochemical processes on theobserved soil pH changes, the MICP reactions are summarized asfollows. The hydrolysis of urea is
NH2 − CO − NH2 þ H2 O → 2NH3 þ CO2

ð5Þ

Decomposition of NH3 and creation of hydroxyls is
2NH3 þ 2H2 O → 2NH4 þ þ 2OH−

ð6Þ

Creation of carbonic acid as a result of CO2 production by bacteria is
CO2 þ OH− → HCO−
3

ð7Þ

CaCO3 precipitation is−Ca2þ þ HCO−
3 þ OH → CaCO3 þ H2 O

ð8ÞNet urea hydrolysis reaction is
NH2 − CO − NH2 þ 3H2 O → 2NH4 þ þ HCO3 − þ OH−

ð9Þ

As Fig. 11 shows for the soil treated with the full bacterial solution, the pH first rises to 8.5, which can be attributed to decomposition of urea and the production of OH− . But after this, adecrease in the trend occurs due to consumption of OH− during
the precipitation process [Eqs. (5)–(9)], leading to a pH of 7.8 after
10 days. For the sample mixed only with the culture media, thedecrease in pH is considerably less than that of the fully treatedsample. The presence of calcium cations in the solution containing
the culture media and CaCl2 (treatment solution without bacteria)causes a consumption of hydroxyl ions. The formation of calcite
crystals, with the accompanying consumption of hydroxyl ionsand decreasing pH values, is intensified by the presence of Bacillus
sphaericus, which is indicative of the catalyzing role of bacteria
and their potential impact on clay-water interactions.

04019028-7

J. Geotech. Geoenviron. Eng., 2019, 145(7): 04019028

J. Geotech. Geoenviron. Eng.

L-SE1

EHT=20.0 KV WD=11 mm MAG = X 600. PHOTO=0

L-SE1

Downloaded from ascelibrary.org by Rahim Saffari on 05/14/19. Copyright ASCE. For personal use only; all rights reserved.

EHT=20.0 KV WD=12 mm MAG = X 600. PHOTO=050.0 µm50.0 µm(a)(b)
L=SE1 EHT=20.0 KV WD=10 mm

MAG= X 600.

PHOTO=050.0 µm(c)
Fig. 12. SEM micrographs of (a) untreated sample; (b) treated sample after 4 days (treated with 14% biological solution, OD ¼ 2.3); and (c) treatedsample after 60 days (treated with 14% biological solution, OD ¼ 2.3).

Imaging with Scanning Electron Microscopy
SEM images were used to investigate the presence of CaCO3 crystals. SEM monographs of, respectively, the untreated sample andsamples after treatments of 4 and 60 days (using the 14% biologicalsolution, OD ¼ 2.3) are presented in Figs. 12(a–c). The plots showthat the amount of produced calcite crystals increases as time progresses. Calcite precipitation on the soil grains (biocementation)and into the pore space (bioclogging) causes a decrease in the
pore-size distribution, the primary mechanism for the observedchanges in the soil water retention curves. The resulting change
in water retention of the fine-grained soils is mainly a functionof the competition between the two proposed mechanisms of biocementation as well as changes in a double-layer thickness of thefine-grained soils.
X-Ray Diffraction
XRD analyses were further carried out to identify the compositionof the precipitates. Both treated and untreated samples were investigated for this purpose to explore the relative changes in soil
© ASCE

composition. Particular attention was paid to identifying calcite,which presumably is the main material produced by biologicaltreatment.
To determine whether or not a certain compound is present in asample, three main peaks of that compound in the standard XRD
patterns should be compared with the peaks found in the XRD
pattern of that sample (Sanches et al. 2003; Sun et al. 2015). If they
are present, the soil contains that compound. Increasing and decreasing peak values then demonstrate an increase or decreasein the amount of that compound in the soil sample, respectively.
Comparing the XRD monographs of the sample treated with thebiological solution (14%, OD ¼ 2.3) and the untreated sample(Fig. 13) indicates that calcite crystals are present in both samples,but that the amount had increased remarkably in the treated sample.
Possible Experimental Errors and Their Importance
Performing soil water retention tests, especially on fine-grainedsoils is, per se, a time-consuming task. In addition, investigating

