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Received: 20 December 2024
Revised: 13 January 2025
Accepted: 17 January 2025
Published: 19 January 2025
Citation: Li, X.; Cong, S.; Tang, L.;
Ling, X. Effect of Freeze–Thaw Cycles
on the Microstructure Characteristics
of Unsaturated Expansive Soil.
Sustainability 2025,17, 762. https://
doi.org/10.3390/su17020762
Copyright: © 2025 by the authors.
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Article
Effect of Freeze–Thaw Cycles on the Microstructure
Characteristics of Unsaturated Expansive Soil
Xinyu Li 1,2, Shengyi Cong 1,2,3,* , Liang Tang 1,2,3 and Xianzhang Ling 1,2,3
1School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China;
18b933025@stu.hit.edu.cn (X.L.); tangliang@hit.edu.cn (L.T.); lingxianzhang@hit.edu.cn (X.L.)
2Heilongjiang Research Center for Rail Transit Engineering in Cold Regions, Harbin 150090, China
3Chongqing Research Institute, Harbin Institute of Technology, Chongqing 401135, China
*Correspondence: congshengyi@hit.edu.cn
Abstract: The term “engineering cancer” refers to expansive soil, whose properties threaten
the stability and safety of structures. As a result, appropriate steps must be taken to guaran-
tee the sustainable development of buildings. To explore the impact of freeze–thaw cycles
(FTCs) on the microscopic characteristics of unsaturated expansive soil in the cold region,
the mineralogical composition and microstructure were analyzed using X-ray diffraction
(XRD), thermogravimetric analysis, and scanning electron microscopy (SEM). The influence
of repeated FTCs on the characteristics of particle morphology and pore structure in expan-
sive soil was quantitatively examined. The findings indicate that, in comparison to other
expansive soil samples, the Yanji expansive soil is particularly susceptible to failures due to
its high sand content and low liquid limit. The FTCs significantly alter the microstructure,
leading to increased complexity in the particle edge shapes, a transition in particle distri-
bution from dispersed to more concentrated, a reduction in larger particles, and a more
intricate spatial arrangement of particles. As moisture content rises, the impact of FTCs
becomes increasingly pronounced. The particle distribution’s area probability index and
fractal dimension are identified as medium-variability parameters, with a high-variation
coefficient before the 3rd FTC, which then gradually decreases. The repeated FTCs result
in particle breakage and agglomeration, causing the particle size to become more uniform
and the soil’s microstructure to stabilize after 3–5 FTCs. These findings contribute to
understanding the FTC behavior of expansive soils, provide theoretical support and scien-
tific guidance for disaster prevention and control measures, as well as for the sustainable
development of engineering projects involving expansive soil sites.
Keywords: freeze–thaw cycle; unsaturated expansive soil; microstructure; pore characteristics
1. Introduction
In engineering practice, expansive soil exhibits special deformation properties, namely,
swelling and shrinking when in contact with water, repeated expansion and contraction,
and a sharp attenuation of immersed bearing capacity, which cause great harm to the
sustainable development of engineering construction [
1
]. These characteristics make it
easy for roads to settle unevenly, for the roadbed slope to collapse, and for foundations
and building structures to deform and crack [
2
]. Consequently, “engineering cancer” is
another name for this [
3
]. However, expansive soil is widely distributed in China and
abroad, mainly in Yunnan, Guangxi, Guizhou, Hubei, and other provinces in China, as
well as in southeast Africa and southeast Asia [
4
–
10
]. In recent years, engineering practice
has found that large-scale expansive soil sites frequently occur in seasonal frozen soil areas
Sustainability 2025,17, 762 https://doi.org/10.3390/su17020762
Sustainability 2025,17, 762 2 of 24
such as Jilin, Liaoning, and Heilongjiang, and this has an important impact on engineering
construction and the prevention of damage caused by freezing [11–14].
To effectively implement sustainable disaster prevention and control measures for ex-
pansive soil sites, it is essential to address the issue at its root. The special microstructure of
expansive soil is the essential cause factor that induces geological disasters. Understanding
the microstructure characteristics of expansive soil is the basis of studying its deformation
behavior (Al-Rawas and McGown, 1999 [
15
]; Katti and Shanmugasundaram, 2001 [
16
]).
Due to its special mineral composition and pore structure, unsaturated expansive soil is
prone to significant volume change under the conditions of moisture and temperature
changes [
17
–
19
]. During freeze–thaw cycles (FTCs), processes such as water migration,
ice crystal formation, and subsequent melting result in dynamic alterations to the soil’s
microstructure, which in turn affect its mechanical properties. Lu and Liu [
20
] performed
the apparent expansive soil cracking induced by FTCs. Li et al. [
21
] and Zhang et al. [
22
]
studied the mechanisms affecting the bearing strength under FTCs, in terms of the CBR, of
weakly expansive soil that could be used as embankment filler. Olgun [
23
] evaluated the
geotechnical properties of an expansive soil subject to a freeze–thaw effect. Yu et al. [
24
]
investigated the freezing characteristics of the expansive soil and the important factors
affecting the freezing characteristic curve. Sun et al. [
25
] discussed the expression degree
and influencing factors of the structural strength of expansive soil. Consequently, when
investigating the cracking behavior of expansive soils in cold regions, it is essential to
account for the impact of FTCs to mitigate potential project risks. Currently, while some
researchers have investigated the mechanical behavior of unsaturated expansive soil under
freeze–thaw action, most research has mainly focused on the macroscopic changes on the ex-
pansive soil site [
26
]; an in-depth exploration of the dynamic changes in the microstructure
is lacking. Numerous researchers have carried out comprehensive analyses and studies on
the impact of dry–wet cycles on expansive soil (Han et al., 2024 [
27
]; Zhao et al., 2021 [
28
];
Ding et al., 2021 [
29
]). The freeze–thaw cycle effect, as a powerful weathering process, has
had a notable impact on the expansion behavior and crack formation in expansive soils
in deep seasonally frozen regions in recent years. However, it is noteworthy that research
on the coupling effects of moisture content and FTCs for unsaturated expansive soil has
not been carried out in depth. In response to this, expansive soils from northeast China
were chosen as the focus of the study. To address the demand for sustainable construction
using expansive soil in northeast China, this study integrated local climatic conditions
and geological characteristics and investigated the changes in the microscopic structure
and the fracture characteristics of the expansive soil while considering the FTCs’ action
and moisture content. The research findings further elucidate the underlying causes of
disaster-proneness at the Yanji expansive soil site and uncover the micro-mechanisms
governing the effects of freeze–thaw cycles.
In this research, the FTC experiments and scanning electron microscope (SEM) analy-
ses were conducted on unsaturated expansive soil with varying initial moisture contents.
The primary goals of the study were (1) to examine the impact of FTCs on the morphol-
ogy of unsaturated expansive soil particles; (2) to elucidate how initial moisture content
and FTCs influence the spatial arrangement of the microstructure; and (3) to develop a
micro-mechanism to explain the role of FTCs in engineering failures related to expansive
soil. These findings contribute to understanding the FTC behavior of expansive soils,
provide theoretical support and scientific guidance for disaster prevention and control mea-
sures, and enable the sustainable development of engineering projects involving expansive
soil sites.
Sustainability 2025,17, 762 3 of 24
2. Test Materials and Methods
2.1. Basic Physical Properties of Soil
The expansive soil used in the experiments was sourced from the Yanji section of the
Jilin–Tumen–Hunchun high-speed railway, a location representative of the region (Figure 1).
The topography is mainly intermountain erosion hilly landform, and the groundwater is
rich and mainly fissure water. The strata are mainly interbedded sandstone and mudstone
of the upper Cretaceous Longjing Formation. The surface Quaternary Holocene residual
slope silty clay is hard-plastic. The upper Cretaceous Longjing Formation sandstone and
mudstone are interbedded. The rock is soft and the argillaceous cementation is poor. In
its natural state, the soil is characterized by a hard-plastic state, with a natural moisture
content of 19.17% and a dry density of 1.49 g/cm
3
(Figure 2). After air drying and grinding,
using a 2 mm sieve, the large particles were removed from the expansive soil collected
from the site. The basic physical properties of the soil are outlined in Table 1, with the
grain size distribution shown in Figure 3, the compaction curve shown in Figure 4, and the
proportions of each fraction presented in Table 2. The characteristic curve of soil and water
was determined by the filter paper method, and the moisture content–suction relationship
of expansive soil is shown in Figure 5. The free expansion rate test revealed that the soil’s
free swelling rate is 50%. According to the Code for Soil Test of Railway Engineering (TB
10102-2023; China’s Ministry of Transport, 2023), this soil is classified as weak expansive
soil and categorized as sandy fine-grained soil, a type of low liquid limit clay (CL-S).
