Study on the Influence of Initial Water Content on One-Dimensional Freezing Process of the Qinghai-Tibet Silty Clay by Using Digital Image Technology

Wang, Yongtao; Wang, Boyuan; Pang, Shoude; Hua, Weihang; Ji, Zhiqiang; Wang, Dayan

doi:https://doi.org/10.1155/2022/9994240

Geofluids

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Abstract Introduction Methods and Materials Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

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Thermal, Mechanical Behavior and Engineering Response of Frozen Soil

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Research Article | Open Access

Volume 2022 | Article ID 9994240 | https://doi.org/10.1155/2022/9994240

Study on the Influence of Initial Water Content on One-Dimensional Freezing Process of the Qinghai-Tibet Silty Clay by Using Digital Image Technology

Yongtao Wang,¹Boyuan Wang,¹Shoude Pang,²Weihang Hua,¹Zhiqiang Ji,³and Dayan Wang⁴

Academic Editor: Dongdong Ma

Received21 Apr 2022

Accepted20 Jun 2022

Published23 Jul 2022

Abstract

The water content of the foundation soil will change dynamically as a result of rainfall, snowfall, and ground surface evaporation, leading to a significant change in frost heave properties of the foundation soil in cold regions. One-dimensional freezing tests of the Qinghai-Tibet silty clay with three different initial water contents in an open system were carried out using CCD image acquisition technology and computed tomography (CT) scanning technology in combination with the traditional soil freeze-thaw test system. The heat conduct process, cryostructure formation, frost heave development, unfrozen zone consolidation, ice segregation, water migration, and redistribution during soil freezing are studied comprehensively. The results show that increasing the sample’s initial water content will lead to the increase of frozen depth, ladder-like cryostructures in vertical sections, more structural polygons in horizontal sections, and also more obvious ice lens segregation, unfrozen zone consolidation, and water migration during the freezing of soil samples.

1. Introduction

Permafrost accounts for about 23% of the global land area, and the seasonally frozen soil is widely distributed in areas above 24° north latitude [1]. Frost heave damage occurs due to frost heave of the foundation soil [2–5]. The factors affecting frost heave of the foundation soil include soil particle, temperature, water content, salinity, and external loads [6, 7]. Studies found that if the frost heave sensitive soil freezes under moderate freezing temperature gradient conditions, suitable initial water content, and external water supply, the water from outside will continuously migrate from unfrozen zone to the frozen zone and segregate to ice lenses, resulting in severe frost heave of the foundation soil, which is far greater than the phase transformation expansion of the original water in the foundation soil [8–10]. Researchers have carried out a lot of studies on the effects of soil particle, temperature gradient, salinity, and external load on the frost heave characteristics of foundation soil, but there are relatively few studies on the effect of initial water content on the frost heave performance of foundation soil.

The researchers found that the initial water content in the foundation soil has a great influence on frost heave of the soil. Wu et al. [11] found that there exists an initial frost heave water content in the foundation soil. Ning and Likos William, Xu et al., Zhang et al., and Xu et al. [12–15] believed that the water content of unsaturated soil greatly affected the pore water pressure, stress-strain behavior, thermal conductivity, and dielectric constant. Qing-zhi et al. [16] revealed that the influence of initial water content on frost heave was the greatest among many factors through the frost heave test of graded crushed stone in laboratory. Gao et al. [17] conducted a series of one-dimensional freezing experiments with atmospheric pressure water supplement and found that with the increase of initial water content, the frost heave amount of coarse-grained soil will increase. Based on thermodynamic equilibrium theory and hydrothermal coupling theory, He et al. [18] proposed a new method for calculating the unfrozen water content and ice content and obtained that the initial water content would affect the water vapor transport in the frozen soil, thereby affecting the development process of frost heave. Mo et al. [19] performed one-dimensional soil freezing experiments in a closed system under different initial conditions and found that the initial water content has a significant effect on the changes of shallow soil temperature and distribution of water content. Although these studies have carried out to study the effect of initial water content on frost heave, some frost heave models also have the ability to analyze the influence of initial water content on frost heave, such as hydrothermal coupling model [20], segregation potential model [21], thermodynamic model [22, 23], rigid ice model [24, 25], and hydrothermal-mechanical coupling model [26], but their main purpose is to predict the frost heave displacement.

