Experimental Study on the Damage Properties of Mechanical Properties of Saline Soil Under Different Influencing Factors

Abstract

Influenced by factors such as the freeze–thaw cycle and water–salt migration, road construction in Uzbekistan’s highway project areas is prone to dissolution and subsidence, salt swelling, corrosion, and other engineering diseases. To investigate how various factors impact saline soils in Uzbekistan’s monsoon freezing zone, we conducted analyses of stress–strain curves, failure strength, and shear strength parameters of these soils through freeze–thaw (F-T) cycle tests and unconsolidated and undrained (UU) triaxial shear tests. The findings indicate that with the increase of salt content, the average reduction in the failure strength of saline soil was 15.8%, 6.3%, and 5.7%; with the increase of water content, the average reduction in cohesion was 10.8%, 44.1%, and 32.6%; and the internal friction angle increased with the increase of the number of F-T cycles and decreased with the increase of freezing temperature. Ultimately, we defined the rates of failure strength deterioration and cohesion damage in saline soil due to various factors, analyzing the destructive impacts of these factors. The results demonstrate a strong correlation between the curves of failure strength deterioration and cohesion damage ratios, indicating that the significant degradation of saline soil due to salt is primarily influenced by F-T cycles, with the extent of damage closely linked to water content.

1. Introduction

As the One Belt and One Road initiative continues to advance, there has been a notable enhancement in the development of transportation infrastructure within saline land regions of Central Asia and other nations situated along the Silk Road. Therefore, the construction of special soils is increasing at this stage [1,2]. Existing studies have shown that the presence of salt often negatively affects the strength of rock or soil [3,4,5]. When salt water penetrates a mine, it causes erosion by dissolving the salt rock, leading to mine collapse [6]. When sulfides are present in the cement rock, the cement rock is softened, resulting in reduced strength [7]. In addition, the interactions between salt and water in saline soils located in seasonally frozen regions, along with their distinctive behaviors concerning water and salt migration during F-T cycles [8], can lead to a deterioration [9,10,11,12,13] of the physical and mechanical properties of the soil, including density, permeability, and strength. This degradation is particularly pronounced under the repeated influence of F-T cycles during construction activities in these areas. Engineering projects frequently encounter challenges, such as dissolution, salt heaving, and corrosion [14,15], which can result in substantial damage to highway subbases. Consequently, comprehending the mechanical properties of saline soil in the context of F-T cycle conditions represents a significant technical challenge in the construction of roadbeds composed of saline soil in high-altitude and cold environments.

The F-T cycle process induces continuous alterations in the proportions of the three phases of saline soil, resulting in modifications to the soil structure that subsequently influence its mechanical properties [16,17]. Over the years, scholars have investigated the physical and mechanical characteristics of saline soil exposed to F-T cycles from three primary perspectives: the microstructural composition of the soil, uniaxial compressive strength, and shear strength parameters.

With advancements in science and technology, computed tomography (CT) scanning, mercury injection, scanning electron microscopy, and other techniques have been widely utilized in the investigation of soil’s microstructure characteristics. You et al. [18] and Wang et al. [19] performed F-T cycle experiments on saline soils characterized by different levels of salt concentration, and both examined the pore changes in soil samples through mercury injection tests. In their F-T cycle experiments, Zhang et al. [20] illustrated that samples characterized by elevated chloride salt concentrations and reduced sulfate content experienced negligible volume contraction. Conversely, samples with diminished chloride salt levels and increased sulfate content displayed considerable volume expansion. Previous studies have demonstrated correlations between the microscopic characteristics and the macroscopic physical and mechanical properties. Wang et al. [21] established a correlation between the distribution of pore sizes and the content of unfrozen water. Additionally, Wang et al. [22] identified a correlation between the micro-pore characteristics of saline soil and its failure strength through the application of principal component regression analysis. This research offers a quantitative framework for elucidating the mechanisms underlying alterations in the mechanical characteristics of saline soil via the microscopic viewpoint. The uniaxial compressive strength of saline soil exposed to F-T cycles is an essential factor in evaluating the mechanical characteristics of saline soil. Nan et al. [23] conducted an investigation to assess the impact of quicklime curing and F-T cycles on the engineering properties of saline soil, utilizing unconfined compression tests as their primary methodology. Their research focused on the impact of curing duration, the F-T cycle frequency, and the conditions of constraint on the mechanical characteristics of saline soil. Relevant studies have shown that cohesion and internal friction angle are two essential parameters influencing soil’s shear strength [24,25]. In addition, the shear strength of soil is influenced by several factors, including moisture content, salt concentration, type of salt, level of compaction, frequency of freeze–thaw cycles, and temperature conditions during these cycles [26,27,28]. Han et al. [29] used the number of F-T cycles and salt content as variables and conducted triaxial shear tests on reshaped mixed salt samples (sodium sulfate:sodium carbonate = 1:1) to explore how these variables impact the shear strength parameters of saline soil. Cheng et al. [30] examined the effects of the F-T cycle process on the mechanical properties of reshaped saline soil, including the critical state line, stress–strain relationship, effective stress path, and shear strength index, through F-T cycling tests and consolidated undrained triaxial shear tests.

