Review of the influence of freeze-thaw cycles on the physical and mechanical properties of soil

Dan Chang, JianKun Liu

School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China

1 Introduction

Seasonally frozen soil is defined as soil that has a temperature lower than 0 °C. There is about 35,760,000 km2of frozen soil in the world, which is about 24% of the world’s land area.Frozen soil accounts for more than 70% in Russia, about 50% in Canada, 60% in Alaska, 65% in Mongolia, 22% in China, and is also distributed in other areas, such as Norway, the Arctic islands,Greenland and Antarctic. With the development of engineering activities, such as building highways, railways and tunnels in frozen soil regions, the strength and deformation characteristics of frozen soils are becoming very important for these activities(Laiet al., 2009, 2010). Frozen soil is made up of solid mineral particles, liquid water (unfrozen water and tightly bound water)and gaseous inclusions (water vapor and air). The difference between frozen soil and unfrozen soil is existing ice in the frozen soil (Laiet al., 2009, 2010). The physical properties of frozen soil are affected by the freezing process, permafrost melting, as well as seasonal or long-term temperature changes and other factors.In the frozen state, most soils are relatively impermeable and of high strength. The aforementioned characteristics are very important and must be taken into account in designing frozen soil engineering (Andersland and Ladanyi, 2011).

Soil strength is the key to the analysis of building foundation stability. The mineral composition, graduation, density and moisture content of the soil are among the important factors affecting soil strength. In cold region engineering construction,the impact on soil strength brought by freezing and thawing process should be considered. A volume expansion occurs when soil water freezes, which increases the voids between soil particles; when the ice melts, the voids decrease due to consolidation.The freezing and thawing process will change the structure of the soil particles configuration, which will change the mechanical properties of the soil (Wanget al., 2005). Also, probability theory can be used to discuss significance and interaction of factors,such as freeze-thaw cycles and freezing temperature on physical-mechanical property change of frozen soil.

2 Freezing and thawing process of the soil

The freezing and thawing process includes the appearance of frost heave when the freezing front moves down and loss of strength when the soil melts. When the pore ice melts, the soil usually cannot absorb all the water immediately, thus weakening the soil which was previously frozen.

Typically, unstable heat is formed on the surface of the foundation, which is the necessary condition for alternating change of soil and ice, accompanied by generation and growth of ice-crystal nucleus, as shown in Figure 1. The reason is that the rate of heat outflow exceeds the rate of heat supply. The melting process conducts from top to bottom and/or upwardly from the bottom. During accelerated melting process in spring, the melting is almost entirely from the ground surface to the bottom.Because of the presence of the lower frozen soil, downward drainage is prevented, thus melting occurs only along the side and ground surface. Figure 1 shows the freezing process of seasonal soil in the lower part of a pavement structure (Andersland and Ladanyi, 2011). During the process of freezing, with the formation of ice, the volume of the soil-water system increases and/or the phenomenon of soil consolidation appear. When water migrates to the freeze-thaw line and freezes, the volume growth amount is greater than the increase in density.

Figure 1 Freezing process of seasonal soil in the lower part of a pavement structure

3 Physical properties due to freeze-thaw cycle

Permafrost is a four-phase material and its physical properties are more complex compared to ordinary soil. The sensitivity to temperature of permafrost leads to soil disturbance in a stable state in cold regions engineering construction. Periodic temperature changes of the air leads to the occurrence of frost heave and thaw in the permafrost foundation. The soils physical properties will change during the freeze/thaw process and repeated freeze-thaw cycles change soil traits, thus the soil develops from an unstable state to a new dynamic equilibrium state. In a closed system, the physical properties, such as moisture content and dry density, are influenced by freeze-thaw cycles and there is a balance of freeze-thaw cycles, that is when the freeze-thaw cycles exceed it, the properties of the soil reach a stable and equilibrium state. In a closed system, the freeze-thaw cycles redistribute the soil water. Previous research has found that for freezing and thawing under an open system, the maximum volume changes were as high as 30% under constant loading.

Bingand He (2009) studied the influence of freeze-thaw cycles on physical and mechanical properties of salty soil. During the test, Lanzhou loess is prepared with different water contents and salt content for exposure to a maximum of ten closed-system freeze-thaw cycles. Results indicate that with the freeze-thaw cycles, salty soil has no yielding capacity, thus it becomes brittle and fractures after reaching the ultimate stress load. Also, the soil is strengthened and weakened due to the mixture of sodium sulfate; and to achieve a new dynamic equilibrium of the loess with sodium sulfate, six freeze-thaw cycles are required.

Wanget al.(2005) researched the physical property changes of Qinghai-Tibet clay to cyclic freezing and thawing. In the test,fine-grained clay was compacted in the laboratory and was thereafter exposed to a maximum of 21 closed-system freezing and thawing cycles. The height and water content of the sample were measured in unfrozen soil as well as in thawed soil. Results show that the height of the studied sample increased and water content decreased before seven freezing/thawing cycles and remained invariable afterwards.

