Experimental study on shear strength and deterioration behavior of silty clay under dry-wet-freeze-thaw cycles

Abstract

To investigate the influence of dry-wet, freeze-thaw, and their coupled cyclic effects on shear strength and deterioration behavior of silty clay, direct shear tests were performed on silty clay with different initial moisture content under the above cyclic conditions. A shear strength deterioration model of silty clay was established based on the indoor test data by using the Logistic function to predict shear strength parameters under dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles. Results are below. Under dry-wet, freeze-thaw, and their coupled cycles, shear stress-displacement curves can be classified into strain-softening and strain-hardening types. Increasing the moisture content, decreasing the vertical stress, and increasing the cycle times of the three cycles mentioned above all promote the transition of the curve from strain-hardening to strain-softening. As dry-wet, freeze-thaw, and their coupled cycles increase, the cohesion and internal friction angle deteriorate rapidly in the early stage and slowly in the later stage. The maximum degree of deterioration of shear strength parameters decreases as the moisture content increases during dry-wet cycles and dry-wet-freeze-thaw cycles, while it increases during freeze-thaw cycles, with the highest during dry-wet-freeze-thaw cycles. The research findings can predict and evaluate the shear performance of silty clay in seasonal frozen soil regions.

Introduction

Permanent and seasonal frozen soil collectively make up approximately 25% of the total land area. The cyclic freezing and thawing phenomenon, which gradually occurs in parts of the permafrost region due to human activities and global warming, is causing the region of seasonal frozen soil to expand gradually. Seasonal frozen soil undergoes freeze-thaw cycles repeatedly due to seasonal and diurnal changes, and the soil mass undergoes dry-wet effects that cause water migration due to sunshine and rainfall¹. In geotechnical engineering practice, silty clay is an extremely common foundation soil². Due to its high cohesion and low permeability, the shear resistance of this type of soil will change significantly when subjected to freezing and thawing and dry-wet cycling, which will lead to a series of engineering problems^3,4. With the energy and transport construction of the China-Mongolia-Russia Economic Corridor and the development of the Sino-Nepalese Transport Corridor, a large number of linear engineering projects and construction are raised in the cold region. The strength of seasonal frozen soil under dry-wet-freeze-thaw cycles directly affects the construction and operation of projects in cold regions.

Researchers from all over the world have conducted a large number of tests to examine the changes in the mechanical properties of rock-soil mass during dry-wet cycles and freeze-thaw cycles at both the macroscopic and microscopic levels. Based on the soil-water characteristic test and shear test, Ma. et al.⁵ studied the remodeled unsaturated loess under the action of different dry-wet cycles, and the results showed that an increase in the number of dry-wet cycles transformed the shear stress-displacement curve from a hardening type to a softening type. Wang et al.⁶ systematically investigated the effects of dry-wet cycling on the mechanical properties of loess by carrying out anisotropic compression tests and triaxial shear tests. The study shows that the loess cohesion tends to decay as the number of dry-wet cycles increases, with the most significant decrease in cohesion after the first cycle; in contrast, the internal friction angle parameter does not produce an observable change. Liu et al.⁷ discovered that the strength of expansive soils decreases in stages when they go through dry-wet cycles: during the first three cycles, the strength drops significantly, with the cohesive force falling by 51.69%, which is the main reason for the loss of shear strength in the soil. Researchers led by Liu⁸ looked at what happened to the mechanical properties of Shu soil when the number of dry-wet cycles was changed. They found that as the cycle times increased, the surface of the soil sample developed permanent cracks, and the shear strength first went up and then slowly went down until it reached a stable level. Mahya et al.⁹ found that the dynamic shear modulus of silty clay increased with the increase of the peripheral pressure and decreased with the increase of the number of freeze-thaw cycles. Xiao et al.¹⁰ used the damage mechanics theory to establish the damage equations of rocks under freezing and thawing cycles and peripheral pressure, which was used for the study of the shear stress-displacement relationship of rocks in the triaxial compression process. Wang et al.¹¹ designed a coupling test between five types of freeze-thaw cycles and eight types of water content to reveal the nonlinear coupling decay mechanism of cohesion with water content and the number of freeze-thaw cycles, and constructed a regression model to quantify the contribution of the synergistic effect of the two to the deterioration of soil strength. Feng et al.¹² analyzed the evolution law of deformation characteristics and obtained the deformation prediction model through the dynamic triaxial test of the roadbed clay under different bias stress, confining pressure, moisture content, loading times, and freeze-thaw cycle times. In summary, scholars have made important progress in the evolution of physical properties and the law of fissure expansion of soil under the action of the dry-wet cycle or freeze-thaw cycle, which has laid a theoretical foundation for revealing the mechanism of strength deterioration of soil. Nonetheless, three primary limitations exist within the current studies: (1) Most existing studies focus solely on either the dry-wet cycle or the freeze-thaw cycle; a comparative analysis of the impacts of dry-wet, freeze-thaw, and their combined cycles on the physical and mechanical properties of soils requires further investigation; (2) The complex working conditions addressed in current models seldom account for the comprehensive effects of varying initial water content and the frequency of dry-wet-freeze-thaw cycles; (3) Research predominantly targets specific soil types, such as loess and expansive soils, while the deterioration analysis of silty clay is inadequate.

