Effects of freeze-thaw cycle on permeability and compression properties of aeolian soil-bentonite mixture

： Since all the soil materials within 200 kilometers of the proposed dam site are aeolian sedimentary soil, 9 the permeability is not in line with the engineering performance of the core dam materials. Moreover, in the 10 construction of a large core dam in a high altitude area, the core wall dam materials will be subjected to repeated 11 freeze-thaw action during the winter construction of the dam, resulting in changes in the particle distribution and 12 particle connection inside the filled dam materials. Thus, the engineering properties and filling quality of dam 13 materials are changed. Therefore, in this paper, two types of bentonite were used to improve the existing aeolian 14 soil material, and the aeolian soil-Na/Ca-bentonite mixture (ANB/ACB) with different bentonite content (0%, 2%, 15 4%, 8%, 16%, 32%) was subjected to different freezing through particle separation test, variable head permeability 16 test and consolidation compression test. The influence of the number of thawing cycles (0, 1, 5, 10) on the 17 permeability and compression properties was studied. The test results show that: (1) the increase of bentonite content, 18 the inhomogeneity coefficient, curvature coefficient and clay content of ANB/ACB increase gradually, and the 19 inhomogeneity coefficient, curvature coefficient and clay


Introduction
With the rapid development of China's economy and its increasing energy demand, it is planned to build a large core wall dam in a high-altitude area in the western region.After detailed geological exploration, it is found that there is aeolian sedimentary soil within 200 kilometers from the proposed site of the dam, and its permeability coefficient is greater than 10 -5 cm/s, which cannot meet the permeability requirements of the dam core wall materials (Jafarzadeh et al.2009).It is necessary to improve the eolian soil materials to meet the seepage prevention requirements of the dam core wall materials.In addition, due to that the low temperature of the proposed site in winter will reach -12 ℃, it is necessary to study the influence of different types of bentonite on the improvement of aeolian soil and the freeze-thaw cycle on the permeability and compression properties of aeolian soil-bentonite mixed soil for the construction of large core wall dam in this high altitude cold area.
At present, the use of bentonite to improve the permeability, compression strength of different types of soil is mainly concentrated in the construction of cutoff wall engineering (Malusis et al.2008, Malusis et al.2009, Hong et al.2012, Britton et al.2004, and Malusis et al.2001).Abeele et al. (1986) studied the effects of bentonite content on the consolidation coefficient, compression index, expansion coefficient and permeability coefficient of a sandy siltbentonite mixture, and the results showed that the consolidation coefficient of sandy silt-bentonite mixture decreased with the increase of the square of bentonite content, while its compression index, expansion index, and permeability coefficient increased with the increase of bentonite content.Kenney et al.(1992) studied the permeability of the compacted sand-bentonite mixture and found that the low permeability of the sand-bentonite mixture mainly depended on the continuity of the bentonite matrix in the mixture and the salt solution also had a great influence on the permeability of the sand-bentonite mixture.Kaoser et al.(2006) discussed the influence of internal erosion of the mixture on the permeability coefficient under a certain hydraulic gradient.It was found that the internal erosion of the mixture was mainly affected by the porosity, and the porosity could be reduced by the appropriate selection of the sand grain size distribution and bentonite content.David and Charles (2009) found that the permeability coefficient of the sand-bentonite mixture was highly correlated with the porosity ratio of bentonite.Watabe et al.(2011) studied the effect of the ratio of bentonite to sand on the permeability and compressibility of sand-bentonite through a series of incremental loading tests and microscopic tests.The results showed that adding sand could reduce compressibility, but did not affect permeability under the same consolidation pressure.The addition of bentonite helped to reduce the overall permeability coefficient of the mixed soil.Wang et al.(2013) characterized the unsaturated hydraulic properties of the sand-bentonite mixture through a series of tests including water retention test, permeability test, and microstructure observation.It was found that under constant volume conditions, with the decrease of suction, the water conductivity decreased first and then increased.When the suction was greater than 12.6 MPa, hydration caused the clay particles to flake off, leading to the blockage of large pores.
On the contrary, when the suction was close to saturation (suction below 4.2 MPa), the number of macropores increased due to the formation of two-dimensional pores.In the process of hydration, the variation trend of soil permeability coefficient was the same as that of macroporosity.Mukherjee and Mishra. (2017) GCL was used to reduce the permeability coefficient of the sand-bentonite mixture to improve the impermeability of the sandbentonite mixture.Mukherjee and Mishra. (2019) studied the addition of tire fiber as a reinforcing material in the sand-bentonite mixture, and found that with the increase of tire fiber, the permeability coefficient of the mixture containing more bentonite increased significantly and the effective strength of the mixture also increased.Gupt et al.(2021) found that no matter the compacted state and the amount of fly ash, the UCS of fly-bentonite mixed soil increased compared with pure bentonite.Lu et al. (2022) studied the effect of sand particle content and particle size on the permeability of bentonite-sand mixed soil, and the results showed that under the condition of the same sand particle content, the particle size of sand had no significant influence on the permeability area and permeability path length of the mixed soil.With the in-depth study of soil-bentonite mixed soils, many scholars have established a prediction model for the permeability coefficient of mixed soils from the aspects of specific surface area, pore ratio, basic physical properties and compressibility (Chapuis RP 1990, Sällfors and Öberg-Högsta 2002, Chapuis RP 2004, Hansen 2004, Yeo et al.2005, Fan et al.2014, and Khalid et al.2023).
