Effects of cyclic freezing and thawing on mechanical properties of Qinghai Tibet clay

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1 Cold Regions Science and Technology 48 (2007) Effects of cyclic freezing and thawing on mechanical properties of Qinghai Tibet clay Da-yan Wang, Wei Ma, Yong-hong Niu, Xiao-xiao Chang, Zhi Wen State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, CAS, Lanzhou , China Received 19 June 2006; accepted 7 September 2006 Abstract A fine-grained clay was compacted in the laboratory and was thereafter exposed to a maximum of 21 closed-system freezing and thawing cycles. The sample height, water content, stress strain behavior, failure strength, elastic modulus, cohesion and friction angle were measured in initial unfrozen soil as well as in subsequent thawed soil. The results show that the physical mechanical characteristic of investigated soils changed after it was subjected to freeze thaw cycles. The height of sample increased and water content decreased before the sample exposed to 7 10th cycle of freeze thaw, but both the height and water content remained constant after the 7th freeze thaw. The shape of the stress strain behavior curves were not affected by the freeze thaw process, but the resilient modulus and the failure strength were heavily influenced by the number of freeze thaw cycles. The minimal values for both the resilient modulus and the failure strength were frequently achieved after the specimen was exposed to about 3 7 freeze thaw cycles, therefore, the resilient modulus and the failure strength of soils experienced seven freeze thaw cycles could be applied to the process of cold region engineering design. For the soil investigated, the cohesion decreased with the increasing number of freeze thaw cycles, and the friction angle exhibit an increasing trend Elsevier B.V. All rights reserved. Keywords: Freeze thaw cycle; Failure strength; Elastic modulus; Cohesive force; Friction angle 1. Introduction A prerequisite for the pavement design, stability analysis and calculation is the availability of mechanical characteristic of subgrade soils. It is widely proved, however, that the factors such as soil structure, water content and bulk density, grain-size distribution, shape and mineralogy, the degrees of soil-grain interlocking, grain cementation and chemical weathering, the particle-bonding mechanisms in clay soils, and the presence of vegetation determine soil mechanical properties (e.g., Huang, 1983; Corresponding author. Tel.: address: dywang@lzb.ac.cn (D. Wang). Chen et al., 1994; Thevanayagam et al., 2002; Coop, 1991; Allman and Atkinson, 1992; Georgiannou et al., 1990; Jafari and Shafiee, 2004). In cold regions, a further dimension is added to soil mechanical properties through the introduction of freezing and thawing for the process of freeze thaw changing the micro structure of the soil (e.g., Tsytovich, 1973; Konrad, 1989; Andersland and Ladanyi 2004). In this case, the soil mechanical properties are also severely affected by ice bonding between particles during freezing and excess moisture during thawing. These effects may in turn substantially reduce the foundation capacity of structure (Tsytovich, 1973; Qi et al., 2004). A review of the published literature reveals that, although considerable research has been carried out on X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.coldregions

2 D. Wang et al. / Cold Regions Science and Technology 48 (2007) freezing and thawing of fine-grained soils and its effect on engineering properties, most researches have taken it as hydraulic barriers in liners and covers for landfills or caps for remediation of contaminated sites, and emphasized the freeze thaw effects on the void ratio, permeability and hydraulic conductivity (e.g., Chamberlain and Gow, 1979; Wong and Haug, 1991; Othman and Benson, 1993; Eigenbrod, 1996; Viklander, 1998a; Viklander, 1998b; Konrad, 2000; Simonsen and Isacsson, 2001; Van Klaveren, 1987), only limited information is available regarding the effects of freeze thaw on the soil mechanical characteristic (e.g., Ma et al., 1999; Zhang et al., 2004; Simonsen et al., 2002; Lee et al., 1995). However, the fine-grained soils are often used as foundation soils along the Qinghai Tibet Railway. If soil undergoes internal fabric changes after freeze thaw cycles, most likely the bearing capacity of the foundation will be affected. An increased knowledge of the mechanical behavior of the soil experienced freeze thaw cycle is therefore needed. In view of this point, a few researches have been conducted to investigate the effect of freeze thaw cycle on soil mechanics characteristic. For example, Johnson et al. (1978) analyzed the stress-deformation data for silt and clay subgrade soils from in situ tests and laboratory tests, and presented the resilient modulus of tested soils in the frozen, thawed, and recovering condition. Van Klaveren (1987) showed that the critical shear strength of a soil subjected to freeze thaw cycle is less than that of the same soil that has never been frozen. Formanek et al. (1984) found that the shear strength of a silt loam was reduced to less than half its original value after one freeze thaw cycle, but second and third cycles resulted in little additional change. Aoyama et al. (1985) found a reduction in soil cohesion that increased as frozen soil temperature dropped but found little change in the soil friction angle after the freeze thaw cycle. Simonsen et al. (2002) suggested that after a completed freeze thaw, the soil displays a decrease of approximately 20 60% in resilient modulus depending on the soil type. Yong et al. (1985) observed that significant changes in the undrained shear strength of clay from Matagami, Quebec, occur after one freeze thaw cycle and that additional cycles did not reduce the shear strength as much as occurred after the first cycle. Despite all that, the consistent quantitative correlations that would be applied for predicting the changes in engineering properties subject to freezing and thawing have not been established. Therefore, a series of cyclic freeze thaw tests for Qinghai Tibet clay was undertaken, to expand and supplement data from previous research on changes in soil mechanical properties. 