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Experimental study on the freezing–thawing deformation of a silty clay
Accepted Manuscript Experimental study on the freezing–thawing deformation of a silty clay
Jianguo Lu, Mingyi Zhang, Xiyin Zhang, Wansheng Pei, Jun Bi PII: DOI: Reference:
S0165-232X(17)30478-0 https://doi.org/10.1016/j.coldregions.2018.01.007 COLTEC 2513
To appear in:
Cold Regions Science and Technology
Received date: Revised date: Accepted date:
9 October 2017 12 December 2017 16 January 2018
Please cite this article as: Jianguo Lu, Mingyi Zhang, Xiyin Zhang, Wansheng Pei, Jun Bi , Experimental study on the freezing–thawing deformation of a silty clay. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Coltec(2017), https://doi.org/10.1016/j.coldregions.2018.01.007
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ACCEPTED MANUSCRIPT Experimental study on the freezing-thawing deformation of a silty clay
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Jianguo Lu1, 2, Mingyi Zhang1, 2*, Xiyin Zhang1, 3, Wansheng Pei1, 2, Jun Bi4
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1. State Key Laboratory of Frozen Soil Engineering, Northwest Institute of
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Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China
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2. University of Chinese Academy of Sciences, Beijing 100049, China
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3. School of Civil Engineering, Lanzhou Jiaotong University, Lanzhou 730000, China 4. School of Civil Engineering and Mechanical, Lanzhou University, Lanzhou 73000,
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China *
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Corresponding author. [email protected] (Mingyi Zhang)
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Abstract: Freezing-thawing deformation is a key factor in determining damages for engineering structures in cold regions. Based on the laboratory experiments, the
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characteristics of the freezing-thawing deformation were analyzed, and the variation of the matric suction was discussed. The results show that for each freeze-thaw cycle, the freezing-thawing deformation can be divided into five stages, i.e., cold shrink, fast frost heave, slow frost heave, thermal bulge and thaw settlement. At the onset of each cooling process, the cold shrink occurs and the freezing-thawing deformation decreases slightly; and then, with the decreased temperature, the freezing-thawing deformation rapidly increases at the beginning of freezing (fast frost heave); and
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ACCEPTED MANUSCRIPT subsequently slowly increases with freezing up to the expected value. During warming process, the inflation occurs with the rapid increase of the ambient temperature; later, with the thawing of ice, the freezing-thawing deformation decreases. Besides, there are similar changing trends for the freezing-thawing deformation and the matrix suction during the freezing-thawing processes. However,
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their changing mechanisms are completely different, and the soil matric suction has a
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stronger sensitivity to the variation of the ambient temperature and volumetric
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unfrozen water content. Furthermore, the effect of freeze-thaw cycles on the characteristic variables of the freezing-thawing deformation, such as cold shrink, net
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deformation, frost heave coefficient and thaw-settlement coefficient, mainly occurs in the first freeze-thaw cycle, which is due to the fact that the first freeze-thaw cycle has
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a significant influence on the soil structure and pore distribution for the remolded
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sample.
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Key words: freezing-thawing deformation; unfrozen water content; matric suction;
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freeze-thaw cycle; silty clay
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ACCEPTED MANUSCRIPT 1 Introduction Freezing-thawing behavior is a complex physical process, and the structure of soils will be significantly changed after freezing-thawing processes, which will cause deformation of soils, and these effects may in turn substantially change the foundation bearing capacity of structures (Cheng, 2005; Li et al., 2014; Özgan et al., 2015). At
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present, more and more engineering constructions are built in cold regions. Most of
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the regions related to frozen soil can produce large deformation during cyclic
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freezing-thawing (Nixon and Mcroberts, 1976). The freezing-thawing problem of soils is key in determining the stability of railways and highways in cold regions, e.g.,
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Qinghai-Tibet Railway and Qinghai-Tibet Highway (Cheng, 2005). The physical changes during the freeze-thaw cycles, such as water migration, heat transfer, and
Lai et al., 2013; Lai et al., 2014).
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stress redistribution, etc. can all cause the freezing-thawing deformation (Cheng, 2005;
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Freeze-thaw cycles are complicated processes which considerably change the
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physical structure and characteristics of soils (Yao et al., 2009). Some research about the influence of freeze-thaw cycles on the physical and mechanical characteristics of
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soils have been carried out. Chamberlain and Gow (1979) found that the freeze-thaw cycles can cause the reduction of porosity and the increase of hydraulic conductivity.
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After that Viklander (1998) tested the volume change of a fine-grained soil during the freezing-thawing processes, and found that the volume typically decreases for a loose soil and increases for a dense soil. This phenomenon has been confirmed by other studies under different experimental conditions (Qi et al., 2008; Qi et al., 2006). Konrad (2000) studied the influence of freeze-thaw cycles on the hydraulic conductivity of a Kaolinite-silt mixture, and developed a simplified porosity model to evaluate the hydraulic conductivity; and later based on the microstructure experiment 3
ACCEPTED MANUSCRIPT of a soft soil subjected to freeze-thaw cycles, Tang and Yan (2015) found that the porosity and specific pore area decreases, while the average pore diameter increases, which causes the increase of the hydraulic conductivity. With variation in the physical characteristics, the mechanical characteristics changes at the same time. Wang et al. (2007) and Xie et al. (2015) studied the effect of freeze-thaw cycles on the strength
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characteristics and mechanical behavior of a silty clay, and concluded that the
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freeze-thaw cycles make the parameters of soil a new dynamic equilibrium. Besides,
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it is found that other physical and mechanical parameters, such as thermal conductivity (Orakoglu et al., 2016; Pei et al., 2013), elastic modulus (Cui et al., 2014;
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Lee et al., 1995; Simonsen et al., 2002), damping ratio (Kweon and Hwang, 2013), and soil cohesion (Simonsen et al., 2002), etc. have been changed after freeze-thaw
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cycles. The variations of the physical and mechanical characteristics due to the freeze-thaw cycles would bring adverse effect in cold region engineering, which could
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cause the differential deformation and even engineering accidents.
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The freezing-thawing deformation is a key factor in determining the stability of engineering structures in cold regions, which would cause many engineering problems,
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including crack of pavement, damage for foundation, and fracture of pipelines (Cui et al., 2014; Yu et al., 2017). The main reason is that part of the water in the soils will be
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frozen into ice in freezing periods, and the volume increased by ice will have a significant effect on the soil characteristics, and the action of ice will greatly disrupt the soil structure. However, during thawing process, the thawing of ice will cause the reduction of bearing capacity, and the thaw settlement will occur simultaneously. In the study of freezing-thawing deformation, Li et al. (2014) studied the freezing-thawing deformation mechanism of a canal in seasonally frozen regions through a moisture-heat-mechanical model, and found that the repetitive 4
ACCEPTED MANUSCRIPT freezing-thawing deformations are mainly from the dramatic water migration and redistribution in the seasonal freezing-thawing layer. Zhang et al. (2016) established a coupled heat-moisture-deformation model to simulate the influence of seepage on temperature and deformation of subgrades. Kweon and Hwang (2013) studied the deformation characteristics of the subgrade soils and subbase materials in freeze-thaw
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cycles, and concluded that the deformation characteristics are related to the initial
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water content. Table (1930) expounded the influence of water migration on
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deformation during the freezing-thawing processes. Through the multi-layer deformation observation, Yu et al. (2017) concluded that the deformation of the
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embankment in permafrost regions is from multiple sources, and other research related to the source of the freezing-thawing deformation have been studied (Guo et
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al., 2016; Yu et al., 2016; Zhang et al., 2016a). In the field scale experiments, Chou et al. (2010) studied the effect of sunny-shady slopes on the stability of the highway
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embankment in warm permafrost regions, and pointed out that embankment stability
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have been destroyed with longitudinal cracks primarily caused by transverse asymmetric settlement. Based on the field monitoring data, the characteristics of
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embankment deformation were summarized along the Qinghai–Tibet Railway in four permafrost regions with different mean annual ground temperatures (Ma et al., 2011).