On the other hand, most of the research on the effect of the freeze-thaw cycle on fine-grained soil is focused on a single fine-grained soil.Edwin J. et al. (1979) studied the permeability of four kinds of fine-grained soil after the freeze-thaw cycle, and found that the freeze-thaw cycle effect would lead to a decrease in the porosity ratio of soil and an increase in permeability.Chamberlain et al.(1990Chamberlain et al.( ,1991) ) and Viklander et al.(1997Viklander et al.( ,1998) ) found through research that freeze-thaw would improve the permeability of fine-grained soil.The first freeze-thaw cycle has the greatest influence on the permeability, and then with the increase in the number of freeze-thaw cycles, the soil permeability will gradually become stable.At the same time, it is found that the freezing and melting process will lead to a decrease in the porosity ratio of the relatively loose soil and the compaction of the soil.But for more dense soil, the effect of the freeze-thaw cycle on the density of soil is the opposite.The concept of residual pore ratio considering the freeze-thaw cycle is put forward.Wang et al.(2007) measured the height, water content and mechanical properties of samples before and after freeze-thaw and found that: Before the 7th to 10th freeze-thaw cycle, the height of samples gradually increased and the water content decreased, but after the 7th freeze-thaw cycle, the height and water content of samples remained unchanged.The freeze-thaw effect has little influence on the stress-strain relationship of soil but has a great influence on the modulus and strength of the soil.Chang et al.(2013) found through research that after the freeze-thaw cycle, agglomeration occurs among fine particles of soil, resulting in a gradually smaller plastic index of soil, and the change of soil pores caused by the freeze-thaw effect leads to the increase of soil permeability.Tian et al.(2015) studied the effects of freeze-thaw cycles and water content changes on the compression characteristics of loess, and the results showed that the change in compression coefficient was positively correlated with water content and the number of freeze-thaw cycles, while the change of compression modulus was inversely correlated with water content and the number of freeze-thaw cycles, and the compressibility was enhanced.Ozgan et al.(2015) used the triaxial test and consolidation test to determine the influence of the freeze-thaw cycle on soil consolidation , and determined that after a certain number of freeze-thaw cycles, the consolidation time point would increase to a fixed value and the mechanical properties would gradually stabilize after a certain number of freeze-thaw cycles.Cui et al.(2017) took silty sandy soil in the seasonal frozen soil area as the research object, conducted freeze-thaw cycle tests and consolidation compression tests with water content, compaction degree and number of freeze-thaw cycles as the control variables, and performed feasibility analysis and summary of the test results and rules.It is concluded that the compression deformation of silty sand samples tends to be stable after repeated freezing and thawing 10 times.When the freeze-thaw cycle is 15 times, the compressive deformation rate increases with the increase of consolidation pressure.Under the same water content, the compressive deformation rate decreases with the increase of the degree of compaction, and the compressive deformation rate increases with the increase of the water content under the same degree of compaction.
The compression modulus decreases with the increase of water content, increases with the increase of compaction degree and freeze-thaw cycles.Li et al.(2018) concluded through laboratory tests that the unconfined compressive strength, cohesion and elastic modulus of loess decreased with the increase in the number of freeze-thaw cycles.Wei et al.(2019) conducted an unconfined compressive strength test on loess under repeated freeze-thaw action and pointed out that after 10 freeze-thaw cycles, the strength of the loess was reduced by about 30%-50%.Zhao et al.(2020) conducted a triaxial consolidation permeability test on loess to explore the influence of the freeze-thaw cycle on its permeability coefficient.The results show that, there is a strong negative linear correlation between porosity and confining pressure in undisturbed and remodeled loess, and the permeability coefficient shows a typical exponential attenuation with the increase of confining pressure, and the permeability coefficient and porosity have a similar change trend, and the change of soil porosity is the main reason for the change of permeability properties.
In summary, most scholars study the effect of freeze-thaw on fine-grained soil mainly on single fine-grained soil, but there are few studies on mixed soil, and the related studies on the effect of freeze-thaw effect on the permeability and compression properties of aeolian soil-bentonite have not been fully carried out.Therefore, based on previous research experience, this paper designed a test scheme with different bentonite types, bentonite content and freeze-thaw cycles as influencing indexes.The effects of freeze-thaw cycles on the permeability and compression properties of aeolian and bentonite mixed soils are obtained through permeability and compression tests, which is of great significance to the construction safety and cost savings of large core dams located in the high altitude seasonal freezing areas.