In recent years, with the application of digital image processing technology on research of frozen soil, some scholars begun to carry out dynamic research on the physical and mechanical processes involved in the soil freezing process. Takeda and Okamura [27] were the first to observe the microstructure of the freezing front during soil freezing by using the digital image technology. Arenson et al. [28–31] used the methods of ion tracing and particle velocity measurement to study the local displacement of soil samples. Jin-sheng et al. and Zhou et al. [32, 33] used a camera to record the growth process of the ice lens during the freezing process and divided it into two stages. Zhou et al. [34] used an infrared thermal imaging system to obtain the exact temperature when ice segregation occurred. Wang et al. [35] used a CCD camera to capture and analyze the development process of cryostructures and frost heave deformation during the one-dimensional freezing process of Qinghai-Tibet silty clay. Zhang et al. and Yuzhi et al. [36, 37] also used a CCD camera to study the water migration in the coarse-grained soil under freeze-thaw cycles. These studies have preliminary explored the relevant physical and mechanical processes in the process of soil freezing. However, no systematic research has been carried out on the heat conduction, the formation of cryostructures, the development of frost heave and consolidation of unfrozen zone, the segregation of ice lenses, the migration, and redistribution of water during the process of soil freezing.

Affected by the action of rainfall, snowfall and ground surface evaporation, etc., the initial water content and saturation state in the foundation soil are often changing dynamically, especially in the capillary water zone. This will inevitably lead to different frost heave responses in the freezing process of the foundation soil [38, 39]. Therefore, in this paper, the CCD image acquisition technology and the computed tomography (CT) scanning technology are combined with the conventional soil freeze-thaw test system to study the heat and mass transfer process of Qinghai-Tibet silty clay under different initial water content conditions during the one-dimensional freezing process. The research results can provide data support for revealing of frost heave laws and prediction of frost heave of the foundation soil.

2. Methods and Materials

2.1. Testing System

CCD image acquisition technology and the computed tomography (CT) scanning technology are combined with the conventional soil freeze-thaw test system XT5405 in the State Key Laboratory of Frozen Soil and Engineering. Schematic of the test system is shown in Figure 1. The test system consisted of six parts: (1) the soil container, (2) temperature control system, (3) temperature and deformation sensors, (4) water supply system, (5) image acquisition system, and (6) lighting system. The detailed description of the system can be found in the paper [35].

2.2. Preparation and Index of the Soil Samples

The soil used in the experiments was collected from the Beiluhe Basin in the Qinghai-Tibet Plateau. The soil is dried, crushed, and sieved to obtain the particle gradation, as shown in Table 1. The liquid limit and the plastic limit water content of the soil are measured as 23.5% and 14%, respectively. As a result, the soil can be named as silty clay by considering the results of size distribution and limit water content analysis comprehensively according to the Standard for Geotechnical Testing Method of China (GB/T 50123-2019).

Three different initial water contents and the initial dry density were designed for the samples and numbers as SW1, SW2, and SW3 to make sure the sample SW1 was under unsaturated state, and the sample SW2 and SW3 were under saturated state. The initial and boundary conditions of these three samples are shown in Table 2. The samples were made by using layered tamping method and stood for 24 hours before the test to obtain a uniform initial state. The temperature of different heights in the samples, the images of the samples in vertical profile, and the displacement of the top plate were recorded dynamically. The water contents on different heights were measured before and after the tests. The horizontal sections of the samples were scanned using the CT before and after the tests. The samples were frozen under the designed boundary temperature in an open system with the water supplied from outside with the atmospheric pressure.

