- Email: [email protected]
Freeze–thaw performance of clayey soil reinforced with geotextile layer
Cold Regions Science and Technology 89 (2013) 22–29
Contents lists available at SciVerse ScienceDirect
Cold Regions Science and Technology journal homepage: www.elsevier.com/locate/coldregions
Freeze–thaw performance of clayey soil reinforced with geotextile layer Mahmoud Ghazavi ⁎, Mahya Roustaei 1 Civil Engineering Department, K. N. Toosi University of Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 1 September 2012 Accepted 7 January 2013 Keywords: Freeze–thaw cycling Clay Geotextile layer UU triaxial test CT images
a b s t r a c t In cold climates, freeze–thaw cycling is an important issue in engineering. In freeze cycles, translocation of water and ice that can be caused by thermodynamic conditions at temperatures just below 0 °C, changes engineering properties of soils. In previous studies, changes in physical, chemical and mechanical properties of soils were investigated. In this study, UU Triaxial compressive tests have been performed to investigate the effect of freeze–thaw cycles on strength properties of soil reinforced with geotextile layer. A clayey soil, reinforced with a geotextile layer located at mid-height of the sample, was compacted in the laboratory and subjected to a maximum of 9 closed-system freeze–thaw cycles. Computerized tomography (CT) images have also been taken from samples. It was found that for the investigated soil, unconsolidated undrained triaxial compressive strength of unreinforced soil decreased with increasing the number of freeze–thaw cycles, whereas reinforced samples showed better performance and the strength reduction amount decreased from 43% to 14% by reinforcing the soil. CT images have shown that free water moved through the soil particles toward the lower part of the soil samples. In addition, it was found that sample reinforcement can reduce the effect of freeze–thaw cycles on changes of cohesion and resilient modulus of the soil. © 2013 Elsevier B.V. All rights reserved.
1. Introduction It has been recognized that freeze–thaw cycling is a weathering phenomenon which is normal in cold climates and considerably changes the structure of soils and thus has great influence on engineering properties of soils such as physical features including hydraulic permeability, densification, unfrozen water content, and mechanical features involving strength, compressibility, and bearing capacity. In the permafrost regions like Canada, it has been found that the embankment constructed on soil which has never experienced freeze–thaw cycles was damaged in just one year due to the loss of bearing capacity (Leroueil et al., 1991). Therefore, newly constructed highway embankments that are left unpaved for a few years may be subjected to possible damages by freeze–thaw cycles (Eigenbrod, 1996). Qi et al. (2006) reviewed the latest efforts which were done to investigate the influence of freeze–thaw cycles on soil properties. They summarized these influences in two parts: physical properties such as density and hydraulic permeability and mechanical properties such as ultimate strength, strain–stress behavior and resilient modulus. According to this research, loose soils tend to be densified and dense soils become looser after freeze–thaw cycles and both loose and dense soils may attain the same void ratio after a number of cycles (Konrad, 1989). Having increased the large pores that are left after the thaw of ice crystals, permeability will increase (Chamberlain et al., ⁎ Corresponding author. Tel.: +98 21 88779623; fax: +98 21 88779476. E-mail addresses: [email protected] (M. Ghazavi), [email protected] (M. Roustaei). 1 Tel.: +98 912 5604076; fax: +98 21 88779476. 0165-232X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.coldregions.2013.01.002
1990). These cycles reduce the ultimate strength of soils. All over consolidated soils exhibit a peak on the triaxial stress–strain curve that is reduced or may even disappear (Graham and Au, 1985). Resilient modulus is one of the most important factors in pavement designs that will be reduced significantly by even a small number of freeze–thaw cycles (Simonsen and Isacsson, 2001). In addition these cycles decrease the undrained shear strength considerably which is an important factor in engineering properties of fine-grained soils (Graham and Au, 1985). Along with many applications for soil improvement, there come several widely varied methods. Taking the influence of freeze–thaw cycles on soils into consideration, just a few researchers have contemporarily focused on using additives which can control the effects of these cycles. Yarbesi et al. (2007) stabilized two granular soils by silica fume– lime, fly ash–lime, and red mud–cement additive mixtures. Their experimental results show that stabilized samples with silica fume– lime, fly ash–lime, and red mud–cement additive mixtures have high freezing–thawing durability as compared to unstabilized samples. These additive mixtures have also improved the dynamic behavior of the soil samples. Therefore silica fume–lime, fly ash–lime, and red mud–cement additive mixtures, particularly silica fume–lime mixture, can be successfully used as an additive material to enhance the freeze–thaw durability of granular soils for road constructions and earthwork applications. Kalkan (2009) used a fine grained soil stabilized by adding silica fume which was generated during silicon metal production. The test results show that the stabilized fine-grained soil samples containing silica fume exhibit high resistance to the freezing and thawing effects