Test Materials
The samples of aeolian soil were extracted from a high-altitude area in southwest China.The soil is yellow and belongs to the soil group formed by weathering and denudation of sedimentary rocks.The aeolian soil sample was dried and crushed through a 2mm screen.Only the aeolian soil that passed the 2mm screen was taken as the sample.
To explore the difference in the improvement effect of different types of bentonite on the permeability and compression properties of aeolian soil, two types of bentonite, Na-bentonite and Ca-bentonite, were used as admixtures.Firstly, the mineral compositions of aeolian soil, Na-bentonite and Ca-bentonite were determined by Xray diffraction (XRD).The results of the three tests are shown in Fig. 1.Then, the results of XRD were analyzed, as shown in Table 1.The physical property parameters of aeolian soil and two kinds of bentonite are shown in Table 2, and the grading curves are shown in Fig. 2.   1 shows that the main mineral components of the aeolian soil used in the test are quartz, albitite, muscovite and illite whose percentage contents are 46.3%, 22.4%,20.5% and 10.8%, respectively.Because X-ray diffraction analysis is difficult to reflect the content of less than 5 percent of the secondary minerals, so its low content of clay minerals are basically ignored.The two bentonites are mainly composed of clay minerals such as montmorillonite and illite, and the main mineral composition of the two is different in that the main mineral composition of the Na-bentonite contains albiar and does not contain calcite while the Ca-bentonite is the opposite, and the proportion of clay minerals of the Ca-bentonite is greater than that of the Na-bentonite.

Method for preparing samples
The permeability and compression properties of the mixture of aeolian soil-Na/Ca-bentonite under the freezethaw cycle were studied through permeability and compression tests.According to the results of the compaction test(see Fig. 3), the aeolian soil-Na-bentonite mixture (ANB) and aeolian soil-Ca-bentonite mixture (ACB) with the mass percentage of 0%, 2%, 4%, 8%, 16% and 32% of bentonite were prepared according to the dry density of 1.55g/cm 3 and then were mixed evenly.After the sample is evenly mixed, the sample is then prepared to the predetermined moisture content by spraying water in a small watering can, and the moisture content of the sample is prepared by 26.3%.Then, the container containing the sample mixed soil material is sealed with plastic wrap and left for at least 24 hours to make the water completely mixed with the soil material.The penetration sample with a diameter of 61.8 mm and a height of 40mm and the compression sample with a diameter of 61.8mm and a height of 20mm were prepared by compression method.Finally, the prepared samples for the permeability and the compression tests were saturated with air and left for 24 hours.Fig. 3 Compaction curve of aeolian soil

Freeze-thaw cycles test
The saturated sample is sealed with the plastic wrap, and then the sample is placed in the freeze-thaw machine for a closed freeze-thaw cycle test (see Fig. 4).Regarding the annual maximum and minimum temperature distribution map of the project site, the freezing temperature is set at -10℃ and the melting temperature is set at 20℃.During the freezing process, the temperature in the freeze-thaw machine quickly drops from 20 ° C to -10 ° C, and then remains stable for 12 hours; during the thawing process, the temperature rises rapidly from -10 ° C to 20 ° C for 12 hours, which is a freeze-thaw cycle.The number of freeze-thaw cycles of geomaterials in the construction process is generally not more than 10 times, so four groups of freeze-thaw cycles of 0, 1, 5 and 10 times are set.

Fig.4
Freeze-thaw process diagram of mixed soil materials

Permeability tests
In this experiment, an automatic soil permeation instrument was used to conduct variable head permeation tests on the mixed soil samples of aeolian soil-Na/Ca-bentonite containing different bentonite types, different bentonite content and different freeze-thaw cycles.According to ASTM D2434 , before loading the intended sample, the inner wall of the container sleeve is coated with petroleum jelly, and then the ring knife containing the sample is pushed into the sleeve and pressed into the water seal, and the screw is tightened to prevent air leakage.Then the exhaust valve is turned on to remove the air from the bottom of the permeating container until there are no bubbles in the overflow water and we close the air valve.Subsequently, the sample is allowed to stand for some time under the action of a certain water head, and when there is water overflow at the outlet of the container, the test and determination begin.

Compression tests
In this experiment, an automatic pneumatic consolidation instrument was used to determine the compression index of the mixed soil samples with different bentonite types, different bentonite content and different freeze-thaw cycles by consolidation test.According to ASTM D2435 (2020), the vertical stresses of 50, 100, 200, 400, 800, and 1600kPa are applied to the mixed soil sample step by step, and the pressure at all levels is maintained for 24 hours.
The vertical deformation of the sample is measured by the dial at the top of the sample.According to the relationship between the deformation and the normal pressure obtained by the test, the e-logσ' curve is drawn to obtain the parameters that can reflect the compressibility of the mixed soil.The particle size distribution of soil is closely related to the engineering performance of soil such as permeability and compression characteristics.It can be seen from Fig. 5 and 6 that with the continuous incorporation of bentonite, its influence on the composition of particles in the mixed soil is very obvious.Since bentonite is dominated by clay particles, the continuous incorporation of Na/ Ca-based bentonite into the aeolian soil can significantly increase the clay content of its mixed soil.Since the clay content of the Ca-based bentonite is higher than that of the Na-based bentonite under the same mass, the clay content of ACB is higher than that of ANB under the same bentonite content.The inhomogeneity coefficient and curvature coefficient of ACB are greater than that of ANB, mainly because the fine particle content of Ca-based bentonite is greater than that of Na-based bentonite.