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It can be seen that considerable researches have been carried out to find out the freezing-thawing
effect
on
engineering
characteristics
of
soils.
And
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freezing-thawing deformation, as a significant index to judge the stability of buildings, plays an important role in cold region engineering. However, only a few studies are focused on the freezing-thawing deformation characteristics of soils during freeze-thaw cycles, and some research of freezing-thawing deformation existed were mainly the numerical simulations and the in-situ experiments on large scales 5
ACCEPTED MANUSCRIPT quantitatively. At the same time, it lacks systematic experiments of the two abnormal deformation phenomenon quantitatively, i.e., cold shrink and thermal bulge, although, Qi et al. (2006) have explained the phenomenon from the aspects of effective stress principle. Therefore, in this study, a series of freeze-thaw tests of a silty clay were performed to study the freezing-thawing deformation characteristics of the soil
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subjected to freeze-thaw cycles, such as cold shrink, thermal bulge, frost heave, and
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thaw settlement, etc. Besides, the variation of the matrix suction was also discussed.
2 Experimental setup
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2.1 Experimental apparatus
In this study, a freezing-thawing cabinet (Fig. 1(a)) was applied to test the soil
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sample, whose inner circumstance temperature can be set from -20℃ to +20℃ with
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an accuracy of ±0.1℃. The soil sample barrel (Fig. 1(b)) was placed in the middle of
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the freezing-thawing cabinet. Fig. 1(c) is the sensors arrangement in the soil sample. The moisture sensor (Fig. 2(a)), 52 mm in length with an accuracy of ±2%, based on the frequency domain reflection principle, together with a temperature sensor
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(precision: ±0.05℃, range: -30℃~+30℃), was embedded into the middle of the soil
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sample (Fig. 1(c)), the moisture sensor was horizontally placed, and the three rods were arranged in a vertical direction (Fig. 2(a)). The matric potential sensor (Fig. 2(b)), with the probe of 8 mm in diameter and 30 mm in length (precision: ±0.1kPa), was placed in 5cm from the bottom of the soil sample (Fig. 1(c)). And the deformation sensor (Fig. 2(c)), with the accuracy of 0.01mm, was placed on the top of the soil sample to measure the freezing-thawing deformation (Fig. 1(c)). All sensors were connected to a data logger, which was, in turn, connected to a computer, and the data acquisition interval was 1 minute. 6
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Fig. 1 Experiment apparatus (a) Freezing-thawing cabinet; (b) Soil sample barrel; (c) Sensors arrangement
Displacement sensor
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2.2 Experimental method
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Fig. 2 Experiment sensors (a) Moisture sensor; (b) Matric potential sensor; (c)
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The silty clay from the Qinghai-Tibet Plateau was used in the experiment, and its
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physical parameters are given in Table 1. Before the experiments, the soil was washed with distilled deionized water to eliminate the influence of salts, and then dried in an
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oven for 24 h with the temperature of 105℃. Later, the dried soil was crushed and sieved over 0.20 cm, and the grain-size distribution of the silty clay is shown in Fig. 3.
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Firstly, the soil was prepared with an initial volumetric water content of 27.21%, and sealed in a plastic bag for 24h to ensure adequate moisture redistribution. And then the
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soil was compacted into the soil sample barrel with a hammer to the target height of 300 mm and a dry density of 1.55×103 kg/m3. Finally, a thin plastic film was covered
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at the top surface of the sample to prevent moisture evaporation during freezing and thawing.
Table 1 Physical parameters of the soil sample
Fig. 3 Grain-size distribution of the silty clay
In the experiment, five freeze-thaw cycles were carried out (Fig. 4). For each 7
ACCEPTED MANUSCRIPT freeze-thaw cycle, the soil sample was maintained to a relative steady temperature of 2℃ 1℃ after cooling for more than 24h, which was to eliminate the effects of the initial water distributions and temperature gradients on the freezing characteristics, and to make the soil moisture and temperature in an equilibrium state. In order to make sure that the soil sample was super-cooled completely and the water was fully
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frozen, the small step temperature (0.2 ~ 0.5℃) was used to cool down the soil
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sample step by step from the initial steady temperature (about 2℃ 1℃) to -1℃, and
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then the desired temperature was applied (Ishizaki et al., 1996). In the five freeze-thaw cycles (Table 2), the controlled temperature patterns of the first two
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freeze-thaw cycles (1 and 2) were the same, i.e. step cooling and rapid warming; then
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the third freeze-thaw cycle was carried out under rapid cooing and rapid warming; finally, the two same cycles (4 and 5) were performed with step cooling, rapid cooling
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and rapid warming. In the controlled temperature patterns(Fig. 4), the step cooling
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was that the temperature was decreased step by step, namely, -1.5℃, -2℃, -6℃, -10℃, -14℃, and -18℃, and the rapid cooling was that the temperature was
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decreased to -18℃, directly. Additionally, the step cooling and rapid cooling was the cooling pattern that the temperature was decreased step by step initially, namely,
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-1.5℃, -2℃, and then, the temperature was decreased to -18℃, directly. The controlled temperature patterns were designed to study the effect of variation of unfrozen water content due to the decreased temperature on the freezing-thawing deformation of the soil. Furthermore, in each cycle, the ambient temperature was increased to warm the sample until the volumetric unfrozen water content was almost steady in the final freezing temperature (-18℃), and all the warming patterns were the rapid warming, namely, the temperature was increased to 0℃ from -18℃, directly 8
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Table 2 Controlled temperature patterns
Fig. 4 Controlled temperature of the freeze-thaw cycles
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3 Experiment results and analysis
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3.1 Variation of freezing-thawing deformation
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Fig. 5(a-e) is the freezing-thawing deformation of the soil sample during the freezing-thawing processes. From the figures, it can be seen that no matter what
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controlled temperature pattern is adopted, each freezing-thawing deformation can be
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divided into five stages, i.e. cold shrink, fast frost heave, slow frost heave, thermal bulge and thaw settlement. The experiment results are similar with the previous
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findings (Qi et al., 2006; Qi et al., 2008), which show that in the freeze-thaw cycles of
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Lanzhou silty soil, the phenomenon of cold shrink is not clearly found, while the phenomenon of thermal bulge distinctly occurs.
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At the onset of cooling, the soil sample is compressed (settlement) rather than
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expanded (heave), namely, the cold shrink occurs and the deformation decreases slightly in the soil sample. With the decrease of the soil temperature, the soil sample starts to freeze and the ice lenses are formed, at the same time, the expansion occurs and the deformation rapidly increases (fast frost heave), and subsequently the deformation slowly increases with freezing up to the expected value (slow frost heave). Because the ambient temperatures are similar (about -18℃) at the end of each cooling process in the five freeze-thaw cycles, the volumetric unfrozen water contents 9
ACCEPTED MANUSCRIPT are almost the same (about 9.8%) at the end of all cooling processes (Fig. 5). However, the value of thaw settlement is greater than that of the frost heave, therefore, a downward net deformation occurs for each freeze-thaw cycle, which causes the decrease of the volume and the increase of the dry density for the soil sample. This is
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mainly from the fact that the dry densities of loose and dense soils subjected to
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freeze-thaw cycles change in opposite, namely, the freeze-thaw cycles cause the
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densification of loose soils, while the opposite to occur for dense soils (Viklander, 1998).