2. Method and materials 2.1. Theoretical considerations For criteria considered in setting up this test program for subgrade or foundation soils were (i) the nature of the freezing environment, (ii) the direction of freezing, and (iii) the temperature of freezing and thawing, respectively. Typically, the soil freezing can be undertaken either in open or closed systems. In an open system, water is drawn up from a free surface through the soil as the freezing front moves downward. The movement of water to the freezing level, or frost front, creates ice lenses in the soil. In a closed system, a redistribution of moisture to the freezing front occurs. The redistribution of moisture causes an increase in water content and a decrease in dry density near the freezing front (Wong and Haug, 1991). However, in many conditions, the clay could be taken as a low-permeability material and was used as liners and soil covers, the rate of moisture transport is generally slower than the rate of frost penetration. Thus, insufficient time is available during freezing to permit a continuous supply of water to reach the freezing front. In addition, moist clay does not have a supply of surplus water available for transport as required in an open system. For these reasons, a closed system was adopted for the present test program. With respect to the effect of direction of freezing, Zimmie et al. (1991) investigated hydraulic conductivity changes for two clays compacted at different water contents by one-dimensional or three-dimensional freezing. The authors observed that changes in hydraulic conductivity were almost identical in both freezing methods. So, we take the three-dimensional freezing and thawing as freeze thaw cycle model used in this experiment. As for the temperature setting of the frozen soil and thawed soil, we think that the temperature too close to 0 C should be avoided owing to the possibility of incomplete freezing or even no freezing at all because of the absence of ice nucleation. So 7 C is chosen as the tested temperature for frozen soil, and +14 C the tested temperature for thawed soil. To reduce sample disturbance during the freezing and thawing cycle, sample temperature was not measured in the process of freeze thaw cycles. However, prior to the test program, a specimen with the same physical properties, instrumented three thermocouples, was used to investigate the soil temperature status and the time attaining the abovementioned soil temperature. Then, the time needed by the soil completely arriving at 7 Cor +14 C was used to determine soil freeze thaw status

3 36 D. Wang et al. / Cold Regions Science and Technology 48 (2007) for the remaining experiments. In the current experiment, the time for completely freezing the soil sample to 7 C is 18 h and for completely thawing the soil sample to +14 C is 6 h Tested material and procedure Fig. 1. Grain-size distribution of studied Qinghai Tibet clay. Table 1 Sample height and water content of the different number of freeze thaw cycles The number of freeze thaw cycles The soil used in the current study was fine-grained clay, which has been used as subgrade material and foundation soil in the process of building Qinghai Tibet Railway. The main physical parameter such as liquid limit is 36.8% and plastic limit is 20.7%. The grain-size distribution is shown in Fig. 1. The procedure of preparing samples is generally performed according to the Specification of Soil Test (GB/T ) issued by the Ministry of Water Resources, PRC. All the specimens, typically 61.8 mm in diameter and 125 mm in height, were compacted in six layers, with a density of 1.8 g/cm 3 and water content of 17.78%. Then all specimens were covered with rubber sleeves to prevent the evaporation of water, and separated into eight groups, each group has five specimens (one is spare). Among these eight groups, one group was tested in triaxial compression test without undergoing freeze thaw cycle to establish a typical mechanical behavior pattern for clay not subjected to freeze thaw cycles. The other seven groups were tested in triaxial compression after undergoing freeze thaw cycles. The freeze thaw cycle for tested specimens took place in a thermo-tank. Adjusting the temperature of the thermo-tank governs the number of freeze thaw cycles for specimens. After the specimens experienced a series number of 1, 3, 5, 7, 10, 15, and 21 cycles of freezing and thawing, the triaxial compression experiments were conducted. The confining pressures of triaxial compression are 200, 400, 600, and 800 kpa, respectively, with the rate of axial loading of 0.5 mm/min. 3. Results and discussion Sample height/ cm Water content/ % 3.1. Effect of the freeze thaw cycle on the height and water content of the specimen The freeze thaw process is a process that the soil in an unstable state develops into a dynamic stable state, and repeated freeze thaw cycles will change the soil's structure towards a new dynamic stable balance state (Yang et al., 2004). In order to learn the effect of the freeze thaw cycles on the soil physical