Results of particle size tests
As a result, under the same bentonite content, the increased ratio of fine particle content in ACB is greater than that of ANB, resulting in a rapid increase in particle size difference and particle contact area of ACB.

Results of permeability tests
Fig. 7 shows the variation curve of the permeability coefficient of ANB with the content of Na-bentonite under different freeze-thaw cycles.It can be seen from Fig. 7 that with the increase of the content of Na bentonite, the permeability coefficient of the soil sample gradually decreases, and the decline rate also gradually decreases with the increase of the content of Na-bentonite, and the decline rate inflection point occurs at about 10% of the content of Na-bentonite.Fig. 8 shows the variation curve of the permeability coefficient of ANB with the number of freezethaw cycles under the condition of different Na-bentonite content.It can be seen from Fig. 8 that with the increase in the number of freeze-thaw cycles, the permeability coefficient of the mixed soil sample under different dosages gradually increases and the increasing trend gradually tends to be flat.

Fig. 7
The relationship between the permeability coefficient of aeolian soil-Na bentonite mixture and bentonite content under different freeze-thaw conditions Fig. 8 The relationship between the permeability coefficient of aeolian soil-Na bentonite mixed soil and freezethaw cycles under different bentonite contents The permeability of soil is closely related to the particle size distribution of soil particles and the pores formed by the particle contact.Due to the incorporation of Na-bentonite, the clay content of ANB increases significantly, and the pores inside the soil gradually change from the original pores formed by the contact of silt particles to the pores formed by the contact of silt particles and clay particles.In addition, the large pores formed between soils are gradually filled by clay particles, resulting in a rapid decline in the overall porosity of the mixed soil.Therefore, with the gradual increase of the content of Na-bentonite, the permeability coefficient decreases rapidly.However, when the bentonite content in the mixed soil reaches a certain extent, the bulk density of the soil particles will approach the critical value because the macropores of the mixed soil are filled with clay particles and most of the pores in the mixed soil are formed by the contact between silt and clay particles and clay particles.At this time, the continuous addition of bentonite has a very limited effect on reducing the permeability of the soil mixture compared with the low content.
When ANB is subjected to a freeze-thaw cycle, the most direct influence on its particle size is the internal pore type.The essence of the influence of the freeze-thaw effect on soil is that the water migration and the outwash phase transformation between the internal pores of soil during the freezing and thawing process result in the change of the size, distribution and type of the internal pores of soil and the agglomeration and decomposition of soil particles, thus affecting the overall permeability of soil.Generally speaking, when the soil is frozen at a negative temperature, as the temperature of the soil is lower than the freezing point of the pore water, the liquid water in the internal pores of the soil will gradually change into solid ice.At the same time, under the driving force of water migration, the unfrozen water in the soil will gradually migrate to the cold end of the soil and converge in the pores to form large pore ice, resulting in the expansion of the pores of the soil by solid ice.When the soil temperature rises, the pore ice in the soil pores will also melt into a large amount of pore water with the increase of temperature.Previously, the frost heave force generated by the formation of a large amount of pore ice will destroy and expand the pores to form connected micro-cracks.Meanwhile, although the freeze-thaw cycle makes the soil melt and compact, with the progress of the freeze-thaw cycle, the free water between the particles repeatedly freezes and melts, affecting the particle and pore structure, and the weak bound water also changes correspondingly under the influence, resulting in the reduction of the thickness of the bound water film on the surface of the bentonite particles in the mixed soil.The resistance of free water flowing between soil particles is reduced, increasing the soil permeability coefficient.
Fig. 9 shows the variation curve of the permeability coefficient of ACB with the content of Ca-bentonite under different freeze-thaw cycles.It can be seen from Fig. 9 that whether ACB goes through the freeze-thaw cycle or not, its permeability coefficient gradually decreases with the increase of Ca-bentonite content, and the decline rate shows an exponential trend of decreasing with the increase of Ca-bentonite content.At the same time, it can also be obviously seen that with the increase of freeze-thaw cycles at low bentonite content, the relationship curve between the permeability coefficient and bentonite content moves upward gradually.However, when the bentonite content is high, the curve shows a gradual downward movement, which is opposite to that when the bentonite content is low, and the turning point of the trend is about 3.7%.Fig. 10 shows the variation curve of the permeability coefficient of ACB with the number of freeze-thaw cycles under different Ca-bentonite contents.Fig. 10 clearly shows that the permeability coefficient of ACB varies with the number of freeze-thaw cycles under different bentonite content levels.When the bentonite content is low (less than 3.7%), the permeability coefficient of the mixed soil gradually increases with the increase of the number of freeze-thaw cycles, and the increasing range gradually tends to be flat.
However, when the bentonite content is high (about more than 3.7%), the permeability coefficient of the mixed soil gradually decreases with the increase of the number of freeze-thaw cycles, and the decrease rate also decreases with the increase of freeze-thaw cycles.