To address the above problems, this paper takes remodeled silty clay as the research object, reveals the deterioration law of shear strength based on indoor tests and theoretical analysis, establishes a unified analytical framework that can compare the effects of drying and wetting, freezing and thawing, and their coupled cycles, and proposes a new Logistic deterioration model. We analyzed the maximum degree of deterioration under three types of cycling, based on the model parameters. Exploring the evolution law of shear resistance of silty clay under different cycling modes and initial moisture content is of great practical significance, as it enables the prediction and evaluation of the impact of seasonal climate change on actual projects, thereby enhancing the safety and reliability of engineering projects.

Test preparation and test program

Test material

The test soil samples were taken from the silty clay between a two-station interval of the Changchun Railway Line 1 South Extension Project in China; the sampling location is shown in Fig. 1, and the color of the soil sample is brownish-yellow. We prepared the remodeled soil samples, measured the basic physical indices according to ASTM standards¹³–¹⁶, and put the results in Table 1. We used the standard soil test to determine the granulometric composition of the soil samples, which is shown in Fig. 2. It can be seen that the natural moisture content of the soil sample is higher than the optimum moisture content, which is in a hard plastic state with good and continuous particle grading.

Table 1 Basic physical indices of soil sample.

Fig. 1

Sampling location.

Fig. 2

Granularity distribution diagram.

On the basis of the natural moisture content, the soil samples were fully dried in a drying oven for 24 h and then passed through a 0.25 mm sieve to obtain them. The amount of water required to be added to the soil samples was calculated according to Eq. (1), and the soil samples were moistened with a sprayer to reach the required moisture and then covered and closed with plastic wrap, and then left to stand for 24 h. After equilibrating to the initial moisture content, respectively, two 20 g soil samples were taken from the center of the specimen, and the maximum allowable difference between the two moisture contents was ± 1%.

$${m_w}={m_d} \times 0.01w^{\prime}$$

(1)

where m_w: Amount of water to be added to soil samples, m_d: Dry soil quality, w′: Required moisture content of soil samples.

The initial moisture content is based on the optimum moisture content, and the values are 10%, 14%, 18%, 22%, and 26%.

Test methods

The specimens were subjected to cyclic effects of dry-wet, freeze-thaw, and dry-wet-freeze-thaw environments using a vacuum pump, a stacked saturator, and a freeze-thaw cycle test chamber, and the test equipment is shown in Fig. 3. Given that the deformation of soil mass and pore structure stabilized after 3 to 8 dry-wet cycles¹⁷ and the mechanical properties stabilized after 5 to 10 freeze-thaw cycles¹⁸, we set the cycle times in each environment to 0, 1, 2, 4, 6, 8, and 10, respectively.