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
as compared to natural fine-grained soil samples. The silica fume decreases the effects of freeze–thaw cycles on unconfined compressive strength and permeability. Hazirbaba and Gullu (2010) performed CBR tests on a fine-grained soil to investigate the influence of freeze–thaw conditions and also no freeze–thaw conditions on soil samples which were treated with the inclusion of geofiber and synthetic fluid in soaked and unsoaked conditions. The results indicate that the addition of geofiber together with synthetic fluid is generally successful in providing resistance against freeze–thaw weakening, and that the addition of synthetic fluid alone isn't very effective against the detrimental impact of freeze–thaw. The results from soaked samples subjected to a freeze–thaw cycle showed poor CBR performance for treatments involving synthetic fluid while samples improved with geofibers alone generally offer better performance. Liu et al. (2010) conducted dynamic triaxial tests on cement and lime-modified soils with different blend ratios in freeze–thaw cycles. The results show that after repeated freeze–thaw cycles, the modified soils exhibit better performance than before modification, the cement-modified clay is superior to the lime modified clay, and all soil mechanical properties are visibly improved. Zaimoglu (2010) investigated the effect of randomly distributed polypropylene fibers on strength and durability behavior of a fine-grained soil subjected to freezing–thawing cycles. The content of polypropylene fiber was varied between 0.25% and 2% by dry weight of soil in the tests. It was observed that the mass loss in reinforced soils was almost 50% lower than that in unreinforced soil. It was also found that the unconfined compressive strength of specimens subjected to freezing–thawing cycles generally increases with increasing fiber content. In addition, the results indicated that the initial stiffness of the stress–strain curves is not affected significantly by the fiber reinforcement in the unconfined compression tests. Ghazavi and Roustaie (2010) reinforced a caolinite clay with steel and polypropylene fibers and exposed the soil samples to a maximum of 10 closed-system freeze–thaw cycles. They found that increasing the number of freeze–thaw cycles results in the decrease of unconfined compressive strength of clay samples by 20–25%. Moreover, the inclusion of fiber in clay samples increases the unconfined compressive strength of soil and decreases the frost heave. Furthermore, the results of the study indicate that the addition of 3% polypropylene fibers results in the increase of unconfined compressive strength of the soil before and after applying freeze–thaw cycles by 60% to 160% and decrease of frost heave by 70%. The strength of cohesive soils may be enhanced using geosynthetics. Most laboratory and theoretical investigations have been conducted on granular soils reinforced with geosynthetic material, while limited studies have concentrated on geosynthetic-reinforced cohesive soils. Krishnaswamy and Srinivasula (1988) reported the influence of the distance between the reinforced materials as well as moisture content of samples tested in undrained condition by triaxial apparatus. The soil was silty clay and reinforced with geotextile. Srivastava et al. (1988) studied the behavior of silty soil reinforced with geotextiles by using unconfined and triaxial tests. By analyzing the confining pressure, the number of reinforcing layers and the ratio of height to the diameter of the sample were evaluated. To evaluate the behavior of cohesive soil reinforced with a geotextile, Mirmoradi and Noorzad (2010) examined soil samples with different moisture contents, relative compaction, soil type, confining pressure, type and number of geotextile layers. They found that as the moisture content increases, the peak strength of both the reinforced and unreinforced samples decreases and the axial strain at failure increases. Moreover, by increasing relative compaction, the peak strength of the sample and the axial strain at failure increase, whereas the peak strength ratio decreases. The peak strength ratio is the ratio of the peak strength of the reinforced samples to that of the unreinforced samples. For soils with low plasticity, it was found that
23