As can be seen from the studies above, most scholars study the influence of single factors on the mechanical properties of saline soils from a microscopic point of view, and there is still a lack of research on the change of mechanical properties of saline soils under the coupling of multiple fields. In addition, the quantitative correlations between the extent of soil strength deterioration and the effects of salt content, moisture content, the number of freeze–thaw (F-T) cycles, and the temperature during these cycles, along with their interactive effects, are not well understood. This research involved conducting F-T cycle experiments on artificially created saline soils characterized by differing levels of salt content, moisture content, durations of F-T cycles, and temperatures. Following the F-T cycle tests, the samples were analyzed through unconsolidated undrained (UU) triaxial shear tests. The aim of this study was to investigate the effect of different factors on saline soils in Uzbekistan and to obtain the effects of multifactorial damage and degradation of saline soils. This study provides a theoretical basis for the design and construction of subgrade engineering in saline soil, applicable to similar conditions in future projects.

2. Materials and Test Scheme

2.1. Overview of the Study Area

This study focused on northwestern Uzbekistan (Figure 1), which has a continental climate with large temperature differences between the four seasons and an abundance of wind resources. Figure 2 shows detailed temperature data for the last three years in northwestern Uzbekistan, in which it can be seen that the region has high and arid summers, and the highest temperature reaches 40 °C. The region has the former fourth largest lake in the world, the Aral Sea, and since the completion of the Karakum Canal in 1967, the water area of the lake has been significantly reduced, resulting in the formation of a special hazard, the “Salt Dust Storm”. The occurrence of salt and dust storms breaks the single pattern of soil salinization salt source (salt comes with water) in the arid zone, and the superposition of “salt comes with wind” and “salt comes with water” forms double salinization, which inevitably causes problems during the construction of roadbeds on saline soils under extreme F-T cycles.

2.2. Soil Materials of the Study Area

According to the regulations of the Ministry of Agriculture and Rural Development of the People’s Republic of China, soil and organic cultivation media are prohibited to be brought or sent into the country, and the project studied in this paper is located in Uzbekistan, so it is difficult to obtain saline soils from this study area. In order to obtain the basic properties of the local soil, our team arrived at the project site to conduct a field study. At the construction site, our team conducted geotechnical tests to obtain the physical and chemical properties of the soils at the site (

Table 1. Physical properties of the soil at the engineering site.

Natural Moisture Content, w%	LIQUID Limit, wL/%	Plastic Limit, wP/%	Plastic Index, IP	Maximum Dry Density, ρdmax/(g/cm3)	Optimum Moisture Content, wop/%
14.83	29.79	17.61	12.18	1.79	17.56

shows the physical properties of the soils, and

Table 2. Ion content in the soil.

Depth of Soil Extraction	Content of Each Ion/(mmol/100 g)						Total Salt Content	${\mathit{S}}_1=\frac{\mathit{c}\left({\mathbf{Cl}}^{-}\right)}{2\mathit{c}\left({\mathbf{SO}}_4^{2-}\right)}$
	${\mathbf{SO}}_4^{2-}$	${\mathbf{Cl}}^{-}$	${\mathbf{HCO}}_3^{-}$	${\mathbf{Ca}}^{2+}$	${\mathbf{Na}}^{+}$	${\mathbf{Mg}}^{+}$
0~30 (cm)	9.561	1.815	0.377	3.048	1.986	0.650	1.921%	0.0949

shows the chemical properties of the soils). In addition, through a grain size analysis test, we obtained the grain-size distribution of the local soils (shown in Figure 3). Table 2 indicates that the highest anion content in the saline soil of the highway project area in Uzbekistan is associated with sulfate ions, followed by chloride ions. Additionally, the highest cation content is associated with calcium ions, followed by sodium ions. The calculation results showed S₁ = 0.0949, and the average salt content is 1.921%. According to the relevant specifications, the saline soil in the highway project area in Uzbekistan can be classified to as medium sulfuric acid saline soil. Therefore, this study concentrated on the mechanical properties of sulfuric acid saline soil.

2.3. Preparation of the Artificial Saline Soil Samples

2.3.1. Materials of the Artificial Saline Soil

To conduct follow-up research and ensure the comparability and representativeness of the soil used in the tests, loess and standard sand were mixed in a certain proportion to prepare saline soil samples based on the basis of the grain-size distribution, liquid and plastic limits, and optimal moisture content of the saline soil at the engineering site [31]. Considering the large mass of soil used in this test, a site closer to the laboratory was chosen to facilitate the taking of loess. Loess was collected from the foundation pit of a construction site in Xi’an, with a sampling approximately 4 m, classified as Q₃ loess. The soil sample was brown–yellow, with both pinhole-sized and large pores developed, iron and manganese spots and calcareous striations, and occasional snail shells and calcareous nodules. Moreover, standard sand was used. To determine the appropriate loess-to-standard sand ratio, three sets of sand-to-soil ratios were considered, namely, 1:1, 2:3, and 3:7. The sampled loess and standard sand were divided into various particle size groups according to the particle grading curve by a high-frequency vibrating screen machine and mixed evenly according to the proposed sand ratio, as shown in Figure 4.