Fig. 3

Vacuum pump, stack saturator, and freeze-thaw cycle test chamber.

A comprehensive dry-wet cycle entails humidifying the specimen from its initial moisture content to the saturated moisture content, followed by drying it back to the beginning moisture content. The humidification method entails fixing the specimen in the stacking saturator for vacuum saturation, with a pumping vacuum duration of 2 h and a saturation soaking period of 12 h. The drying process involves placing the specimen in a cold, well-ventilated location to return it to its original moisture content. Figure 4 depicts the soil samples throughout the drying process, whereas Fig. 5 demonstrates the variation in moisture content during the dry-wet cycle test.

Fig. 4

Specimens during drying.

Fig. 5

Variation of moisture content of soil samples during dry-wet cycle test.

The temperatures for the freeze-thaw cycle test chamber were established at -20℃ and 20℃, with each phase lasting 24 h. The “freeze-thaw” procedure concluded a freeze-thaw cycle. We encased and secured the specimens in plastic wrap to avert testing inaccuracies due to moisture evaporation. We checked the seal after each cycle and weight to ensure the moisture content didn’t change by over 0.5%. Based on the dry-wet cycle and freeze-thaw cycle test processes, we subjected the soil sample to a single humidification-humidity reduction-freezing-thawing cycle to complete the dry-wet-freeze-thaw cycle test. Figure 6 displays the soil samples after undergoing dry-wet-freeze-thaw cycles.

Fig. 6

Soil samples after dry-wet-freeze-thaw cycles.

The specimens, both those that underwent dry-wet, freeze-thaw, and their coupled cycling and those that did not, were placed in a quadruple strain-controlled straight shear apparatus and subjected to an unconsolidated undrained shear test at vertical stresses of 100 kPa, 200 kPa, 300 kPa, and 400 kPa, respectively. The shear rate was maintained at 8 mm/min, and the shear displacement was 6 mm throughout the shear operation.

Test results and analysis

Shear stress-displacement curve

The shear stress-displacement curves of silty clay are categorized into two mechanisms, strain softening and strain hardening, according to the slope characteristics of the tangent line following the apex of the curve. Strain softening refers to the phenomenon where the shear stress of a material gradually decreases with increasing shear displacement after reaching its peak value in a direct shear test; strain hardening refers to the phenomenon where the shear stress continuously increases or remains stable after the peak value with increasing shear displacement¹⁹. The tests were carried out in groups according to the initial moisture content, and in order to avoid redundancy, a specimen with a moisture content of 22% (close to the natural moisture content) was used in Fig. 7 to reveal the evolution of the direct shear stress-displacement curves of the pulverized clay that underwent the dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles under different vertical stresses. Figure 7 illustrates the variation of shear stress-shear displacement curves corresponding to the number of dry-wet cycles, freeze-thaw cycles, and dry-wet-freeze-thaw cycles during the straight shear test at various vertical stresses for silty clay specimens exposed to these conditions.

The evolution of shear stress-displacement curves in silty clay is influenced by the cycle times and vertical stress following numerous cycles of dry-wet, freeze-thaw, and their combined effects. The silty clay underwent destruction during the dry-wet cycle due to the repeated ingress and egress of water, which leads to the dissolution or migration of the cementing material within the pore structure. In the freeze-thaw cycle, the expansion of freezing pore water exerted pressure on the soil skeleton, resulting in a looser soil body upon thawing. After several cycles, the soil body changed from a compact state to a loosened state, showing more stress-softening characteristics. These conditions reduced the shear strength at the shear surface, and post-peak stress couldn’t be maintained. As the cycle times increased, the curve transitioned from strain-hardening to strain-softening type, with the softening effect becoming increasingly pronounced. Softening deformation was more probable at low vertical stress, while increased vertical stress could postpone the onset of strain softening. Analysis indicates that at a low vertical stress condition, the spacing between clay particles is substantial, resulting in the soil exhibiting elevated plasticity and mobility, hence facilitating extrusion and deformation²⁰.