the main cause of increase in the strength is the increase in the cohesion of the reinforced sample. However, in soils with higher plasticity index, as the number of geotextile layers increases, the internal friction angle of the reinforced samples increases as well. The aim of the present study is to investigate the influence of reinforcement with geotextile layer on reducing the freeze–thaw cycles effects. For this purpose, the physical and mechanical behavior of cohesive soils reinforced with geotextile have been evaluated before and after applying freeze–thaw cycles in triaxial tests. 2. Materials In this study, laboratory tests were carried out on a clayey soil classified as CL in the Unified Soil Classification System. It is noted that the effects of freeze–thaw cycles are more considerable in fine-grain soils in comparison with sand or gravel (Qi et al., 2006). The soil properties are presented in Table 1 and its grain size distribution is shown in Fig. 1. Standard Proctor Compaction tests were performed on the soil, and a maximum dry mass density of approximately 1.78 g/cm 3 at optimum moisture content (OMC) of approximately 17.4% was obtained. The specimens were reinforced with one layer of geotextile which was placed in the middle of sample height. The physical and mechanical properties of geotextile reported by the manufacturer are presented in Table 2. 3. Testing procedure The scope of this investigation is to study the effects of application of geotextile layer on the strength changes of highly compressible clay compacted at maximum dry density with the optimum moisture content and subjected to 0–9 freeze–thaw cycles. To evaluate the soil changes in these cycles, four main following steps should be taken for each sample. Sufficient amount of extra samples (app. thirty percent of the whole) was prepared and tested in order to examine the repeatability of test results. 3.1. Specimen preparation All specimens having 50 mm diameter and 100 mm height were prepared with the maximum dry unit weight and optimum water content. For sample preparation, initially, the necessary OMC was determined and mixed with the soil and the mixture was then placed within double layered plastic bags and sealed for 24 h to achieve uniform water content within the soil mass. The moisture content was then checked before sample compaction. During the soil compaction procedure in the mold, when half of the sample was compacted, one geotextile layer was placed in the middle height of the sample. After removal of each sample from the mold, it was immediately covered with a plastic layer to protect its moisture content. 3.2. Freeze–thaw cycles To prepare the samples for the closed system freezing and thawing cycles, specimens were placed in a digital refrigerator at −20 °C for 6 h and then at +20 °C for thawing phase for 6 h. These temperatures had been previously used in some research works (Qi et al., 2006). Six hours is a proportional period after which the alteration of specimens' Table 1 Properties of soil. Gs
2.657
Plastic limit Liquid limit Plastic index
36% 20% 16%
24
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
Fig. 1. Grain size distribution of clay.
height would become constant. This means that the height increase in freeze phase and the height decrease in thaw phase stop. The cycles were continued up to 9 cycles. This number of cycles was chosen since most soil strength reduction would occur in primary cycles and after 5–10 cycles a new equilibrium condition would become predominant on samples (Ghazavi and Roustaie, 2010).
As mentioned in the previous section, several geotextilereinforced and unreinforced samples were tested to study the strength change of reinforced soil subjected to freeze–thaw cycles. The variations of stress–strain response of samples are named UnRe-NC for unreinforced samples under N freeze–thaw cycles and Re-NC for geotextile-reinforced samples subjected to N freeze–thaw cycles.
3.3. Strength testing 4.1. Effect of the freeze–thaw cycles on stress strain behavior Strength features of soil were measured in unconsolidated– undrained (UU) triaxial compression tests after 0, 1, 3, 6, and 9 cycles. The strain rate was constant at 1 mm per minute throughout the testing program and according to ASTM D2850-03a. Three different confining pressures of 30, 60, and 90 kPa had been selected for triaxial tests in order to model the real condition of freeze and thaw cycles which occur practically in the ground surface. 3.4. CT images For more investigation, the soil samples were scanned with computerized tomography (CT) apparatus from various sections. In recent years, CT has been widely used in geo-engineering studies (Alshibli et al., 2000; Homem et al., 1999; Qi et al., 2003). CT has two main advantages over the microscope as a means of describing the microstructure. Firstly, the information on the internal fabric of specimens can be obtained quantitatively and dynamically without damaging the specimen during testing procedure. Secondly, three-dimensional images can be obtained with no overlap. In this study, two reinforced and unreinforced frozen samples after 9 freeze and 8 thaw phases were placed in the CT apparatus and have been scanned from 20 sections each of which was 5 mm thick. 4. Results In order to learn the detailed changes of mechanical behavior influenced by freeze–thaw cycles, triaxial compression tests have been conducted on unfrozen clay and thawed clay under three different confining pressures after the soil was subjected to 1, 3, 6, and 9 cycles of freeze–thaw.