04019028-8

J. Geotech. Geoenviron. Eng., 2019, 145(7): 04019028

J. Geotech. Geoenviron. Eng.

104

Calcite (00-001-0837)
Treated
Untreated

104

103

CountsCounts

103

102

102

101

101
100

100
35

40

45

55

60

65

70

34

4

10

3

10

2

10

1

34.5

35

35.5

(b)

36

36.5

37

37.5

38

Position[°2Theta]

10

4

Calcite (00-001-0837)
Treated
Untreated

Calcite (00-001-0837)
Treated
Untreated

CountsCounts50

Position[°2Theta]

0

10

3

10

2

10

1

0

10
38.5

39

(c)

39.5

40

40.5

10
42.5

43

43.5

(d)

Position[°2Theta]

44

Position[°2Theta]

Fig. 13. (a) XRD patterns of the untreated samples and samples treated with 14% biological solution, OD ¼ 2.3, and calcite; (b) XRD patterns of thefirst main peak; (c) XRD patterns of the second main peak; and (d) XRD patterns of the third main peak.

104
SWRC experimental datapoints (Mean ± Standard Deviation)(Treatment solution=12% ,OD=2.3)

0.3

Water content

Matric suction (kPa)

Downloaded from ascelibrary.org by Rahim Saffari on 05/14/19. Copyright ASCE. For personal use only; all rights reserved.

(a)10

Calcite (00-001-0837)
Treated
Untreated

103

102

101
0.1

0.25
0.2
0.15
0.1
0.05

0.12

0.14

0.16

0.18

0.2

0.22

0.24

0
0
10

Water content

Fig. 14. Extent of errors in the soil water retention curve of the finegrained samples mixed with 12% treatment solution as obtained fromfilter paper tests at OD ¼ 2.3.

various factors considered in this study requires a multitude of experiments, which added to the complexity of the experimental design. Therefore, given the available laboratory and time resources,
several control experiments were performed to inspect the extent towhich experimental errors could influence the obtained parameters
and, consequently, this study’s conclusions. For this purpose, extra
filter paper tests were performed on the fine-grained soil sample
treated with 12% biological solution having an OD value of 2.3.
Furthermore, pressure-plate tests were performed on the sand sample treated with 14% biological solution having an OD value of 2.
Because the moisture content of the sample can be controlled
and the suction is measured for filter paper tests, whereas for© ASCESWRC experimental datapoints (Mean ± Standard Deviation)(Treatment solution=14%, OD=2)

10

1

10

2

10

3

Matric suction (kPa)

Fig. 15. Extent of errors in the soil water retention curve of the coarsegrained samples mixed with 14% treatment solution as obtained frompressure-plate tests at OD ¼ 2.

the pressure-plate test, the intended suction value is imposed
and the water content is measured, the error bars are considered forsuction and water content for these two tests, respectively (Figs. 14
and 15). Furthermore, the relative errors between the obtained retention parameters of the original and control experiments (replicates) were obtained and are presented in Tables 9 and 10. As
can be seen, the relative errors in the retention model parameters
are fairly low (maximum of 3.4%), which indicates a good levelof repeatability (reliability) of the obtained retention data andparameters. However, given the complex interplay of soil mineral

04019028-9

J. Geotech. Geoenviron. Eng., 2019, 145(7): 04019028

J. Geotech. Geoenviron. Eng.

Table 9. Soil water retention parameters obtained for biologically treatedfine-grained soil sample mixed with 12% treatment solution (OD ¼ 2.3)
VG parameter

Original
experiment

Repeated
test case

Relative
error (%)

wswrn
α (kPa−1 )0.23020
1.30
0.002640.230801.29
0.00273

0.3
0
0.8
3.4

Downloaded from ascelibrary.org by Rahim Saffari on 05/14/19. Copyright ASCE. For personal use only; all rights reserved.