Sustainability 2025, 17, x FOR PEER REVIEW 4 of 25
Figure 1. Sample site along the high-speed railway in Yanji.
Figure 2. Macroscopic characterization of expansive soil at the site.
Figure 1. Sample site along the high-speed railway in Yanji.
Sustainability 2025,17, 762 4 of 24
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Figure 1. Sample site along the high-speed railway in Yanji.
Figure 2. Macroscopic characterization of expansive soil at the site.
Figure 2. Macroscopic characterization of expansive soil at the site.
Table 1. Physical properties of expansive soil.
Plastic Limit (%)
Liquid Limit (%)
Plasticity Index
(%)
Optimum
Moisture
Content (%)
Maximum Dry
Density (g⁄cm3)
Free Swelling
Ratio (%)
19.10 37.39 18.29 19.83 1.69 50
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Figure 3. Grain-size distribution of expansive soil.
Figure 4. Compaction curve of expansive soil.
Figure 5. Soil–water characteristic curve of expansive soil.
2.2. Sample Preparation and Tests
X-ray diffraction (XRD) and thermogravimetric analyses were carried out on the un-
disturbed cuing ring sample from the site. The wet sieve analysis method was employed
to separate the expansive soil particles into different size fractions, allowing for the
Figure 3. Grain-size distribution of expansive soil.
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Figure 3. Grain-size distribution of expansive soil.
Figure 4. Compaction curve of expansive soil.
Figure 5. Soil–water characteristic curve of expansive soil.
2.2. Sample Preparation and Tests
X-ray diffraction (XRD) and thermogravimetric analyses were carried out on the un-
disturbed cuing ring sample from the site. The wet sieve analysis method was employed
to separate the expansive soil particles into different size fractions, allowing for the
Figure 4. Compaction curve of expansive soil.
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Table 2. Proportions of each fraction.
General Designation of
Fraction Fine Grain Coarse Grain
Name of fraction Clay Silt Sand Gravel
Particle size range (mm) d ≤0.005 mm 0.005 mm < d ≤0.075 mm 0.075 mm < d ≤2 mm 2 mm < d ≤20 mm
Proportion (%) 21.41 33.05 41.58 3.96
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Figure 3. Grain-size distribution of expansive soil.
Figure 4. Compaction curve of expansive soil.
Figure 5. Soil–water characteristic curve of expansive soil.
2.2. Sample Preparation and Tests
X-ray diffraction (XRD) and thermogravimetric analyses were carried out on the un-
disturbed cuing ring sample from the site. The wet sieve analysis method was employed
to separate the expansive soil particles into different size fractions, allowing for the
Figure 5. Soil–water characteristic curve of expansive soil.
2.2. Sample Preparation and Tests
X-ray diffraction (XRD) and thermogravimetric analyses were carried out on the undis-
turbed cutting ring sample from the site. The wet sieve analysis method was employed to
separate the expansive soil particles into different size fractions, allowing for the examina-
tion of the micro-morphologies of the particles within these ranges after the agglomeration
of the dispersed particles.
Microscopic observation samples were prepared to undergo different FTCs with
different moisture content. The preparation method for a microstructure observation
sample is as follows. Using the sample percussion method, cutting ring samples were
prepared with dimensions of
Φ
39.1 mm
×
80 mm. Microscale samples were then created,
representing both untreated soil and soil subjected to varying FTCs. To ensure uniform
expansion potential, the dry density of the samples was consistently controlled at 1.52
g/cm
3
, and it was ensured that the compaction degree reached 90%. To prevent moisture
loss, the prepared loose samples with different moisture contents were wrapped in plastic
wrap and were maintained for one day before the FTC tests were conducted. The moisture
content was controlled to 14% (Group X), 20% (Group Y), and 26% (Group Z), and the
20% condition was the condition closest to the optimum moisture content. According to
the local climate conditions and the FTC test scheme used in a previous study [
4
], the
FTC test was designed with a freezing temperature of
−
15
◦
C for 12 h, followed by a
thawing temperature of 15
◦
C for 12 h. According to the previous research conducted by
our research group [
12
], the number of freeze–thaw cycles (N
FTC
) for each sample is set
to five distinct FTCs (i.e., 0, 1, 3, 7, and 11 cycles). The samples were labeled based on the
group letter and the N
FTC
. For example, the sample name X11 indicates that the sample
has a moisture content of 14% (Group X) and undergoes 11 FTCs.
Scanning electron microscopy was employed to examine the micro-morphology of the
expansive soil. The sample was dried and gently broken along a shallow tank to expose
the fresh structural surface of the soil sample. A relatively flat fracture surface was chosen
for scanning observation, and a small SEM sample approximately 1 cm
×
1 cm
×
0.5
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cm (length
×
width
×
height) was carefully cut using a sharp blade. Loose particles on
the surface were gently removed with an ear bulb syringe. The sample surface was then
coated with gold using an SBC-12 ion-sputtering device, and the gold-coated sample was
transferred to the observation chamber for analysis. A Phenom Pro bench scanning electron
microscope was used for the electron microscope scanning to avoid structural singularity
of the soil samples, and photographs of a representative area were taken from high power
to low power to obtain images of the microstructure. Images with a magnification of 1000
×
provided a clearer view of the overall microstructure, while images at 8000
×
magnification
were used to analyze the local, representative microstructural characteristics of the particles.
2.3. Microscopic Image Analysis Methods
The particles (pores) and crack analysis system (PCAS) was employed for the quanti-
tative microscopic image analysis [
30
]. The SEM images of the soil samples were imported,
and the particles were automatically identified through binarization, with black as particles
and white as pores (Figure 6b). The results after vectorization are shown in Figure 6c, and
different colors represent different pores. Statistical parameters, including the average form
factor, area–circumference fractal dimension, area probability distribution index, fractal
dimension of the particle distribution, and probabilistic entropy were obtained via quan-
titative analysis, and the particle morphology, grain-size distribution, and arrangement
were quantitatively analyzed. The pore characteristics were analyzed according to the
pore classification after the PCAS analysis. Based on the microscopic study of the samples,
the particles and pores belong to the same control system. According to the particle size
classification standard, the pores in expansive soil can be categorized into four types [
31
].
The pore type of coarse pores (d > 75
µ
m) is inter-aggregate pores; the pore type of fine
pores is inner-aggregate pores (5
µ
m<d
≤
75
µ
m); the pore type of micropores (0.1
µ
m
< d
≤
5
µ
m) is inter-particle pores; and the pore type of ultramicropores (d
≤
0.1
µ
m) is
inner-particle pores. The fine pores can be subdivided into three categories. The large
pores (20
µ
m < d
≤
75
µ
m) consist of inter-aggregate pores and some inner-aggregate
pores; the medium pores (10
µ
m<d
≤
20
µ
m) are inner-aggregate pores; and the small
pores (5
µ
m<d
≤
10
µ
m) consist of inner-aggregate pores and some inter-particle pores
(Table 3). According to the PCAS operation step [
32
], the obtained soil microscopic image
was imported, and the threshold value was repeatedly adjusted until the particles in the
pores were visible. The average value of the obtained threshold value for each of three
measurements was utilized as the segmentation threshold in the segmentation program.
Regarding the quantitative study of the particle characteristics, the following statistical
parameters were calculated and analyzed to obtain more accurate quantitative relations.
Sustainability 2025, 17, x FOR PEER REVIEW 7 of 25
(d ≤ 0.1 µm) is inner-particle pores. The fine pores can be subdivided into three categories.
The large pores (20 µm < d ≤ 75 µm) consist of inter-aggregate pores and some inner-
aggregate pores; the medium pores (10 µm < d ≤ 20 µm) are inner-aggregate pores; and
the small pores (5 µm < d ≤ 10 µm) consist of inner-aggregate pores and some inter-particle
pores (Table 3). According to the PCAS operation step [32], the obtained soil microscopic
image was imported, and the threshold value was repeatedly adjusted until the particles
in the pores were visible. The average value of the obtained threshold value for each of
three measurements was utilized as the segmentation threshold in the segmentation pro-
gram. Regarding the quantitative study of the particle characteristics, the following statis-
tical parameters were calculated and analyzed to obtain more accurate quantitative rela-
tions.
