1. Introduction
Saline and alkaline soil, as well as the soil following salinization and alkalization, are referred to as saline soil [
1,
2]. Due to the large amount of salt in pore water, the structural and strength changes of salinized soil in seasonal permafrost regions are more complex than the changes in the strength and structure of saline soil in non-saline soil [
3]. When the external temperature, moisture, and other conditions change, the pores inside the saline soil will increase, the soil will soften, or the volume will increase. The mechanical properties of saline soil in seasonal permafrost areas are usually poor. If this material is directly used as a basic filling material, a variety of engineering problems may occur [
4], such as road grouting, melting settlement, subgrade settlement, and slope stability reduction. Therefore, to maintain the sustainability of engineering construction in seasonal frozen soil areas and to improve the durability of engineering in these areas, it is of great engineering significance to study the influence of the freeze-thaw cycle on the strength characteristics of saline soil and to improve the properties of saline soil.
Current studies have shown that the unconfined compressive strength of undisturbed soil and improved soil will decrease with the increase in the number of freeze-thaw cycles [
5,
6]. The cohesion comprehensively reflects various physical and chemical forces, and the internal friction angle reflects the slip or interlocking effect between soil particles [
7]. Therefore, these two parameters, cohesion and the internal friction angle, are also widely used in the study of the properties of saline soil after freeze-thaw cycles. Wang et al. [
8], Xu et al. [
9], and Zhang et al. [
10] showed that with the increase in the number of freeze-thaw cycles, both cohesion and the internal friction angle showed a downward trend on the whole, but the internal friction angle may fluctuate in the process of change. The change in the soil engineering properties under freeze-thaw conditions is mainly due to the destruction of the soil structure. Freeze-thaw cycles can change the grain size distribution of soil [
11,
12] and can also lead to changes in the internal void structure and skeleton of soil [
8,
13]. Aldaood et al. [
14] revealed the effects of water penetration and crack propagation on soil structure through mercury injection and scanning electron microscopy (SEM). Qi et al. [
15] studied the shear strength and microstructure of silty clay and loess before and after freeze-thaw cycles through mechanical and SEM tests. They pointed out that the microstructure of soil was an important characteristic for explaining its mechanical properties. Through SEM analysis, Wang et al. [
10] provided a quantitative basis for explaining the change mechanism of the mechanical properties of saline soil from a microscopic perspective. Previous studies have shown that the strength of soil is closely related to its microstructure [
10,
14,
15], and the study of the microstructure of soil under freeze-thaw cycles is conducive to the study of the mechanism of the influence of freeze-thaw cycles with respect to the properties of saline soil.
While a great deal of research has been done on the basic properties of salted soil, some achievements have been made in the concrete construction of salted soil areas and the application of modifiers to improve salted soil road performance under freeze-thaw conditions [
16,
17,
18]. Takeshi et al. [
16] used recycled kyanite obtained from gypsum waste as a stabilizing material for soft clay, which improved the durability and strength of saline soil under freeze-thaw cycles. LV et al. [
17] used lime, fly ash, and sodium silicate to solidify fine-grained salted sulfate soil, and their study showed that the addition of a curing agent could effectively inhibit the salt-swelling characteristics of salted sulfate soil and improve its strength. Li et al. [
18] used fly ash produced by a local power plant to solidify salinized soil and obtain an environmentally friendly, improved, and solidified super-salinized soil. At present, most of the studies on saline soil characteristics and the effect of the application of modifiers on these characteristics have reported on the moisture content of the salt, the modifier dosage, and the influence of various factors on the saline soil strength. However, there is limited research on saline soil that is seasonally frozen (under the effect of the freezing and thawing cycles), especially with respect to the effect and mechanism of action of added modifiers on the quantitative changes in the relevant parameters, microstructure, and strength of the saline soil.