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During the warming processes, with the rapid increase of ambient temperature, the
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thaw settlement does not happen immediately as is expected, instead, the inflation occurs (Fig. 5). This is mainly because during freezing, although the soil sample
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expands, the expanding potential, caused by growth of ice crystals, is not exerted
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completely. At the same time, the suction generated in the freezing process works as a restraining force among the particles. When the cooling source is withdrawn, the
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restraining force suddenly greatly decreases (Fig. 6), the sample expands due to the
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stress release, which causes a thermal bulge at the beginning of thawing (Qi et al., 2008). Later, with the thawing of ice, the freezing-thawing deformation decreases. From Fig. 5, it can also be seen that there is a close relationship between the freezing-thawing deformation and the volumetric unfrozen water content and soil temperature. During the freezing processes, the rapid increase of freezing-thawing deformation (fast frost heave) mainly occurs at the freezing point, during which is the beginning of the reduction of volumetric unfrozen water content. This is mainly from 10
ACCEPTED MANUSCRIPT the fact that the water in the large pores is frozen firstly, and the rapid increase of freezing-thawing deformation (fast frost heave) is mainly attributed to the ice formation in large pores, and then with the decreased temperature, the water in small pores is frozen, but the freezing-thawing deformation increases slightly. During the
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warming processes, there is a good relationship between the decreased
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freezing-thawing deformation and the increased volumetric unfrozen water content,
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but in the initial thawing stage, the change of freezing-thawing deformation is small even though the volumetric unfrozen water content increases, and then with increased
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ambient temperature, the soil temperature reaches to the thawing point, the rapid
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reduction of freezing-thawing deformation and the increase of volumetric unfrozen water are all in the sharp phase change stage, during which the thaw settlement occurs
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rapidly. This may be because that the ice in large pores begins to thaw, and the
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variations of the freezing-thawing deformation are mainly related to the phase change
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of water/ice in large pores.
(a) Freeze-thaw cycle 1
(b) Freeze-thaw cycle 2
(c) Freeze-thaw cycle 3
(d) Freeze-thaw cycle 4
(e) Freeze-thaw cycle 5 Fig. 5 Freezing-thawing deformation of the soil sample during the
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3.2 Variation of matric suction During the freezing-thawing processes, the volumetric unfrozen water content,
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changed with the temperature, causes the change of the freezing-thawing deformation as well as the variation of the soil matric suction. Fig. 6 (a-e) shows the variation of
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the matrix suction of the soil sample during the freezing-thawing processes.
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During the cooling process, with the decreased temperature (above 0℃), the cold
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shrink occurs but the matric suction keeps invariable; and then, when the expected temperature (below 0℃) is applied to the soil sample, the fast frost heave appears and
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the matric suction increases rapidly (Fig. 5); with the deformation reaching to the
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slow frost heave, the matric suction also changes slowly (Fig. 6).
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The variation of matric suction in the thawing process is quite different from that in the freezing process, which might be due to the remarkable hysteresis effect of
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volumetric unfrozen water content during the freezing and thawing processes (Wen et al., 2012). And this may be determined by several factors, e.g. the super-cooling effect
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of pore water (metastable nucleation), the pore-blocking effect (ink-bottle mechanism), the capillarity effect and electrolyte effect, etc. (Bittelli et al., 2003; Overduin et al., 2006). During the warming process, a constant ambient temperature (0℃) is applied to the surface of the soil sample, the thermal bulge occurs and the matric suction drops slightly (Fig. 5). With the increase of temperature, the freezing-thawing deformation and matric suction drop quickly in a short time, and the
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ACCEPTED MANUSCRIPT decrease rate of the matric suction is larger than that of the freezing-thawing deformation. Generally, although there are similar changing trends for the freezing-thawing deformation and the matrix suction during the freezing-thawing processes; however,
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their changing mechanisms are completely different, namely, the variations of the
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freezing-thawing deformation are resulted in the frozen heave or thaw settlement
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related to the phase change of ice/water during the freezing-thawing processes; while the variations of the matric suction are attributed to the different constraining force of
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soil particles to water and ice in pores, as well as related to the destroyed soil structure
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by the ice formation during the freezing-thawing processes (Wen et al., 2012; Peng et al., 2016). Furthermore, compared with the freezing-thawing deformation, the soil
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matric suction has a stronger sensitivity to the volumetric unfrozen water content
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(Figs. (5-6)).
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(a) Freeze-thaw cycle 1
(b) Freeze-thaw cycle 2
(c) Freeze-thaw cycle 3
(d) Freeze-thaw cycle 4
(e) Freeze-thaw cycle 5 Fig. 6 Variation of the matrix suction of the soil sample during the freezing-thawing processes
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ACCEPTED MANUSCRIPT 3.3 Effect of freeze-thaw cycle Cyclic freezing-thawing processes change the soil structure from a steady state to another steady state (Wang et al., 2007; Yang et al., 2004). The freezing-thawing deformation process during a freeze-thaw cycle is schematically shown in Fig. 7.
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From the discussion above, a whole freezing-thawing deformation includes five stages,
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namely, cold shrink, fast frost heave, slow frost heave, thermal bulge, and thaw
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settlement. In order to study the variations of deformation in the freeze-thaw cycles (e.g. Fig. 7), some characteristic variables related to the freezing-thawing deformation,
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i.e. net deformation hi , cold shrink deformation hc , thermal bulge deformation
deformation h , are introduced.
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ht , frost heave coefficient i , thaw-settlement coefficient A0 , and cumulative
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The net deformation hi of each freeze-thaw cycle reflects the effect of
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freezing-thawing process on the soil structure during a freeze-thaw cycle (Fig. 7), expressed as:
hi hi0 hit
(1)
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Where subscript i denotes the number of freeze-thaw cycle; hi0 is the initial height
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of soil sample; hit is the height of soil sample after freezing. The cold shrink deformation hc and thermal bulge deformation ht are introduced to reflect the effecting degree of freeze-thaw cycle on the two phenomenon, irrespectively (Fig. 7), which can be calculated by Eq. (2) and Eq. (3), respectively.
hc hi0 hc
(2)
ht hb hi
(3)
Where hc and hb are the height of soil sample after cold shrink and thermal bulge, 14
ACCEPTED MANUSCRIPT respectively; hi is the height of soil sample after thawing. And another deformation variable, the cumulative deformation h , is used to show the effect of freeze-thaw cycles on the soil structure, calculated by Eq. (4):
h hi
(4)
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At the same time, the frost heave coefficient is introduced to quantify the
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maximum frost heave of the soil sample in each freeze-thaw cycle, which is defined
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as the ratio of the increased height in the steady freezing temperature to the initial height for each freeze-thaw cycle, namely, as Eq.(5) shown (Qiu et al., 1994): hi hi0 hi0
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i
(5)
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The thaw-settlement coefficient, as a dimensionless parameter, has been introduced to evaluate the effect of freeze-thaw cycles on the thaw settlement, which
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is defined as the ratio of the decreased height after a thawing process to the steady height of the sample after a freezing process. It can be expressed as:
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A0
hi hit hi
(6)
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Fig. 7 Freezing-thawing deformation process during a freeze-thaw cycle
The characteristic variables of the soil sample during the freezing-thawing processes are listed in Table 3. Fig. 8 presents the cold shrink deformation and thermal bulge deformation versus number of freeze-thaw cycles. From Table 3 and Fig. 8, it can be seen that the cold shrink deformation increases with the number of freeze-thaw cycles, the value of the first freeze-thaw cycle is maximum (0.36 mm), and from the 15
ACCEPTED MANUSCRIPT second freeze-thaw cycle, the cold shrink deformation increases gradually with the number of freeze-thaw cycles. However, the thermal bulge deformation has a slight change (ranging from 0.10 mm to 0.20 mm) with the number of freeze-thaw cycles.