(a) (b) (c)
Figure 6. SEM image processing results: (a) Original image; (b) Binarization result; and (c) Vectori-
zation result.
Table 3. Classification of pore types (after Ye et al., 2019 [31]).
Pore Type Pore Size Range (µm) Pore Composition
Coarse pores d > 75 µm Inter-aggregate pores
Fine pores
Large pores 20 µm < d ≤ 75 µm Inter-aggregate pores and some inner-aggregate pores
Medium pores 10 µm < d ≤ 20 µm Inner-aggregate pores
Small pores 5 µm < d ≤ 10 µm Inner-aggregate pores and some inter-particle pores
Micropores 0.1 µm < d ≤ 5 µm Inter-particle pores
Ultramicropores d ≤ 0.1 µm Inner-particle pores
(a).
The average form factor 𝑓𝑓 of different particles in the same plane is used to de-
scribe the roundness of soil particles and the shape of particle edges. The closer the
value is to 0, the rougher and more uneven the shape of particle edges;
(b).
According to the fractal characteristics of the particle shape, the area-circumference
fractal dimension D is between 1 and 2. The smaller the D value, the simpler the par-
ticle structure and the greater the degree of smoothness of surface of the spatial mor-
phology of the particles;
(c).
The area probability distribution index b represents the trend in particle count as the
particle area increases, with the relationship between the two following a power
function. A higher value of the area probability distribution index indicates a pre-
dominance of smaller particles and fewer larger ones;
(d).
The fractal dimension of the particle distribution D
d
characterizes the degree of par-
ticle homogenization and indicates the variability in particle size. A larger fractal di-
mension signifies poorer particle uniformity, a more concentrated distribution, and
a higher degree of collectivization;
(e).
The probabilistic entropy H
m
is a parameter that reflects the arrangement of the par-
ticles. The value range of H
m
is [0, 1]. A higher value indicates a more disordered
particle arrangement, with lower levels of order.
Figure 6. SEM image processing results: (a) Original image; (b) Binarization result; and (c) Vectoriza-
tion result.
Sustainability 2025,17, 762 7 of 24
Table 3. Classification of pore types (after Ye et al., 2019 [31]).
Pore Type Pore Size Range (µm) Pore Composition
Coarse pores d > 75 µm Inter-aggregate pores
Fine pores
Large pores 20 µm<d≤75 µm Inter-aggregate pores and some inner-aggregate pores
Medium pores 10 µm<d≤20 µm Inner-aggregate pores
Small pores 5 µm<d≤10 µm Inner-aggregate pores and some inter-particle pores
Micropores 0.1 µm<d≤5µm Inter-particle pores
Ultramicropores d ≤0.1 µm Inner-particle pores
(a).
The average form factor
f f
of different particles in the same plane is used to describe
the roundness of soil particles and the shape of particle edges. The closer the value is
to 0, the rougher and more uneven the shape of particle edges;
(b).
According to the fractal characteristics of the particle shape, the area-circumference
fractal dimension Dis between 1 and 2. The smaller the Dvalue, the simpler the
particle structure and the greater the degree of smoothness of surface of the spatial
morphology of the particles;
(c).
The area probability distribution index brepresents the trend in particle count as
the particle area increases, with the relationship between the two following a power
function. A higher value of the area probability distribution index indicates a predom-
inance of smaller particles and fewer larger ones;
(d).
The fractal dimension of the particle distribution D
d
characterizes the degree of
particle homogenization and indicates the variability in particle size. A larger fractal
dimension signifies poorer particle uniformity, a more concentrated distribution, and
a higher degree of collectivization;
(e).
The probabilistic entropy H
m
is a parameter that reflects the arrangement of the
particles. The value range of H
m
is [0, 1]. A higher value indicates a more disordered
particle arrangement, with lower levels of order.
3. Test Results and Analysis
3.1. XRD Results
The composition and structural morphology of the clay minerals was obtained via X-
ray diffraction analysis (Figure 7). Based on the XRD analysis of the unsaturated expansive
soil, the impact of these cycles on the material composition of the expansive soil from Yanji
was investigated. The characteristic peaks of montmorillonite with a center of 6.03
◦
(2
θ
)
appear in the XRD patterns, and the diffraction peaks of the montmorillonite phase in
the system decrease under the FTCs. The characteristic peaks of the albite phase appear
between 26
◦
and 30
◦
(2
θ
). By comparing the characteristic peaks of albite after 0 and
11 FTCs, the diffraction intensity decreases. As the N
FTC
increases, the diffraction peak
intensity of quartz strengthens. The XRD analysis results indicate that the tested expansive
soil from Yanji is mainly composed of montmorillonite, albite, quartz, potassium feldspar,
illite, and hematite. After 11 FTCs, the XRD diffraction peak was shifted to the left at a
small angle due to the increase in the lattice constant, which may be due to the change
in the lattice constant caused by the doping of atoms. This can also be caused by lattice
distortion. The contents of the various substances in the samples subjected to 0 and 11
FTCs are presented in Table 4. With increasing N
FTC
, the contents of quartz, illite, and
hematite increase, while the contents of montmorillonite, albite, and potassium feldspar
decrease. The FTC exerts a form of frozen differentiation effect, which is more intense than
the common physical differentiation effect and has a greater effect on the soil structure.
Sustainability 2025,17, 762 8 of 24
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3. Test Results and Analysis
3.1. XRD Results
The composition and structural morphology of the clay minerals was obtained via X-
ray diffraction analysis (Figure 7). Based on the XRD analysis of the unsaturated expansive
soil, the impact of these cycles on the material composition of the expansive soil from Yanji
was investigated. The characteristic peaks of montmorillonite with a center of 6.03° (2θ)
appear in the XRD paerns, and the diffraction peaks of the montmorillonite phase in the
system decrease under the FTCs. The characteristic peaks of the albite phase appear be-
tween 26° and 30° (2θ). By comparing the characteristic peaks of albite after 0 and 11 FTCs,
the diffraction intensity decreases. As the NFTC increases, the diffraction peak intensity of
quar strengthens. The XRD analysis results indicate that the tested expansive soil from
Yanji is mainly composed of montmorillonite, albite, quar, potassium feldspar, illite, and
hematite. After 11 FTCs, the XRD diffraction peak was shifted to the left at a small angle
due to the increase in the laice constant, which may be due to the change in the laice
constant caused by the doping of atoms. This can also be caused by laice distortion. The
contents of the various substances in the samples subjected to 0 and 11 FTCs are presented
in Table 4. With increasing NFTC, the contents of quar, illite, and hematite increase, while
the contents of montmorillonite, albite, and potassium feldspar decrease. The FTC exerts
a form of frozen differentiation effect, which is more intense than the common physical
differentiation effect and has a greater effect on the soil structure.
Figure 7. XRD paerns of expansive soil after 0 and 11 FTCs.
Table 4. Mineral compositions of the expansive soil based on XRD analysis results.
Sample Montmorillonite
(%)
Illite
(%)
Albite
(%)
Potassium Feldspar
(%)
Quar
(%)
Hematite
(%)
FT-0 51.9 2.8 26.4 3.8 14.9 0.2
FT-11 51 4 26 2 16 1
3.2. Thermogravimetric Analysis
The STA449F3 Jupiter synchronous thermal analyzer was utilized for thermogravi-
metric analysis of the samples, and two parallel measurements were carried out in this
experiment. Finally, the thermogravimetric (TG)–derivative thermogravimetric (DTG)
Figure 7. XRD patterns of expansive soil after 0 and 11 FTCs.
Table 4. Mineral compositions of the expansive soil based on XRD analysis results.
Sample Montmorillonite
(%)
Illite
(%)
Albite
(%)
Potassium
Feldspar
(%)
Quartz
(%)
Hematite
(%)
FT-0 51.9 2.8 26.4 3.8 14.9 0.2
FT-11 51 4 26 2 16 1
3.2. Thermogravimetric Analysis
The STA449F3 Jupiter synchronous thermal analyzer was utilized for thermogravi-
metric analysis of the samples, and two parallel measurements were carried out in this
experiment. Finally, the thermogravimetric (TG)–derivative thermogravimetric (DTG)
curves were obtained by measuring the temperature and weight loss during the experiment
for mineral analysis.