At present, most sections of the Suihua to Daqing expressway (Sui-Da Expressway) of China under planning and construction are saline soil sites, and this area is a seasonal frozen soil area, with a long winter, a large temperature difference between day and night, and long-term temperature below zero degrees Celsius in winter. According to data from the China Meteorological Network, the lowest temperature in Suihua since 2011 reached −34 °C, and the average temperature in January, February, and December has been −20 °C, −15 °C, and −17 °C, respectively. This special environment exposes the local subgrade soil to freeze-thaw cycles over a long period. Such periodic changes will lead to subgrade loosening and will threaten the operation of the road.
Using fly ash to improve the salinized soil subgrade can improve the soil shear strength and stability. Compared with other improvement methods, this approach can reduce the construction cost and accelerate the comprehensive utilization process of fly ash, which has good economic and environmental significance.
In this study, we studied the suitable use of fly ash-modified sulfuric saline soil for roads in cold regions. Fly ash was used as a modifier considering the influence of different freeze-thaw cycles on the saline soil; analysis was conducted on the effect of different dosages of fly ash applied during the freezing and thawing cycle on the unconfined compressive strength and shear strength of the saline soil; the influence of freeze-thaw cycles on the saline soil characteristics was determined; finally, the influence of Image-Pro Plus (IPP) on the soil microstructure was determined using SEM experiments.
We found that the strength characteristics of subgrade salinized soil improved by fly ash, and the shear strength, cohesion, and internal friction angle of soil with 15% fly ash content obtained the highest values. These results provided a reference for the design and construction of saline soil roadbed engineering in seasonal frozen areas and the construction of saline land belts in seasonal and winter areas.
3. Test Scheme Design
3.1. Fly Ash Mix Ratio Design
Existing studies have shown that soil strength can be improved by adding 10%–20% fly ash to the soil [
18,
21,
22]. In order to further compare the improvement effect of 10%, 15%, and 20% fly ash content on the strength of saline soil, the improved test scheme is shown in
Table 4.
3.2. Sample Preparation
The collected natural saline soil was dried and passed through a 2 mm sieve. The fly ash was passed through the same sieve before use. According to the soil samples collected in the early exploration phase, the average natural moisture content of the soil samples was 12.1% based on laboratory tests. In order to make the test conditions closer to the actual working conditions and make the test data more comparable, the water content was controlled at 12.1% in the test process. The dry density of soil samples measured by the compaction test is shown in
Table 5.
The fly ash and salted soil were mixed according to the ratio relationship, and distilled water was added according to the mass volume relationship so that the moisture content reached 12.1%. After curing the configured soil sample under standard conditions for 24 h, a hydraulic press was used to press the soil sample into a standard triaxial specimen [
23] with a diameter of 39.1 mm and a height of 80 mm in accordance with 95% compaction [
23], and the unconfined compression specimen [
23] had a diameter of 50 mm and a height of 50 mm. The specimen was wrapped in plastic film, sealed in a bag, cured in a humid environment for 24 h, and then placed inside a cryogenic box.
According to the statistical data of the China Weather Network and the local meteorological bureau, the local winter is from October to March of the following year. The average daytime temperature in winter is −4.3 °C and the average night temperature is −13.9 °C, and thus −13.9 °C was selected as the freezing test temperature. The samples were frozen in a cryogenic chamber at −13.9 °C for 6 h and then melted in a cryogenic chamber at 20 °C for 6 h.
3.3. Unconfined Compression Test
The unconfined compressive test was carried out at room temperature (20 °C) following the Test Rules for Stabilized Materials of Highway Engineering Inorganic Binding Materials (JTG E51-2009). The cylindrical soil samples (Φ50 mm × 50 mm) were compared using a microcomputer-controlled electronic testing machine (WDW-50, Changchun Kexin Test Instrument Co., Ltd., Changchun, China). During the test, the deformation rate was 1 mm/min, and the loading data and displacement were automatically recorded by the data recording instrument to calculate the corresponding stress and strain and draw the stress-strain curve.