In the 100–200 kPa vertical stress interval, 22-26% of high water content soils exhibited significant softening susceptibility, with strain softening induced by only 1–2 cycles of freezing and thawing or wet and dry cycles. This phenomenon is attributed to the continuous reduction of the cohesion between soil particles by high pore water pressure, superimposed on the insufficient structural constraints of the low-stress environment, which together lead to the rapid destabilization of the cemented system. In contrast, there is a significant threshold delay in the hardening-softening transition in 10-18% low to medium moisture content soils, which requires a higher cycle times to be triggered. When the vertical stress was elevated to 300–400 kPa, the 10-14% low water content soils maintained stable hardening due to effective stress compression of the pore channels, whereas the 18-26% high water content soils changed their damage mode from brittle destabilization in the low stress region to gradual damage, even though they still softened. It is noteworthy that the coupled dry-wet-freeze-thaw cycles produced significant synergistic effects in the 18-22% water content interval, resulting in a 50-70% reduction in the number of softening critical cycles compared with that of the single action, confirming the nonlinear coupling mechanism of moisture migration and freezing and swelling effects.

Fig. 7

Sheer stress-displacement curves representing silty clay under vertical stresses of (a) 100 kPa, (b) 200 kPa, (c) 300 kPa, and (d) 400 kPa under dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles.

The shear stress-displacement curves of each initial moisture content specimen after different cycles are shown in Fig. 8. Given that the interaction effect between water content and the number of dry-wet-freeze-thaw cycles was most significant at 100 kPa, this stress level was selected in Fig. 8 to highlight the key variable relationship. To avoid duplication with the 0–10 cycles data in Fig. 7, the middle 6 cycle conditions were selected for comparative analysis of water content groups in Fig. 8. It can be seen that, with the increase of moisture content, the softening phenomenon of reduced strength is more likely to occur in the process of shear deformation of the soil body, which is attributed to the fact that moisture lubrication weakens the cohesion and makes the soil body more susceptible to plastic deformation²¹. In addition, at high moisture content, moisture penetrates into the crevices of the soil body, resulting in an increase in the pore-specific surface area of the clay and a weakening of the connections between the clay particles, which strengthens the tendency of strain softening.

Fig. 8

Plots of moisture content and shear strength of silty clay under 6 dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles under 100 kPa vertical stress.

Shear strength and its parameters

The specimen’s shear strength was ascertained from the shear stress-shear displacement curves under various test settings depicted in Fig. 7, with the peak strength of the curves representing the soil’s shear strength. Figure 9 illustrates the shear strength curves of silty clay subjected to dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles in relation to the cycle times for each vertical stress. The results show that the shear strength of soil samples under dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles decreases with the cycle times of dry-wet, freeze-thaw, and their coupled cycles and stabilizes after 6 cycles. In contrast, the shear strength under dry-wet-freeze-thaw cycling is the lowest. Dry-wet cycling destroys the cementitious material through cyclic changes in moisture, freeze-thaw cycling destroys the structure through physical expansion, and coupled cycling combines the effects of both, leading to even more severe deterioration. after six cycles, coupled cycling results in a loss of strength that is 1.3 to 1.8 times greater than that of single-cycle action. With the increase of vertical stress, the relationship between the cycle times and shear strength of dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles tended to flatten out, with the curves of the three cyclic activities converging progressively. When the vertical stress increases, particle fragmentation and rearrangement form a deterioration-resistant soil skeleton, weakening further damage to the pore structure by dry-wet-freeze-thaw cycling, resulting in a lower rate of decay of shear strength with the cycle times; and the effective stresses borne by the soil skeleton increase, inhibiting water migration and frost heave, so that the difference in deterioration between different cycling types is reduced.