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Table 3 Characteristic variables during the freezing-thawing processes
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Fig. 8 Cold shrink deformation and thermal bulge deformation versus
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number of freeze-thaw cycles
The net deformation and cumulative deformation versus number of freeze-thaw
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cycles is shown in Fig. 9. From the figure, it can be seen that the net deformation of the soil sample is the maximum (-7.57mm) at the first cycle, but gradually becomes
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small from the second freeze-thaw cycles. At the same time, the cumulative
tends to be steady.
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deformation increases with the number of freeze-thaw cycles but the increase rate
freeze-thaw cycles
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Fig. 9 Net deformation and cumulative deformation versus number of
Fig. 10 shows the frost heave coefficient versus number of freeze-thaw cycles. From the figure, it can be seen that the frost heave coefficient in the first freeze-thaw cycle (1.62 10-2) is the maximum, and decreases with the number of freeze-thaw cycle. The thaw-settlement coefficient versus number of freeze-thaw cycles is shown in Fig. 11. As the change of the frost heave coefficient, the thaw-settlement coefficient is 16
ACCEPTED MANUSCRIPT also the maximum (4.07 10-2) in the first freeze-thaw cycle, but then gradually becomes small from the second freeze-thaw cycles. As analyzed above, the effect of freeze-thaw cycles on the characteristics variables of the freezing-thawing deformation, such as the cold shrink, net deformation, frost heave coefficient, and thaw-settlement coefficient, mainly occurs in
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the first freeze-thaw cycle. This is mainly from the fact that the freeze-thaw cycles
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have a significant influence on the soil structure and pore distribution (Cui et al., 2014;
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Qi et al., 2008), and generally, this effect is greatest in the first freeze-thaw cycle for the remold soil sample.
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Fig. 10 Frost heave coefficient versus number of freeze-thaw cycles
Fig. 11 Thaw-settlement coefficient versus number of freeze-thaw
5 Conclusions
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cycles
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Based on the above analysis and discussion for the effect of freeze-thaw cycles on the freezing-thawing deformation of a silty clay, some preliminary conclusions are
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drawn:
(1) For each freeze-thaw cycle, the freezing-thawing deformation could be divided into five stages, i.e., cold shrink, fast frost heave, slow frost heave, thermal bulge and thaw settlement. (2) There are similar changing trends for the freezing-thawing deformation and the matrix suction during the freezing-thawing processes. However, their changing mechanisms are completely different, and the soil matric suction has a stronger sensitivity to the variations of the ambient temperature and volumetric unfrozen water 17
ACCEPTED MANUSCRIPT content. (3) The effect of freeze-thaw cycles on the characteristics variables of the freezing-thawing deformation, such as cold shrink, net deformation, frost heave coefficient, and thaw-settlement coefficient, mainly occurs in the first freeze-thaw cycle, which is due to the fact that the first freeze-thaw cycle has significant influence
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on the soil structure and pore distribution of the remolded soil sample.
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Acknowledgements
This research was supported by the National Natural Science Foundation of China
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(Grant No. 41471063), the Program of the State Key Laboratory of Frozen Soil Engineering (Grant No. SKLFSE-ZT-23), the 100-Talent Program of the Chinese
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Academy of Sciences (Granted to Dr. Mingyi Zhang), the Key Research Program of Frontier Sciences of Chinese Academy of Sciences (QYZDY-SSW-DQC015), and the
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STS Program of the Chinese Academy of Sciences (Grant No. HHS-TSS-STS-1502).
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Lai, Y.M., Pei, W.S., Yu, W.B., 2013. Calculation theories and analysis methods of thermodynamic stability of embankment engineering in cold regions. Chinese Science Bulletin, 59(3): 261-272. Lai, Y.M., Pei, W.S., Zhang, M.Y., et al., 2014. Study on theory model of hydro-thermal–mechanical interaction process in saturated freezing silty soil. International Journal of Heat and Mass Transfer 78 (5): 805–819. Lee, W., Bohra, N.C., Altschaeffl, A.G., et al., 1995. Resilient modulus of cohesive 19
ACCEPTED MANUSCRIPT soils and the effect of freeze–thaw. Canadian Geotechnical Journal, 32(4): 559-568. Li, S.Y., Lai, Y.M., Pei, W.S., et al., 2014. Moisture–temperature changes and freeze– thaw hazards on a canal in seasonally frozen regions. Natural Hazards, 72(2):
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287–308.
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Ma, W., Mu, Y.H., Wu, Q.B., et al., 2011. Characteristics and mechanisms of
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embankment deformation along the Qinghai–Tibet Railway in permafrost regions. Cold Regions Science and Technology, 67(3): 178-186.
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Nixon, J.F., Mcroberts, E.C., 1976. A design approach for pile foundations in
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permafrost. Canadian Geotechnical Journal, 13(1): 40-57. Orakoglu, M.E., Liu, J.K., Niu, F.J., 2016. Experimental and modeling investigation
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of the thermal conductivity of fiber-reinforced soil subjected to freeze-thaw
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freezing and thawing soil using the soil temperature response to heating, Cold
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Regions Science and Technology, 45, 8–22. Özgan, E., Serin, S., Ertürk, S. and Vural, I., 2015. Effects of Freezing and Thawing Cycles on the Engineering Properties of Soils. Soil Mechanics and Foundation Engineering, 52(2): 95-99. Pei, W.S., Yu, W.B., Li, S.Y., et al., 2013. A new method to model the thermal conductivity of soil–rock media in cold regions: An example from permafrost regions tunnel. Cold Regions Science and Technology, 95: 11-18. 20
ACCEPTED MANUSCRIPT Peng, Z.Y., Tian, F.Q., Wu, J.W., et al., 2016. A numerical model for water and heat transport in freezing soils with nonequilibrium ice-water interfaces. Water Resources Research, 52(9): 7366-7381. Qi, J.L., Vermeer, P.A., Cheng, G.D., 2006. A review of the influence of freeze-thaw
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Qi, J.L., Ma, W., Song, C.X., 2008. Influence of freeze–thaw on engineering properties of a silty soil. Cold Regions Science and Technology, 53(3):
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Qiu, G.Q., Liu, J.R. and Liu, H.X., 1994. Geocryological Glossary. Gansu Science and Technology Press, Lanzhou.
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Simonsen, E., Janoo, V.C., Isacsson, U., 2002. Resilient Properties of Unbound Road
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Materials during Seasonal Frost Conditions. Journal of Cold Regions Engineering, 16(1): 28-50.
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Taber, S., 1930. The Mechanics of Frost Heaving. The Journal of Geology, 38(4):
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303-317.
Tang, Y.Q., Yan, J.J., 2015. Effect of freeze–thaw on hydraulic conductivity and microstructure of soft soil in Shanghai area. Environmental Earth Sciences, 73(11): 7679-7690. Viklander, P., 1998. Permeability and volume changes in till due to cyclic freeze/thaw. Canadian Geotechnical Journal, 35(3): 471-477. Wang, D.Y., Ma, W., Niu, Y.H., et al., 2007. Effects of cyclic freezing and thawing on 21
ACCEPTED MANUSCRIPT mechanical properties of Qinghai–Tibet clay. Cold Regions Science and Technology, 48(1): 34-43. Wen, Z., Ma, W., Feng, W.J., et al., 2012. Experimental study on unfrozen water content and soil matric potential of Qinghai-Tibetan silty clay. Environmental
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Earth Sciences, 66(5): 1467–1476.
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Xie, S.B., Qu, J.J., Lai, Y.M., et al., 2015. Effects of freeze-thaw cycles on soil
Mountain Science, 12(4): 999-1009.
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mechanical and physical properties in the Qinghai-Tibet Plateau. Journal of
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Yang, C.S., He, P., Cheng, G.D., et al., 2004. Testing study on influence of freezing
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and thawing on dry density and water content of soil. Chinese Journal of Rock Mechanics and Engineering, 22((supp. 2)): 2695–2699.