3.4. Triaxial Test
Considering practical engineering, due to the rapid construction speed, the water and air in the soil pores are not completely eliminated. In order to better simulate the actual working conditions and at the same time avoid the negative effect of the triaxial consolidation test on the original soil structure, the use of an automatic triaxial apparatus (TSZ-6, Nanjing NingXi soil instrument factory, Nanjing, China) for cylindrical soil samples (φ 39.1 mm×80 mm) was not consolidated in an undrained triaxial compression test to obtain the shear strength. In order to reduce the disturbance to the soil during the process of placing the sample, the sample was placed in the triaxial apparatus after the frozen component of the last freeze-thaw cycle was completed and thawing had occurred for 6 h. In order to ensure the consistency of the thawing process and reduce the influence of the ambient temperature on the test data, the ambient temperature of the triaxial instrument was set to 20 °C during the thawing test. The confining pressures applied to the soil samples were set at 100, 200, 300, and 400 kPa, and the shear strain rate was 0.8 mm/min. When there was no obvious failure in the test specimens, the principal stress difference at a strain of 15% was adopted as the failure value, and the temperature remained unchanged during the test. Based on triaxial test data, the cohesion and internal friction angle of the soil samples were obtained using the Mohr-Coulomb criterion.
3.5. SEM Test
SEM was used to observe the surface microstructure of the soil samples, and the microstructure of the soil samples was quantitatively analyzed by image processing software. In the experiment, SEM (Apreo 2 SEM, Thermo Scientific, Suzhou, China) was used to observe the soil samples under different freeze and thaw cycles. In order to prevent disturbance of the soil structure, the dried pieces were peeled off along the crack, and soil samples with relatively flat surfaces were selected for cutting to ensure that the length and width of the samples after cutting were 10 mm. The cut sample was fixed on a conductive copper sheet and placed in a 105 °C oven for drying for 48 h. The dry samples were sprayed with gold to ensure good electrical conductivity. The soil samples were then observed by SEM.
3.6. Acquisition of Microscopic Parameters
In this study, IPP software was used to process the micrographs of soil samples under different conditions to obtain microscopic pore parameters. To reduce the data differences caused by human factors during image processing and data extraction, the same processing method was used for each soil sample photo. The processing steps of the SEM photos were as follows: image conversion and cropping, brightness and contrast adjustment, image morphology processing and binarization processing, and image parameter processing. To ensure that the calculation results of different photos were comparable, the first three steps of image processing were processed by the same Matlab calculation code. In the process of obtaining microscopic parameters, the selection of the image magnification factor and the setting of the threshold have an important influence on the image processing results under SEM [
10]. Magnification refers to the conclusions of existing studies [
10,
24,
25]. Magnification was combined with the characteristics of the soil used in the test to ensure that the microstructures and pore changes of the samples were relatively clearly reflected in the photos taken. Finally, a micrograph with a magnification of 500 times was selected for analysis. The same image was observed, results with poor observation effects were discarded, and the average was taken as the final threshold of image binarization. IPP software can be used to obtain the basic micropore parameters, including the perimeter, area, and diameter and to select the parameters required by the test to characterize the micropore of the sample. The specific meaning and calculation method of each parameter follow.
Since the total pore area extracted from the SEM image is a concept on the plane, the porosity obtained is the porosity on the two-dimensional plane. Porosity is expressed as the percentage pore area relative to the total size of the SEM image. The specific expression is as follows:
where
S0 represents the total pore area, and
S represents the total area of the microscopic image. Porosity can reflect the pore content of the sample under certain circumstances.
The average pore diameter of each pore in the SEM image can be directly calculated using IPP software. For a single pore, the average pore diameter refers to the arithmetic average of each line segment’s length connecting the two points of the outer contour of the measured object and passing through the center of the mass. For the samples being tested, the average pore diameter of all pores was calculated as the average pore diameter of the sample .
The fractal dimension reflects the spatial effectiveness occupied by a complex form, and it is a measure of the irregularity of a complex form. At present, a large number of studies have shown that the soil fractal structure can be used to characterize soil pores, and the microstructure of soil can be studied based on fractal theory [
26,
27]. The fractal dimension of pore surface fluctuation is generally expressed by
F, which can vividly reflect the morphological characteristics, arrangement characteristics, and particle size distribution characteristics of soil particles to a certain extent. The specific expression is as follows:
where
ε is the length of the grid in image segmentation, and
is the response to the total number of grid microstructure pictures.