3. Results and Discussion

3.1. Temperature Changes in the Samples

Figure 2 shows the movement process of the freezing fronts in the samples under different initial water contents. The positions of the freezing fronts in samples SW1, SW2, and SW3 were determined according to their respective freezing points measured before the tests, which were -0.61°C, -0.31°C, and -0.30°C, respectively. It can be seen from the figure that the freezing front moves down rapidly in the first 10 hours after the start of samples freezing, moves down slowly between 10 hours and 24 hours, and then tends to be stable after 24 hours. At the same time, the initial water content has a certain influence on the stable position of the freezing front in the samples. The position of the stable freezing front moves downward with the increase of the initial water content; in other words, the freezing depth increases with the increase of the initial water content. After freezing for 120 hours, the positions of the stabilized freezing fronts in the samples SW1, SW2, and SW3 are 10.05 cm, 9.44 cm, and 9.07 cm, respectively. The reason for this phenomenon is because the initial water content directly affects the degree of saturation in the soil sample. When the initial water content increases, the pores between the soil particles will be filled with more water, which will inevitably lead to an increase in the thermal conductivity of the sample. Therefore, the thickness of the frozen zone in the soil sample increases with the increase of the initial water content at the same boundary temperature.

Figure 3 shows the temperature distribution in the vertical section of the samples frozen for 120 hours with different initial water contents. It can be seen from the figure that the temperature with different initial water contents after freezing for 120 hours basically shows a piecewise linear distribution with sample height, and the average temperature gradient in the frozen zone is significantly larger than in the unfrozen zone. Ice segregation leads to thermal conductivity increase of the frozen zone. The overall average temperature gradients of the three samples were -0.56°C/cm, -0.59°C/cm, and -0.60°C/cm, respectively, which indicated that the bigger the initial water content, the greater the overall temperature gradient in the samples after frozen. The temperature gradients in the frozen zone of the samples SW1, SW2, and SW3 were -0.66°C/cm, -0.76°C/cm, and -0.77°C/cm, respectively. It means that more water segregated to ice when the initial water content increased. The temperature gradients of the unfrozen zone of the samples SW1, SW2, and SW3 were -0.53°C/cm, -0.52°C/cm, and -0.51°C/cm, respectively. It is because that the thermal conductivity of water is smaller than that of soil particles, so the water content in the unfrozen zone of the sample SW1 is smaller than that of the samples SW2 and SW3.

3.2. Cryostructure Formation in the Samples

3.2.1. Cryostructure in Vertical Section of the Samples

Figures 4–6 show the freezing process of the vertical profile of the samples SW1, SW2, and SW3 in the height direction of 0-14 cm and the width direction of 9-11 cm, respectively. It can be seen from the figures that the freezing process of the samples SW1, SW2, and SW3 are all reached stability within 24 hours, which verifies the conclusion obtained in Section 3.1. After freezing stable, the cryostructures on the vertical profile can be divided into the microthin layered structural area between the yellow dash lines L1 to L2, the thin layered structural area between the yellow dash lines L2 to L3, and the warmest thick layered structural area between the yellow dash lines L3 to L4 [35]. However, there are obvious differences in the cryogenic cracks and ice segregation processes in the cryostructure areas with the change of the initial water content.

It can be seen from Figure 4 that for the sample SW1, the shape of ice lenses is curved in the microthin layered structural area, the distances between the ice lenses are larger than that in the same area in the samples SW2 and SW3, and there are also large irregular vertical cryogenic cracks formed in this area. There are network-like cryogenic cracks distributed in the thin layered structural area, dividing the horizontal sections of the sample into polygonal shapes. The ice lenses in the warmest thick layered structural area are flat and well developed. However, unlike the saturated samples SW2 and SW3, there are also cracks formed in the unfrozen zone where there exist the initial cracks and also developed and expanded during the freezing process. The reason is that the water in the unfrozen zone has migrated to the frozen zone during the freezing process. When the water in the unfrozen zone decreased, the pores between the soil particles contracted and resulted in the expansion of the shrinkage crack.

In the microthin layered structural area of the sample SW2, ice lenses have developed, as shown in Figure 5, and the distances between each adjacent ice lenses are extremely small and parallel to each other. The ice lenses in the thin layered structural area are distributed in a curved shape. The ice lenses in the warmest thick layered structural area are single and well developed.