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Yao, X.L., Qi, J.L., Ma, W., 2009. Influence of freeze–thaw on the stored free energy
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in soils. Cold Regions Science and Technology, 56(2-3): 115-119. Yu, F., Qi, J.L., Lai, Y.M., et al., 2016. Typical embankment settlement/heave patterns
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of the Qinghai–Tibet highway in permafrost regions: Formation and evolution.
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Engineering Geology, 214: 147-156. Yu, F., Zhang, M.Y., Lai, Y.M., et al., 2017. Crack formation of a highway embankment installed with two-phase closed thermosyphons in permafrost regions: Field experiment and geothermal modelling. Applied Thermal Engineering, 115: 670-681. Zhang, J.M., Ruan, G.F., Su, K., et al., 2016. Estimation on settlement of precast tower footings along the Qinghai–Tibet Power Transmission Line in warm 22
ACCEPTED MANUSCRIPT permafrost regions. Cold Regions Science and Technology, 121: 275-281. Zhang, M.L., Wen, Z., Xue, K., et al., 2016. Temperature and deformation analysis on slope subgrade with rich moisture of Qinghai–Tibet railway in permafrost regions. Chinese Journal of Rock Mechanics and Engineering, 35(8): 1677–
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ACCEPTED MANUSCRIPT Table 1 Physical parameters of the soil sample Plastic limit (by gravity)/%
Liquid limit (by gravity)/%
Dry density /(kg/m3)
Porosity
27.21
17.44
31.84
1.55×103
0.43
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Initial volumetric water content/%
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Freeze-thaw cycles
Cooling pattern
Warming pattern
1
Step cooling
Rapid warming
2
Step cooling
Rapid warming
3
Rapid cooling
Rapid warming
4
Step cooling and rapid cooling
Rapid warming
5
Step cooling and rapid cooling
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Table 2 Controlled temperature patterns
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Rapid warming
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ACCEPTED MANUSCRIPT Table 3 Characteristic variables during the freezing-thawing processes 2
3
4
5
Net deformation/mm
-7.565
-1.283
-0.542
-0.474
-0.228
Cold shrink deformation/mm
-0.360
-0.129
-0.143
-0.104
-0.075
Thermal bulge deformation/mm
0.120
0.081
0.102
0.180
0.104
Frost heave coefficient/(×10-2)
1.620
0.920
0.911
0.816
0.823
Thaw-settlement coefficient/(×10-2)
4.076
1.321
0.982
0.967
0.971
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Freeze-thaw cycles
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ACCEPTED MANUSCRIPT Highlights
The characteristics of the deformation of a silty clay during the freeze-thaw processes were studied.
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The effect of freeze-thaw cycles on the characteristics variables of the
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freezing-thawing deformation was analyzed.
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The variation of the matric suction during the freezing-thawing processes was
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discussed.
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Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
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Jianguo Lu, Mingyi Zhang, Xiyin Zhang, Wansheng Pei, Jun Bi PII: DOI: Reference:
S0165-232X(17)30478-0 https://doi.org/10.1016/j.coldregions.2018.01.007 COLTEC 2513
To appear in:
Cold Regions Science and Technology
Received date: Revised date: Accepted date:
9 October 2017 12 December 2017 16 January 2018
Please cite this article as: Jianguo Lu, Mingyi Zhang, Xiyin Zhang, Wansheng Pei, Jun Bi , Experimental study on the freezing–thawing deformation of a silty clay. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Coltec(2017), https://doi.org/10.1016/j.coldregions.2018.01.007
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ACCEPTED MANUSCRIPT Experimental study on the freezing-thawing deformation of a silty clay
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Jianguo Lu1, 2, Mingyi Zhang1, 2*, Xiyin Zhang1, 3, Wansheng Pei1, 2, Jun Bi4
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1. State Key Laboratory of Frozen Soil Engineering, Northwest Institute of
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Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China
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2. University of Chinese Academy of Sciences, Beijing 100049, China
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3. School of Civil Engineering, Lanzhou Jiaotong University, Lanzhou 730000, China 4. School of Civil Engineering and Mechanical, Lanzhou University, Lanzhou 73000,
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China *
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Corresponding author. [email protected] (Mingyi Zhang)
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Abstract: Freezing-thawing deformation is a key factor in determining damages for engineering structures in cold regions. Based on the laboratory experiments, the
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characteristics of the freezing-thawing deformation were analyzed, and the variation of the matric suction was discussed. The results show that for each freeze-thaw cycle, the freezing-thawing deformation can be divided into five stages, i.e., cold shrink, fast frost heave, slow frost heave, thermal bulge and thaw settlement. At the onset of each cooling process, the cold shrink occurs and the freezing-thawing deformation decreases slightly; and then, with the decreased temperature, the freezing-thawing deformation rapidly increases at the beginning of freezing (fast frost heave); and
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ACCEPTED MANUSCRIPT subsequently slowly increases with freezing up to the expected value. During warming process, the inflation occurs with the rapid increase of the ambient temperature; later, with the thawing of ice, the freezing-thawing deformation decreases. Besides, there are similar changing trends for the freezing-thawing deformation and the matrix suction during the freezing-thawing processes. However,
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their changing mechanisms are completely different, and the soil matric suction has a
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stronger sensitivity to the variation of the ambient temperature and volumetric
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unfrozen water content. Furthermore, the effect of freeze-thaw cycles on the characteristic variables of the freezing-thawing deformation, such as cold shrink, net
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deformation, frost heave coefficient and thaw-settlement coefficient, mainly occurs in the first freeze-thaw cycle, which is due to the fact that the first freeze-thaw cycle has
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a significant influence on the soil structure and pore distribution for the remolded
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sample.
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Key words: freezing-thawing deformation; unfrozen water content; matric suction;
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freeze-thaw cycle; silty clay
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ACCEPTED MANUSCRIPT 1 Introduction Freezing-thawing behavior is a complex physical process, and the structure of soils will be significantly changed after freezing-thawing processes, which will cause deformation of soils, and these effects may in turn substantially change the foundation bearing capacity of structures (Cheng, 2005; Li et al., 2014; Özgan et al., 2015). At
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present, more and more engineering constructions are built in cold regions. Most of
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the regions related to frozen soil can produce large deformation during cyclic
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freezing-thawing (Nixon and Mcroberts, 1976). The freezing-thawing problem of soils is key in determining the stability of railways and highways in cold regions, e.g.,
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Qinghai-Tibet Railway and Qinghai-Tibet Highway (Cheng, 2005). The physical changes during the freeze-thaw cycles, such as water migration, heat transfer, and
Lai et al., 2013; Lai et al., 2014).
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stress redistribution, etc. can all cause the freezing-thawing deformation (Cheng, 2005;
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Freeze-thaw cycles are complicated processes which considerably change the
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physical structure and characteristics of soils (Yao et al., 2009). Some research about the influence of freeze-thaw cycles on the physical and mechanical characteristics of
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soils have been carried out. Chamberlain and Gow (1979) found that the freeze-thaw cycles can cause the reduction of porosity and the increase of hydraulic conductivity.