Liquid and plastic limit tests of the three groups of mixed loess and standard sand samples with different proportions were conducted. It was revealed that the plastic limit of the 30% standard sand and 70% loess mixed samples is 18.1%. This value is notably similar to the plastic limit of 17.61% observed in the saline soil at the engineering site, and the optimal moisture content (w_op) of the soil was close to the plastic limit. Therefore, the optimal moisture content (w_op) and maximum dry density (ρ_dmax) of the soil samples were determined via compaction tests at a sand-to-soil ratio of 3:7. Finally, the prepared saline soil with a sand-to-soil ratio of 3:7 exhibited the closest soil properties to those at the engineering site. The physical properties of the artificial saline soil are presented in

Table 3. Physical properties of the artificial saline soil.

Liquid Limit, wL/%	Plastic Limit, wP/%	Plastic Index, IP	Maximum Dry Density, ρdmax/(g/cm3)	Optimal Moisture Content, wop/%
31.05	18.1	12.95	1.83	17.73

.

2.3.2. Sample Preparation Process

According to the determined particle size combination and sand-to-soil ratio, a suitable loess and standard sand mixture was prepared for the preparation of artificial sulfuric acid saline soil. The required water and salt contents were weighed and mixed to make a sodium sulfate solution according to the water and salt contents designed for the test protocol. The sodium sulfate solution was sprayed into the mixture of loess and standard sand and mixed thoroughly to make sulfate salty soil. The required mass of sulfate salty soil was weighed and filled into a triaxial mold in three batches to prepare a cylindrical standard triaxial specimen (39.1 mm in diameter, 80 mm in height). A flow chart of the sample preparation is shown in Figure 5. In accordance with the relevant design specification requirements, the subgrade compaction degree of the secondary highway should be ≥95%, so the design compaction degree is 95%, and the calculated dry density (ρ_d) of the triaxial sample is 1.74 g/cm³.

2.4. F-T Tests

2.4.1. Boundary Condition

Based on the relevant temperature data for the project area presented in Figure 2, the freezing and melting temperatures for the F-T cycle test were established at −30 °C and 40 °C, respectively. An F-T cycle was structured to include 12 h of freezing, followed by 12 h of melting. A total of 6 sets of F-T cycles were designed, incorporating 0, 1, 2, 5, 10, and 15 cycles.

2.4.2. F-T Cycle Device

The instrument used is the XT5402-TC400-R60 high- and low-temperature test chamber produced by Hangzhou Xuezhongcheng Temperature Technology Co. (Hangzhou, China). The instrument has a newly designed dynamic thermostatic control system, with precise temperature control over the entire range and optimal thermostatic fluctuation of ±0.5 °C. The unit can reduce the temperature from 40 °C to −30 °C in 41 min and increase the temperature from −30 °C to 40 °C in 18 min.

2.4.3. F-T Cycle Process

Prepared standard triaxial samples were meticulously encased in plastic wrap, with each standard triaxial sample encased in three layers of plastic wrap. After the wrapping of the plastic wrap was completed, a layer of transparent tape was wrapped around the side of the triaxial samples, followed by a layer of tape on the upper and lower surfaces of each triaxial sample. The triaxial samples were wrapped in the manner above to mitigate water loss throughout the freeze–thaw (F-T) cycles. The samples were exposed to alternating conditions in high- and low-temperature test chambers to facilitate the F-T cycle testing.

2.5. UU Triaxial Shear Test

To avoid damage to the structural state of the sample in the consolidation and drainage process after the F-T cycle test, the UU triaxial shear test method was adopted. Upon completion of the specified number of F-T cycles, the samples were extracted for the subsequent triaxial shear testing.

The confining pressures in the tests were 100, 200, and 300 kPa. The test scheme is detailed in

Table 4. Test scheme.

Influencing Factors	Moisture Content/%	Salt Content/%	Cyclic Temperature/°C	Number of F-T cycles	Confining Pressure/kPa
Moisture content	11.7	1.92	−30~40	0, 1, 2, 5, 10, 15	100, 200, 300
	14.7
	17.7
	20.7
Salt content	17.7	0.00	−30~40	0, 1, 2, 5, 10, 15	100, 200, 300
		1.00
		1.92
		3.00
Cycle temperature	17.7	1.92	−30~40	0, 1, 2, 5, 10, 15	100, 200, 300
			−20~40
			−10~40
			−20~25

. The shear rate was established at 0.4 mm/min. In instances where the stress–strain curve demonstrated strain-hardening characteristics, the experiment was concluded upon the attainment of an 18% strain. Conversely, if the stress–strain curve indicated strain-softening behavior, the test was terminated upon reaching a shear strain of 3% to 5% subsequent to the peak value.