As shown in Figure 8, the TG–DTG curve was derived from the temperature and
weight loss measured from the thermogravimetric test data. The TG curve of the expansive
soil shows that the amount of weight damage and the loss rate were greater after 11 FTCs,
indicating that the damage effect of the lattice structure of the montmorillonite was more
obvious. There are three obvious endothermic valleys in the DTG curve of the expansive
soil. The first endothermic valley appears between 80
◦
C and 150
◦
C, and it is a compound
valley. The second endothermic valley appears between 400
◦
C and 500
◦
C. The third
endothermic valley is between 750
◦
C and 800
◦
C, and it is followed by an exothermic
peak. The curves above demonstrate that montmorillonite is the predominant mineral in
the expansive soil from Yanji. Furthermore, the first endothermal valley is a complex type
of valley, which indicates that the montmorillonite is calcium-based montmorillonite. With
increasing temperature, the free water in the sample first evaporated and absorbed heat,
and a valley appeared in the TG curve at about 100
◦
C, which was combined to form the
first composite endothermic valley. This was caused by the removal of the adsorbed water
and interlayer water according to the distribution of the calcium montmorillonite. It can be
seen from the temperature values marked in Figure 8that the FTCs had a weak influence
on the adsorbed water and interlayer water in the soil sample. The FTCs had minimal
impact on the hydration capacity of the Ca
2+
. The second endothermic valley appears
near 430
◦
C. The crystal water was removed in this temperature zone, the original lattice
was transformed, and the temperature values of the bottoms of the two curves were only
Sustainability 2025,17, 762 9 of 24
slightly different. The third endothermic valley is located within the temperature range of
750–800
◦
C. This valley exhibits a shallow shape, and the mass loss is relatively minor due
to the lower content of structural water. The high temperatures in this range induced lattice
damage, leading to the release of structural water and resulting in more or less irreversible
changes within the structure. The subsequent exothermic peak indicates the formation
of new phases, specifically, spinel and quartz. However, there is a large difference in the
bottom temperature. After 11 FTCs, the temperature is higher compared to that of the
untreated samples, with the valley shape becoming more pronounced. This indicates that
the FTCs increased the content of the structural water in the soil samples and enhanced
the stability of the microstructure. From the above analysis, it can be concluded that the
FTCs resulted in great changes in the mineral composition and microstructure, which are
mainly reflected in the decrease in the montmorillonite content and the weakening of the
expansibility. The FTCs enhanced the structural thermal stability of the expansive soil
and also amplified the weight loss characteristics and structural damage effects of the soil
samples at high temperatures.
Sustainability 2025, 17, x FOR PEER REVIEW 10 of 25
Figure 8. TG–DTG curves of the expansive soil after different NFTC. Note: TG−0 and TG−11, DTG−0
and DTG−11 represent the TG curves and DTG curves of Yanji expansive soil samples with 0 and
11 FTCs, respectively.
3.3. Characteristics of Microstructure
3.3.1. Microscopic Characteristics of Particle Separation
For the soil sample utilized in this study, the silt content is 33.05% and sand content
is 41.58%. These two components collectively form the skeletal structure of the expansive
soil. The clay cements and encapsulates the single grains. The mineral composition is in-
tricately linked to the particle size. Consequently, both the shape and structure of the soil
particles vary according to their sizes. To further investigate the properties of the expan-
sive soil particles, we conducted segmented analysis on the samples collected from Yanji
according to the different particle size ranges. To ensure complete dispersion of these in-
terbonded particles, wet sieve analysis was employed to separate them into distinct par-
ticle size fractions. The results of the expansive soil particle separation are shown in Figure
9, where Group A consists of soil particles above 1mm-sieve; Group B consists of soil par-
ticles above 0.5mm-sieve; Group C consists of soil particles above 0.25mm-sieve; Group
D consists of soil particles above 0.1mm-sieve; Group E consists of soil particles above
0.075mm-sieve; Group F consists of soil particles under 0.075mm-sieve; and Group H is
unclassified expansive soil particles.
(a) (b)
Figure 9. Particle separation results for the expansive soil: (a) Unclassified expansive soil; and (b)
Expansive soil stored in tubes after separation. Note: Group A, B, C, D, E, F, and H represent differ-
ent groups of expansive soil particles.
Figure 8. TG–DTG curves of the expansive soil after different N
FTC
. Note: TG
−
0 and TG
−
11, DTG
−
0
and DTG
−
11 represent the TG curves and DTG curves of Yanji expansive soil samples with 0 and 11
FTCs, respectively.
3.3. Characteristics of Microstructure
3.3.1. Microscopic Characteristics of Particle Separation
For the soil sample utilized in this study, the silt content is 33.05% and sand content is
41.58%. These two components collectively form the skeletal structure of the expansive
soil. The clay cements and encapsulates the single grains. The mineral composition is
intricately linked to the particle size. Consequently, both the shape and structure of the
soil particles vary according to their sizes. To further investigate the properties of the
expansive soil particles, we conducted segmented analysis on the samples collected from
Yanji according to the different particle size ranges. To ensure complete dispersion of these
interbonded particles, wet sieve analysis was employed to separate them into distinct
particle size fractions. The results of the expansive soil particle separation are shown in
Figure 9, where Group A consists of soil particles above 1 mm-sieve; Group B consists of
soil particles above 0.5 mm-sieve; Group C consists of soil particles above 0.25 mm-sieve;
Group D consists of soil particles above 0.1 mm-sieve; Group E consists of soil particles
above 0.075 mm-sieve; Group F consists of soil particles under 0.075 mm-sieve; and Group
H is unclassified expansive soil particles.
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Figure 8. TG–DTG curves of the expansive soil after different NFTC. Note: TG−0 and TG−11, DTG−0
and DTG−11 represent the TG curves and DTG curves of Yanji expansive soil samples with 0 and
11 FTCs, respectively.
3.3. Characteristics of Microstructure
3.3.1. Microscopic Characteristics of Particle Separation
For the soil sample utilized in this study, the silt content is 33.05% and sand content
is 41.58%. These two components collectively form the skeletal structure of the expansive
soil. The clay cements and encapsulates the single grains. The mineral composition is in-
tricately linked to the particle size. Consequently, both the shape and structure of the soil
particles vary according to their sizes. To further investigate the properties of the expan-
sive soil particles, we conducted segmented analysis on the samples collected from Yanji
according to the different particle size ranges. To ensure complete dispersion of these in-
terbonded particles, wet sieve analysis was employed to separate them into distinct par-
ticle size fractions. The results of the expansive soil particle separation are shown in Figure
9, where Group A consists of soil particles above 1mm-sieve; Group B consists of soil par-
ticles above 0.5mm-sieve; Group C consists of soil particles above 0.25mm-sieve; Group
D consists of soil particles above 0.1mm-sieve; Group E consists of soil particles above
0.075mm-sieve; Group F consists of soil particles under 0.075mm-sieve; and Group H is
unclassified expansive soil particles.
(a) (b)
Figure 9. Particle separation results for the expansive soil: (a) Unclassified expansive soil; and (b)
Expansive soil stored in tubes after separation. Note: Group A, B, C, D, E, F, and H represent differ-
ent groups of expansive soil particles.
Figure 9. Particle separation results for the expansive soil: (a) Unclassified expansive soil; and
(b) Expansive soil stored in tubes after separation. Note: Group A, B, C, D, E, F, and H represent
different groups of expansive soil particles.
Figure 10 presents SEM images of the particles with different particle sizes. It can be
seen that the dispersed particles are mainly subangular, debris and lamellar clay minerals
are attached to their surfaces, and the colors within each particle size range are significantly
different. According to the characteristics of the particles, the particles of the expansive soil
from Yanji are more subangular, with a small amount of debris and lamellar clay minerals
on their surfaces, and nano-pores are present locally [
33
]. There are pores and detrital
complexes between the particles, and micron-level debris particles and clay minerals form
aggregate particles.
Sustainability 2025, 17, x FOR PEER REVIEW 11 of 25
Figure 10 presents SEM images of the particles with different particle sizes. It can be
seen that the dispersed particles are mainly subangular, debris and lamellar clay minerals
are aached to their surfaces, and the colors within each particle size range are signifi-
cantly different. According to the characteristics of the particles, the particles of the ex-
pansive soil from Yanji are more subangular, with a small amount of debris and lamellar
clay minerals on their surfaces, and nano-pores are present locally [33]. There are pores
and detrital complexes between the particles, and micron-level debris particles and clay
minerals form aggregate particles.