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After that Viklander (1998) tested the volume change of a fine-grained soil during the freezing-thawing processes, and found that the volume typically decreases for a loose soil and increases for a dense soil. This phenomenon has been confirmed by other studies under different experimental conditions (Qi et al., 2008; Qi et al., 2006). Konrad (2000) studied the influence of freeze-thaw cycles on the hydraulic conductivity of a Kaolinite-silt mixture, and developed a simplified porosity model to evaluate the hydraulic conductivity; and later based on the microstructure experiment 3
ACCEPTED MANUSCRIPT of a soft soil subjected to freeze-thaw cycles, Tang and Yan (2015) found that the porosity and specific pore area decreases, while the average pore diameter increases, which causes the increase of the hydraulic conductivity. With variation in the physical characteristics, the mechanical characteristics changes at the same time. Wang et al. (2007) and Xie et al. (2015) studied the effect of freeze-thaw cycles on the strength
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characteristics and mechanical behavior of a silty clay, and concluded that the
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freeze-thaw cycles make the parameters of soil a new dynamic equilibrium. Besides,
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it is found that other physical and mechanical parameters, such as thermal conductivity (Orakoglu et al., 2016; Pei et al., 2013), elastic modulus (Cui et al., 2014;
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Lee et al., 1995; Simonsen et al., 2002), damping ratio (Kweon and Hwang, 2013), and soil cohesion (Simonsen et al., 2002), etc. have been changed after freeze-thaw
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cycles. The variations of the physical and mechanical characteristics due to the freeze-thaw cycles would bring adverse effect in cold region engineering, which could
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cause the differential deformation and even engineering accidents.
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The freezing-thawing deformation is a key factor in determining the stability of engineering structures in cold regions, which would cause many engineering problems,
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including crack of pavement, damage for foundation, and fracture of pipelines (Cui et al., 2014; Yu et al., 2017). The main reason is that part of the water in the soils will be
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frozen into ice in freezing periods, and the volume increased by ice will have a significant effect on the soil characteristics, and the action of ice will greatly disrupt the soil structure. However, during thawing process, the thawing of ice will cause the reduction of bearing capacity, and the thaw settlement will occur simultaneously. In the study of freezing-thawing deformation, Li et al. (2014) studied the freezing-thawing deformation mechanism of a canal in seasonally frozen regions through a moisture-heat-mechanical model, and found that the repetitive 4
ACCEPTED MANUSCRIPT freezing-thawing deformations are mainly from the dramatic water migration and redistribution in the seasonal freezing-thawing layer. Zhang et al. (2016) established a coupled heat-moisture-deformation model to simulate the influence of seepage on temperature and deformation of subgrades. Kweon and Hwang (2013) studied the deformation characteristics of the subgrade soils and subbase materials in freeze-thaw
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cycles, and concluded that the deformation characteristics are related to the initial
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water content. Table (1930) expounded the influence of water migration on
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deformation during the freezing-thawing processes. Through the multi-layer deformation observation, Yu et al. (2017) concluded that the deformation of the
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embankment in permafrost regions is from multiple sources, and other research related to the source of the freezing-thawing deformation have been studied (Guo et
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al., 2016; Yu et al., 2016; Zhang et al., 2016a). In the field scale experiments, Chou et al. (2010) studied the effect of sunny-shady slopes on the stability of the highway
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embankment in warm permafrost regions, and pointed out that embankment stability
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have been destroyed with longitudinal cracks primarily caused by transverse asymmetric settlement. Based on the field monitoring data, the characteristics of
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embankment deformation were summarized along the Qinghai–Tibet Railway in four permafrost regions with different mean annual ground temperatures (Ma et al., 2011).
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It can be seen that considerable researches have been carried out to find out the freezing-thawing
effect
on
engineering
characteristics
of
soils.
And
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freezing-thawing deformation, as a significant index to judge the stability of buildings, plays an important role in cold region engineering. However, only a few studies are focused on the freezing-thawing deformation characteristics of soils during freeze-thaw cycles, and some research of freezing-thawing deformation existed were mainly the numerical simulations and the in-situ experiments on large scales 5
ACCEPTED MANUSCRIPT quantitatively. At the same time, it lacks systematic experiments of the two abnormal deformation phenomenon quantitatively, i.e., cold shrink and thermal bulge, although, Qi et al. (2006) have explained the phenomenon from the aspects of effective stress principle. Therefore, in this study, a series of freeze-thaw tests of a silty clay were performed to study the freezing-thawing deformation characteristics of the soil
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subjected to freeze-thaw cycles, such as cold shrink, thermal bulge, frost heave, and
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thaw settlement, etc. Besides, the variation of the matrix suction was also discussed.
2 Experimental setup
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2.1 Experimental apparatus
In this study, a freezing-thawing cabinet (Fig. 1(a)) was applied to test the soil
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sample, whose inner circumstance temperature can be set from -20℃ to +20℃ with
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an accuracy of ±0.1℃. The soil sample barrel (Fig. 1(b)) was placed in the middle of
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the freezing-thawing cabinet. Fig. 1(c) is the sensors arrangement in the soil sample. The moisture sensor (Fig. 2(a)), 52 mm in length with an accuracy of ±2%, based on the frequency domain reflection principle, together with a temperature sensor
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(precision: ±0.05℃, range: -30℃~+30℃), was embedded into the middle of the soil
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sample (Fig. 1(c)), the moisture sensor was horizontally placed, and the three rods were arranged in a vertical direction (Fig. 2(a)). The matric potential sensor (Fig. 2(b)), with the probe of 8 mm in diameter and 30 mm in length (precision: ±0.1kPa), was placed in 5cm from the bottom of the soil sample (Fig. 1(c)). And the deformation sensor (Fig. 2(c)), with the accuracy of 0.01mm, was placed on the top of the soil sample to measure the freezing-thawing deformation (Fig. 1(c)). All sensors were connected to a data logger, which was, in turn, connected to a computer, and the data acquisition interval was 1 minute. 6
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Fig. 1 Experiment apparatus (a) Freezing-thawing cabinet; (b) Soil sample barrel; (c) Sensors arrangement
Displacement sensor
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2.2 Experimental method
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Fig. 2 Experiment sensors (a) Moisture sensor; (b) Matric potential sensor; (c)
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The silty clay from the Qinghai-Tibet Plateau was used in the experiment, and its
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physical parameters are given in Table 1. Before the experiments, the soil was washed with distilled deionized water to eliminate the influence of salts, and then dried in an
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oven for 24 h with the temperature of 105℃. Later, the dried soil was crushed and sieved over 0.20 cm, and the grain-size distribution of the silty clay is shown in Fig. 3.
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Firstly, the soil was prepared with an initial volumetric water content of 27.21%, and sealed in a plastic bag for 24h to ensure adequate moisture redistribution. And then the
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soil was compacted into the soil sample barrel with a hammer to the target height of 300 mm and a dry density of 1.55×103 kg/m3. Finally, a thin plastic film was covered
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at the top surface of the sample to prevent moisture evaporation during freezing and thawing.
Table 1 Physical parameters of the soil sample
Fig. 3 Grain-size distribution of the silty clay
In the experiment, five freeze-thaw cycles were carried out (Fig. 4). For each 7
ACCEPTED MANUSCRIPT freeze-thaw cycle, the soil sample was maintained to a relative steady temperature of 2℃ 1℃ after cooling for more than 24h, which was to eliminate the effects of the initial water distributions and temperature gradients on the freezing characteristics, and to make the soil moisture and temperature in an equilibrium state. In order to make sure that the soil sample was super-cooled completely and the water was fully
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frozen, the small step temperature (0.2 ~ 0.5℃) was used to cool down the soil
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sample step by step from the initial steady temperature (about 2℃ 1℃) to -1℃, and
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then the desired temperature was applied (Ishizaki et al., 1996). In the five freeze-thaw cycles (Table 2), the controlled temperature patterns of the first two
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freeze-thaw cycles (1 and 2) were the same, i.e. step cooling and rapid warming; then
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the third freeze-thaw cycle was carried out under rapid cooing and rapid warming; finally, the two same cycles (4 and 5) were performed with step cooling, rapid cooling
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and rapid warming. In the controlled temperature patterns(Fig. 4), the step cooling
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was that the temperature was decreased step by step, namely, -1.5℃, -2℃, -6℃, -10℃, -14℃, and -18℃, and the rapid cooling was that the temperature was
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decreased to -18℃, directly. Additionally, the step cooling and rapid cooling was the cooling pattern that the temperature was decreased step by step initially, namely,
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-1.5℃, -2℃, and then, the temperature was decreased to -18℃, directly. The controlled temperature patterns were designed to study the effect of variation of unfrozen water content due to the decreased temperature on the freezing-thawing deformation of the soil. Furthermore, in each cycle, the ambient temperature was increased to warm the sample until the volumetric unfrozen water content was almost steady in the final freezing temperature (-18℃), and all the warming patterns were the rapid warming, namely, the temperature was increased to 0℃ from -18℃, directly 8
ACCEPTED MANUSCRIPT (Fig. 4).