3. Stress–Strain Characteristics

3.1. Effect of Salt Content

In order to investigate the influence of salt concentration on the soil’s stress–strain curve, the water content was consistently maintained at 17.7%. The temperature throughout the freeze–thaw cycle varied between −30 °C and 40 °C. The stress–strain relationships of the soil sample, after undergoing a single F-T cycle under three different confining pressures and varying levels of salt content, are depicted in Figure 6.

Under varying salt concentrations and three different confining pressures, the stress–strain curves obtained from the saline soil samples are largely consistent. As the axial strain increases, the deviatoric stress of the samples correspondingly rises, ultimately resulting in plastic failure. The soil’s stress–strain curve demonstrates a strain hardening behavior, aligning with the Duncan–Chang hyperbolic model. This curve can be segmented into three distinct phases. The initial phase is marked by linear elastic behavior, during which the deviatoric stress exhibits a linear increase in response to axial strain. At the beginning, the axial strain is low, and the soil sample is gradually compacted without damaging the soil structure [32]. The second phase represents a strain-hardening stage of elastic–plastic deformation, where the soil particles are largely compromised, and the growth rate of the deviatoric stress significantly decreases as axial strain increases [33]. The third phase is the failure stage, in which the deviatoric stress progressively approaches the limit of deviatoric stress with increasing axial strain, at which point the soil’s internal structure is compromised. An increase in salt concentration, while maintaining a constant number of F-T cycles, results in a notable decrease in the peak deviatoric stress of saline soil. Furthermore, the initial yield point of the stress–strain curve for the sample exhibits a continuous decline, suggesting that elevated salt levels correspond to diminished soil strength.

3.2. Effect of Moisture Content

In order to investigate the influence of moisture content on the stress–strain curve, the salt concentration in the samples was consistently maintained at 1.92%. The temperature during the F-T cycles varied between −30 °C and 40 °C. The stress–strain relationships of the soil samples, subjected to three different confining pressures and varying moisture levels after one F-T cycle, are depicted in Figure 7. The findings indicate that an increase in moisture content did not modify the nature of the stress and strain curves, which consistently demonstrates strain hardening behavior. As axial strain increases, the deviatoric stress of the samples also increases, ultimately culminating in plastic failure. Figure 7 further illustrates that elevated moisture content led to a significant reduction in the peak deviatoric stress across a comparable number of F-T cycles. Distinct variations in the soil’s stress–strain curves are observed at moisture contents of 11.7%, 14.7%, and 17.7%, which are accompanied by a corresponding decrease in the peak deviatoric stress. Notably, when the moisture content surpasses the optimal threshold of 17.7%, a further decline in the peak deviatoric stress is recorded. The continuous advancement of the initial yield point with increasing water content suggests that higher moisture levels substantially diminish the soil’s resistance to deformation.

3.3. Effect of F-T Cycle Temperature

To examine how the temperature during the F-T cycles affects the stress–strain curve, Figure 8 illustrates the stress–strain relationships of soil samples after 10 F-T cycles, considering three different confining pressures and various F-T temperatures, with a salt content of 1.92% and moisture content of 17.7%. At a melting temperature of 40 °C, the sample demonstrated the lowest peak deviatoric stress at a freezing temperature of −10 °C, whereas the highest peak deviatoric stress was observed at −30 °C for an equivalent number of freeze–thaw cycles. This suggests that, with a constant melting temperature, a lower freezing temperature results in higher peak deviatoric stress. At a freezing temperature of −20 °C, the stress–strain curve at a melting temperature of 25 °C was significantly lower than that at a melting temperature of 40 °C. This indicates that as the melting temperature decreases, the peak value of the deviatoric stress exhibits a reduction. As a result, the stress–strain relationships exhibit a strong correlation with the temperature variations during the F-T cycle. The volume increase diminishes after water transitions into ice, when the freezing temperature is lowered, causing the frost heave phenomenon to reach its maximum state earlier than the shrinkage phenomenon. Notably, once the frost heave phenomenon reaches its peak, the soil volume continues to decrease, leading to an increase in effective stress and average dry density of the soil sample, as well as an increase in its peak value of deviatoric stress.

3.4. Effect of Number of F-T Cycles

To examine how the number of F-T cycles affects the stress–strain curve, the salt concentration was maintained at 1.92%, and the moisture content was kept at 14.7%. The temperature during the F-T cycles varied between −30 and 40 °C. Figure 9 illustrates the stress–strain relationships of the samples that underwent different numbers of F-T cycles.

Figure 9 demonstrates that the stress–strain relationships of samples with identical salt content exhibited considerable variation in response to differing numbers of F-T cycles. The stress–strain behavior of saline soil samples subjected to various F-T cycles revealed significant discrepancies. Notably, the peak value of the stress–strain curve that had not experienced any F-T cycles was markedly higher than that of the sample exposed to the F-T process. As the number of F-T cycles increased, the axial strain at the initial yield point of the saline soil sample consistently decreased. Concurrently, the peak value of the deviatoric stress in the saline soil sample also showed a reduction, with an increasingly pronounced attenuation trend. These findings collectively suggest that the F-T process can substantially impact the strength of saline soil. Importantly, the peak value of the deviatoric stress exhibited a significant decline following the application of two F-T cycles. By the time the samples reached 10 and 15 cycles, the stress–strain curves indicated considerable stabilization, suggesting that the effects of the F-T cycle process had become negligible and that the soil structure had undergone consistent adjustment and gradual stabilization.

