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1.INTRODUCTIONSilty clay is a type of soil that is ubiquitous in foundation soils and has a significant impact on the design and construction of infrastructure such as buildings, roads, and bridges1,2. Due to its special physical and engineering properties, silty clay may undergo changes such as dry-wet cycles, expansion and contraction under different seasons and climatic conditions. Therefore, special consideration needs to be given to these effects in engineering design and implementation, and in-depth research on dry-wet cycle conditions is required. Engineering issues in silty clay are crucial to ensure the safety, stability and sustainable development of infrastructure. Seasonal rainfall and drought influence soil moisture levels, and these periodic dry-wet cycles impact the soil’s physical and mechanical characteristics. At present, most investigations are centered on the impact of dry-wet cycle frequency and intensity on the shear strength index, and jointly study the influence mechanism of dry-wet cycles on soil strength from the characteristic parameters such as moisture content, dry density, and water sensitivity of soil3,4. Kholghifard et al. focused on how compacted laterite samples deform under dry-wet cycles to explore the potential hazards of collapsibility in laterite foundations5. The SWCC is important indicator associated with moisture content, volume change, strength, etc. In the current research on the SWCC, relevant scholars focus on the impact of initial dry density and pore structure on the SWCC, and perform function fitting on the obtained the SWCC6,7. Liu et al. studied the hysteresis phenomena in the SWCC of loess subjected to varying dry densities and dry-wet cycles8. Nevertheless, existing research on the effects of dry-wet cycles on the shear strength and SWCC of silty clay is still limited, indicating a need for more in-depth studies. Therefore, this paper explores the effects of dry-wet cycles on both the SWCC and shear strength of silty clay to elucidate how these cycles influence these characteristics. NMR T2 relaxation spectrum was employed to study the pore size distribution of soil subjected to dry-wet cycles, focusing on how changes in soil structure at a microscopic level influence the SWCC and shear strength. Finally, the commonly used models are used to fit and analyze the test results in this article, providing a useful data basis for engineering numerical calculations in the field of geotechnical engineering. 2.TEST OVERVIEW2.1Basic propertiesSilty clay has a specific gravity value of 2.73, it is known that the clay component less than 0.005mm in the particle size gradation accounts for 15.2%, the silt soil of 0.075-0.005mm accounts for 46.7%, and the silty clay is mainly silty and clay. The liquid limit of silty clay is 31.5%, the plasticity index is 19.2%, and the free expansion rate is 22.8%. 2.2Test planThis study investigates the SWCC and shear strength of silty clay through various dry-wet cycles, which involve processes of saturation, air-drying, and re-saturation, and the SWCC and direct shear strength tests were carried out after reaching the target number of cycles. In order to study the change of pore volume of soil after drying-wet cycle, nuclear magnetic resonance test analysis was carried out, and the specific test process is as follows.