properties, the height and the water content of each specimen were measured before and after freeze thaw. In Table 1 the height and water content of each specimen exposed to a series of freeze thaw cycle are summarized. As the values presented in the table were measured after the thawed specimen reached the stable state, the effect of uncontrollable factors such as fluctuated environmental temperature or preparing sample errors could be eliminated. To evaluate the change of specimen height, a dimensionless parameter, B, has been introduced, which is defined as the ratio of the increment of height obtained in the thawed soil after N cycles (ΔH ) to the height measured initially in the unfrozen soil (H 0 ), namely, Eq. (1). B ¼ DH ð1þ H 0 Also, another dimensionless parameter, F, has been introduced to evaluate the effect of freeze thaw cycle on specimen's water content as follows: F ¼ Dw ð2þ w 0 where, Δw is the increment of water content obtained in the thawed soil after N cycles, w 0 is the initial water

4 D. Wang et al. / Cold Regions Science and Technology 48 (2007) content in the unfrozen soil. In Fig. 2 the ratios B and F are plotted versus the number of freeze thaw cycles for the tested Qinghai Tibet clay. Fig. 2 reflects that during each cycle the height of specimen increases slightly at the beginning but stabilized after 7 cycles, even though the soil was exposed to 21 cycles of freeze thaw. At the same time, the water content of the specimen decreased slightly with increased number of freeze thaw cycles at the beginning but gradually stabilized after ten cycles. The minimum water content in the specimen was found after ten cycles. Taking into account both of the features of the height change and the minimum water content, it implies that a new dynamic balance in the internal fabric of soil would be reached when the soil has been undergone for 7 10 cycles of freeze thaw. In general, when prepared specimen is placed into the thermo-tank, a sudden constant temperature below freezing is applied to the surface of a soil sample, unsteady heat flow is initiated. The freezing front progresses into the soil as a function of the imbalance of the heat supplied to the heat removed and pore water freezes in situ. At the same time, a temperature gradient is formed between the frozen surface and unfrozen zone, a suction gradient develops in the frozen zone in response to any temperature gradient and water migrates from the unfrozen soil through the continuous unfrozen water films into the frozen zone where it freezes some what behind the freezing front. Hence, the ice crystals appeared on the surface of the sample. But in a closed system, not enough pore water is available for migrating under temperature gradient and Fig. 2. B and F versus the number of freeze thaw cycles. a: B versus the number of freeze thaw cycles. b: F versus the number of freeze thaw cycles. pore water migration will soon stop (Andersland and Ladanyi, 2004). A lot of ice lenses in soil may form due to freezing in situ water. During in situ water freezing, water expands about 9% as phase transition, which will increase the soil volume and increase the height of the soil. When the temperature of the thermo-tank increases, the sample surface, impacted by surrounding air, first begin to melt. During the process of thawing, the core temperature of sample is always lower than that of the surface, which leads the melted pore water in the surface layer migrating into the center zone of the sample. However, this process will stop sooner due to the core temperature of the sample increasing. With the increase of the sample temperature, the soil ice melts, and the pore volume decreases as the self-weight of overlying soil, thus cause the specimen volume decreasing. This process is expressed by the specimen height decreasing after thawing. The magnitude of the decrease, however, is always less than the swell increment brought by pore water freezing, owing to the cohesion in the soil. After several cycles of freeze thaw, the volume variety, expressed by specimen height, arrives at a constant value, reflecting that a dynamic equilibrium is reached in the specimen internal fabric. In the present experiment, the needed cyclic number is 7. Provided the pore water migrating from the center to the surface of the specimen, during the freezing process, as forward migration, the migration from the surface to the center, during the melting process, is called converse migration, the amount of forward migration is always much more than that of the converse migration. After several cycles of freeze thaw, the amount of migration between forward and converse migration will also reach a dynamic balance. In the present study, the needed cyclic number is 7 10 as well. Several investigations conducted on the effect of the number of freeze thaw cycles on the hydraulic conductivity of compacted clays (e.g., Wong and Haug, 1991; Othman and Benson, 1993; Eigenbrod 1996; Viklander 1998b) have shown that the hydraulic conductivity of the compacted clay may increased significantly after a single freeze thaw cycle, and further increases in hydraulic conductivity occur with more cycles of freeze thaw. After a few freeze thaw cycles of around 3 to 9, however, the change in hydraulic conductivity ceases, hence a new balance state will be achieved by freeze thaw. When soil is in the abovementioned balance state, the additional freeze thaw process will not influence the soil physical properties such as water content, volume, permeability, hydraulic conductivity, and porosity. Our experimental results are consistent with the results obtained by previous researcher.