Fig. 9

Plots of cycle times and shear strength of silty clay with moisture content of 22% under dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles under 100 kPa vertical stress.

Figure 10 illustrates the relationship between shear strength and initial moisture content for silty clay subjected to dry-wet, freeze-thaw, and combined dry-wet-freeze-thaw cycles at various vertical stress levels. Since Fig. 10 focuses on the water content group comparison, to avoid redundancy of information, combined with the law that the intensity decay tends to stabilize after 6 cycles shown in Fig. 9, the 6-cycle condition was chosen to be shown.

Macroscopic shear strength deterioration is essentially a reflection of accumulated microstructural damage. Under the dry-wet cycle, the expansion of clay minerals during the wetting phase enlarges the primary pores, and shrinkage cracks (changes in pore size) are formed during drying and shrinkage, destroying the inter-particle cementation²². Under the freeze-thaw cycle, the volume expansion during the freezing phase leads to the extrusion of soil particles to form ice lenses, making the pores visible during the thawing phase²³. Under dry-wet-freeze-thaw cycling, the dry-wet cycling phase triggers initial cracking and provides a rapid moisture migration pathway, and the freeze-thaw cycling phase expands the cracks, eventually leading to structural collapse. When the vertical stress is 100 kPa, the shear strength of the freeze-thaw cycle increases and then decreases with the increase of moisture content, and the peak strength appears at 14% moisture content, which is slightly below optimal moisture content. The optimum moisture content corresponds to the state of maximum dry density of the soil, when the inter-particle bonding and friction are in equilibrium and theoretically high shear strength is achieved. It is worth noting that the peak shear strength is slightly lower than the optimum moisture content determined from the compaction test without dry-wet-freeze-thaw cycling, indicating that dry-wet-freeze-thaw cycling may have changed the optimum state of the soil microstructure. ry-wet-freeze-thaw cycling resulted in structural damage to the soil, which shifted the actual peak strength toward the lower moisture content.At low stress (100 kPa), the water content-shear strength curve shows an “S” shape. The appearance of this inflection point marks the transition of soil microstructure from stability maintenance to accelerated deterioration. When the moisture content is lower than 14%, the water enhances the inter-particle bonding through capillary action²⁴, while the low cycle times does not significantly damage the soil structure; after exceeding 14%, the shear strength rises with the moisture content; the pore water pressure dominates and weakens the effective stress, and the freezing and expansion aggravate the bond damage, and the strength continues to decline. The peak moisture content (14%) is slightly lower than the optimal moisture content because the expansion of the water phase change during the freeze-thaw cycle needs to reserve microscopic pore space, and the dense structure close to the optimal moisture content will amplify the ice crystal extrusion damage. Under the high-stress condition (≥ 200 kPa), the vertical stress compresses the pore space to inhibit capillary action²⁵, and the increase in moisture content directly exacerbates the pore water pressure and frost damage, resulting in a monotonically decreasing strength²⁶.

When the vertical stress was increased from 200 to 400 kPa, the shear strengths under the dry-wet cycle, freeze-thaw cycle, and dry-wet-freeze-thaw cycle decreased with the increase of moisture content, among which the strengths under the freeze-thaw cycle decreased more, which indicated that the freeze-thaw cycle responded more significantly to the increase of moisture content. After analysis, it can be seen that, due to the silty clay containing a higher proportion of fine-grained particles, the contact area between the particles is larger. When the silty clay undergoes the freeze-thaw cycle, the water in the soil body occurs freezing and thawing alternately. When the water freezes, the contact area between the clay particles decreases, and the particles are rearranged to organize themselves, and the repeated process of this particle rearranging destroys the internal structure of the clay, so that the soil body undergoes freezing, expansion, thawing, and sinking, which in turn leads to the deterioration of shear strength. The strength reduction from just the freeze-thaw cycle was greater than that from the combined dry-wet-freeze-thaw cycle because the dry-wet phase created more microfractures. These cracks allowed water to move more easily, which helped drain water and lower the amount of unfrozen water during the freeze-thaw phase, weakening the strength against freeze damage. These cracks became preferential channels for water migration, promoting water migration and drainage and reducing the unfrozen water content in the freeze-thaw phase, thus weakening the freeze damage strength.