3.TEST RESULTS AND ANALYSIS DISCUSSION3.1SWCCFigure 2 illustrates the SWCC of silty clay across various dry-wet cycles, with saturation data reflecting volume changes during water loss. The SWCC of silty clay shows a typical S shape. Figure 2 indicates that with more dry-wet cycles, the water-holding ability of the sample decreases, and the saturation levels follow the order N=1 > N=3 > N=5 > N=7 at the same matrix suction, with minimal difference between the SWCCs for N=5 and N=7. The smaller the number of cycles, the larger the inlet value of the specimen, which corresponds to 35 kPa, 26 kPa, 18 kPa, and 12 kPa, respectively. If the matrix suction is higher than the air entry value, the soil saturation starts to decline sharply in the low matrix suction range, and the water loss rate is significantly increased. When the suction exceeds 600-800 kPa, the rate of the saturation reduction begins to slow down. 3.2Direct shear strengthFigure 3 depicts the shear strength versus vertical pressure for silty clay samples subjected to different dry-wet cycles. The figure shows that while shear strength increases with vertical pressure for a constant dry-wet cycle number, it decreases as the dry-wet cycles progress. This trend highlights that repeated dry-wet cycles result in fatigue damage that reduces the soil’s shear strength. Figure 4 demonstrates the shear strength indicators for soil, comparing peak and residual strength across varying dry-wet cycles. With an increase in dry-wet cycles, cohesion and friction angle both decline, with a more pronounced decrease in cohesion. The shear strength index for peak strength exceeds that of residual strength. 3.3NMR curvesFigure 5 illustrates the NMR curves of the sample across various dry-wet cycles, according to the nuclear magnetic resonance curve distribution map, the pore size structure of the silty clay exhibits a bimodal configuration. According to the bimodal structural relaxation time distribution characteristics of the NMR curve, T2=7.0ms was taken as the cut-off value for small and medium-sized pores and macropores, that is, the part with T2<7ms is regarded as small and medium pores, and the part with T2>7ms is regarded as large pores. As the frequency of dry-wet cycles increases, the peak area corresponding to T2>7ms progressively expands, while the peak area corresponding to T2<7ms gradually contracts, indicating a rise in large pore structures and an enlargement of small and medium-sized pores within the sample. The NMR results demonstrate that the expansion of large pores within the sample confirms that an increase in dry-wet cycles induces fatigue damage. Consequently, the sample experiences more severe internal damage with a higher number of dry-wet cycles, leading to a reduction in final shear strength. The size of soil pores determines the water-holding characteristics of soil9,10. The correlation between soil matric suction and pore size in soil can be calculated using the Young-Laplace equation (1). It can be seen from equation (1) that as the pore diameter increases, the matrix suction of the corresponding pore will gradually become smaller. Therefore, the pore diameter expands with the increase in dry-wet cycles, driven by the fatigue damage to the soil, and the proportion of large pores in the soil increases, and a large amount of pore water will be discharged when the matrix suction is low, and the soil’s water retention ability at N=1 is greater than that when N=7. In the formula, s is the matrix suction; d is the diameter of the pore; T is the surface tension of the pore water meniscus; θw is the contact angle of the air-liquid interface. 4.MODEL FITTINGEngineering numerical calculations often involve the application of SWCC, in this paper, the commonly used Van Genuchten model is used to fit the SWCC of silty clay across various dry-wet cycles. The detailed functional expression can be found in equation (3), and the least squares method is used in Origin software for model fitting. In the formula, m, n, α, Sre are the material parameters related to the soil. Figure 6 is the SWCC result of Van Genuchten model fitting. As shown in Table 1, the fitting parameters indicate an R2 value for the model that is greater than 0.99, effectively approaching 1.0. The fit improves with more dry-wet cycles, showing that the Van Genuchten model accurately fits the SWCC of silty clay under varying dry-wet cycles. Table 1.Fitting parameters.
4.2Fitting of the shear strength decay indexIn order to quantitatively analyze the deterioration characteristics of shear strength parameters under different dry-wet cycles, the deterioration degree of cohesion and internal friction angle deterioration of silty clay were calculated. According to the variation law of each data point, For the deterioration of cohesion and internal friction angle as the number of cycles increased, hyperbolic and linear functions were fitted respectively. The fitting models are described in equation (3), and the detailed results are presented in Table 2 and Figure 7. Table 2.Fitting parameters.
Based on the fitting results presented in Table 2, it is evident that equation (3) provides a good representation of the deterioration behavior of the shear strength index in silty clay under various dry-wet cycles. Figure 7 illustrates that the deterioration trend of the shear strength index of peak strength and residual strength is basically the same. The degree of deterioration of cohesion during dry-wet cycles gradually increases with the number of cycles and then becomes stable, and the degree of deterioration of the internal friction angle increases linearly with the number of dry-wet cycles. Comparing the change amplitudes of the cohesion and internal friction angle deterioration, Under identical dry-wet cycle conditions, it was found that the cohesion deteriorates more severely than the internal friction angle, reflecting that cohesion is more susceptible to the effects of dry-wet cycles in silty clay. 5.CONCLUSION
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