5 38 D. Wang et al. / Cold Regions Science and Technology 48 (2007) Effect of the freeze thaw cycle on stress strain behavior As the temperature of the sample fluctuates below 0 C, the soil pore water changed into ice. These ice can displace soil particles and separate soil aggregates, often disrupting the interlocking of soil grains and changing the soil structure, void ratio, density, soil fabric, saturated water-holding capacity, and hydraulic conductivity, resulting in decreased soil cohesion and mechanical strength (e.g., Aoyama et al., 1985; Benoit and Voorhees, 1990; Chamberlain and Gow, 1979; Andersland and Ladanyi, 2004). In order to learn the detailed changes of mechanical behavior influenced by freeze thaw, here, triaxial compression tests have been conducted on unfrozen clay and thawed clay under four different confining pressures after the soil experienced 3, 7, 10, 21 cycles of freeze thaw, respectively. The results are presented in Fig. 3. Fig. 3 shows that the stress strain curves of the thawed clay will transfer from strain-softening type to strain-hardening type with the confining pressure increase from 200 kpa to 800 kpa. In other words, the stress strain behavior of thawed clay expresses strain-softening type under lower confining pressure and strain-hardening type under higher confining pressure. At the same time, the type of the stress strain curves is not affected by freeze thaw process, but the elastic modulus and the failure strength are heavily influenced by the number of freeze thaw cycles Effect of the number of freeze thaw cycles on resilient modulus Lee et al. (1995) investigated the resilient properties of cohesive soils and found that the stress level at 1% (ε u1.0% ) strain in the unconfined compression test could be used to estimate the resilient modulus. They concluded that cohesive soils with ε u1.0% lower than 55 kpa would exhibit negligible freeze thaw effects. In contrast, soils with ε u1.0% higher than 103 kpa would exhibit a decrease of over 50% in resilient modulus due to freeze thaw. Simonsen et al. (2002) conducted tests on various coarse and fine-grained subgrade soils at selected temperatures from room temperature down to 10 C and back to room temperature and obtained a continuous resilient modulus during full freeze thaw cycling. The result indicated that after completed freeze thaw, the resilient modulus displayed decreases of approximately 20 60% depending on soil type. The volume in a very dense soil might increase due to freeze thaw, making the soil structure slightly looser than it was prior to freezing. However, a common factor Fig. 3. Stress strain behavior of unfrozen clay and clay soils experienced freeze thaw cycles under different confining pressure.