Fig. 9 The relationships between the permeability coefficient of aeolian soil-Ca bentonite mixture and bentonite content under different freeze-thaw conditions Fig. 10 The relationships between the permeability coefficient of aeolian soil-Ca bentonite mixed soil and freezethaw times under different bentonite content The permeability coefficient of ACB also gradually decreases with the increase of Ca-bentonite content.The main reason is that with the continuous incorporation of Ca-bentonite, the clay content in the soil material gradually increases, resulting in the gradual filling of pores by fine particles, which rapidly reduces the overall porosity of the soil.In addition, since the hydration degree of Ca-bentonite in the mixed soil is lower than that of Na-bentonite, the Ca-bentonite that is not fully hydrated is easy to form a Ca-film covering the surface of the soil particles to form a water barrier after absorbing water, which also leads to the decrease of the permeability of ACB.
It can be seen from Fig. 9 and 10 that the permeability coefficient of ACB varies with the number of freezethaw cycles and the bentonite content to different degrees.This is mainly because under the condition of low Cabentonite content, the overall permeability and freeze-thaw sensitivity of soil are dominated by the original aeolian soil, and when the bentonite content reaches a certain level, most of the pores in the mixed soil are filled by Cabentonite.At this time, the overall permeability and freeze-thaw sensitivity of soil begins to be dominated by Cabentonite.This results in the differentiation of the permeability of the mixture in response to the freeze-thaw effect with different levels of Ca bentonite content.
Fig. 11 shows the contrast curve of the permeability coefficient of ANB and ACB with bentonite content under different freeze-thaw cycles.It can be seen from the comparison figure that the permeability coefficient of ANB is smaller than that of ACB under the same freeze-thaw cycle condition when the content is less than 4.5%.However, with the continuous increase of bentonite content, the permeability coefficient of ACB is lower than that of ANB, which indicates that Ca-bentonite has a better improvement effect on anti-seepage performance.It can also be found from the comparison figure that the permeability coefficient of the two different mixed soil samples increases gradually with the increase of freeze-thaw cycles at a lower bentonite content; however, when the bentonite content is higher, the permeability coefficient of the two soils reflects the freeze-thaw effect completely.That is, the permeability coefficient of ACB decreases with the increase of freeze-thaw times.The permeability coefficient of ANB increases with the increase of freezing and thawing cycles.The curve relationship between the two shows an "X" type differentiation.Under different freeze-thaw cycle conditions, the relationship curves between the permeability coefficients of ANB and ACB and the bentonite content show an "X" type differentiation.The main reasons are as follows: Firstly, at low bentonite content, the hydration degree of Na-bentonite is higher than that of Ca-bentonite, and the clay particles of Na-bentonite rapidly diffuse and fill the internal pores of the soil, while Ca-bentonite is not fully hydrated.The diffusion filling speed is much lower than that of Na-bentonite and the calcium film formed by incomplete hydration of Ca-bentonite has a smaller coverage area due to its low content.As a result, the permeability coefficient of ANB is lower than that of ACB under the condition of the same bentonite content and the same number of freeze-thaw cycles, that is when the content of bentonite is low.ANB has better anti-seepage performance than ACB.Second, when the bentonite content is low, the permeability of the two mixtures reflects the same freeze-thaw effect, and the permeability coefficient increases with the increase of freeze-thaw cycles.This is mainly because when the bentonite content is low, the influence of the freeze-thaw effect on the permeability of mixed soil is dominated by the aeolian soil in the mixed soil.Third, when the bentonite content is high, the permeability coefficient of ANB and ACB has an opposite trend with the changing trend of freeze-thaw cycles.This is due to the complete hydration of the Na-bentonite in ANB, which quickly integrates with the aeolian soil and mixes to form fine-grained soil with a high clay content.In the process of the freeze-thaw cycle, the internal pore structure of this kind of fine-grained soil gradually expands and breaks down to form micro-cracks with the progress of the freezethaw cycle, resulting in the permeability coefficient of ANB increasing with the increase of freeze-thaw cycle.
However, the Ca-bentonite in ACB has a low hydration degree when it has not undergone the freeze-thaw cycle.
With the alternating process of freezing and melting, the hydration degree of the calcium-based bentonite filled in the pores of ACB is further improved, and its clay particles can better expand and diffuse into various pores of the soil.In addition, due to the improvement of the hydration degree of Ca-bentonite, the calcium film formed by Cabentonite also gradually expands and extends to cover a larger range, playing a better role in water insulation, which led to the permeability coefficient of ACB gradually decreases with the increase of the number of freeze-thaw cycles.different bentonite content conditions when the number of freeze-thaw cycles is 0, 1, 5 and 10 respectively.It can be seen from Fig. 14 that, regardless of the number of freeze-thawing cycles, the compression coefficients of the two mixed soil materials show a gradual trend of decline with the increase of normal pressure, and the decline rate becomes smaller and smaller.When the normal stress is low, the compression coefficient of ACB is greater than that of ANB for low bentonite content, while that of ACB is smaller than that of ANB for high bentonite content.