Table 10. van Genuchten parameters obtained for biologically treatedcoarse-grained samples mixed with 14% treatment solution (OD ¼ 2)
VG parameter

Original
experiment

Repeated
test case

Relative
error (%)

wswrn
α (kPa−1 )

0.249
0
5.31
0.00973

0.252
0
5.14
0.01002

1.2
0
3.2
2.98

characteristics, the soil-water-bacteria environment, and the biochemical factors involved in MICP, further studies with various
clay mineralogical properties and soil types are recommended
so as to broaden the scope and applicability of the results and toprovide a more in-depth analysis of the observed patterns andmechanisms involved.

Conclusions
This study investigated the effects of biological treatment using
Bacillus sphaericus on the water retention properties of a finegrained soil (low-plasticity clay) and a coarse-grained soil (sand).
Results demonstrated that biological treatment alters the soil water
retention curves of the fine-grained and coarse-grained samples indifferent ways. By increasing the bacterial concentration, the airentry value of the SWRCs (or van Genuchten α-parameter) of thecoarse-grained soil monotonically increased (decreased), whereas
for the fine-grained soil, the air-entry value first increased and thendecreased. For the latter case, at a certain saturation, the valueof suction required to drain the soil at that saturation hence firstincreased and then decreased.
It was hypothesized that two mechanisms govern the observed
behavior in fine-grained soils: biocementation, which clogs poresof smaller radii and thus causes SWRCs to shift forward and up,
and biogeochemical processes caused by the additional presence ofbacteria, which lead to a decrease in the diffuse double-layer thickness and lower matric suctions at the same fluid saturation. Atterberg limit tests further confirmed the effect of bacteria on the water
adsorption tendency of soil, and XRD and SEM tests indicatedcalcite precipitation and changes in the microfabric of the soil
samples.
The conducted tests were limited to one relatively fine-textured
and one relatively coarse-textured sample. More extensive tests
are required to investigate the effects of microbial-induced calcite
precipitation on soils with different grain-size distributions anddifferent mineralogy and to investigate in depth other treatment scenarios. Finally, the use of environmental scanning electron microscopy (ESEM) and X-ray microtomography on the treated samples
is recommended to further explore the different processes discussed
in this study.
© ASCE

Appendix. Durability of MICP Effects: Limitations,
Scope of Application, and Future Perspectives
One may ask the extent to which the effects induced by MICP are
durable. Although the primary focus of this study was not the
durability of MICP, this aspect has practical significance and isworthy of some comments. The use of MICP as a soil improvement
technique aims for changes in soil fabric and structure in order toimprove soil engineering properties (e.g., shear strength, collapse
potential, and unconfined compressive strength). For such applications, the main concern is not necessarily the longevity of the bacteria but the durability of the calcite precipitated in the sample (andthe structure formed therein) against severe environmental conditions. Cheng et al. (2016) examined biologically treated sand columns where only 0.7 g weight loss was observed when the samples
were exposed to 12 L of acid rain, corresponding to 5 years of rainfall (1,000 mm=year). Salifu et al. (2016) investigated the erosion
mitigation of sandy soil foreshore slopes against tidal waves. MICP
treatment of the slopes resulted in negligible erosion and no change
in the slopes, whereas the untreated slopes collapsed to the angle ofrepose at the first tidal event, followed by the erosion of 0.2° of theslope angle per tidal event during subsequent tidal events. Moravej
et al. (2018) further investigated the potential of using MICP
to improve dispersive soils. They found that MICP was capableof decreasing the level of dispersivity of the soils involved. Thesestudies clearly demonstrate the durability of MICP effects on thesoil structure for different environmental conditions. The use of
MICP as a protective layer in the exterior of historical monuments
adds further evidence of its potential level of durability (Perito
et al. 2014).
There are of course optimal environmental conditions for MICP
process, which include appropriate temperature and pH ranges.
Kim et al. (2018) showed that different calcite precipitation–inducing
microorganisms express different sensitivity levels to environmental
conditions (notably pH and temperature). They found that the optimal temperature for MICP processes was 30°C. However, calcite
precipitation did not stop at higher temperatures. Moreover, once calcite precipitates are formed, they are durable under extreme environmental conditions (rainfall and temperature, among others).
MICP has also shown considerable success in field trials aimed
at ground reinforcement (van Paassen et al. 2009, 2010; DeJonget al. 2014; Gomez et al. 2015; Esnault-Filet et al. 2016; Terzis
and Laloui 2017). However, applications of MICP face a great challenge in terms of the nonheterogeneous distribution or penetrationof treatment across and into soil volumes. New procedures have to
be formulated and further studies are required when improving soil
mechanical and hydraulic properties for relatively large soil volumes is concerned. Furthermore, in order to treat large volumesof soil, more economic methods for producing the ingredientsof the culture media are needed. A possible alternative to deal withcomplex on-site bacteria cultivation is to use MICP utilizing
lyophilized cells instead of bacterial cells in their vegetative state(Terzis and Laloui 2018). A thorough review of MICP, and limitations encountered when upscaling to field conditions has been
given by DeJong et al. (2013) and Haouzi and Courcelles (2018).