As shown in Figure 6, for the sample SW3, the ice lenses in the microthin layered structural area are horizontal and close to each other. The vertical cryogenic cracks are developed more obviously in this area in sample SW3 than that in samples SW1 and SW2. The vertical cracks are connected by horizontal cracks and formed a ladder-like structure. The cryostructure in the thin layered structural area in the sample SW3 is as same as that of the sample SW2, but the ice lenses are curved and do not parallel to each other. The ice lens in the warmest thick layered structural area is approximately flat and well developed. But it is worth noting that during the freezing process, the crack in this layer, which is between the yellow dash line L3 to L4, is filled with the ice lens before 36 hours but melted partly after 36 hours, and a crack also changed from expansion to closure. The melting and closing process of the ice lens is accompanied by the continuous downwards movement of line L4. This process is mainly induced by the downward movement of the original segregated ice lens and leads to an increase in temperature in this area, so the ice lens is melted and the cracks are closed.

3.2.2. Cryostructure in the Horizontal Section of the Samples

Figure 7 is the CT value distribution images obtained by CT scanning of the horizontal sections of the samples SW1, SW2, and SW3 before and after the test. The subfigures a0, b0, and c0 are the CT values distribution images of the initial states of three samples, respectively. It can be seen that the CT value distribution, in other words, the density distribution of the samples, is relatively uniform under the given initial water content. However, when the initial water content increases, a large number of pores appear in the image obtained by CT scanning. These pores are filled with water, resulted in the overall reduction of CT values. Subfigures a1, a2, a3, and a4 are the CT value distribution images of the sample SW1 scanned after freezing for 120 hours; b1, b2, b3, and b4 are the CT value distribution images of the sample SW2 scanned after freezing for 120 hours, and c1, c2, c3, and c4 are the CT value distribution images of sample SW3 after freezing for 120 hours. Their height positions in the respective sample are located in the microthin layered structure area, thin layered structure area, warmest thick layered structure area, and unfrozen zone of the vertical section of the samples from top to bottom.

AV in the notes of Figure 7 represents the average CT value of the corresponding scanned section. Generally speaking, in a specific sample, the average CT value of the frozen zone (a1 to a3, b1 to b3, and c1 to c3) is less than the initial average CT value, while the average CT value of the unfrozen zone is greater than the initial average CT value. The reasons are the density decreases due to ice segregation in the frozen zone, while it increases due to the consolidation of unfrozen zone since the water migrates from unfrozen zone to frozen zone during freezing of the samples. After the sample with the same initial water content is frozen, the average CT values from the top to the bottom of the sample first decrease and then increase, and the average CT value in the warmest thick layered structure area is the smallest, because the massive ice lens segregated in this area. The large cracks in the samples are expanded and connected, and the small polygonal structure gradually becomes larger and larger from the microthin layered structure area to the warmest thick layered structure area of the samples from the frozen zone to the unfrozen zone. These cracks and polygons are potential channels for water migration. With the increase of the initial water content of the sample, the large cracks gradually decrease and the polygonal structure gradually increases after the sample is frozen. It can be seen that the initial water contents of the samples have a great influence on the microstructure of the samples, which directly affect the water migration and frost heaving process during the freezing process of the sample.

3.3. Sample Frost Heaving and Consolidation

3.3.1. Sample Frost Heaving and Consolidation Process

Figure 8 shows the frost heave and the displacement process of the boundary lines of the cryostructure areas in the samples SW1, SW2, and SW3. The cryostructure areas’ boundary lines are marked by the yellow dash line in Figures 4–6. L1 is the upper boundary of the microthin layered structure area, L2 is the upper boundary of the thin layered structure area, L3 is the upper boundary of the warmest thick layered structure area, and L4 is the lower boundary of the warmest thick layered structure area.

It can be seen from the frost heave curve of sample SW1 in Figure 8 that the frost heave process of the sample can be divided into two linear stages. The average frost heave rate was 0.070 mm/hour before 36 hours and 0.029 mm/hour after 36 hours. The total frost heave displacement of the sample SW1 is 4.927 mm.