(a) Group A: 1 mm ≤ d < 2 mm (b) Group B: 0.5 mm ≤ d < 1 mm
(c) Group C: 0.25 mm ≤ d < 0.5 mm (d) Group D: 0.1 mm ≤ d < 0.25 mm
(e) Group E: 0.075 mm ≤ d < 0.1 mm (f) Group F: d < 0.075 mm
Figure 10. Characteristics of particles of different sizes in the expansive soil.
Since the Yanji expansive soil is a fine-grained soil with a sand content of as high as
41.58%, which constitutes the main soil skeleton of the expansive soil, a detailed analysis
of the shape characteristics of the particles larger than 0.075 mm was essential. The basic
geometric parameters of the particle shape calculated using image-processing software
were used to calculate the near-sphericity, elongation, and equivalent diameter. The cal-
culation results for all the particles were statistically calculated (Table 5). The average
elongation is 0.57, indicating that the overall shape of the particles is inclined to be block
shaped. The average near-sphericity of 0.33 is low, indicating that the surfaces of the par-
ticles have edges and that the shape of the particles is angular or subangular. The average
roundness is 0.53, indicating a nearly rectangular shape. Therefore, we used a partial block
shape, which is mainly subround or subangular, as the shape of the soil particles in the
test.
Figure 10. Characteristics of particles of different sizes in the expansive soil.
Sustainability 2025,17, 762 11 of 24
Since the Yanji expansive soil is a fine-grained soil with a sand content of as high as
41.58%, which constitutes the main soil skeleton of the expansive soil, a detailed analysis
of the shape characteristics of the particles larger than 0.075 mm was essential. The basic
geometric parameters of the particle shape calculated using image-processing software were
used to calculate the near-sphericity, elongation, and equivalent diameter. The calculation
results for all the particles were statistically calculated (Table 5). The average elongation is
0.57, indicating that the overall shape of the particles is inclined to be block shaped. The
average near-sphericity of 0.33 is low, indicating that the surfaces of the particles have
edges and that the shape of the particles is angular or subangular. The average roundness
is 0.53, indicating a nearly rectangular shape. Therefore, we used a partial block shape,
which is mainly subround or subangular, as the shape of the soil particles in the test.
Table 5. Statistics of the particle shape parameter calculation results.
Maximum Minimum Range Mean Mid-Value
Elongation 0.95 0.13 0.79 0.51 0.57
Proximal sphericity 0.78 0.01 0.77 0.33 0.36
Roundness 0.93 0.16 0.87 0.53 0.47
3.3.2. Qualitative Analysis of Microscopic Characteristics
SEM was performed to examine the microstructure of expansive soil samples with
varying moisture contents, subjected to different N
FTC
. The microstructure image of the
Group X expansive soil sample is shown as an example (Figure 11) to illustrate the mi-
crostructure characteristics of the expansive soil. It was observed that the N
FTC
significantly
influenced the arrangement and contact relationships of the expansive soil particles, thereby
impacting the fractal characteristics of the microstructure. The SEM results revealed that
the overall state was relatively stable after one FTC. After three FTCs, the signs of surface
layer shedding were obvious and appeared within a wide range. After seven FTCs, in
addition to some micro-cracks that began to cause material to fall off layer by layer, the
cracks extended to a certain depth around the silt, and the cementation of the clay began to
break down at this time. After 11 FTCs, the layered shedding still existed in a large area, the
vertical penetration pores gradually deepened to form holes with a width of about 10
µ
m,
and the horizontal penetration pores gradually became more abundant.
Sustainability 2025, 17, x FOR PEER REVIEW 12 of 25
Table 5. Statistics of the particle shape parameter calculation results.
Maximum Minimum Range Mean Mid-Value
Elongation 0.95 0.13 0.79 0.51 0.57
Proximal sphericity 0.78 0.01 0.77 0.33 0.36
Roundness 0.93 0.16 0.87 0.53 0.47
3.3.2. Qualitative Analysis of Microscopic Characteristics
SEM was performed to examine the microstructure of expansive soil samples with
varying moisture contents, subjected to different N
FTC
. The microstructure image of the
Group X expansive soil sample is shown as an example (Figure 11) to illustrate the micro-
structure characteristics of the expansive soil. It was observed that the N
FTC
significantly
influenced the arrangement and contact relationships of the expansive soil particles,
thereby impacting the fractal characteristics of the microstructure. The SEM results re-
vealed that the overall state was relatively stable after one FTC. After three FTCs, the signs
of surface layer shedding were obvious and appeared within a wide range. After seven
FTCs, in addition to some micro-cracks that began to cause material to fall off layer by
layer, the cracks extended to a certain depth around the silt, and the cementation of the
clay began to break down at this time. After 11 FTCs, the layered shedding still existed in
a large area, the vertical penetration pores gradually deepened to form holes with a width
of about 10 µm, and the horizontal penetration pores gradually became more abundant.
Figure 11. SEM images of the expansive soil after different N
FTC
.
Expansive soils exhibit varying characteristics depending on the moisture content.
When the natural moisture content approaches the plastic limit, the expansive soil is in a
dry state and becomes hard and prone to cracking. Conversely, when this type of soil is
saturated, its volume increases significantly with increasing moisture content, demon-
strating pronounced expansibility. The cracks may narrow or close as the soil structure
becomes denser. As the moisture content decreases, the volume of the soil inevitably
shrinks, leading to shrinkage phenomena and contraction stress. In this study, for the
remolded expansive soil sample, the soils with different moisture contents also exhibited
distinct characteristics (Figure 12). The microstructure of the low-moisture-content sam-
ples revealed that the clay particles initially aggregated around the sand or silt particles
while still maintaining a flocculent structure. During the sample preparation, particle for-
mation occurred due to compaction efforts. With increasing moisture content, the clay
particles continued to aggregate. However, the Group Y samples achieved the most dense
state. Additionally, more fragmentation cracks developed after the FTCs. In the Group X
samples (i.e., a lower moisture content), the contacts between the particles remained
Figure 11. SEM images of the expansive soil after different NFTC.
Expansive soils exhibit varying characteristics depending on the moisture content.
When the natural moisture content approaches the plastic limit, the expansive soil is
in a dry state and becomes hard and prone to cracking. Conversely, when this type
of soil is saturated, its volume increases significantly with increasing moisture content,
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demonstrating pronounced expansibility. The cracks may narrow or close as the soil
structure becomes denser. As the moisture content decreases, the volume of the soil
inevitably shrinks, leading to shrinkage phenomena and contraction stress. In this study,
for the remolded expansive soil sample, the soils with different moisture contents also
exhibited distinct characteristics (Figure 12). The microstructure of the low-moisture-
content samples revealed that the clay particles initially aggregated around the sand or
silt particles while still maintaining a flocculent structure. During the sample preparation,
particle formation occurred due to compaction efforts. With increasing moisture content,
the clay particles continued to aggregate. However, the Group Y samples achieved the
most dense state. Additionally, more fragmentation cracks developed after the FTCs. In
the Group X samples (i.e., a lower moisture content), the contacts between the particles
remained relatively sparse. Consequently, numerous large pores persisted after compaction.
In contrast, for the Group Z samples, after compaction, a more pronounced agglomeration
phenomenon was observed in the dry soil mixed with water. This resulted in an increase
in the presence of large pores and enhanced penetration pore development under FTC
conditions.
Sustainability 2025, 17, x FOR PEER REVIEW 13 of 25
relatively sparse. Consequently, numerous large pores persisted after compaction. In con-
trast, for the Group Z samples, after compaction, a more pronounced agglomeration phe-
nomenon was observed in the dry soil mixed with water. This resulted in an increase in
the presence of large pores and enhanced penetration pore development under FTC con-
ditions.
Figure 12. Microstructures of expansive soil samples with different moisture contents and N
FTC
.