Table 2 Controlled temperature patterns
Fig. 4 Controlled temperature of the freeze-thaw cycles
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3 Experiment results and analysis
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3.1 Variation of freezing-thawing deformation
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Fig. 5(a-e) is the freezing-thawing deformation of the soil sample during the freezing-thawing processes. From the figures, it can be seen that no matter what
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controlled temperature pattern is adopted, each freezing-thawing deformation can be
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divided into five stages, i.e. cold shrink, fast frost heave, slow frost heave, thermal bulge and thaw settlement. The experiment results are similar with the previous
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findings (Qi et al., 2006; Qi et al., 2008), which show that in the freeze-thaw cycles of
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Lanzhou silty soil, the phenomenon of cold shrink is not clearly found, while the phenomenon of thermal bulge distinctly occurs.
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At the onset of cooling, the soil sample is compressed (settlement) rather than
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expanded (heave), namely, the cold shrink occurs and the deformation decreases slightly in the soil sample. With the decrease of the soil temperature, the soil sample starts to freeze and the ice lenses are formed, at the same time, the expansion occurs and the deformation rapidly increases (fast frost heave), and subsequently the deformation slowly increases with freezing up to the expected value (slow frost heave). Because the ambient temperatures are similar (about -18℃) at the end of each cooling process in the five freeze-thaw cycles, the volumetric unfrozen water contents 9
ACCEPTED MANUSCRIPT are almost the same (about 9.8%) at the end of all cooling processes (Fig. 5). However, the value of thaw settlement is greater than that of the frost heave, therefore, a downward net deformation occurs for each freeze-thaw cycle, which causes the decrease of the volume and the increase of the dry density for the soil sample. This is
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mainly from the fact that the dry densities of loose and dense soils subjected to
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freeze-thaw cycles change in opposite, namely, the freeze-thaw cycles cause the
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densification of loose soils, while the opposite to occur for dense soils (Viklander, 1998).
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During the warming processes, with the rapid increase of ambient temperature, the
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thaw settlement does not happen immediately as is expected, instead, the inflation occurs (Fig. 5). This is mainly because during freezing, although the soil sample
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expands, the expanding potential, caused by growth of ice crystals, is not exerted
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completely. At the same time, the suction generated in the freezing process works as a restraining force among the particles. When the cooling source is withdrawn, the
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restraining force suddenly greatly decreases (Fig. 6), the sample expands due to the
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stress release, which causes a thermal bulge at the beginning of thawing (Qi et al., 2008). Later, with the thawing of ice, the freezing-thawing deformation decreases. From Fig. 5, it can also be seen that there is a close relationship between the freezing-thawing deformation and the volumetric unfrozen water content and soil temperature. During the freezing processes, the rapid increase of freezing-thawing deformation (fast frost heave) mainly occurs at the freezing point, during which is the beginning of the reduction of volumetric unfrozen water content. This is mainly from 10
ACCEPTED MANUSCRIPT the fact that the water in the large pores is frozen firstly, and the rapid increase of freezing-thawing deformation (fast frost heave) is mainly attributed to the ice formation in large pores, and then with the decreased temperature, the water in small pores is frozen, but the freezing-thawing deformation increases slightly. During the
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warming processes, there is a good relationship between the decreased
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freezing-thawing deformation and the increased volumetric unfrozen water content,
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but in the initial thawing stage, the change of freezing-thawing deformation is small even though the volumetric unfrozen water content increases, and then with increased
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ambient temperature, the soil temperature reaches to the thawing point, the rapid
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reduction of freezing-thawing deformation and the increase of volumetric unfrozen water are all in the sharp phase change stage, during which the thaw settlement occurs
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rapidly. This may be because that the ice in large pores begins to thaw, and the
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variations of the freezing-thawing deformation are mainly related to the phase change
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of water/ice in large pores.
(a) Freeze-thaw cycle 1
(b) Freeze-thaw cycle 2
(c) Freeze-thaw cycle 3
(d) Freeze-thaw cycle 4
(e) Freeze-thaw cycle 5 Fig. 5 Freezing-thawing deformation of the soil sample during the
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3.2 Variation of matric suction During the freezing-thawing processes, the volumetric unfrozen water content,
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changed with the temperature, causes the change of the freezing-thawing deformation as well as the variation of the soil matric suction. Fig. 6 (a-e) shows the variation of
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the matrix suction of the soil sample during the freezing-thawing processes.
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During the cooling process, with the decreased temperature (above 0℃), the cold
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shrink occurs but the matric suction keeps invariable; and then, when the expected temperature (below 0℃) is applied to the soil sample, the fast frost heave appears and
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the matric suction increases rapidly (Fig. 5); with the deformation reaching to the
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slow frost heave, the matric suction also changes slowly (Fig. 6).
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The variation of matric suction in the thawing process is quite different from that in the freezing process, which might be due to the remarkable hysteresis effect of
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volumetric unfrozen water content during the freezing and thawing processes (Wen et al., 2012). And this may be determined by several factors, e.g. the super-cooling effect
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of pore water (metastable nucleation), the pore-blocking effect (ink-bottle mechanism), the capillarity effect and electrolyte effect, etc. (Bittelli et al., 2003; Overduin et al., 2006). During the warming process, a constant ambient temperature (0℃) is applied to the surface of the soil sample, the thermal bulge occurs and the matric suction drops slightly (Fig. 5). With the increase of temperature, the freezing-thawing deformation and matric suction drop quickly in a short time, and the
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ACCEPTED MANUSCRIPT decrease rate of the matric suction is larger than that of the freezing-thawing deformation. Generally, although there are similar changing trends for the freezing-thawing deformation and the matrix suction during the freezing-thawing processes; however,
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their changing mechanisms are completely different, namely, the variations of the
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freezing-thawing deformation are resulted in the frozen heave or thaw settlement
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related to the phase change of ice/water during the freezing-thawing processes; while the variations of the matric suction are attributed to the different constraining force of
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soil particles to water and ice in pores, as well as related to the destroyed soil structure
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by the ice formation during the freezing-thawing processes (Wen et al., 2012; Peng et al., 2016). Furthermore, compared with the freezing-thawing deformation, the soil
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matric suction has a stronger sensitivity to the volumetric unfrozen water content
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(Figs. (5-6)).
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(a) Freeze-thaw cycle 1
(b) Freeze-thaw cycle 2
(c) Freeze-thaw cycle 3
(d) Freeze-thaw cycle 4
(e) Freeze-thaw cycle 5 Fig. 6 Variation of the matrix suction of the soil sample during the freezing-thawing processes
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ACCEPTED MANUSCRIPT 3.3 Effect of freeze-thaw cycle Cyclic freezing-thawing processes change the soil structure from a steady state to another steady state (Wang et al., 2007; Yang et al., 2004). The freezing-thawing deformation process during a freeze-thaw cycle is schematically shown in Fig. 7.