The shear strength parameters were obtained according to the Moore-Cullen theory. Figure 11 shows the variation of shear strength parameters with the cycle times for silty clay specimens subjected to dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles, and Fig. 12 shows the variation with moisture content.

The cohesion and internal friction angle of silty clay gradually deteriorate with the cycle times, exhibiting a fast deterioration rate in the early stage, slowing down in the later stage, and tending to stabilize after 4 cycles. Dry-wet, freeze-thaw, and their coupled cyclic action process in the soil body of the moisture cyclic changes, resulting in changes in the relative position of soil particles and pore structure damage, increasing the proportion of large pores in the soil body, decreasing the contact area between soil particles, resulting in the deterioration of cohesion and the internal friction angle.

Fig. 10

Curves of shear strength and moisture content of silty clay under 6 dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles.

Fig. 11

Plots of cycle times and shear strength parameters of silty clay under dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles.

Figure 12a reveals that the internal friction angle of the soil samples diminishes progressively with increasing moisture content. Furthermore, elevated moisture levels may increase the porosity of the soil, thereby reducing the effective stress per unit volume. This leads to a decrease in the shear strength and internal friction angle of silty clay. As the moisture content rises, the cohesion initially increases and subsequently declines. As previously discussed, the optimum condition of the soil microstructure may change and be slightly below the optimum moisture content, and cohesion follows a similar tendency. Figure 12b reveals that the interaction between moisture and soil particles attains optimal conditions at the 14% moisture content; at this juncture, the soil and its internal moisture facilitate the bonding of clay particles while preventing excessive lubrication forces among them.

Fig. 12

Curves of internal friction angle and moisture content and shear strength parameters of silty clay under dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles.

Deterioration behavior analysis

In order to uniformly compare and analyze the deterioration behaviors of shear strength and its parameters of silty clay under different dry-wet and freeze-thaw conditions, the degree of deterioration “D” is used as a metric according to the damage mechanics theory²⁷. The degree of deterioration “D” can be defined as Eq. (2).

$$D=\left( {1 - \frac{I}{{{I_0}}}} \right) \times 100\%$$

(2)

where D: the degree of deterioration (%), I: magnitude of mechanical parameters of the specimen after cyclic action, I₀: magnitude of mechanical parameters of the specimen before cyclic action.

The analysis of the test data indicates that as the number of dry-wet, freeze-thaw, and their coupled cycles grows, the degree of deterioration progressively escalates, exhibiting a rapid deterioration rate in the initial phase, which subsequently slows down. The mechanical properties of silty clay demonstrate a strong correlation with dry-wet cycles, freeze-thaw cycles, as well as their combined effects during the deterioration process and Logistic deterioration model. Compared with the common exponential and hyperbolic decay functions, the Logistic model characterizes the nonlinear process of initial rapid deterioration and later saturation, which is suitable for the scenario that there is an upper limit to the damage threshold for silty clay under the dry-wet-freeze-thaw cycle. The exponential decay shows a steady and quick drop without considering the point where damage levels off, which can make long-term predicted result seem worse than it is. The physical meaning of the hyperbolic decay parameter is ambiguous, and it cannot be directly related to the microscopic mechanism of the soil body. The Logistic shear strength parameters deterioration model of silty clay under the action of dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles was established as shown in Tables 2 and 3, and 4. Where the independent variables “${N_{DW}}$”, “${N_{FT}}$”, “${N_{DWFT}}$” represented the cycle times in which the silty clay experiences dry-wet cycles, freeze-thaw cycles, and dry-wet-freeze-thaw cycles, respectively. The linear fitting curves of the Logistic internal friction angle and cohesion deterioration model parameter in relation to moisture content were presented in Fig. 13. Also, the table provided the goodness of fit “R²”.