The morphologies of the soil particles of the expansive soil were observed under
2000× magnification (Figure 13), including granular and agglomerate types. The granular
particles can be subdivided into subrounded, angular, strips, flaky, and mostly angular
and flaky. The particle agglomerates can be subdivided into compositionary, accretionary
and superimpositionary, and mainly compositionary and superimpositionary. The com-
positionary particle agglomerates were formed by the complete cementation of the small
particles to the large particles, which belong to the outer clay-like particles. The accretion-
ary particle agglomerates were formed by the accumulation of particles. Due to the large
clay content of the expansive soil, the accumulation form is relatively small, and the accu-
mulation is a weak zone. The superimpositionary particle agglomerates were formed in
the form of edge–edge, edge–face, or face–face contacts with the flake particles. Overall,
the structural unit of the expansive soil sample is largely composed of compositionary
and superimpositionary particle agglomerates, and a clay particle matrix is present in
these structures, thus forming irregular aggregates. Following Ye et al. [31], loess contains
a relatively high percentage of silt and exhibits weaker cementation. In contrast, this study
finds that the particles in expansive soil have a higher clay content and stronger cementa-
tion, resulting in a more closely connected arrangement and a more robust structure.
Moreover, the lower the moisture content, the more pronounced this characteristic be-
comes. This observation also explains why expansive soil was historically misidentified
as a high-quality foundation material due to its high strength and low compressibility
under drought conditions.
Figure 12. Microstructures of expansive soil samples with different moisture contents and NFTC.
The morphologies of the soil particles of the expansive soil were observed under 2000
×
magnification (Figure 13), including granular and agglomerate types. The granular particles
can be subdivided into subrounded, angular, strips, flaky, and mostly angular and flaky.
The particle agglomerates can be subdivided into compositionary, accretionary and super-
impositionary, and mainly compositionary and superimpositionary. The compositionary
particle agglomerates were formed by the complete cementation of the small particles to
the large particles, which belong to the outer clay-like particles. The accretionary particle
agglomerates were formed by the accumulation of particles. Due to the large clay content
of the expansive soil, the accumulation form is relatively small, and the accumulation is
a weak zone. The superimpositionary particle agglomerates were formed in the form of
edge–edge, edge–face, or face–face contacts with the flake particles. Overall, the structural
unit of the expansive soil sample is largely composed of compositionary and superimpo-
sitionary particle agglomerates, and a clay particle matrix is present in these structures,
thus forming irregular aggregates. Following Ye et al. [
31
], loess contains a relatively high
percentage of silt and exhibits weaker cementation. In contrast, this study finds that the
particles in expansive soil have a higher clay content and stronger cementation, resulting in
a more closely connected arrangement and a more robust structure. Moreover, the lower
the moisture content, the more pronounced this characteristic becomes. This observation
Sustainability 2025,17, 762 13 of 24
also explains why expansive soil was historically misidentified as a high-quality foundation
material due to its high strength and low compressibility under drought conditions.
Sustainability 2025, 17, x FOR PEER REVIEW 14 of 25
Figure 13. Microscopic features of the expansive soil sample.
3.3.3. Quantitative Analysis of Microscopic Characteristics
The quantitative analysis of the microstructure parameters was mainly carried out to
assess the impact of the moisture content and the N
FTC
on the morphology, grain-size dis-
tribution, and arrangement of the soil particles [30].
Figure 14a shows the variations in the average form factor, which decreases as the
moisture content and N
FTC
increase. The greater the moisture content, the greater the effect
of the N
FTC
. Under the action of FTCs, the particle edge shape tends to be complicated, and
the collected grain shape tends to be irregular. Figure 14b shows the variations in the area–
circumference fractal dimension o. As the moisture content and N
FTC
increase, the area–
circumference fractal dimension of the particles increases until it becomes stable. It can be
seen that the complexity of the morphology of the skeleton particles increases as the N
FTC
increases. Figure 14c shows the variations in the area probability distribution index with
the N
FTC
, and it increases as the N
FTC
and moisture content increase. This indicates that the
structural units with large areas decrease under the action of FTCs. With increasing mois-
ture content, the adhesive effect of the clay particles is enhanced, and the structural units
with large areas increase. Figure 14d shows the fractal dimension of the particle distribu-
tion. As the N
FTC
increases, the fractal dimension of the particle distribution increases.
With increasing moisture content, the fractal dimension of the particle distribution grad-
ually decreases. Under the strong physical weathering caused by FTCs, large particles are
broken due to the effect of the freezing of the water in the soil, and the homogenization
and concentration of the particles are enhanced. Figure 14e shows the probabilistic en-
tropy, which initially increases and then decreases as the N
FTC
increases. It also increases
with increasing moisture content. After the FTCs, it is evident that the particle arrange-
ment changes from chaotic to ordered, with three FTCs marking a turning point. It is chal-
lenging for the particle distribution to achieve a uniform state during the compaction pro-
cess because the bonding effect between the particles is increased by the increase in mois-
ture content.
Figure 13. Microscopic features of the expansive soil sample.
3.3.3. Quantitative Analysis of Microscopic Characteristics
The quantitative analysis of the microstructure parameters was mainly carried out
to assess the impact of the moisture content and the N
FTC
on the morphology, grain-size
distribution, and arrangement of the soil particles [30].
Figure 14a shows the variations in the average form factor, which decreases as the
moisture content and N
FTC
increase. The greater the moisture content, the greater the effect
of the N
FTC
. Under the action of FTCs, the particle edge shape tends to be complicated,
and the collected grain shape tends to be irregular. Figure 14b shows the variations in the
area–circumference fractal dimension o. As the moisture content and N
FTC
increase, the
area–circumference fractal dimension of the particles increases until it becomes stable. It
can be seen that the complexity of the morphology of the skeleton particles increases as
the N
FTC
increases. Figure 14c shows the variations in the area probability distribution
index with the N
FTC
, and it increases as the N
FTC
and moisture content increase. This
indicates that the structural units with large areas decrease under the action of FTCs. With
increasing moisture content, the adhesive effect of the clay particles is enhanced, and
the structural units with large areas increase. Figure 14d shows the fractal dimension
of the particle distribution. As the N
FTC
increases, the fractal dimension of the particle
distribution increases. With increasing moisture content, the fractal dimension of the
particle distribution gradually decreases. Under the strong physical weathering caused by
FTCs, large particles are broken due to the effect of the freezing of the water in the soil, and
the homogenization and concentration of the particles are enhanced. Figure 14e shows the
probabilistic entropy, which initially increases and then decreases as the N
FTC
increases.
It also increases with increasing moisture content. After the FTCs, it is evident that the
particle arrangement changes from chaotic to ordered, with three FTCs marking a turning
point. It is challenging for the particle distribution to achieve a uniform state during the
compaction process because the bonding effect between the particles is increased by the
increase in moisture content.
Sustainability 2025,17, 762 14 of 24
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(a) (b)
(c) (d)
(e)
Figure 14. Microscopic characteristics of the expansive soil samples: (a) Average form factor; (b)
Area–circumference fractal dimension; (c) Area probability distribution index; (d) Fractal dimension
of particle distribution; and (e) Probabilistic entropy.
To visualize the changes in the characteristics of the different types of pore distribu-
tions after FTCs, the PCAS V2.3 software was used to calculate the classified pore charac-
teristics. According to Table 3, the pores can be divided into coarse pores (d > 75 µm),
large pores (20 µm < d ≤ 75 µm), medium pores (10 µm < d ≤ 20 µm), small pores (5 µm <
d ≤ 10 µm), micropores (0.1 µm < d ≤ 5 µm), and ultramicropores (d ≤ 0.1 µm). A stacked
Figure 14. Microscopic characteristics of the expansive soil samples: (a) Average form factor; (b) Area–
circumference fractal dimension; (c) Area probability distribution index; (d) Fractal dimension of
particle distribution; and (e) Probabilistic entropy.
To visualize the changes in the characteristics of the different types of pore distributions
after FTCs, the PCAS V2.3 software was used to calculate the classified pore characteristics.
According to Table 3, the pores can be divided into coarse pores (d > 75
µ
m), large pores
(20
µ
m < d
≤
75
µ
m), medium pores (10
µ
m < d
≤
20
µ
m), small pores (5
µ
m < d
≤
10
µ
m),
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micropores (0.1
µ
m<d
≤
5
µ
m), and ultramicropores (d
≤
0.1
µ
m). A stacked bar plot
of the pore classification area ratio changes is shown in Figure 15, and the changes in the
fractal dimension are shown in Figure 16.
Sustainability 2025, 17, x FOR PEER REVIEW 16 of 25
bar plot of the pore classification area ratio changes is shown in Figure 15, and the changes
in the fractal dimension are shown in Figure 16.
Figure 15. Pore area ratio of the expansive soil samples: (a) Moisture content of 14%; (b) Moisture
content of 20%; and (c) Moisture content of 26%.