Results of Consolidation Tests
When the normal stress is large, the compression coefficient of ACB will gradually be greater than that of ANB.At When the two kinds of mixed soil are compressed under the normal stress, all kinds of pores in the soil will be compressed gradually with the gradual increase of normal stress.As a result, the overall porosity of the soil decreases rapidly and the contact between soil particles becomes closer.As a result, the overall compactness of the soil is greatly improved, that is, the soil is substantially compacted under a high normal stress.The whole body is in a state of high density, and its compressibility is greatly attenuated.However, when the normal stress on the soil increases to a certain extent, it is difficult for the internal particles to continue to mismove and occlude to further compress the soil.Therefore, the compression coefficient of the two mixed soil materials gradually decreases with the increase of the normal stress under high normal stress state.
Under the condition of low normal stress, the particles slip inside the soil, and the thickness of water film between the particles and the size of pores have a dominant influence on the compressibility of the soil.Since the hydration degree of Ca-bentonite is smaller than that of Na-bentonite, the proportion of hydration filling internal pores of the bentonite particles of ACB is smaller than that of ANB when the bentonite content is low.As a result, under the same number of freeze-thaw cycles, the porosity of ACB is slightly greater than that of ANB at low bentonite content, that is, its compressibility is greater than that of ANB at this time.When the bentonite content reaches a certain level, although the hydration degree of Ca-bentonite is lower than that of Na-bentonite, the clay content of ACB is greater than that of ANB under the same mass, and the proportion of clay filled into the pores formed by the contact of aeolian silt particles is greater than that of ANB.As a result, under the condition of low stress and high bentonite content, the compactness of ACB is greater than that of the ANB, that is, the compressibility of ACB is less than that of ANB.Under the condition of high normal stress, the main factors that determine the compressibility of soil are particle dislocation, filling and pore compression transformation stress level and mechanical properties of particles themselves.As the clay content of Ca-bentonite is larger, the clay has a lamellar structure and stronger compressibility, resulting in stronger overall compressibility, and the compression coefficient of ACB is greater than that of ANB.under different freeze-thaw cycles when bentonite content is 2%, 4%, 8%, 16% and 32% respectively.It can be clearly seen from Fig. 15 that, regardless of normal pressure, the compression coefficient of the two mixtures presents a gradual upward trend with the number of freeze-thaw cycles, and the greater the normal pressure, the smaller the influence of the freeze-thaw cycle effect on the compression coefficient.Meanwhile, with the increase in the number of freeze-thaw cycles, no matter how much bentonite content, the compression coefficient of ACB and ANB showed a trend of gradually approaching.
(a) B=2% Fig. 15 The relationship between compression coefficient and normal stress under different freeze-thaw cycles In the process of freeze-thawing, the soil particles mismove, the pore volume expands, and the connection between clay particles is destroyed.As a result, the compression coefficient of the two kinds of mixed soil becomes larger, the compression modulus decreases, and the compressibility increases after repeated freeze-thawing.When the normal stress is at a high level, the particles inside the soil have been tightly compacted.At this time, the compression performance of the soil depends on the stress level and the compressibility of the particles themselves, rather than the connection between the pore structure and clay particles changed by the freeze-thaw effect to affect the compression coefficient.Therefore, at the high normal stress level, the freeze-thaw effect has little influence on the compression coefficient.With the freeze-thaw cycle, Ca-bentonite is fully hydrated, and bentonite particles fill the pores between the silt particles.Although the soil density of the two mixtures gradually converges, the high contact ratio between clay particles in ACB will lead to its compression coefficient is still slightly higher than that of ANB.Fig. 16(a) and (b) show the relationship between ANB/ACB compression index and bentonite content under different freeze-thaw conditions respectively.It can be seen from Fig. 15(a) and (b) that the compression index of ANB/ACB increases with the increase of the content of Na/ Ca-bentonite, but neither the content of Na-bentonite nor Ca-bentonite has a limited effect on the compression index.The compression index of the two mixtures increases linearly with the bentonite content, meanwhile, the linear relationship under the condition of the same number of freeze-thaw cycles can be expressed by the formula in Fig. 15.It can also be clearly seen from Fig. 16 that the greater the number of freeze-thaw cycles, the lower the curve of the relationship between the compression index and the bentonite content, which indicates that the freeze-thaw effect reduces the compressibility of soil.16 that the compression index of the two soil mixtures presents a linear increase with the bentonite content.When the bentonite content is low, the compression index of ACB is about equal to that of ANB under the condition of the same content and the same freezing and thawing times.However, when the content reaches a certain value (about 10%), it can be clearly seen that the compression index of ANB is significantly smaller than that of ACB, and the slope of the curve of the relationship between the compression index of ANB and the bentonite content is also significantly smaller than the slope of the curve of the relationship between the compression index of ANB and the bentonite content, that is, the growth rate of the compression index of ANB with the content of bentonite is smaller than that of ACB.The compressibility of soil depends mainly on the particle size, composition and structure of soil, and is also affected by the external environment, such as temperature.Compressibility of fine soil mainly comes from three main factors: i.The water film between particles is compressed and thinned; ii.relative slippage occurs between particles; and iii.flat clay particles are elastic and flex under pressure.Fig. 16 shows that the compressive deformation performance of ANB/ACB gradually increases with the increase of bentonite content.This is mainly due to the fact that most of the clay particles are flat and scaly, with a large specific surface area and high surface activity.Therefore, the compression of clay particles is the result of the synthesis of three factors.However, the aeolian soil formed by wind erosion, transportation and accumulation belongs to the soil mass with a dispersive structure, which is nearly parallel.The compression deformation of this kind of soil mass is only caused by the squeezing and drainage of water between particles, resulting in the compressibility of clay particles is greater than that of aeolian soil particles, that is, the compressibility of mixed soil mixed with a large number of clay particles is greater than that of aeolian soil.As can be seen from Fig. 16, when the bentonite content is low, the compression index of the two mixed soils is roughly equal under the same freeze-thaw and bentonite content.When the bentonite content is high, the compression index of ANB is significantly lower than that of ACB.This is mainly because when the bentonite content is low, the compression characteristics of the mixed soil are dominated by the aeolian soil, which accounts for the vast majority of the mixed soil, so the compression index of the two mixed soil materials is close when the bentonite content is low.However, with the increase of bentonite content, the proportion and rate of clay increase caused by the incorporation of Ca-based bentonite into aeolian soil is greater than that of Na bentonite.
As a result, the compression index of ACB is greater than that of aeolian soil-Na bentonite mixed soil at a higher bentonite content.Fig. 18(a) and (b) show the relationship curves between the compression index of ANB/ACB and the number of freeze-thaw cycles under different bentonite content.As can be seen from Fig. 18, the compression index of ANB/ACB decreases with the increase of freeze-thaw cycles, and the decreasing range gradually becomes flat with the increase of the number of freeze-thaw cycles.At the same time, it can also be clearly seen from Fig. 16 that the first freeze-thaw cycle has the greatest impact on the compressibility of ANB/ACB, that is, after the first freezethaw cycle, the compression index of the mixed soil material decreases the most.It can be seen from Fig. 17(c) that under the condition of the same bentonite content, the relationship curve between the compression index of ACB and the number of freeze-thaw cycles is above ANB, indicating that the compressibility of ACB is greater than that of ANB under the same bentonite content.Moreover, with the increase of bentonite content, it can be obviously found that the relationship between the compression index of ACB and the number of freeze-thaw cycles is more and more different from that between the compression index of ANB and the number of freeze-thaw cycles, which indicates that Ca-bentonite can better increase the compressibility of the mixed soil than that of Na-based bentonite.After the freeze-thaw cycle, the internal structure and particle size of the fine soil will be greatly changed, which results in the compression properties of ANB/ACB will also be affected by the freeze-thaw effect.With the increase in the number of freeze-thaw cycles, the freeze-thaw effect changes the size and stability of the soil aggregate.Through the alternating process of freezing and melting, the fine particles in the soil gradually agglomerate and form small aggregates.At the same time, the freeze-thaw action also effectively breaks the large and medium-sized aggregates inside the soil into small aggregates, that is, the particles inside the soil tend to aggregate into medium-sized particles, resulting in the soil skeleton being dominated by clay-powder particles to be dominated by powder-powder particles so that with the increase of the number of freeze-thaw cycles, the compression index of ANB/ACB gradually decreases and its compressibility gradually decreases.