Table 2 Properties of geotextile. Geotextile
Bontec-NW8-nonwoven
Tensile strength (kN/m) Water permeability (m/s) Opening size (μm) Thickness (mm)
8 0.125 117 1
Separation of soil aggregates which is caused by ice lenses, made of soil pure water at temperatures just below 0 °C, disrupts the interlocking of soil grains and changes the mechanical properties of soil. In this study, UU triaxial tests were conducted to investigate the changes of soil mechanical features after freeze–thaw cycles. Figs. 2 to 4 show the test results for samples subjected to 0, 1, 3, 6, and 9 freeze–thaw cycles in three different confining pressures. As seen, in all reinforced and unreinforced samples, by increasing freeze–thaw cycles, the soil strength decreases. However, the strength decrease is more pronounced in unreinforced samples than in geotextile-reinforced ones. The stress– strain variation of thawed clay tends to vary from strain-softening type to strain-hardening type when reinforced with geotextile. The strength reduction for unreinforced soil subjected to freeze–thaw cycles was also observed in previous studies (Ghazavi and Roustaie, 2010; Graham and Au, 1985; Wang et al. 2007). 4.2. Effect of the freeze–thaw cycles on resilient modulus Resilient modulus is a fundamental material property used to characterize unbound pavement materials. It is a measure of material stiffness and provides a mean to analyze stiffness of materials under different circumstances, such as moisture content, density, and stress level. It is also a required input parameter to mechanistic-empirical pavement design method. 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 E¼
Δσ σ 1:0% −σ 0 ¼ Δε ε1:0% −ε0
ð1Þ
where Δσ is the increment of deviator stress, Δε is the increment of axial strain; σ1.0% is the deviator stress corresponding to the axial strain of 1.0% (ε1.0%); and σ0 and ε0 are the initial stress and strain, respectively (Wang et al., 2007). Lee et al. (1995) investigated the resilient properties of cohesive soils and found that cohesive soils with resilient modulus lower than 55 kPa would exhibit negligible freeze–thaw effects. In contrast,
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
1.4
0C 1C
a
0.7
Deviator Stress (MPa)
Deviator Stress (MPa)
0.8
3C
0.6
6C
0.5
9C
0.4 0.3 UnRe-0C UnRe-3C UnRe-9C
0.2 0.1 0 0
5
UnRe-1C UnRe-6C
1C
a
1.2
3C 0C
1 0.8
15
0.4
UnRe-0C
UnRe-1C
0.2
UnRe-3C
UnRe-6C
UnRe-9C 0
20
5
b
0.7
1C
0C
3C
6C
0.6
9C
0.5 0.4 0.3 Re-0C Re-3C Re-9C
0.2 0.1 0 0
5
Re-1C Re-6C
10
15
20
Strain (%)
20
soils with resilient modulus higher than 103 kPa would exhibit a decrease of over 50% in this parameter due to freeze–thaw. Simonsen and Isacsson (2001) conducted tests on various coarse and finegrained 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 results 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–
1.2
a
1C
b
1
0C
0.8
9C
3C
6C
0.6 0.4 0.2
Re-0C
Re-1C
Re-3C
Re-6C
Re-9C
0 0
5
10
15
20
Strain (%)
Fig. 2. Stress–strain variation of: a) unreinforced and b) reinforced samples during the freeze–thaw cycles under 30 kPa confining pressure.