6 D. Wang et al. / Cold Regions Science and Technology 48 (2007) in the previously mentioned research is the absence of the effect of repeated freeze thaw cycle on soil resilient modulus. Here, the variety of resilient modulus for clays subjected to 0 21 freeze thaw cycles was investigated. According to Lee's approach, the resilient modulus is defined as a ratio of the deviator stress increment at 1% axial strain to the axial strain increment, which can be expressed by E ¼ Dr De ¼ r 1:0k r 0 ð3þ e 1:0k e 0 where, Δσ is the increment of deviator stress, Δε is the increment of axial strain; σ 1.0% is the deviator stress corresponding to the axial strain of 1.0% (ε 1.0% ); σ 0 and ε 0 are the initial stress and strain, respectively. Using the data obtained from the triaxial compression test, the resilient modulus of the tested clay subjected to each freeze thaw cycle can be calculated by Eq. (3), as shown in Fig. 4. Fig. 4, indicates that the general developing trend of resilient modulus decreases firstly, then gradually recovers with the number of freeze thaw cycle increasing, and the greatest changes in resilient modulus in all of the four different confining pressure conditions were obtained after the first cycle. This point suggests that a significant disturbance was took up during the first freeze thaw cycle process, which is in agreement with the other studies reported by Othman and Benson (1993). After the specimen exposed to seven cycles of freeze thaw, the minimum resilient modulus is reached, despite different confining pressures. When the number of freeze thaw cycle goes beyond the limit of seven times, the resilient modulus will recover to a certain degree and remain constant with Fig. 4. Resilient modulus of soils versus the number of freeze thaw cycles. further freeze thaw cycles. Fig. 4 also indicates that the magnitude of the resilient modulus decreases by 18 27% of unfrozen soil resilient modulus depending on confining pressure exerted on the triaxial compression tests Effect of the number of freeze thaw cycles on failure strength As the soil is exposed to freezing, the pore water is turned into ice. Ice forces will separate the soil particle from each other and cause the pore volume increases. When the ice melts, the increased pore volume that developed during freezing will not fully recover after soil thawing, owing to the cohesion in the soil (Andersland and Ladanyi, 2004). Therefore, many people think that the freeze thaw process could reduce the soil strength. Will this decreasing process develop with additional freeze thaw cycles? To investigate this issue, the triaxial compression tests have been conducted on the soil in never-frozen and thawed state experiencing 1, 3, 5, 7, 10, 15, and 21 cycles, respectively. Considering the shape of the stress strain curves is different under the different confining pressure, the judgment criterion for soil failure is different. If the soil under lower confining pressure exhibits a strain-softening stress strain curve, the peak stress indicates the maximum level of shearing stress requiring to rupture a majority of the inter-particle contacts and to slide some of the particles over each other. So the peak stress could be taken as failure strength of the studied specimen. If the specimen under higher confining pressure exhibits a strain-hardening stress strain curve, namely, which demonstrates a constant shearing resistance to shear, the stress corresponding to 15% strain could be regard as failure strength. Typical results are shown in Fig. 5a for the studied soils where the failure strength is plotted versus the number of freeze thaw cycles for various test stages, along with the results of a ratio of the soil strength after each freeze thaw cycle to the strength of never-frozen and thawed soil (Fig. 5b). With increased number of freeze thaw cycles, the failure strength decreased slightly after each cycle. The smallest failure strength was measured after 3 7th cycle. For a soil under the same confining pressure, with the number of freeze thaw cycle increasing, the failure strength decreased slightly at the beginning, then increased to a certain level, and maintained constant without of influenced by cyclic freeze thaw. The number of freeze thaw cycles needed to achieve the minimum failure strength is about seven cycles, basically equal to that of sample height, water content and resilient modulus needed. Therefore,

7 40 D. Wang et al. / Cold Regions Science and Technology 48 (2007) tenth cycle is about 2.24 MPa, which is approaching 2.25 MPa of never-frozen thawed soil strength under the same confining pressure, while it is 1.2 MPa under the confining pressure of 200 kpa, and is less than 1.69 MPa of never-frozen thawed soil strength. Therefore, the higher confining pressure could increase the strength of soils disturbed by freeze thaw cycle to an initial level, where the soil is never exposed to freeze thaw Effect of the number of freeze thaw cycles on cohesion and internal friction angle Fig. 5. Failure strength of soils versus the number of freeze thaw cycles. the failure strength as the soil experienced for seven freeze thaw cycles could be applied to the engineering design in the cold region. As the confining pressure applied to the soil causes soil particles to move, rearrange, and consolidate, and recover soil strength in a sense, the confining pressure is another factor which influences the soil strength. From Fig. 5, we may find how the failure strength is influenced by the confining pressure. For the specimen under higher confining pressure, the recovery failure strength after ten freeze thaw cycles is higher than that of the specimen under lower confining pressure, which indicates that the soil disturbance caused by freeze thaw process predominates over soil strength for the lower confining pressure, but for the specimen subjected to higher confining pressures, the soil particle rearrangement caused by higher confining pressure will close the cracks and fissures produced by freeze thaw and enhance the soil strength. In Fig. 5, the failure strength under confining pressure of 800 kpa after the Cohesion and internal friction angle are two key factors to evaluate the shearing strength of the soil from the microcosmic view. In general, they are constant for any single soil or group of soils. The internal friction angle represents the slipping and interlocking of one particle with respect to another, and the cohesion reflects the synthesis action of all kinds of physical chemical forces between particles, such as Coulomb force, Van der Waals forces and the ions of adjacent particles, bonding action and so on. The magnitude of cohesion is frequently influenced by the space between particles and bonding force caused by bonding material. The aim of studying the number of freeze thaw cycle's effect on cohesive force and internal friction angle is to learn the effect of freeze thaw action on the shear strength of the soil. According to soil strength obtained from triaxial shearing testing at four levels of confining pressure, namely σ 3 =200, 400, 600, and 800 kpa, the failure principle stress line of the specimen exposed to 3 freeze thaw cycle can be plotted in p q space, as shown in Fig. 6. In this figure the deviator stress is q= (σ 1 -σ 3 )/2 and the mean stress is p=(σ 1 -σ 3 )/2, where Fig. 6. Failure envelop in p q space.