According to the movement of the cryostructure boundary lines of the sample SW1 in Figure 4, it can be seen that the displacement of the line L1 and the frost heave curve of the sample are consistent; the displacement difference between the line L1 and L2 is slightly enlarged during the freezing process of the sample, indicating that there is a small content of ice lenses segregated in the microthin layered structure area of the sample during the freezing process and causes a small frost heave of the sample. The lines L2 and L3 moved upward, and the line L4 remained basically unchanged during the freezing process of the sample. The distances between the lines L2 and L3 and between the lines L3 and L4 were gradually expanding during the freezing process of the sample, which indicates that the ice lens segregation in the thin layered structural area and the warmest thick layered structural area of the sample also caused the frost heave of the sample SW1. Quantitatively, after the sample was freezing for 120 hours, the distance between the lines L2 and L3 was 2.415 mm, and the distance between the lines L3 and L4 was 2.044 mm, which indicated that the ice lens segregation in the thin layered structural area had a bigger effect on the frost heave of the sample than other cryostructure areas. The fact that the line L4 remained basically unchanged during the freezing process indicated that the unfrozen zone of the unsaturated sample SW1 was basically unconsolidated during the freezing process. The average frost heave rate of the sample before freezing for 36 hours was significantly greater than the average frost heave rate after 36 hours. This is because the ice lenses in the warmest thick layered structural area have not been fully formed in the first 36 hours. Therefore, the permeability coefficient in this area was little influenced and water can migrate from unfrozen zone to frozen zone freely and resulting in a larger frost heave rate. In contrast, the water migration process was reduced by the ice lens formation in the warmest thick layered structural area after 36 hours freezing of the sample, so the frost heave rate decreases.

From the displacement change process of the cryostructure boundary lines of the sample SW2, it can be seen that the displacement process of the line L1 is basically consistent with the change process of the whole sample’s frost heave curve. The total frost heave displacement of the sample SW2 is 3.081 mm. The frost heave process of the sample SW2 can be divided into three stages: rapid frost heave (0 to 2 hours), stable frost heave (2 hours to 36 hours), and linear frost heave (36 hours to 120 hours). The average frost heave rate of the linear frost heave stage was 0.028 mm/h. In addition, the change process of the line L2 is basically the same as that of the line L1. The position of the line L3 did not change during the frost heave process of the sample. The position of the line L4 moved downward for the first 36 hours during the freezing process of the sample and remained unchanged after that, which indicated that the unfrozen zone of the sample was consolidated during the frost heave process. From the change of the boundary lines of the cryostructure areas, it is presented that the frost heave in the sample was induced by the combined action of the frost heave in the frozen zone and the consolidation in the unfrozen zone before the sample was frozen for 36 hours, and the consolidation effect in the unfrozen zone was basically completed after 36 hours. According to the movement of the lines L1, L2, and L3 and the freezing process of the sample SW2 in Figure 5, it can be seen that the main frost heave of the sample SW2 is caused by the segregation of the ice lenses in the thin layered structural area. According to position movement of L4, it can be seen that the unfrozen zone consolidation is caused by the segregation of the ice lenses in the warmest thick layered structural area.

From the frost heave curve of the sample SW3, it was found that the frost heave displacement of the sample SW3 changes little during the whole freezing process, and the total frost heave displacement of the sample is 0.188 mm. The movement of the lines L1 and L2 and the sample frost heave curve are basically same. The lines L3 and L4 move downward during the freezing process of the sample. After the sample is frozen for 120 hours, the total displacements of the lines L3 and L4 are 0.894 mm and 3.664 mm downwards, respectively, which indicated that only a small part of the ice lens segregation in the thin layered structural area has led to the frost heave of the sample, and the big part has led to the compression of the warmest thick layered structural area. The ice lens segregation in warmest thick layered structural area all causes the consolidation of the unfrozen zone of the sample. Combined with the freezing process of the sample SW3 in Figure 6, it indicated that large effect of the consolidation of unfrozen zone of the sample can be attributed to the high initial water content and soft state in the unfrozen zone of the sample.

3.3.2. Change Rule of the Frost Heave Ratio

As mentioned above, segregation of the ice lenses not only induces the frost heave of the whole sample but also causes the consolidation of the unfrozen zone of the sample. However, the conventional method for frost heave ratio calculation did not consider the consolidation. In this context, two different frost heave ratio’s calculation methods are proposed: one is , which does not include the amount of consolidation, is the whole frost heave amount of the sample measured through the displacement of the top plate, and is the frozen depth determined by the temperature data; the other one is , which considers the consolidation amount obtained by analysis of the lines L4 of warmest thick layer areas in all samples. By calculating these two frost heave ratios of samples SW1, SW2, and SW3 after frozen for 120 hours, a relationship between frost heave ratios and the initial water contents is established as shown in Figure 9. It was found that decreases linearly with the increase of the initial water content. The frost heave ratio of the samples SW1, SW2, and SW3 are 13.14%, 6.91%, and 0.39%, respectively. The frost heave ratio of the samples SW1, SW2, and SW3 are 13.26%, 7.92% and 8.01%, respectively. The frost heave ratio is greater than the frost heave ratio because includes the consolidation amount.