Yanget al.(2003) studied the influence of freezing and thawing on dry density and water content of soils through the testing method. Under an open system environment, sandy clay and sandy loam, which are the two typical kinds of soils along the Qinghai-Tibet Railway, are tested in repeated freezing and thawing circulation experiments. All the soil samples are tested at different dry density, water content and temperature in these experiments. The magnitude of the dry density of soils is an important factor when measuring the frost heave and subsidence amounts during the freeze-thaw test. Numerous tests have shown that frost heave and thaw settlement of soil have a good relationship with dry density. In an open system with replenishment, repeated freezing and thawing is the process by which density, water and stress fields are rearranged. Soils with a larger density will become loose and have a lower density under the freezing and thawing cycles, while soils with a smaller density will become denser and settlement during the melting process is greater than the frost heaving amount at the beginning of the test.For soils with large or small density, the final settlement will be equal to the frost heaving amount, and the soil reaches a new dynamic equilibrium state after repeated freeze-thaw cycles, then the dry density of soil will not change with the different freeze-thaw cycles.

The results from Yanget al. (2003) also indicate that re-peated freezing and thawing circulation can change the soil water content. After the repeated freezing and thawing circulation,soil water content will increase and the water content at the top part is greater than that at the bottom part due to water migration from bottom to top. Water content distribution is different between soils in freezing state and in thawing state. The water content at the top of the freeze-thaw zone in freezing state is greater than in the thawing state, while the water content at the bottom of freeze-thaw zone in thawing state is greater than in the freezing state. Also, the water content at the freezing front in thawing state is greater than in the freezing state.

Suet al.(2008) conducted tests on physical properties of Qinghai-Tibet slope clay under freeze-thaw cycles. The liquid limit of the soil in the test is 31.52%, while the plastic limit is about 16.08%. Results indicate that sample water is re-distributed during the freeze-thaw cycles in the closed system and the water content decreases with an increase of the freeze-thaw cycles, and then reaches a steady state after 10 freeze-thaw cycles. The greater the initial water content, the greater the variation rate of water content. For different initial water content, there is different stable water content and the final equilibrium state is influenced by the initial water content. For soils with natural water content of 13.21% under 10 freeze-thaw cycles, the saturated water content is about 16.32%.

Test results from Suet al.(2008) also indicate that soil with small initial density will become dense and reach a stable state,while soil with great initial density will become loose and reach a stable state. The density of soil reaches a stable state after 10 freeze-thaw cycles for both the loose and dense soils. The final density that the soil reaches is related to the types of soil and not to the initial density.

There are also other experiments that studied the influence of freeze-thaw cycles. Biet al. (2010) experimented on the impact of freeze-thaw cycle on physical properties of loess. Test result shows that repeated freeze-thaw actions will gradually increase the water content in samples from bottom to top. The water content changed sharply near the interface between frozen and unfrozen areas. At the start of the freeze-thaw cycle, frost heave of loess samples is stronger. Then the overall deformation became stable,which meant that deformation induced by frost heave was equal to that induced by thaw settlement. At the end of the freeze-thaw cycle, there was a small settlement in samples, probably from soil particle rearrangement caused by strong freezing and thawing action. In addition, freezing and thawing actions gradually decreased the dry density of the loess sample. The dry density in the upper part was larger than in the bottom part.

Ice crystals occur on the soil surface conducted by low temperature and it will destroy the original phase equilibrium, causing the internal moisture to migrate to the freezing front and freeze. This causes an increase of water content on the surface,ice crystals precipitated on the surface until the soil is completely frozen, and the volume expands. The surface also began to melt,influenced by heated air during the melting procedure of soil.Internal temperature is always lower than surface temperature,thus water migrates from the surface inward. As the thawing time is less than the freezing time, the amount of water migrating internally during the melting procedure is less than migrating to the surface during the freezing procedure.

For soils with small dry density, the porosity is great and particle cementation is weak. Some frost-heave capacity occurs during the first few freezing procedures. Soil settlement appears during melting while part of the pore water discharges and the total thaw-settlement capacity is larger than the total frost-heave capacity, producing a denser soil. The thaw-settlement capacity is equal to the frost-heave capacity and dry density reaches a stable state after repeated freeze-thaw cycles. For soils with large dry density, it will become loose and the dry density decreases and reaches a stable state after repeated freeze-thaw cycles. Particle cementation is strong, with the ability to resist thaw-settlement. The thaw-settlement capacity is smaller than the frost-heave capacity during the first few freezing procedures and the soil expands. The water content reaches a steady state and the frost-heave capacity decreases which make the frost-heave capacity and the thaw-settlement capacity equal after repeated freeze-thaw cycles.

4 Mechanical properties due to freeze-thaw cycle

In cold regions engineering construction, the impact of seasonal freezing and thawing process on soil strength should be considered. The process of freezing and thawing of the soil is actually the freezing and thawing process of water. When soil water freezes, the volume expands, so the pore size between the soil particles increases, and when the ice melts, the pore size decreases. Thus, the freezing process will change the structure coupled between soil particles, which will change the mechanical properties of the soil.