For the moisture content of 14% (Figure 15a), the influence of the FTCs on the differ-
ent pore types was dynamic and persistent, and the proportion of each pore type fluctu-
ated to a certain extent under different NFTC. In particular, the proportions of the coarse
pores and ultramicropores changed greatly, while the proportion of the fine pores in-
creased significantly after the first FTC and then gradually stabilized. The proportion of
the large pores exhibited an increasing trend in general. For the moisture content of 20%
(Figure 15b), the coarse pores fluctuated significantly during the FTCs, initially increasing
and then decreasing. The large pores increased continuously. The medium pores fluctu-
ated greatly during the FTCs. The small pores also fluctuated greatly during the FTCs, but
they tended to decrease in general. In general, the number of fine pores increased gradu-
ally during the FTCs. The micropores and ultramicropores tended to decrease in general.
For the moisture content of 26% (Figure 15c), the proportions of the coarse pores and ul-
tramicropores decreased after multiple FTCs. The proportion of the pores of each category
of fine pores fluctuated, and that of the large pores initially decreased and then increased.
Generally speaking, the percentages of small and medium pores rose in the early stages
and marginally declined in the later stages. There was a general upward trend in the per-
centage of micropores, which decreased in the early stages and rebounded in the laer
stages.
Figure 15. Pore area ratio of the expansive soil samples: (a) Moisture content of 14%; (b) Moisture
content of 20%; and (c) Moisture content of 26%.
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Figure 16. Variations in pore fractal dimension for the samples.
Figure 16 shows the variations in the pore fractal dimension with NFTC. As the NFTC
increased, the pore fractal dimension varied slightly but generally dropped, suggesting
that the pore structure’s complexity reduced. Overall, the fractal dimensions of the sam-
ples with low moisture contents and high moisture contents decreased significantly with
increasing NFTC, indicating that the pore structure of these samples became more uniform
or less porous during the FTCs. The fractal dimension of the optimal moisture content
sample changed lile, indicating that its structure was relatively stable during the FTCs.
The findings demonstrate that the effect of the FTCs was evident and that the cementation
of the samples with low moisture levels was poor as a result of their low water content.
The FTCs had a bigger effect on samples with higher moisture levels because of the in-
creased moisture content. As a result, the FTCs had a greater influence in the beginning
stage and stability in the later stage. The FTCs had the least impact on the sample with the
optimum moisture level, which also had a more compact structure and higher cementa-
tion strength.
4. Discussion
4.1. Effect of Freeze
–
Thaw Cycles on Microstructure
The variability of the microscopic characteristics of the expansive soil under the ac-
tion of FTCs needs to be combined with the FTC and parameter changes before and after
the FTCs, so the two parameters of the amount of change and the coefficient of variation
are introduced. The formula for calculating the amount of change is
𝛥𝑀 = 𝐵−
𝐴
, (1)
where ΔM is the variations in the property parameters during the FTCs; Bn is the param-
eter values of the material properties after FTCs; A is the parameter values of the material
properties that have not experienced FTC; and n is the NFTC. ΔM is the difference in the
corresponding index under the action of the different FTCs. If ΔM > 0, the corresponding
index value increases; otherwise, the index value decreases.
The formula for calculating the coefficient of variation K is
1
KAB
n
=−
, (2)
where K is the coefficient of variation, that is, the frequency and severity of the changes in
the various indicators under FTC, which can be used to judge the strengths of the changes
in the various indicators under FTC. When 0 ≤ K ≤ 0.1, the variability is low. When 0.1 ≤ K
≤ 1, the variability is medium. When K > 1, the variability is high.
Figure 16. Variations in pore fractal dimension for the samples.
For the moisture content of 14% (Figure 15a), the influence of the FTCs on the different
pore types was dynamic and persistent, and the proportion of each pore type fluctuated
to a certain extent under different N
FTC
. In particular, the proportions of the coarse pores
and ultramicropores changed greatly, while the proportion of the fine pores increased
significantly after the first FTC and then gradually stabilized. The proportion of the
large pores exhibited an increasing trend in general. For the moisture content of 20%
(Figure 15b), the coarse pores fluctuated significantly during the FTCs, initially increasing
Sustainability 2025,17, 762 16 of 24
and then decreasing. The large pores increased continuously. The medium pores fluctuated
greatly during the FTCs. The small pores also fluctuated greatly during the FTCs, but they
tended to decrease in general. In general, the number of fine pores increased gradually
during the FTCs. The micropores and ultramicropores tended to decrease in general.
For the moisture content of 26% (Figure 15c), the proportions of the coarse pores and
ultramicropores decreased after multiple FTCs. The proportion of the pores of each category
of fine pores fluctuated, and that of the large pores initially decreased and then increased.
Generally speaking, the percentages of small and medium pores rose in the early stages and
marginally declined in the later stages. There was a general upward trend in the percentage
of micropores, which decreased in the early stages and rebounded in the latter stages.
Figure 16 shows the variations in the pore fractal dimension with N
FTC
. As the N
FTC
increased, the pore fractal dimension varied slightly but generally dropped, suggesting that
the pore structure’s complexity reduced. Overall, the fractal dimensions of the samples with
low moisture contents and high moisture contents decreased significantly with increasing
N
FTC
, indicating that the pore structure of these samples became more uniform or less
porous during the FTCs. The fractal dimension of the optimal moisture content sample
changed little, indicating that its structure was relatively stable during the FTCs. The
findings demonstrate that the effect of the FTCs was evident and that the cementation of
the samples with low moisture levels was poor as a result of their low water content. The
FTCs had a bigger effect on samples with higher moisture levels because of the increased
moisture content. As a result, the FTCs had a greater influence in the beginning stage and
stability in the later stage. The FTCs had the least impact on the sample with the optimum
moisture level, which also had a more compact structure and higher cementation strength.
4. Discussion
4.1. Effect of Freeze–Thaw Cycles on Microstructure
The variability of the microscopic characteristics of the expansive soil under the action
of FTCs needs to be combined with the FTC and parameter changes before and after the
FTCs, so the two parameters of the amount of change and the coefficient of variation are
introduced. The formula for calculating the amount of change is
∆M=Bn−A, (1)
where
∆
Mis the variations in the property parameters during the FTCs; B
n
is the parameter
values of the material properties after FTCs; Ais the parameter values of the material
properties that have not experienced FTC; and nis the N
FTC
.
∆
Mis the difference in the
corresponding index under the action of the different FTCs. If
∆
M> 0, the corresponding
index value increases; otherwise, the index value decreases.
The formula for calculating the coefficient of variation Kis
K=
1
n|A−B|, (2)
where Kis the coefficient of variation, that is, the frequency and severity of the changes in
the various indicators under FTC, which can be used to judge the strengths of the changes
in the various indicators under FTC. When 0
≤
K
≤
0.1, the variability is low. When 0.1
≤
K≤1, the variability is medium. When K> 1, the variability is high.
The changes in the microscopic characteristics of the expansive soil samples with
different moisture contents and subjected to different N
FTC
are shown in Figure 17, in
which the changes in the average form factor were negative, indicating that the FTCs
increased the complexity of the particle edge shapes. The changes in the area–circumference
Sustainability 2025,17, 762 17 of 24
fractal dimension, area probability distribution index, and fractal dimension of the particle
distribution were positive, indicating that the number of large particles decreased, the
spatial morphology of the particles became complicated, and the particle distribution
changed from dispersed to concentrated under the FTCs. The variation characteristics
of the above four indexes all indicate that, the greater the moisture content, the more
significant the effect of the FTCs. With increasing N
FTC
, the change in the probabilistic
entropy gradually decreased from positive to negative, indicating that, compared with
the samples not subjected to FTCs, those subjected to FTCs initially exhibited an increase
and then a decrease in the probabilistic entropy index. Moreover, the probabilistic entropy
values of the group X and group Y samples were lower than those of the no-FTCs samples,
indicating that the particle arrangement’s order declined and that it became chaotic under
the FTCs. The occurrence of multiple FTCs formed new connections and structures inside
the soil samples. Under the fragmentation and agglomeration of the soil particles, the
grain size homogenized, and eventually the changes in the soil samples’ characteristics
progressively stabilized and established a new equilibrium.