As the sample SW1 is in an unsaturated state, due to the small amount of consolidation of the unfrozen zone of the sample, the segregation of the ice lenses in the sample basically is acted on the frost heave of the sample, so there is little difference between the two frost heave ratios. As the samples SW2 and SW3 are in the saturated state, part of the ice lens segregation in the sample acts on the frost heave of the sample, and another part acts on the consolidation of the sample. Therefore, with the increase of the initial water content, the whole frost heave amount of the samples decreases and the whole consolidation amount of the unfrozen zone increases. However, it is interesting that when the initial water content of the sample is greater than a certain degree, the sums of the frost heave amount of the whole sample and the consolidation amount of the whole unfrozen zone in each sample are basically equal. That is to say that the frost heave ratio basically no longer changes with the change of initial water content when it is greater than a certain degree. However, with the increase of initial water content, the difference between the frost heave ratios and becomes larger and larger. This indicated that the consolidation process must be taken into account when calculating the frost heave ratio of samples with a high initial water content in laboratory tests; otherwise, the calculated frost heave ratio will be too small, which will underestimate the frost heave sensitivity of soil samples and may lead dangerous to engineering design.

3.3.3. The Consolidation Process and Consolidation Rate of the Unfrozen Zone

From the displacement process of the boundary lines L4 of the samples in Figure 8, it can be obtained that the unfrozen zone of the sample consolidated during the freezing process. The total consolidation amounts of the samples SW1, SW2, and SW3 are 0.047 mm, 0.446 mm, and 3.664 mm, respectively. For the sample SW1, due to the high initial dry density, the consolidation effect of the unfrozen zone is weak during the freezing process. For the sample SW2, the consolidation of the unfrozen zone develops rapidly before freezing for 36 hours and then tends to stable. For the sample SW3, the unfrozen zone of the sample is always in the consolidation process during the freezing process. It is obvious that with the increase of the initial water content of the sample, the amount of consolidation during the freezing process of the sample is also increasing.

In order to evaluate the degree of consolidation in the unfrozen zone of the samples, this paper not only analyzes the change process of the amount of consolidation but also proposes an evaluation parameter which is the consolidation ratio defined as the consolidation amount of the per unit of the unfrozen zone height, to determine the degree of consolidation in the unfrozen zone. The total consolidation ratios in the unfrozen zone of the samples SW1, SW2, and SW3 are 0.046%, 0.473%, and 4.040%, respectively. The relationship between the consolidation amount and consolidation ratio of the samples frozen for 120 hours and the initial water content of the samples was analyzed, as shown in Figure 10. It can be seen from the figure that the consolidation amounts and consolidation rates in the unfrozen zone of the samples are both increasing with the increase of the initial water content of the samples matching well with exponential functions.

3.4. Water Supplement Process

During the freezing process of the samples, the water in the Mariotte flask was absorbed to the sample by the freezing suction force. Figure 11 reflects the change process of the water supplement amounts during the freezing process of the samples SW1, SW2, and SW3. It is shown that the water supplement process is basically as same as the frost heave process. The water supplement process in the sample SW1 can be divided into two linear stages before and after 36 hours. The average water supplement rate of the first 36 hours stage is 1.061 mL/hour, equals to an average frost heave rate of 0.074 mm/h. The average rate of water supplement after 36 hours was 0.657 mL/h, equals to an average frost heave rate of 0.046 mm/h. Comparing the average frost heave rate calculated by the water supplement with that calculated by the frost heave curve, it is found that the average frost heave rate calculated by water supplement is greater than the average frost heave rate calculated by the frost heave curve of the sample SW1. It is because that a part of suppled water just filled with the pores and did not contribute to frost heave for the unsaturated state of the sample SW1. After the sample SW2 was frozen for 36 hours, which corresponds to the linear frost heave stage, the water supplement rate was 0.374 mL/h, equals to an average frost heave rate of 0.026 mm/h, which was basically consistent with the frost heave rate calculated by the frost heave curve, indicating that the water supplied from outside will fully acts on the frost heave of the sample.