Microscopic analysis under the effects of bentonite contents and FTC
After freeze-thaw cycles of 0, and 1, SEM tests were conducted on samples with pure aeolian soil, ANB and ACB with 4% bentonite content and ANB and ACB with 16% bentonite content to obtain microscopic images, to reveal the effects of bentonite content and freeze-thaw effect on the microstructure of the mixed soil.Fig. 19(a), (b), (c), (d), and (e) show the SEM images of pure aeolian soil, ANB and ACB with 4% bentonite content, and ANB and ACB with 16% bentonite content, respectively, in non-freezing-thawing condition.It can be clearly seen from Fig. 19 that with the addition of Na/ Ca-bentonite, the pores of the soil sample gradually decrease and the number of small particle size particles also gradually increase.As shown in Fig. 19(b) and Fig. 19(c), and Fig. 19(d) and Fig. 19(e), the number of macropores in ACB is significantly smaller than that in ANB under the condition of the same bentonite content, and the ACB soil sample has a layer of gray and white Ca-film covering part of the pores and particle surfaces(As can be clearly observed in Fig. 20).ACB with 16% bentonite content after freezing and thawing cycles of 0 and 1.As can be seen from the horizontal comparison in Fig. 19, with the progress of the freeze-thaw cycle, no matter what kind of mixed soil material, its pore structure is destroyed and the pore scale becomes larger.Moreover, under the influence of the freeze-thaw effect, the Ca-film of ACB gradually decreases due to the intensification of the hydration degree of Ca-bentonite.
On the other hand, observing the changes in particle size before and after the freeze-thaw of these five types of soil samples, it can also be found that with the increase in the number of freeze-thaw cycles, the fine particles in the soil samples begin to agglomerate, and the fine particles decreased significantly, and a large number of medium-sized particles are formed.The skeleton structure of the soil sample changes from the dominant position of silt-clay and clay-clay to the dominant position of silt-powder and powder-aggregate.