Table 2 The Logistic model that depicts the deterioration of the internal friction angle and the cohesion under dry–wet cycles in silty clay.

Table 3 The Logistic model that depicts the deterioration of the internal friction angle and the cohesion under freeze–thaw cycles in silty clay.

Table 4 The Logistic model that depicts the deterioration of the internal friction angle and the cohesion under dry–wet-freeze–thaw in silty clay.

Fig. 13

The fitting curves of the linear relation between the maximum degree of deterioration of shear strength parameters and the moisture content.

Figure 13 shows that as the moisture content increases, frost heave more clearly affects the silty clay specimens during freeze-thaw cycles, leading to a gradual increase in the maximum degree of deterioration of the internal friction angle and cohesion. Conversely, as the moisture content increases, the dry-wet amplitude of the soil specimens decreases during dry-wet cycles, resulting in a gradual decrease in the maximum degree of deterioration of the internal friction angle and cohesion. The maximum degree of deterioration under the coupled dry-wet-freeze-thaw cycle is the largest, compared to the dry-wet and freeze-thaw cycles. These phenomena indicate that for silty clay with high initial moisture content, in the surface layer of the seasonal freezing area or below the groundwater level all year round in natural environments, it is not significantly affected by the action of the dry-wet cycle and is more significantly affected by the action of the freeze-thaw cycle.

We prepared soil samples with 12%, 16%, and 20% moisture content and subjected them to 3, 5, 7, and 9 dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles, respectively, for validation tests of the above model. The predicted and test values were fitted, and error band diagrams were plotted as shown in Fig. 14. The width of the error bands of the predicted and test values of the deterioration is more uniform, and the error bands are narrow, and the gap between the predicted and test values is small. Therefore, the Logistic deterioration model has high accuracy in predicting the mechanical property indexes of silty clay under the effects of dry and wet, freeze-thaw, and their coupled cycles. The validity of the Logistic model for sheer strength deterioration is limited to the interval of 0 to 10 times for ${N_{DW}}$, ${N_{FT}}$, and ${N_{DWFT}}$, beyond which the prediction results need to be verified. The model is applicable to silty clay systems with moisture content ranging from 10 to 26%; the model is only applicable to plastic silty clay in the Changchun area. When the moisture content exceeds 26%, the pore water of the soil body is in a free-water dominant state; the pore water pressure continues to act, and the strength decay is accelerated, so the measured value is lower than that predicted by the model. When ${N_{DW}}$ goes over 10 times, the way cracks form from dry-wet cycles mostly becomes stable, but the model still thinks damage keeps building up, which causes the predicted results to differ from the actual measurements when the cycles are high; when ${N_{FT}}$ goes over 10 times, the soil’s strength starts to weaken faster, and the actual measurements are higher than what the model predicts; the predicted differences in ${N_{DWFT}}$ follow a similar trend. At the same time, the model assumes that there are no effects from stress relaxation or chemical weathering during dry-wet, freeze-thaw, or dry-wet-freeze-thaw cycles, and all tests are done at a constant freezing rate. At the same time, the model assumes that there are no effects from stress relaxation or chemical weathering during the dry-wet, freeze-thaw, or dry-wet-freeze-thaw cycles, and that all conditions are kept at a steady freezing rate.

Fig. 14

Plots of error band between test and predicted shear strength parameters.