Christopher S and Christopher G (1998) studied freeze-thaw effects on Boston blue clay. Undisturbed samples of natural Boston blue clay (BBC) and BBC subjected to one freeze-thaw cycle were studied to investigate the effects of freezing-thawing on BBC. Results indicate that both soils had similar direct shear strength, but there was a significant decrease in undrained shear strength with freezing. Engineering properties of BBC are significantly changed with one cycle of freeze-thaw and this change in BBC behavior may affect geotechnical designs.

Wanget al. (2012) conducted experimental studies on red clay soil in Wuhan, China under freeze-thaw cycle conditions with different water contents. Results indicate that the freeze-thaw action has important influence on the strength of red clay soil. The cohesion and angle of internal friction without freeze-thaw cycle are 29.8 kPa and 25.25°, respectively. After 10 freeze-thaw cycles, they changed to 37.9 kPa and 27.01°, respectively. The reason may be that after freeze-thaw cycles, soil weight loss occurs and the cohesion, and angle of internal friction increase with water loss.

The study from Wanget al. (2012) also indicates that water loss is great and leads to a great strength with an increase of freeze-thaw cycles when the water content is lower than 19.43%,while the strength loss is greater with an increase of freeze-thaw cycles when the water content is greater than 19.43%. After 10 freeze-thaw cycles, the unconfined compressive strength is 12.6 kPa and the strength ratio is only about 7.44%.

Wanget al. (2005) researched the mechanical property changes of Qinghai-Tibet clay to cyclic freezing and thawing. In the test, fine-grained clay was compacted in the laboratory and was thereafter exposed to a maximum of 21 closed-system freezing and thawing cycles. Results indicate that the elastic modulus decrease firstly and then increase with increasing number of freeze-thaw cycles. The first freeze-thaw cycle had the largest influence on the soil modulus and the reduction rate of the elastic modulus is the greatest. The reduction amplitude of the elastic modulus is about 18% to 27%. Almost all the elastic modulus of samples reached a minimum after seven freeze-thaw cycles.

For shear strength, the cohesion decreases with increasing number of freeze-thaw cycles. This indicates that the distance between soil particles increases after every freeze-thaw cycle.The angle of internal friction changed between 15° to 30° after 0 to 21 freeze-thaw cycles and had no regularity.

Suet al.(2008) conducted tests on mechanical properties of Qinghai-Tibet slope clay under freeze-thaw cycles. The liquid limit of the soil in the test is 31.52%, while the plastic limit is about 16.08%. Results show that the change law of shear strength is different during the freeze-thaw cycles for the soil with different initial conditions. For soils with small density, the shear strength increases with an increase of freeze-thaw cycles,while the shear strength for dense soils decreases.

Cohesion and angle of internal friction are two indicators to determine soil shear strength. For the same type of soil, both cohesion and angle of internal friction are constant under the same testing conditions. The angle of internal friction mainly reflects the mutual movement and occlusion effect between the particles, while the cohesion reflects the various physical and chemical interactions between the particles, such as Coulomb force, van der Waals force, cementation force, whose magnitude is determined by distance and the cementation force of the cementing material between the particles. During the freezing procedure,the pore volume increases. When the ice melts, the pore cannot restore to its original state, which will make the soil become loose and cementation force between the particles decreases.

5 Significance and interaction of different factors under freeze-thaw cycles

The physical-mechanical properties of frozen and unfrozen soil depend not only on a single factor, but also on the possible interaction between different factors. For example, the freezing temperature and freeze-thaw cycles may have an interaction for dry density, elastic modulus, and shear strength. The method of significance test can also be used to measure the influence on soil properties for some factors, such as pressure, freezing temperature, freeze-thaw cycles, and moisture content.

Liet al. (2012) studied the significance and interaction of factors on mechanical properties of frozen soil. Factors such as water content, temperature, salt content, confining pressure, and strain rate, as well as the possible interaction between them had been considered in the research. Results show that these factors obviously influenced the mechanical properties of frozen soil,and temperature is the most important factor. Also, the interactions on strength, which originated between temperature, water content and strain rate were found to be important and cannot be neglected. Thus, the most appropriate method is to study overall factors with the interaction taken into consideration. Thus, it is necessary to consider the interaction of different factors for soils under freeze-thaw cycles.

6 Conclusions

The freeze-thaw action has significant influence on the physical-mechanical properties of soil. Under an open system environment, soils with larger density will become loose and have a lower density while soils with smaller density will become denser and the soil water content will increase under the freezing and thawing cycles. Both densities will reach a stable state after repeated cycles. Also, the modulus, cohesion and angle of internal friction of the soil will obviously change under the freeze-thaw cycles. Significance and interaction of different factors under freeze-thaw cycles may have remarkable influence on the soil properties and it should be taken into account in theoretical and experimental research.

The authors wish to acknowledge the support and motivation provided by National 973 Project of China (No.2012CB026104) and the Fundamental Research Funds for the Central Universities (No. 2011JBZ009).

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