4. Strength Characteristics

4.1. Failure Strength

In order to examine the degradation properties of sulfuric acid saline soil strength in relation to varying salt content, the deviatoric stress at axial strain was utilized as a measure of failure strength. The trend of the failure strength in response to salt contents under various confining pressures was analyzed, as illustrated in Figure 10. The figure demonstrates that salt content has a significant impact on salt expansion, erosion, and other issues associated with saline soil. At a salt content of 0%, the mean failure strength is recorded at 171.9 kPa. As the salt concentration rises to 1%, 1.92%, and 3%, the average reductions in failure strength observed are 15.8%, 6.3%, and 5.7%, respectively. The most significant reduction in failure strength occurs at 1% salt content. As the concentration of salt increases, the failure strength of the sulfuric acid saline soil consistently decreases, exhibiting clear signs of deceleration and deterioration. The solubility of sodium sulfate is markedly reduced as a consequence of the F-T cycles. As the salt content rises, the amount of precipitated sodium sulfate crystals increases, leading to a corresponding rise in the degree of damage to the soil structure. Consequently, this results in a continuous decline in failure strength.

Figure 11 illustrates the variation in the failure strength of sulfuric acid saline soil samples as a function of their moisture content. The data indicate that at a water content of 11.7%, the average failure strength is 573.9 kPa. As the water content gradually increases to 14.7%, 17.7%, and 20.7%, the average failure strength decreases by 43%, 62.3%, and 33.1%, respectively. Specifically, when the moisture content increases by 3%, the average reduction in failure strength accounts for 46.9%, 42.5%, and 10.6% of the total reduction, respectively. Between moisture contents of 11.7% and 17.7%, the failure strength of the saline soil sample decreases linearly. However, once the moisture content exceeds the optimal level of 17.7%, the rate of reduction in failure strength significantly diminishes. This analysis concludes that moisture content has a substantial detrimental effect on the failure strength of saline soil when subjected to different levels of confining pressure.

The reason is that as water content increases, the volume of ice crystals formed during freezing also increases, resulting in progressively greater damage to the soil structure. Furthermore, with higher water content, some sodium sulfate crystals dissolve, which increases the spacing between soil particles. This dissolution reduces the bonding strength and cementation between the particles. Additionally, the increased water content leads to a thicker hydration film surrounding the soil particles, enhancing the lubricating effect of the water.

Figure 12 depicts the variation in failure strength of sulfuric acid saline soil samples in relation to the temperature during F-T cycles. An examination of Figure 12 reveals that at a melting temperature of 40 °C, the failure strength exhibits a gradual increase as the freezing temperature decreases from −10 °C to −30 °C, with average growth rates of 4.2% and 1.4%, respectively. These growth rates are relatively modest. In contrast, when the freezing temperature is held constant at −20 °C, an increase in melting temperature from 25 °C to 40 °C leads to a marked enhancement in the failure strength of the saline soil samples across the three confining pressures, with an average growth rate of 12.6%. This observation indicates that the melting temperature exerts a substantial impact on the failure strength.

The analysis indicated that as the freezing temperature continues to decrease, the sulfuric acid saline soil sample expands significantly (the volume increases). In the case of sodium sulfate, a decrease in temperature results in enhanced crystallization, and the precipitated crystals contribute to an increase in the bonding strength of the soil particles, leading to a gradual increase in soil strength. Once the salt crystals cease to precipitate, the particles maintain stability.

Figure 13 illustrates that the frequency of F-T cycles significantly influences the saline soil’s strength. As the number of cycles increases, the failure strength initially decreases rapidly before gradually stabilizing. After 1, 2, 5, 10, and 15 F-T cycles, the average reduction in failure strength for samples with varying moisture contents was 12%, 7.1%, 2.8%, 5.5%, and 3.3%, respectively. The average reduction in damage intensity accounted for 43.9%, 22.3%, 8.9%, 16.7%, and 8.2% of the total reduction, respectively. It can be concluded that the decrease in failure strength is most pronounced after the first and second F-T cycles. The reduction rate of failure strength is highest after one F-T cycle, followed by the rate after two cycles. Upon reaching a total of 15 cycles, the lowest reduction rate in failure strength suggests that the soil structure is continuously adjusting and gradually stabilizing.

4.2. Cohesion and Internal Friction Angle

The relationships between cohesion and the internal friction angle in relation to salt content are illustrated in Figure 14 and Figure 15, respectively. It is demonstrated in Figure 14 that an increase in salt content leads to a continuous decrease in cohesion; however, this decrease is relatively modest overall. As the salt content rises from 0% to 3%, the average reduction in cohesion across different numbers of F-T cycles is observed to be 18.2%. Figure 15 shows that, with the exception of the sample subjected to 0 F-T cycles, the increase in salt content results in a sustained reduction in the internal friction angle, with the rate of decline consistently diminishing. The mean reduction in the internal friction angle observed across the various F-T cycles was 40.8%. This indicates that the effect of salt content on the internal friction angle of saline soil is more significant than its impact on cohesion.