Acknowledgments
The authors would like to thank the journal editors and anonymous
reviewers for their constructive comments. The second author
would like to thank the Environmental Hydrogeology Group of
Utrecht University for hosting him for several research visits that
led to this study.

04019028-10

J. Geotech. Geoenviron. Eng., 2019, 145(7): 04019028

J. Geotech. Geoenviron. Eng.

Downloaded from ascelibrary.org by Rahim Saffari on 05/14/19. Copyright ASCE. For personal use only; all rights reserved.

References
Aldaood, A., M. Bouasker, and M. Al-Mukhtar. 2014. “Soil–water characteristic curve of lime treated gypseous soil.” Appl. Clay Sci.
102 (Dec): 128–138. https://doi.org/10.1016/j.clay.2014.09.024.
Alonso, E. E., J. M. Pereira, J. Vaunat, and S. Olivella. 2010. “A microstructurally based effective stress for unsaturated soils.” Géotechnique
60 (12): 913–925. https://doi.org/10.1680/geot.8.P.002.
Al Qabany, A. 2011. “Microbial carbonate precipitation in soils.” Ph.D.
dissertation, Dept. of Engineering, Geotechnical Engineering Group,
Univ. of Cambridge.
Al Qabany, A., K. Soga, and C. Santamarina. 2012. “Factors affecting
efficiency of microbially induced calcite precipitation.” J. Geotech.
Geoenviron. 138 (8): 992–1001.
ASTM. 2011. Standard practice for classification of soils for engineering
purposes (unified soil classification system). ASTM D2487. WestConshohocken, PA: ASTM.
ASTM. 2012. Standard test methods for laboratory compaction characteristics of soil using standard effort (12,400 ft-lbf = ft3 (600 kN-m = m3)).
ASTM D698. West Conshohocken, PA: ASTM.
ASTM. 2016. Standard test method for measurement of soil potential (suction) using filter paper. ASTM D5298. West Conshohocken, PA:
ASTM.
ASTM. 2017. Standard test methods for liquid limit, plastic limit, and plasticity index of soils. ASTM D4318-17e1. West Conshohocken, PA:
ASTM.
Aubertin, M., M. Mbonimpa, B. Bussière, and R. P. Chapuis. 2003. “Amodel to predict the water retention curve from basic geotechnical properties.” Can. Geotech. J. 40 (6): 1104–1122. https://doi.org/10.1139/t03-054.
Burbank, M., T. Weaver, R. Lewis, T. Williams, B. Williams, and
R. Crawford. 2013. “Geotechnical tests of sands following bioinduced
calcite precipitation catalyzed by indigenous bacteria.” J. Geotech. Geoenviron. Eng. 139 (6): 928–936. https://doi.org/10.1061/(ASCE)GT.1943-5606.0000781.
Cheng, L., M. A. Shahin, and D. Mujah. 2016. “Influence of key environmental conditions on microbially induced cementation for soil stabilization.” J. Geotech. Geoenviron. 143 (1): 04016083. https://doi.org/10.1061/(ASCE)GT.1943-5606.0001586.
DeJong, J. T., et al. 2013. “Biogeochemical processes and geotechnical applications: Progress, opportunities and challenges.” Géotechnique
63 (4): 287–301. https://doi.org/10.1680/geot.SIP13.P.017.
DeJong, J. T., M. B. Fritzges, and K. Nüsslein. 2006. “Microbially induced