Increasing number of freeze-thawing cycles
Based on the results of permeability, compression and SEM tests, the evolution of influences of bentonite incorporation amount and FTC effect on the microstructure of aerolite-bentonite mixture is shown in Fig. 20.During the freezing process, pore water in five different types of soil samples, namely pure aeolian soil, ANB and ACB with 4% bentonite content, and ANB and ACB with 16% bentonite content, gradually formed pore ice inside the pores as the ambient temperature gradually dropped below the freezing point, but not all pore water is converted into ice.Under the action of particle surface energy, there is always a certain amount of unfrozen water in the soil, and the internal unfrozen water migrates from the inside of the soil sample to the freezing front.The water migration of unfrozen water is most intuitively manifested as the continuous growth of pore ice and the trend of gradually connecting to form an ice lens.Because some of the particles and pores in ACB with high bentonite content are covered by a layer of Ca-film, the growth of the ice lens is restricted, so it is difficult for ACB with high content to form an ice lens formed by interconnection during the freezing process.During the thawing process, the gradual thawing of pore ice will produce negative pore pressure, resulting in the compaction of the soil under its weight.
Due to the connected pores formed by ice lenses in pure aeolian soil, ANB and ACB with low bentonite content, connected micro-cracks will be formed after thawing.However, for ACB with high bentonite content, it is difficult to form ice lenses in the freezing stage, so ACB with high bentonite content will not have a large area of microcracks.In addition, the freeze-thaw process will further improve the hydration degree of Ca-bentonite, and bentonite particles will continue to fill the pores that have not been filled by fine particles after continuous hydration during the thawing process.

Conclusions
In this paper, the effects of freeze-thaw actions on the permeability and compression characteristics of ANB/ACB and the effects of bentonite content on the particle size and expansibility of the mixed soil were studied.
The main conclusions are summarized as follows: (1) The inhomogeneity coefficient, curvature coefficient and clay content of ANB/ACB mixed soils increase gradually with the continuous incorporation of bentonite, and the inhomogeneity coefficient and curvature coefficient show an opposite growth trend with the change rate of bentonite content, that is, the growth rate of the inhomogeneity coefficient increases, while the growth rate of the curvature coefficient decreases.Under the same bentonite content, the inhomogeneity coefficient, curvature coefficient and clay content of ACB are greater than of freeze-thaw cycles, the compression index of the two mixtures decreases gradually, and the decreasing rate of both mixtures decreases gradually.At the same time, the compression index of the two soil mixtures decreased the most after the first freeze-thaw cycle.
(6) It is found by SEM that the ratio of macropores in the soil samples gradually decreases with the addition of Na/ Ca-bentonite, and the proportion of small and medium size particles in the soil samples gradually increases.
The proportion of macropores in ANB is significantly higher than that of ACB, and a layer of gray and white calcium film is formed on the surface of the particles and pores of the ACB soil sample.At the same time, it is also found from the microscopic electron microscope photos that after the freeze-thaw cycle, the fine particles in all kinds of mixed soil samples begin to agglomerate, the larger particles are broken, and the particles gradually develope into medium-sized particles agglomerates.

Fig. 5
Fig. 5(a) and (b) respectively show the grading curves of ANB and ACB under different bentonite content

Fig. 11
Fig. 11 Comparison curve of saturation permeability coefficient and dosage of aeolian soil-Na bentonite mixtures and aeolian soil-Ca bentonite mixtures

Fig. 15
Fig.15mainly shows the influence of the freeze-thaw cycle effect on the compression coefficient (a v ) of ANB (a) aeolian soil -Na bentonite mixture (b) aeolian soil -Ca bentonite mixture

Fig. 16
Fig. 16 Relationship between compression index and bentonite content of aeolian soil -bentonite mixture under different freeze-thaw conditions Fig. 17 shows the contrast curve of the compression index of ANB and ACB with bentonite content under

Fig. 17
Fig. 17 Comparison of compression index and bentonite content of aeolian soil-Na/Ca-bentonite mixture under different freeze-thaw conditions (a) aeolian soil -Na bentonite mixture (b) aeolian soil -Ca bentonite mixture (c) comparison chart

Fig. 18
Fig. 18 Relationship between compression index and freeze-thaw times of aeolian soil-bentonite mixture under different bentonite content

Fig. 19
Fig.19 Scanning electron micrographs of aeolian-bentonite mixture specimens under various bentonite contents

Fig. 19
Fig.19 Scanning electron microscopy images of ANB/ACB with different bentonite contents after undergoing freeze-thaw cycles Fig.20 Schematic diagram of freeze-thaw evolution

Table 1 .
Mineralogical composition of three types of soil

Table 2 .
Physical properties of the soils used in the tests