Cohesion is influenced by molecular forces, the cementation forces between structural particles, and the occlusion effect. The molecular forces are predominantly influenced by the mineral composition and density of the soil. Repeated F-T cycles can lead to the continuous processes of crystallization, dissolution, and recrystallization of sodium sulfate. Furthermore, the quantity of sodium sulfate crystals that can precipitate during freezing also rises, with an increase in salt content. This increase contributes to a higher pore ratio in the soil, and a greater number of pores inevitably results in decreased soil compactness, consequently resulting in a decrease in cohesion. In conclusion, the process of salt swelling adversely affects the bonding between soil particles, resulting in the deterioration of the structural integrity of saline soil. Consequently, this leads to a decrease in both cohesion and the internal friction angle.

The relationship between cohesion and the internal friction angle with respect to moisture content is illustrated in Figure 16 and Figure 17, respectively. It is demonstrated in Figure 16 that moisture content exerts a considerable negative impact. As moisture content increases, cohesion gradually decreases, following a slow–quick–slow decline pattern. With an equivalent number of F-T cycles, when the moisture content is increased by 3% increments from 11.7% to 14.7%, 17.7%, and 20.7%, the average reductions in cohesion are 10.8%, 44.1%, and 32.6%, respectively. Notably, the decrease in cohesion when the moisture content increases from 14.7% to 17.7% is significantly greater than in the other cases. As shown in Figure 17, the internal friction angle exhibits a gradual decline as the moisture content increases, indicating an overall reduction in deceleration. When the moisture content increases from 11.7% to 17.7%, the average reductions in the internal friction angle are 37.9% and 44.1%, reflecting an approximately linear attenuation trend. However, when the moisture content increases from 17.7% to 20.7%, the average reduction in the internal friction angle is only 16.3%, which is significantly lower than the optimal moisture content. Consequently, the impact of moisture content on the internal friction angle is diminished.

Figure 18 and Figure 19 depict the correlations between the cohesion and the internal friction angle in relation to the temperature of the F-T cycle. As illustrated in Figure 18, maintaining a constant melting temperature of 40 °C while decreasing the freezing temperature from −10 °C to −30 °C leads to a progressive decline in the cohesion of saline soil. This decline is characterized by average reductions of 4.2% and 5.7%, respectively, culminating in an overall average reduction of 9.7%. Conversely, when the freezing temperature is held constant at −20 °C, the cohesion of the saline soil significantly decreases as the melting temperature is lowered from 40 °C to 25 °C, indicating that the melting temperature is a critical factor influencing changes in cohesion. As shown in Figure 19, when the melting temperature remains constant at 40 °C, reducing the freezing temperature from −10 °C to −30 °C leads to a continuous increase in the internal friction angle of the saline soil, with average growth rates of 10.8% and 9.6%, indicating a linear trend. When the temperature is consistently held at −20 °C, the internal friction angle typically exhibits an increase as the melting temperature is reduced from 40 °C to 25 °C.

A decrease in freezing temperature correlates with an increased severity of deterioration observed during the F-T cycle process. At this stage, ice crystals form at a faster rate between the soil particles. The force exerted by frost heave diminishes the bonding strength between soil particles, which, consequently, exacerbates the degradation of the original structure and reduces the cohesion of soil. However, the internal friction angle is significantly influenced by the freezing temperature rather than cohesion, which is attributed to the low degree of cementation in artificial saline soil. As the freezing temperature decreases, the saline soil sustains greater damage, resulting in an increased number of contact points among the soil particles. As a result, this contributes to a rising trend in the internal friction angle.

The correlations between changes in the cohesion and internal friction angle, as well as the frequency of F-T cycles at varying moisture contents, are depicted in Figure 20 and Figure 21, respectively. It is demonstrated in Figure 20 that as the frequency of F-T cycles increases, cohesion initially decreases rapidly before gradually stabilizing. This decline is primarily concentrated after the first and second F-T cycles. The average decreases in cohesion for different moisture contents are 21.1% and 14.2%, respectively, with these reductions accounting for 36.1% and 19% of the total decrease in cohesion. Furthermore, cohesion eventually stabilizes. The phase transformation of ice to water, along with the salting out of soluble crystals during the F-T cycle, disrupts the structural arrangement among soil particles, resulting in a decrease in cohesion. When the number of freeze–thaw cycles exceeds a specific threshold, the configuration of soil particles starts to achieve stability, leading to a consistent residual bonding strength among the particles. At this juncture, the cohesion within the soil also attains a stable state.