1
Deviator Stress (MPa)
Deviator Stress (MPa)
15
1.2
0.8
Deviator Stress (MPa)
10
Strain (%)
Strain (%)
1C 0C
3C
0.8
6C 0.6
Fig. 4. Stress–strain variation of: a) unreinforced and b) reinforced samples during the freeze–thaw cycles under 90 kPa confining pressure.
thaw, making the soil structure slightly looser than it was prior to freezing. Wang et al. (2007) also reported that the magnitude of the resilient modulus decreases by 18–27% of unfrozen soil depending on the confining pressure in triaxial compression tests. The resilient modulus of unreinforced and reinforced samples subjected to each freeze–thaw cycle can be calculated by Eq. (1), shown in Fig. 5. As seen, by increasing the number of cycles, the resilient modulus of both reinforced and unreinforced samples decrease and minimum values are obtained after 6 cycles. When the number of freeze–thaw cycle exceeds seven, the resilient modulus will reach a certain value and remains constant upon applying further freeze– thaw cycles (Wang et al., 2007). Fig. 5 also indicates that the magnitude of resilient modulus decreases by about 40% of unfrozen unreinforced samples and 60% in geotextile-reinforced samples.
9C
0.4 UnRe-0C UnRe-3C UnRe-9C
0.2 0 0
5
10
UnRe-1C UnRe-6C 15
20
Strain (%) 1
Deviator Stress (MPa)
9C
6C
0.6
0
10
25
1C
b
0.8
3C 0C 6C
0.6
9C
0.4 Re-0C Re-3C Re-9C
0.2
Re-1C Re-6C
0 0
5
10
15
20
Strain (%) Fig. 3. Stress–strain curves of a) unreinforced and b) reinforced samples during the freeze–thaw cycles under 60 kPa confining pressure.
4.3. Effect of the freeze–thaw cycles on failure strength Fig. 6 shows the triaxial strength ratio of reinforced and unreinforced samples versus the number of freeze–thaw cycles for various confining pressures. This ratio is defined as the strength of a reinforced or unreinforced sample at a given cycle divided by that of the same sample which is not subjected to freeze–thaw cycles. The strength values are denoted by qu–N and qu–0, respectively. It is obvious from Fig. 6 that by increasing the number of freeze–thaw cycles, the strength of both reinforced and unreinforced samples decreases. In addition, it is obvious that by increasing the confining pressure, the strength reduction decreases. Therefore, freeze–thaw cycles are more destructive on surface of the ground which is in contact with structure foundations or road pavement. According to these figures, using one geotextile layer in the soil affects the strength reduction caused by freeze–thaw cycles. As seen in Table 3, the compression strength of unreinforced samples decreases by 14–43% due to the application of 9 freeze–thaw cycles while this reduction is about 7–14% for geotextile-reinforced soil. The greatest
26
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
1.2 UnRe -30 kPa
25
1
UnRe -60 kPa
20
UnRe -90 kPa
qu-N/qu-0
Resilient modulus (MPa)
30
15 10 5
a
0.8 0.6 0.4
0
2
4
6
8
10
Re
0
Number Of Cycle
0
2
4
6
8
10
Number Of Cycle
35 Re-30 kPa
1.4
30 Re-60 kPa 25
1.2
Re-90 kPa 1
20
qu-N/qu-0
Resilient modulus (MPa)
UnRe
σ3 = 30 kPa
0.2
0
15 10 5
0.6 0.4
b 2
4
6
8
10
UnRe
σ3 = 60 kPa
0.2
0 0
0.8
Re
0
Number Of Cycle
0
2
4
6
8
10
Number Of Cycle
Fig. 5. Variation of resilient modulus of: a) unreinforced and b) reinforced samples versus freeze–thaw cycles.