Conclusion

The study focused on silty clay prevalent in seasonal frozen regions, conducting direct shear tests on soil samples with varying initial moisture content after undergoing dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycling. Utilizing Moore-Coulomb theory and damage mechanics, the analysis encompassed shear stress-displacement curves, shear strength, cohesion, and the internal friction angle of the silty clay under different dry-wet and freeze-thaw cycling conditions, revealing the corresponding deterioration patterns. Based on the Logistic model, a deterioration model of shear strength parameters was established, and the experiment verified that the deterioration model has high accuracy in predicting the shear strength parameters of silty clay under the action of dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles. The main conclusions are as follows.

• (1) As the number of dry-wet, freeze-thaw, and their coupled cycles increases, the shear stress-displacement curve of silty clay is changed from strain hardening to strain softening, and the moisture content and vertical stress significantly influence their behavior. High moisture content weakens the soil cohesion and increases the pore-specific surface area, which strengthen the trend of strain softening. The increase in vertical stress increases cohesion of the soil sample and makes it less likely to exhibit strain softening behavior. The evolution of shear stress-displacement curves for silty clay under dry-wet-freeze-thaw cycles is similar to that under dry-wet cycles. The evolution of the type of shear stress-displacement curves for silty clay under the action of freeze-thaw cycles is mainly observed for soil samples above the optimum moisture content. Overall, the silty clay is most prone to strain softening under the action of dry-wet-freeze-thaw cycles.

• (2) The overall shear strength of silty clay diminishes and stabilizes with the frequency of dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles. During dry-wet cycles and dry-wet-freeze-thaw cycles, the correlation with moisture content exhibits an “S” pattern. As the moisture content rises, the shear strength decreases in all three cycles when there is high vertical stress. The freeze-thaw cycle responds more strongly to the rise in moisture content than the dry-wet cycles and the dry-wet-freeze-thaw cycles. This is because when silty clay undergoes freeze-thaw cycling, the freezing and thawing of water causes the particles to rearrange themselves, which in turn destroys the internal structure of the clay, ultimately leading to deterioration of the shear strength. The shear strength of soil samples subjected to freeze-thaw cycles peaked slightly below the optimum moisture content under low vertical stress conditions.

• (3) The cohesion and internal friction angle of the silty clay specimens deteriorate with the increase of the number of dry-wet, freeze-thaw, and dry-wet-freeze-thaw cycles, and the deterioration slows down gradually and reaches stability obviously under a limited cycle times. Under the action of dry-wet, freeze-thaw, and their coupled cycles, the cohesion of soil samples tends to go up and then down as the moisture content goes up, and the extreme value appears at positions slightly below the optimal moisture content.

• (4) The degree of deterioration of internal friction angle and cohesion increases with the number of dry-wet, freeze-thaw, and their coupled cycles, being highest under the dry-wet-freeze-thaw coupled cycles. As the soil samples get closer to saturation, the maximum degree of degradation of the internal friction angle and cohesion decreases gradually under dry-wet and dry-wet-freeze-thaw cycles, whereas it increases gradually under freeze-thaw cycles. Compared with the soil samples under the action of dry-wet cycles and freeze-thaw cycles, the maximum degree of deterioration of the soil samples under the dry-wet-freeze-thaw cycles is higher. This shows that for silty clay with high initial moisture content, the effect of dry-wet cycles is more significant than that of freeze-thaw cycles.

• (5) The applicability range of the Logistic degradation model is defined as follows: ${N_{DW}}$, ${N_{FT}}$, and ${N_{DWFT}}$ are all ≤ 10; the moisture content is 10–26%, and the soil is plastic silty clay from the Changchun area. This model quantifies the coupling effect of moisture content and the cycle times, providing a theoretical basis for the stability analysis of engineering in seasonally frozen regions and soil improvement. According to the actual requirements of engineering applications, the model can predict the degree of deterioration, define the range of water content, and determine critical values for the cycle times in different stress zones. In practical engineering, this model can guide the optimal selection of soil improvement materials and, through the coordinated control of in-situ moisture content monitoring and drainage system design, meet the structural stability requirements.