Figure 21 demonstrates that as the number of F-T cycles increases, the internal friction angle experiences a rapid initial increase, followed by a period of stabilization. Similar experimental results were obtained by Qi et al. [34] and Han et al. [29] based on triaxial tests, where the angle of internal friction increased with the number of F-T cycles. The most substantial growth is observed following the initial and subsequent F-T cycles, with average growth rates recorded at 8.1% and 3.9%, respectively. These increases account for 27.3% and 17.3% of the total growth in the internal friction angle. Subsequently, the internal friction angle stabilizes with an average increase of 2.8%. The analysis indicates that the F-T cycle process disrupts the original cementation state of saline soil, causing larger agglomerates to break apart. This results in the rearrangement of soil particles, leading to a more uniform structure and an increased number of contact points among the particles, which ultimately enhances the internal friction angle.

5. Discussion

5.1. Deteriorative Effects of Various Factors on Failure Strength

The results of the triaxial shear tests demonstrate that the failure strength of saline soils is considerably compromised by the combined effects of F-T cycles, salinity, and moisture content. The relative variation in failure strength is utilized to quantify the influence of F-T cycles, as well as salinity levels, on the degree of deterioration experienced by saline soils, defined as follows:

$$T_{N,i}=1-{\displaystyle \frac{{\left({\sigma }_1-{\sigma }_3\right)}_{N,i}}{{\left({\sigma }_1-{\sigma }_3\right)}_{0,0}}}$$

(1)

where $T_{N,i}$ represents the deterioration rate associated with a salt content of i and a specified number of F-T cycles, denoted as N; ${\left({\sigma }_1-{\sigma }_3\right)}_{N,i}$ indicates the failure strength at a salt content of i with F-T cycles of N; and ${\left({\sigma }_1-{\sigma }_3\right)}_{0,0}$ indicates the failure strength at a salt content of 0 with no F-T cycles.

The relative variation in failure strength is utilized to characterize and define the impact of F-T cycles, as well as moisture content, on the degree of deterioration of saline soils, defined as follows:

$$T_{N,j}=1-{\displaystyle \frac{{\left({\sigma }_1-{\sigma }_3\right)}_{N,j}}{{\left({\sigma }_1-{\sigma }_3\right)}_{0,11.7}}}$$

(2)

where $T_{N,j}$ represents the deterioration rate associated with a salt content of i and a specified number of F-T cycles, denoted as N; ${\left({\sigma }_1-{\sigma }_3\right)}_{N,j}$ indicates the failure strength at a water content of j with F-T cycles of N; and ${\left({\sigma }_1-{\sigma }_3\right)}_{0,11.7}$ indicates the failure strength at a moisture content of 11.7% with no F-T cycles.

5.1.1. Deteriorative Effect of F-T and Salinity on Failure Strength

Figure 22 illustrates the correlations between the deterioration rate and the number of F-T cycles, as well as the salt content, while maintaining a confining pressure of 100 kPa, where the X-axis is the number of F-T cycles and salt content, respectively, and the Y-axis is the failure strength deterioration. As depicted in Figure 22a, the X-axis is the number of F-T cycles, the Y-axis is the deterioration rate, and the deterioration rate of failure strength increases corresponding to an increase in the number of F-T cycles. Specifically, after two cycles, the deterioration rate exceeds 29%, and after five cycles, it surpasses 33%. The substantial decline in failure strength is primarily noted during the first two cycles of F-T. This observation suggests that these first two cycles exert a considerable influence on the damage to the soil’s internal structure.

As illustrated in Figure 22b, the X-axis is the salt content, the Y-axis is the deterioration rate, and the rate of deterioration in failure strength of saline soils exhibits an approximately linear increase with rising salt content; however, this increase is relatively modest. In the absence of F-T cycles, the deterioration rate of failure strength in saline soils remains below 21% as the salt content increases. Conversely, when these soils are exposed to F-T cycles, the deterioration rate exceeds 23% with increasing salt content.

5.1.2. Deteriorative Effect of F-T and Moisture on Failure Strength

Figure 23 illustrates the correlation between the deterioration rate of failure strength and the number of F-T cycles, as well as water content, while subjected to pressure of 100 kPa, where the X-axis is the number of F-T cycles and moisture content, respectively, and the Y-axis is the failure strength deterioration. In Figure 23a, it is observed that when the moisture content is below 14.7%, the deterioration rate of failure strength increases with the frequency of freezing and thawing cycles, exhibiting a pronounced increase. Conversely, when the moisture content surpasses 17.7%, the deterioration rate escalates rapidly with additional F-T cycles before stabilizing. It is indicated in Figure 23b that the deterioration rate of failure strength increases approximately linearly with rising moisture content. Notably, the increase is more significant when the moisture content ranges from 11.7% to 17.7%, while the increase is less pronounced when the moisture content ranges from 17.7% to 20.7%.