1.6 1.4
amount of strength reduction obtained after the 9th cycle is 43% for unreinforced soil and 14% for reinforced soil.
qu-N/qu-0
1.2
4.4. Effect of the freeze–thaw cycles on the shear strength parameters of clay To investigate the effect of freeze–thaw phenomenon on the soil shear strength parameters, the variations of p–q are considered where q ¼ ðσ 1 −σ 3 Þ 2 and p ¼ ðσ 1 þσ 3 Þ 2 . Here σ1 and σ3 are failure normal and lateral stresses, respectively. If test specimens are compacted when they are partially saturated, consolidation may occur when the confining pressure and deviatoric stress are applied even drainage is not permitted. Therefore, if several partially saturated specimens of the same material are tested at different confining stresses, they will not have the same undrained shear strength. Thus, the Mohr– Coulomb failure envelope for unconsolidated undrained triaxial tests on partially saturated soils is usually curved and for the soil investigated in this study a small friction angle will be obtained from the curves (ASTM, 2007). The influence of the number of freeze–thaw cycles on the cohesion of unreinforced and reinforced samples is illustrated in Fig. 7. As seen, the soil cohesion decreases with increasing the number of freeze–thaw cycles although there are some points in which the soil cohesion increases. This suggests that voids between clay particles may increase due to forming ice lenses and thus the volume increases. These results are in agreement with the findings of Wang et al. (2007) who performed tests on unreinforced soil. According to Fig. 7, trendlines of reinforced and unreinforced samples show that by reinforcing samples with a geotextile layer, the reduction trend for cohesion decreases to 15.6% compared with unreinforced samples which is 72.3%, after the 9th cycle. Fig. 8 shows the internal friction angle changes of samples during freeze–thaw cycles. As observed, although unreinforced samples experience a negligible increase in the friction angle during freeze–
1 0.8 0.6 0.4
UnRe
σ3 = 90 kPa
0.2
Re
0 0
2
4
6
8
10
Number Of Cycle Fig. 6. Variation of strength ratio of unreinforced and reinforced samples versus freeze–thaw cycles under confining pressure of 30, 60, and 90 kPa.
thaw cycles, the values of friction angle of reinforced samples remain relatively constant. 4.5. Effect of the freeze–thaw cycles on the height of the samples During freeze–thaw cycles, the heights of the specimens have been measured several times. Fig. 9 shows the variation of unreinforced sample height during freeze–thaw cycles. In this figure, ΔH represents the height change for every freeze–thaw cycle and H is the sample initial height. When environment temperature drops below 0 °C, the soil moisture starts to freeze. As a result, ice crystals Table 3 Strength reduction amounts during the freeze–thaw cycles. Confining pressure
Unreinforced samples
Reinforced samples
30 kPa 60 kPa 90 kPa
43% 27% 14%
14% 7% 8%
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
200
27
2.0%
UnRe Re
1.5% 120
Re
80
ΔH/H
Cohesion(kPa)
160
1.0%
UnRe
40
0.5%
UnRe
UnRe Re
Re 0 0
2
4
6
8
0.0%
10
0
2
4
Number Of Cycle
6
8
10
Number Of Cycle
Fig. 7. Variation of soil cohesion of unreinforced and reinforced samples versus freeze– thaw cycles.
Fig. 9. Height changes of reinforced and unreinforced samples during freeze–thaw cycles.
are formed in a freezing procedure and the sample is subjected to volumetric change. The crystals' volume would increase up to 9 times and applies a considerable stress on soil aggregates, resulting in the change of soil characteristic in micro and macro scales (Konrad, 1989). A common influence of freeze–thaw cycles on soil is the frost heave. When samples are frozen, their heights increase and then, in the thawing phase, they decrease. However, the decrease and increase values are not the same and the specimens will not reach their initial height. By cohesion decrease, reinforced sample in thaw condition cannot return to the first position as it was before freezing and this phenomenon leads to increase in height. It is obvious from Fig. 10 that the greatest height increase occurs in the first 6–7 cycles and for unreinforced samples, they are slightly more than those of reinforced samples.