cementation to control sand response to undrained shear.” J. Geotech.
Geoenviron. Eng. 132 (11): 1381–1392. https://doi.org/10.1061/(ASCE)1090-0241(2006)132:11(1381).
DeJong, J. T., B. C. Martinez, T. R. Ginn, C. Hunt, D. Major, and B. Tanyu.
2014. “Development of a scaled repeated five-spot treatment model for
examining microbial induced calcite precipitation feasibility in field applications.” Geotech. Test. J. 37 (3): 20130089. https://doi.org/10.1520/GTJ20130089.
Dello Sterpaio, P. 2012. “The impact of bacteria on the biophysics of water
retention and flow in soil.” Ph.D. dissertation, School of Science,
Engineering and Technology, Abertay Univ.
Dennis, M. L., and J. P. Turner. 1998. “Hydraulic conductivity ofcompacted soil treated with biofilm.” J. Geotech. Geoenviron. Eng.
124 (2): 120–127. https://doi.org/10.1061/(ASCE)1090-0241(1998)
124:2(120).
Dijkema, J., J. E. Koonce, R. M. Shillito, T. A. Ghezzehei, M. Berli, M. J.
van der Ploeg, and M. T. van Genuchten. 2017. “Water distribution in anarid zone soil: Numerical analysis of data from a large weighing lysimeter.” Vadose Zone J. 17 (1). https://doi.org/10.2136/vzj2017.01.0035.
Ercole, C., P. Cacchio, A. L. Botta, V. Centi, and A. Lepidi. 2007. “Bacterially induced mineralization of calcium carbonate: The role of exopolysaccharides and capsular polysaccharides.” Microsc. Microanal.
13 (1): 42–50. https://doi.org/10.1017/S1431927607070122.
Esnault-Filet, A., I. Gutjahr, J. F. Mosser, L. Sapin, and K. Ibrahim. 2016.
“A novel grouting process for the reinforcement of low permeability
soils with the use of biocementation by biocalcis.” In Proc., 19th
© ASCESoutheast Asian Geotechnical Conf. and 2nd AGSSEA Conf. Pathum
Thani, Thailand: Southeast Asian Geotechnical Society.
Ethington, E. F. 1990. Interfacial contact angle measurements of water,
mercury, and 20 organic liquids on quartz, calcite, biotite, andCa-montmorillonite substrates. Open-File Rep. No. 90-409. Reston,
VA: USGS.
François, B., and S. Ettahiri. 2012. “Role of the soil mineralogy on thetemperature dependence of the water retention curve.” In Unsaturated
soils: Research and applications, 173–178. Berlin: Springer.
Fredlund, D. G., and H. Rahardjo. 1993. Soil mechanics for unsaturated
soils. New York: Wiley.
Fredlund, D. G., and A. Xing. 1994. “Equations for the soil-water characteristic curve.” Can. Geotech. J. 31 (4): 521–532. https://doi.org/10.1139/t94-061.
Fredlund, D. G., A. Xing, M. D. Fredlund, and S. L. Barbour. 1996. “Therelationship of the unsaturated soil shear to the soil-water characteristic
curve.” Can. Geotech. J. 33 (3): 440–448. https://doi.org/10.1139/t96-065.
Gomez, M. G., B. C. Martinez, J. T. DeJong, C. E. Hunt, L. A. deVlaming,
D. W. Major, and S. M. Dworatzek. 2015. “Field-scale bio-cementation
tests to improve sands.” Proc. Inst. Civ. Eng. Ground Improv. 168 (3):