5.2. Damage Effects of Various Factors on Cohesion

The test results indicate that F-T cycles, salinity, and moisture all contribute to a reduction in cohesive strength. To quantitatively assess the effects of different variables on the cohesion of saline soil, the damage ratio associated with varying salt concentrations under the influence of F-T cycles was defined as follows:

$$D_{N,i}=1-{\displaystyle \frac{C_{N,i}}{C_{0,0}}}$$

(3)

where $D_{N,i}$ represents the damage rate associated with a salt content of i and a specified number of F-T cycles, denoted as N; $C_{N,i}$ indicates the cohesion at a salt content of i with F-T cycles of N; and $C_{0,0}$ indicates the cohesion at a salt content of 0 with no F-T cycles.

The impact of differing moisture levels on the cohesion of saline soil exposed to F-T cycles was defined by the damage ratio as follows:

$$D_{N,j}=1-{\displaystyle \frac{C_{N,j}}{C_{0,11.7}}}$$

(4)

where $D_{N,j}$ represents the damage rate associated with a moisture content of j and a specified number of F-T cycles, denoted as N; $C_{N,j}$ indicates the cohesion at a moisture content of j with F-T cycles of N; and $C_{0,11.7}$ indicates the cohesion at a moisture content of 11.7% with no F-T cycles.

5.2.1. Damage Effect of F-T and Salinity on Cohesion

Figure 24 illustrates the regular patterns of the damage ratio as it varies in relation to F-T cycles and salt content, where the X-axis is the number of F-T cycles and salt content, respectively, and the Y-axis is the cohesion damage ratio. In comparison to Figure 22, the cohesion damage ratio and the rate of deterioration in failure strength exhibit a remarkably similar trend.

Comparing Figure 22a and Figure 24a, the cohesion damage ratio of saline soils exceeds 34% after being exposed to F-T cycles. Upon reaching 15 F-T cycles, the deterioration rate of failure strength is less than 59%, while the cohesion damage ratio exceeds 58%. This observation indicates that the impact of F-T cycles on the cohesion damage ratio is greater than its effect on the deterioration rate of failure strength. In comparing Figure 22b and Figure 24b, it is evident that when the number of cycles is less than or equal to one, the influence of salt content on the cohesion damage ratio is more significant than its effect on the deterioration rate of failure strength. Nevertheless, when the number of cycles surpasses one, the observed trend undergoes a reversal.

5.2.2. Damage Effect of F-T and Moisture on Cohesion

Figure 25 illustrates the typical curves of damage ratio in relation to the number of F-T cycles, as well as moisture content, where the X-axis is the number of F-T cycles and moisture content, respectively, and the Y-axis is the cohesion damage ratio. When compared to Figure 23, it is evident that both F-T cycles and water content exhibit remarkably similar patterns of change concerning the damage ratio of cohesion and the deterioration rate of failure strength.

A comparison of Figure 23a and 25a indicates that the impact of F-T cycles on the cohesion damage ratio is indistinguishable from its effect on the deterioration rate of failure strength when the moisture content ranges from 11.7% to 14.7%. However, when the moisture content is between 17.7% and 20.7%, the impact of F-T cycles on the damage ratio of cohesion is significantly less than that on the deterioration rate of failure strength. Furthermore, a comparison of Figure 23b and Figure 25b reveals that when the moisture content is greater than or equal to 14.7%, the effect of moisture content on the damage ratio of cohesion is less pronounced than its effect on the deterioration rate of failure strength as the water content increases.

6. Conclusions

In this experiment, the effects of different water content, salt content, number of F-T cycles, and temperature on the mechanical properties of saline soils were investigated through freeze–thaw cycle experiments and triaxial shear tests, and the following conclusions can be drawn:

(1)

The stress–strain curves of saline soil samples exposed to a variety of F-T cycles demonstrate a strain-hardening behavior under differing conditions of salt and moisture content, as well as varying temperatures.

(2)

It has been established that an increase in salt content leads to a continuous decrease in the breaking strength, cohesion, and angle of internal friction of saline soils.

(3)

The optimum moisture content is a threshold for saline soils, where the damage strength of the soil continues to decrease as the water content increases, and the rate of decrease in damage strength decreases significantly once the water content exceeds the optimum threshold. Notably, when the moisture content remains below the optimal level of 17.7%, the internal friction angle decreases linearly. However, once the moisture content exceeds this optimal threshold, the rate of reduction in the internal friction angle significantly decreases.

(4)

When the melting temperature is established at 40 °C, a decrease in the freezing temperature from −10 °C to −30 °C is associated with a progressive increase in failure strength, a gradual decline in cohesion, and a rise in the internal friction angle, which ultimately demonstrates a linear trend. In contrast, when the freezing temperature is set at −20 °C, an elevation in the melting temperature from 25 °C to 40 °C results in a substantial increase in the ultimate deviatoric stress across the three confining pressures, a significant enhancement in cohesion, and a reduction in the internal friction angle.

(5)

The change rule curves for the deterioration rate of failure strength and the cohesion damage ratio exhibit a high degree of similarity. Furthermore, the impact of multiple factors on saline soil is greater than that of a single factor.

(6)

When F-T cycles and salt interact with saline soils, the initial two F-T cycles inflict the most considerable damage. Additionally, the presence of salt exacerbates the detrimental effects on saline soils during these F-T processes.