8 D. Wang et al. / Cold Regions Science and Technology 48 (2007) σ 1 and σ 3 are the failure axial stress and the corresponding radial stress, respectively. Fig. 6 clearly shows that q increases linearly with increasing p. The general behavior of p and q can be expressed as q ¼ a þ ptana ð4þ where a is the intercept of the straight line with q axis and α is the slope. The parameters a and α for the specimen experienced 0 21 freeze thaw cycles are obtained using the abovementioned methods (Table 2). The results are summarized in Table 2. Based on the data of a and α shown in Table 2, the internal friction angle (φ) and cohesive force (c) of the studied soil can be calculated by the following relationship: u ¼ sin 1 ðtgaþ ð5þ c ¼ a cosu ð6þ The influence of the number of freeze thaw cycle on the cohesive forces is illustrated in Figs. 7 and 8. Itis clear from this figure that the cohesion decreases with increasing number of freeze thaw cycle, and suggests that the space between clay particles may increase with increasing of the freeze thaw cycles within the range of the tested cycles. However, the internal friction angle exhibits an increasing trend with increasing freeze thaw cycles even though the data appears more scattered. This result is in agreement with the previous studied results obtained by Ogata et al. (1985). They observed in both undisturbed alluvial soil and consolidated kaolin that freeze thaw cycles reduced cohesion and increased the angle of internal friction. Fig. 7. Cohesion of studied clay versus the number of freeze thaw cycles. the design of buildings exposed to freezing and thawing conditions. An experimental study was performed on compacted Qinghai Tibet clay at different initial confining pressure to investigate the effect of freeze thaw on the mechanical properties such as stress strain behavior, failure strength, resilient modulus, cohesive force and friction angle. The following conclusions are drawn based on this study: (1) The height of the specimen examined in this test program increased and the water content decreased with increasing numbers of freeze thaw cycles during the first seven cycles. However, after seven cycles, they will keep constant with additional freeze thaw cycles. (2) Even though the magnitude of the resilient modulus and the failure strength were influenced 4. Summary and conclusions The possibility of changed mechanical properties subsequent to freezing and thawing is very important for Table 2 Summary of triaxial experiment results under the different number of freeze thaw cycles The number of freeze thaw cycles a (MPa) α ( ) c (MPa) φ ( ) Fig. 8. Friction angle of studied clay versus the number of freeze thaw cycles.

9 42 D. Wang et al. / Cold Regions Science and Technology 48 (2007) by increasing the number of freeze thaw cycles, the type of the stress strain curve was only in response to the variation of confining pressure and regardless of increasing the number of freeze thaw cycles. With respect to resilient modulus, with the number of freeze thaw cycles increasing, it decreases gradually before seven cycles, and after seven cycles, it increases by degrees to a certain level and then remains constant with additional freeze thaw cycles. The greatest decrease of the resilient modulus took place after the first cycles, which indicated that a significant disturbance happened during the first freeze thaw process. In general, the magnitude of the resilient modulus decreases about 18 27% of unfrozen soil resilient modulus depending on the confining pressure. With respect to the failure strength, it reaches the minimum point after the specimen exposed to about 3 7 freeze thaw cycles, and subsequently it will increase with increasing the number of freeze thaw cycles until the cycle number arrives at 10. When the number of cycles exceeds 10, the failure strength of tested soil will keep constant and regardless of freeze thaw cycles increasing. It is suggested that the resilient modulus and the failure strength of the soil experiencing seven freeze thaw cycles could be applied to the engineering design in the cold region. (3) The cohesion in the soil decreases with the number of freeze thaw cycle increases, while the angle of internal friction exhibits an increasing trend with the number of freeze thaw cycle increases. 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