206–216. https://doi.org/10.1680/grim.13.00052.
Gratchev, I., and I. Towhata. 2013. “Stress-strain characteristics of twonatural soils subjected to long-term acidic contamination.” Soils Found.
53 (3): 469–476. https://doi.org/10.1016/j.sandf.2013.04.008.
Haouzi, F. Z., and B. Courcelles. 2018. “Major applications of MICP sandtreatment at multi-scale levels: A review.” In Proc., 71st Canadian
Geotechnical Conf. and 13th Joint CGS/IAH-CNC Groundwater Conf.
Richmond, Canada: Canadian Geotechnical Society.
He, J., J. Chu, and V. Ivanov. 2013. “Mitigation of liquefaction of saturated
sand using biogas.” Géotechnique 63 (4): 267–275. https://doi.org/10.1680/geot.SIP13.P.004.
Khalili, N., M. A. Habte, and S. Zargarbashi. 2008. “A fully coupled flow
deformation model for cyclic analysis of unsaturated soils including hydraulic and mechanical hysteresis.” Comput. Geotech. 35 (6): 872–889.
https://doi.org/10.1016/j.compgeo.2008.08.003.
Kibbey, T. C. 2013. “The configuration of water on rough natural surfaces:
Implications for understanding air-water interfacial area, film thickness,and imaging resolution.” Water Resour. Res. 49 (8): 4765–4774. https://doi.org/10.1002/wrcr.20383.
Kim, G., J. Kim, and H. Youn. 2018. “Effect of temperature, pH, and reaction duration on microbially induced calcite precipitation.” Appl. Sci.
8 (8): 1277. https://doi.org/10.3390/app8081277.
Krauskopf, K. B., and D. K. Bird. 1995. Introduction to geochemistry.
New York: McGraw-Hill.
Lu, N. 2016. “Generalized soil water retention equation for adsorption andcapillarity.” J. Geotech. Geoenviron. 142 (10): 04016051. https://doi.org/10.1061/(ASCE)GT.1943-5606.0001524.
Lu, N., and W. J. Likos. 2004. Unsaturated soil mechanics. Hoboken, NJ:
Wiley.
Mitchell, J. K. 1993. Fundamentals of soil behavior. 2nd ed. New York:
Wiley.
Montoya, B. M., J. T. DeJong, and R. W. Boulanger. 2013. “Dynamic
response of liquefiable sand improved by microbial-induced calcite precipitation.” Géotechnique 63 (4): 302–312. https://doi.org/10.1680/geot.SIP13.P.019.
Moravej, S., G. Habibagahi, E. Nikooee, and A. Niazi. 2018. “Stabilizationof dispersive soils by means of biological calcite precipitation.”
Geoderma 315 (Apr): 130–137. https://doi.org/10.1016/j.geoderma.2017.11.037.
Moravej, S., E. Nikooee, G. Habibagahi, and A. Niazi. 2015. “Biostabilization of dispersive soils.” In Proc., European Conf. Soil
Mechanics and Foundation Engineering. London: Institution of Civil
Engineers.
Mualem, Y. 1976. “A new model for predicting the hydraulic conductivityof unsaturated porous media.” Water Resour. Res. 12 (3): 513–522.
https://doi.org/10.1029/WR012i003p00513.
Okhrimenko, D. V., K. N. Dalby, and S. L. Stipp. 2013. “Adsorption properties of chalk: Contributions from calcite and clays.” Procedia Earth
Planet. Sci. 7: 632–635. https://doi.org/10.1016/j.proeps.2013.03.144.