It can be seen from Figure 11 that the average water supplement rates decrease with the increase of the initial water content of the samples during the freezing process of the samples, and the water supplement amounts increase with the increase of the initial water content of the samples at the same freezing time. This is because with the increase of the initial water content of the sample, the degree of consolidation in the unfrozen zone is greater during the freezing process of the sample. The consolidation of the unfrozen zone will reduce the permeability coefficient of the unfrozen zone, so the water supplement rate is changing smaller with the freezing time.

3.5. Water Redistribution Process

Due to the processes of water migration, ice segregation, and consolidation in the unfrozen zone, the water will redistribute during the freezing of the sample. Figure 12 shows the change of the water content of the samples before and after the tests. It can be seen from the figure that after the samples were frozen for 120 hours, the water content of the frozen zone has increased, while the water content of the unfrozen zone has decreased, and there exists an area where the water content has increased significantly in the frozen zone near the freezing front. The increase of water content in the frozen zone is the result of water supplement from the outside and migration from the unfrozen zone to the frozen zone. The area with a significantly increased water content corresponds to the ice lens segregation in the thin layered structural area and the warmest thick layered structural area. Correspondingly, the decrease of water content in the unfrozen zone corresponds to the consolidation of the unfrozen zone.

The relationships among the average water content in the unfrozen zone after tests, the average water content in the frozen zone after tests, and the maximum water content after tests and the initial water content are analyzed and shown in Figure 13. It presents that the average water content and maximum water content in the frozen zone after tests are higher than the initial water content, and the water content in the unfrozen zone after tests is lower than the initial water content for each of the three samples. The average water content in the unfrozen zone, the average water content in frozen zone, and also the maximum water content are all increased with the increase of the initial water content for each of the three samples. The average water content in the unfrozen zone and that in the frozen zone have a linear relationship with the initial water content, and the maximum water content has an exponential relationship with the initial water content. And with the increase of the initial water content, the increase rate of the water content in the frozen zone, the maximum water content, and the decrease rate of the water content in the unfrozen zone are all increased too.

4. Conclusions

By combining digital image technology with conventional soil freeze-thaw test system, one-dimensional freezing tests of Qinghai-Tibet silty clay under different initial water content were carried out. The heat conduction, the cryostructure formation, the frost heave and consolidation development, and the process of water migration and redistribution during the freezing process of the samples were systematically studied. The main conclusions are as follows: (1)The samples reached freezing stability after freezing for about 24 hours. Given the same boundary temperature conditions, the difference of the initial water content has an important influence on the temperature distribution after the freezing stability of the samples, presenting that the greater the initial water content, the greater the freezing depth of the sample(2)The cryostructures on the vertical sections of the samples after freezing can be divided into microthin layered structural area, thin layered structural area, and warmest thick layered structural area. Cryogenic cracks and polygonal structures in both the unfrozen and unfrozen regions provide channels for water migration(3)The segregation of the ice lenses in the samples not only causes the frost heave of the frozen zone of the sample but also causes the consolidation of the unfrozen zone. The consolidation effect of the unfrozen zone increases with the increase of the initial water content. In the case of high initial water content, the consolidation of the unfrozen zone should also be considered in the calculation of the soil frost heave ratio(4)During the freezing process of the samples, it can absorb water from the outside and migrate through the unfrozen zone to the frozen zone. The area with a significantly increased water content corresponds to the ice segregation in the thin layered structural area and the warmest thick layered structural area(5)This work can provide data support for comprehensive revealing of freezing processing of foundation soil and more factors which have influence on the soil freezing will be considered in future research

Data Availability

The data used to support the findings of this study are included in the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 41901070, No. 41871054, and No. 41771070) and the Open Fund of State Key Laboratory of Frozen Soil Engineering (No. SKLFSE201805).

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