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From the discussion above, a whole freezing-thawing deformation includes five stages,
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namely, cold shrink, fast frost heave, slow frost heave, thermal bulge, and thaw
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settlement. In order to study the variations of deformation in the freeze-thaw cycles (e.g. Fig. 7), some characteristic variables related to the freezing-thawing deformation,
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i.e. net deformation hi , cold shrink deformation hc , thermal bulge deformation
deformation h , are introduced.
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ht , frost heave coefficient i , thaw-settlement coefficient A0 , and cumulative
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The net deformation hi of each freeze-thaw cycle reflects the effect of
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freezing-thawing process on the soil structure during a freeze-thaw cycle (Fig. 7), expressed as:
hi hi0 hit
(1)
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Where subscript i denotes the number of freeze-thaw cycle; hi0 is the initial height
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of soil sample; hit is the height of soil sample after freezing. The cold shrink deformation hc and thermal bulge deformation ht are introduced to reflect the effecting degree of freeze-thaw cycle on the two phenomenon, irrespectively (Fig. 7), which can be calculated by Eq. (2) and Eq. (3), respectively.
hc hi0 hc
(2)
ht hb hi
(3)
Where hc and hb are the height of soil sample after cold shrink and thermal bulge, 14
ACCEPTED MANUSCRIPT respectively; hi is the height of soil sample after thawing. And another deformation variable, the cumulative deformation h , is used to show the effect of freeze-thaw cycles on the soil structure, calculated by Eq. (4):
h hi
(4)
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At the same time, the frost heave coefficient is introduced to quantify the
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maximum frost heave of the soil sample in each freeze-thaw cycle, which is defined
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as the ratio of the increased height in the steady freezing temperature to the initial height for each freeze-thaw cycle, namely, as Eq.(5) shown (Qiu et al., 1994): hi hi0 hi0
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i
(5)
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The thaw-settlement coefficient, as a dimensionless parameter, has been introduced to evaluate the effect of freeze-thaw cycles on the thaw settlement, which
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is defined as the ratio of the decreased height after a thawing process to the steady height of the sample after a freezing process. It can be expressed as:
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A0
hi hit hi
(6)
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Fig. 7 Freezing-thawing deformation process during a freeze-thaw cycle
The characteristic variables of the soil sample during the freezing-thawing processes are listed in Table 3. Fig. 8 presents the cold shrink deformation and thermal bulge deformation versus number of freeze-thaw cycles. From Table 3 and Fig. 8, it can be seen that the cold shrink deformation increases with the number of freeze-thaw cycles, the value of the first freeze-thaw cycle is maximum (0.36 mm), and from the 15
ACCEPTED MANUSCRIPT second freeze-thaw cycle, the cold shrink deformation increases gradually with the number of freeze-thaw cycles. However, the thermal bulge deformation has a slight change (ranging from 0.10 mm to 0.20 mm) with the number of freeze-thaw cycles.
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Table 3 Characteristic variables during the freezing-thawing processes
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Fig. 8 Cold shrink deformation and thermal bulge deformation versus
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number of freeze-thaw cycles
The net deformation and cumulative deformation versus number of freeze-thaw
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cycles is shown in Fig. 9. From the figure, it can be seen that the net deformation of the soil sample is the maximum (-7.57mm) at the first cycle, but gradually becomes
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small from the second freeze-thaw cycles. At the same time, the cumulative
tends to be steady.
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deformation increases with the number of freeze-thaw cycles but the increase rate
freeze-thaw cycles
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Fig. 9 Net deformation and cumulative deformation versus number of
Fig. 10 shows the frost heave coefficient versus number of freeze-thaw cycles. From the figure, it can be seen that the frost heave coefficient in the first freeze-thaw cycle (1.62 10-2) is the maximum, and decreases with the number of freeze-thaw cycle. The thaw-settlement coefficient versus number of freeze-thaw cycles is shown in Fig. 11. As the change of the frost heave coefficient, the thaw-settlement coefficient is 16
ACCEPTED MANUSCRIPT also the maximum (4.07 10-2) in the first freeze-thaw cycle, but then gradually becomes small from the second freeze-thaw cycles. As analyzed above, the effect of freeze-thaw cycles on the characteristics variables of the freezing-thawing deformation, such as the cold shrink, net deformation, frost heave coefficient, and thaw-settlement coefficient, mainly occurs in
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the first freeze-thaw cycle. This is mainly from the fact that the freeze-thaw cycles
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have a significant influence on the soil structure and pore distribution (Cui et al., 2014;
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Qi et al., 2008), and generally, this effect is greatest in the first freeze-thaw cycle for the remold soil sample.
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Fig. 10 Frost heave coefficient versus number of freeze-thaw cycles
Fig. 11 Thaw-settlement coefficient versus number of freeze-thaw
5 Conclusions
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cycles
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Based on the above analysis and discussion for the effect of freeze-thaw cycles on the freezing-thawing deformation of a silty clay, some preliminary conclusions are
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drawn:
(1) For each freeze-thaw cycle, the freezing-thawing deformation could be divided into five stages, i.e., cold shrink, fast frost heave, slow frost heave, thermal bulge and thaw settlement. (2) There are similar changing trends for the freezing-thawing deformation and the matrix suction during the freezing-thawing processes. However, their changing mechanisms are completely different, and the soil matric suction has a stronger sensitivity to the variations of the ambient temperature and volumetric unfrozen water 17
ACCEPTED MANUSCRIPT content. (3) The effect of freeze-thaw cycles on the characteristics variables of the freezing-thawing deformation, such as cold shrink, net deformation, frost heave coefficient, and thaw-settlement coefficient, mainly occurs in the first freeze-thaw cycle, which is due to the fact that the first freeze-thaw cycle has significant influence
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on the soil structure and pore distribution of the remolded soil sample.
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Acknowledgements
This research was supported by the National Natural Science Foundation of China
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(Grant No. 41471063), the Program of the State Key Laboratory of Frozen Soil Engineering (Grant No. SKLFSE-ZT-23), the 100-Talent Program of the Chinese
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Academy of Sciences (Granted to Dr. Mingyi Zhang), the Key Research Program of Frontier Sciences of Chinese Academy of Sciences (QYZDY-SSW-DQC015), and the
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STS Program of the Chinese Academy of Sciences (Grant No. HHS-TSS-STS-1502).
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ACCEPTED MANUSCRIPT Table 1 Physical parameters of the soil sample Plastic limit (by gravity)/%
Liquid limit (by gravity)/%
Dry density /(kg/m3)
Porosity
27.21
17.44
31.84
1.55×103
0.43
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Initial volumetric water content/%
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Freeze-thaw cycles
Cooling pattern
Warming pattern
1
Step cooling
Rapid warming
2
Step cooling
Rapid warming
3
Rapid cooling
Rapid warming
4
Step cooling and rapid cooling
Rapid warming
5
Step cooling and rapid cooling
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Table 2 Controlled temperature patterns
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Rapid warming
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ACCEPTED MANUSCRIPT Table 3 Characteristic variables during the freezing-thawing processes 2
3
4
5
Net deformation/mm
-7.565
-1.283
-0.542
-0.474
-0.228
Cold shrink deformation/mm
-0.360
-0.129
-0.143
-0.104
-0.075
Thermal bulge deformation/mm
0.120
0.081
0.102
0.180
0.104
Frost heave coefficient/(×10-2)
1.620
0.920
0.911
0.816
0.823
Thaw-settlement coefficient/(×10-2)
4.076
1.321
0.982
0.967
0.971
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Freeze-thaw cycles
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ACCEPTED MANUSCRIPT Highlights
The characteristics of the deformation of a silty clay during the freeze-thaw processes were studied.
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The effect of freeze-thaw cycles on the characteristics variables of the
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freezing-thawing deformation was analyzed.
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The variation of the matric suction during the freezing-thawing processes was
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discussed.
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Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11