04019028-11

J. Geotech. Geoenviron. Eng., 2019, 145(7): 04019028

J. Geotech. Geoenviron. Eng.

Downloaded from ascelibrary.org by Rahim Saffari on 05/14/19. Copyright ASCE. For personal use only; all rights reserved.

Perito, B., M. Marvasi, C. Barabesi, G. Mastromei, S. Bracci, M. Vendrell,
and P. Tiano. 2014. “A Bacillus subtilis cell fraction (BCF) inducing
calcium carbonate precipitation: Biotechnological perspectives formonumental stone reinforcement.” J. Cult. Heritage 15 (4): 345–351.
https://doi.org/10.1016/j.culher.2013.10.001.
Puppala, A. J., K. Punthutaecha, and S. K. Vanapalli. 2006. “Soil-water
characteristic curves of stabilized expansive soils.” J. Geotech. Geoenviron. 132 (6): 736–751. https://doi.org/10.1061/(ASCE)1090-0241(2006)132:6(736).
Roberson, E. B., and M. K. Firestone. 1992. “Relationship between desiccation and exopolysaccharide production in a soil Pseudomonas sp.”
Appl. Environ. Microbiol. 58 (4): 1284–1291.
Russo, G. 2005. “Water retention curves of lime stabilised soil.” In Proc.,Int. Workshop on Advanced Experimental Unsaturated Soil Mechanics,
Experus 2005, Trento (I), edited by A. Tarantino, E. Romero, and Y. J.
Cui, 391–396. Rotterdam, Netherlands: A.A. Balkema.
Sadjadi, M., E. Nikooee, and G. Habibagahi. 2014. “Biological treatmentof swelling soils using microbial calcite precipitation.” In Unsaturated
soils: Research and applications, 917–922. London: CRC Press.
Saffari, R., G. Habibagahi, E. Nikooee, and A. Niazi. 2017. “Biologicalstabilization of a swelling fine-grained soil: The role of microstructuralchanges in the shear behavior.” IJST-T Civ. Eng. 41 (4): 405–414.
https://doi.org/10.1007/s40996-017-0066-z.
Salifu, E., E. MacLachlan, K. R. Iyer, C. W. Knapp, and A. Tarantino. 2016.
“Application of microbially induced calcite precipitation in erosion mitigation and stabilisation of sandy soil foreshore slopes: A preliminary
investigation.” Eng. Geol. 201 (Feb): 96–105. https://doi.org/10.1016/j.enggeo.2015.12.027.
Sanches, L. S., S. H. Domingues, A. Carubelli, and L. H. Mascaro. 2003.
“Electrodeposition of Ni-Mo and Fe-Mo alloys from sulfate-citrate acidsolutions.” J. Braz. Chem. Soc. 14 (4): 556–563. https://doi.org/10.1590/S0103-50532003000400011.
Seki, K., T. Miyazaki, and M. Nakano. 1998. “Effects of microorganisms
on hydraulic conductivity decrease in infiltration.” Eur. J. Soil. Sci.
49 (2): 231–236. https://doi.org/10.1046/j.1365-2389.1998.00152.x.
Sisakht, B. N., E. Nikooee, G. Habibagahi, and A. Niazi. 2015. “Stabilization of collapsible soils: A biological technique.” In Proc., European
Conf. Soil Mechanics and Foundation Eng. London: Institution of Civil
Engineers.
Stocks-Fischer, S., J. K. Galinat, and S. S. Bang. 1999. “Microbiologicalprecipitation of CaCO3.” Soil Biol. Biochem. 31 (11): 1563–1571.
https://doi.org/10.1016/S0038-0717(99)00082-6.
Suazo, G., A. Fourie, and J. Doherty. 2016. “Experimental study of theevolution of the soil water retention curve for granular material undergoing cement hydration.” J. Geotech. Geoenviron. 142 (7): 04016022.
https://doi.org/10.1061/(ASCE)GT.1943-5606.0001468.
Sun, Z., Y. Yan, G. Zhang, Z. Wu, and S. Zheng. 2015. “The influence ofcarriers on the structure and photocatalytic activity of TiO2/diatomitecomposite photocatalysts.” Adv. Powder Technol. 26 (2): 595–601.
https://doi.org/10.1016/j.apt.2015.01.007.
Tarantino, A. 2009. “A water retention model for deformable soils.”
Géotechnique 59 (9): 751–762. https://doi.org/10.1680/geot.7.00118.