Based on the above analysis, the coefficient of variation was introduced to analyze
the degrees of change of the quantitative indexes of the particles during the FTC process
(Figure 18). The statistical parameters of microstructure particles can be used to quanti-
tatively analyze the characteristics of particle morphology, arrangement, and grain-size
distribution, among which the average form factor and area–circumference fractal dimen-
sion were used to represent the morphological characteristics, area probability distribution
index and fractal dimension of the particle distribution were used to represent the grain-
size distribution characteristics, and the probabilistic entropy was used to represent the
arrangement characteristics. According to the classification criteria for the variability, al-
though the average form factor, area–circumference fractal dimension, and probability
entropy changed during the FTCs, the variability was small, indicating that these were low-
variability parameters. The variabilities of the area probability distribution index and the
fractal dimension of the particle distribution were medium. Their coefficients of variation
were large before the third FTC, and then the variability gradually decreased. The medium
variation level indicates that the FTC mainly had a great influence on the particle grain-size
distribution in the expansive soil sample, which is also consistent with the conclusion of
Zhang et al. [
34
]. Due to the decreasing effects of the soil particle arrangement structure
and grain-size distribution changes (i.e., the decreases in the degrees of these changes), the
fluctuation frequency of the coefficients of variation decreased, which also indicates that
the soil sample formed a new stable structure after several FTCs. In essence, the coefficients
of variation of these physical properties were also closely related to the changes in the
structure, particle size composition, and mineral composition of the soil, and the internal
changes in the sample were reflected by the various coefficients of variation, so they can be
used to analyze the mechanical properties of the sample to a certain extent.
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(a) (b)
(c) (d)
(e)
Figure 17. Variations in the quantitative indexes for the expansive soil particles: (a) Average form
factor; (b) Area–circumference fractal dimension; (c) Area probability distribution index; (d) Fractal
dimension of the particle distribution; and (e) Probabilistic entropy. Note: The purple, green, and
red in this bar chart represent the results of groups X, Y, and Z, respectively.
Based on the above analysis, the coefficient of variation was introduced to analyze
the degrees of change of the quantitative indexes of the particles during the FTC process
(Figure 18). The statistical parameters of microstructure particles can be used to quantita-
tively analyze the characteristics of particle morphology, arrangement, and grain-size dis-
tribution, among which the average form factor and area–circumference fractal dimension
Figure 17. Variations in the quantitative indexes for the expansive soil particles: (a) Average form
factor; (b) Area–circumference fractal dimension; (c) Area probability distribution index; (d) Fractal
dimension of the particle distribution; and (e) Probabilistic entropy. Note: The purple, green, and red
in this bar chart represent the results of groups X, Y, and Z, respectively.
Sustainability 2025,17, 762 19 of 24
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(a) (b)
(c) (d)
(e)
Figure 18. Coefficients of variation of the quantitative indexes of the expansive soil particles: (a)
Average form factor; (b) Area–circumference fractal dimension; (c) Area probability distribution
index; (d) Fractal dimension of the particle distribution; and (e) Probabilistic entropy. Note: The red,
green, and blue in this line chart represent the results of groups X, Y, and Z, respectively.
4.2. Mechanism of Freeze–Thaw Action in Unsaturated Expansive Soil
The FTCs process leads to the spliing of coarse-grained particles and the agglomer-
ation of fine-grained particles, which occur synchronously. Repeated FTCs cause the soil
particles to break or agglomerate, thus changing the size of the particles. The fragmenta-
tion and agglomeration of particles not only change the size of the particles but also
Figure 18. Coefficients of variation of the quantitative indexes of the expansive soil particles: (a) Av-
erage form factor; (b) Area–circumference fractal dimension; (c) Area probability distribution index;
(d) Fractal dimension of the particle distribution; and (e) Probabilistic entropy. Note: The red, green,
and blue in this line chart represent the results of groups X, Y, and Z, respectively.
4.2. Mechanism of Freeze–Thaw Action in Unsaturated Expansive Soil
The FTCs process leads to the splitting of coarse-grained particles and the agglom-
eration of fine-grained particles, which occur synchronously. Repeated FTCs cause the
soil particles to break or agglomerate, thus changing the size of the particles. The frag-
mentation and agglomeration of particles not only change the size of the particles but also
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change the morphology of the particles. The changes in the particle morphology result
in changes in the soil mechanical properties. After several FTCs, the composition of the
soil will have greatly changed. The mineral particles with larger grain sizes will fragment,
while the smaller particles will agglomerate. After this, the changes in the particle size
of the entire sample exhibit a decreasing trend, and the whole particle size composition
evolves toward uniformity. The changes in the composition cause changes in the structural
connections between the particles. During the freezing process, the structural connections
of the soil particles change from condensation to condensation-agglomeration to crystalliza-
tion. The changes in the soil structure during the FTCs also produce different integrated,
network, or layered structures due to the FTCs. The FTC test revealed that, during the first
and second freezing steps, the soil bodies were divided into layers of different sizes by
ice interlayers, and they recovered their original overall structure after melting occurred
(Figure 19). In the subsequent 3–5 FTCs, the structure was completely integrated, and the
phenomenon in which the layers and integrated structure occurred alternately during the
FTC process no longer occurred [
34
,
35
]. Many scholars have analyzed the influence of
freeze–thaw cycles on the macroscopic mechanical characteristics of expansive soil [
4
,
12
,
14
].
The research shows that the freeze–thaw effect will have a certain weakening effect on the
mechanical characteristics, and, when the number of freeze–thaw cycles exceeds a certain
value, the strength and creep characteristics of expansive soil will tend to be stable. From
the perspective of engineering practice, the characteristics of shallow destruction, easy
disintegration, and low residual strength of expansive soil indicate that the expansive soil
site has a high probability of significant damage in the first spring thawing period after
disturbance, and will stabilize after several freeze–thaw cycles. In this study, the damage
mechanism of an expansive soil site is studied comprehensively from the microscopic level.
The summarized research work will be useful when designing corresponding mitigation
measures for expansive soil sites in seasonally frozen regions.
Sustainability 2025, 17, x FOR PEER REVIEW 21 of 25
change the morphology of the particles. The changes in the particle morphology result in
changes in the soil mechanical properties. After several FTCs, the composition of the soil
will have greatly changed. The mineral particles with larger grain sizes will fragment,
while the smaller particles will agglomerate. After this, the changes in the particle size of
the entire sample exhibit a decreasing trend, and the whole particle size composition
evolves toward uniformity. The changes in the composition cause changes in the struc-
tural connections between the particles. During the freezing process, the structural con-
nections of the soil particles change from condensation to condensation-agglomeration to
crystallization. The changes in the soil structure during the FTCs also produce different
integrated, network, or layered structures due to the FTCs. The FTC test revealed that,
during the first and second freezing steps, the soil bodies were divided into layers of dif-
ferent sizes by ice interlayers, and they recovered their original overall structure after
melting occurred (Figure 19). In the subsequent 3–5 FTCs, the structure was completely
integrated, and the phenomenon in which the layers and integrated structure occurred
alternately during the FTC process no longer occurred [34,35]. Many scholars have ana-
lyzed the influence of freeze–thaw cycles on the macroscopic mechanical characteristics
of expansive soil [4,12,14]. The research shows that the freeze–thaw effect will have a cer-
tain weakening effect on the mechanical characteristics, and, when the number of freeze–
thaw cycles exceeds a certain value, the strength and creep characteristics of expansive
soil will tend to be stable. From the perspective of engineering practice, the characteristics
of shallow destruction, easy disintegration, and low residual strength of expansive soil
indicate that the expansive soil site has a high probability of significant damage in the first
spring thawing period after disturbance, and will stabilize after several freeze–thaw cycles.
In this study, the damage mechanism of an expansive soil site is studied comprehensively
from the microscopic level. The summarized research work will be useful when designing
corresponding mitigation measures for expansive soil sites in seasonally frozen regions.
Figure 19. Schematic diagram of the changes in the expansive soil layer structure during the FTCs
process (after Zhang et al., 2013; Zhan et al., 2019) [34,35].
Akagawa et al. [36] and Lai et al. [37] concluded that, when the macroscopic crystal-
lization stress exceeds the tensile strength of the soil, the pore structure will be destroyed
and cracks will form. The heat and moisture exchange during FTCs causes considerable
changes in the pore, disrupting the arrangement and connection of the soil particles. In
this study, FTCs significantly influence the arrangement mode and contact relationships
of expansive soil particles, thereby affecting the fractal characteristics of the microstruc-
ture. Micro-cracks form in the water dissipation channel during FTCs. With increasing
N
FTC
, the number and scale of the micro-cr