Terzis, D., and L. Laloui. 2017. “On the application of microbially induced
calcite precipitation for soils: A multiscale study”. In Advances in© ASCElaboratory testing and modelling of soils and shales, 388–394. Cham,
Switzerland: Springer.
Terzis, D., and L. Laloui. 2018. “3-D micro-architecture and mechanicalresponse of soil cemented via microbial-induced calcite precipitation.”
Sci. Rep. 8 (1): 1416. https://doi.org/10.1038/s41598-018-19895-w.
Thudi, H. R. 2007. “Assessment of soil-water retention properties of lime
and cement treated clays.” M.Sc. thesis, Dept. of Civil Engineering,
Univ. of Texas.
Tuller, M., and D. Or. 2005. “Water films and scaling of soil characteristic
curves at low water contents.” Water Resour. Res. 41 (9): W09403.
https://doi.org/10.1029/2005WR004142.
Vanapalli, S. K., D. G. Fredlund, D. E. Pufahl, and A. W. Clifton. 1996.
“Model for the prediction of shear strength with respect to soil suction.”
Can. Geotech. J. 33 (3): 379–392. https://doi.org/10.1139/t96-060.
van Genuchten, M. Th. 1980. “A closed-form equation for predicting thehydraulic conductivity of unsaturated soils.” Soil Sci. Soc. Am. J. 44 (5):892–898. https://doi.org/10.2136/sssaj1980.03615995004400050002x.
van Genuchten, M. Th., F. J. Leij, and S. R. Yates. 1991. The RETC
code for quantifying the hydraulic functions of unsaturated soils.
Rep. No. EPA/600/2-91/065. Ada, OK: R.S. Kerr Environmental
Research Laboratory and USEPA.
van Genuchten, M. Th., and D. R. Nielsen. 1985. “On describing and predicting the hydraulic properties of unsaturated soils.” Ann. Geophys.
3 (5): 615–628.
van Olphen, H. 1991. An introduction to clay colloid chemistry. 2nd ed.
Malabar, FL: Krieger.
van Paassen, L. A. 2009. “Biogrout, ground improvement by microbial induced carbonate precipitation.” Ph.D. dissertation, Biotechnology,
Delft Univ. of Technology.
van Paassen, L. A., R. Ghose, T. J. van der Linden, W. R. van der Star, and
M. C. van Loosdrecht. 2010. “Quantifying biomediated ground improvement by ureolysis: Large-scale biogrout experiment.” J. Geotech.
Geoenviron. 136 (12): 1721–1728. https://doi.org/10.1061/(ASCE)GT.1943-5606.0000382.
van Paassen, L. A., M. Pieron, A. Mulder, T. J. M. van der Linden, M. C. M.
van Loosdrecht, and D. J. M. Ngan-Tillard. 2009. “Strength and deformation of biologically cemented sandstone.” In Proc., ISRM Regional
Conf., 405–410. Lisbon, Portugal: International Society for RockMechanics and Rock Engineering.
Wang, Y., Y. J. Cui, A. M. Tang, C. S. Tang, and N. Benahmed. 2015.
“Effects of aggregate size on water retention capacity and microstructure of lime-treated silty soil.” Geotech. Lett. 5 (4): 269–274. https://doi.org/10.1680/jgele.15.00127.
Zhang, X., M. Mavroulidou, and M. J. Gunn. 2017. “A study of the water
retention curve of lime-treated London clay.” Acta Geotech. 12 (1):23–45. https://doi.org/10.1007/s11440-015-0432-6.
Zheng, W., X. Yu, and Y. Jin. 2015. “Considering surface roughness effects
in a triangular pore space model for unsaturated hydraulic conductivity.”
Vadose Zone J. 14 (7). https://doi.org/10.2136/vzj2014.09.0121.
Zheng, W., S. Zeng, H. Bais, J. M. LaManna, D. S. Hussey, D. L. Jacobson,and Y. Jin. 2018. “Plant growth-promoting rhizobacteria (PGPR) reduce
evaporation and increase soil water retention.” Water Resour. Res.
54 (5): 3673–3687. https://doi.org/10.1029/2018WR022656.

04019028-12

J. Geotech. Geoenviron. Eng., 2019, 145(7): 04019028

J. Geotech. Geoenviron. Eng.