determination of water contents in several parts of reinforced sample after 9 freeze–thaw cycles. Fig. 10 shows water content ratio of reinforced and unreinforced samples versus the sections from which this ratio is determined. This ratio is defined as the water content of a reinforced or unreinforced sample at the 9th cycle divided by that of the same sample which is not subjected to freeze–thaw cycles. The water content values are denoted by w(N) and w(0), respectively. This figure shows that the lower part of soil samples has more moisture content than the upper part. However, this trend is more obvious for reinforced sample because geotextile layer can drain water from the upper sample part and water freely flows down toward geotextile layer. Therefore, by increasing the number of freeze–thaw cycles and the number of largened pores left after the thaw of ice crystals, the soil strength decreases. This can be the reason of reduction of freeze–thaw cycle influence on decreasing the strength of reinforced soil in which the upper part of the sample is more resistant during the cycles because of lower water content. As mentioned in the previous section, computerized tomography (CT) images were used in order to describe the microstructure changes during the cycles. Figs. 11 and 12 contain 7 of 20 scanned sections from top to the bottom of the samples. It is worth mentioning that in the reinforced samples, section D is the closest section to geotextile layer. As seen in Fig. 12, on the left column related to unreinforced sample, the appearance of sample sections remained almost identical which shows the homogeneousity of ice distribution, whereas on the right column related to reinforced sample, from top down, the visibility of the soil particles due to greater amount of ice is harder. Thus, the upper part of reinforced sample with lower water content has consequently less ice lenses and offers more resistance to freeze–thaw cycles. This is interesting since the upper part of the soil is close to foundations and road pavements.
4.6. Effect of using geotextile layer on the physical and mechanical characteristics of the soil after freeze–thaw cycles As stated in previous sections, the use of a geotextile layer can reduce the effect of freeze–thaw cycles on the mechanical characteristics of the soil by decreasing the triaxial strength reduction amount from 43% for unreinforced samples to 14% for geotextile reinforced samples. This change in mechanical part of the soil is the result of some changes in its physical part during the cycles. According to properties presented in Table 2, it is obvious that due to the permeability of geotextile layer, water can freely move toward and through it from the soil mass. During freezing phase, ice crystals are formed and then, during thawing phase, these crystals start to melt and free water appears in the sample. The free water moves to lower parts of the sample due to gravity force and geotextile permeability makes water movement easier. This phenomenon can be demonstrated by
1.02 1
60
UnRe
w(N)/w(0)
Friction angle(Degree)
80
Re
40
a
20
0.98
Re
0.96 0.94
UnRe Re-9C
0.92
UnRe Re
UnRe-9C
0.9 0
0 0
2
4
6
8
10
Number Of Cycle Fig. 8. Variation of friction angle of unreinforced and reinforced samples versus freeze– thaw cycles, a) actual changes, and b) trendlines of changes.
1
2
3
4
5
6
7
8
Section number from top to the bottom of the sample Fig. 10. Water content ratio of unreinforced and reinforced samples after the 9th cycle from top to the bottom of the samples.
28
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
Fig. 11. Sections of CT images from frozen unreinforced and reinforced samples after the 8th cycle.
Besides soil particles, the fabric of frozen soil is related to internal ice and unfrozen water. The average density, which characterizes the distribution of particles, ice and unfrozen water across a specimen section, is described by the CT value defined as (Siemens AG, 1991): CT value ¼ 1000
μ i −μ H2 o μ H2 o
ð2Þ
where μi denotes the absorption index of the target, and μ H2 o denotes the absorption index of pure water. The unit of CT value is HU (Hounsfield unit). The CT values of a central point in all sections versus the number of sections were plotted in Fig. 13. The section number starts from 1 at the top and ends with 19. It is clear that by approaching the bottom of the reinforced sample, CT values and average densities increase. This means that the distribution of ice and unfrozen water in reinforced sample is less than those in unreinforced sample at the top of geotextile layer and more than the unreinforced one at the lower part of it. These results are in agreement with what are illustrated from water content determination tests in Fig. 10. 5. Conclusions Laboratory tests were performed to demonstrate the influence of freeze–thaw cycles on compressive strength of a reinforced clayey soil. The results show that: - By increasing the number of freeze–thaw cycles, the strength of all reinforced and unreinforced samples decreases. The effect of freeze– thaw cycles is more pronounced in unreinforced samples compared with the reinforced ones.
Fig. 12. CT images of frozen unreinforced and reinforced samples after 8th cycle.
- By increasing the confining pressure, the strength reduction during freeze–thaw cycles decreases. Therefore, freeze–thaw cycles are more destructive on surface part of the ground which is in contact with structure foundations or road pavement. - The stress–strain variation of thawed clay tends to vary from strain-softening type to strain-hardening type when reinforced with geotextile. - Existence of a nonwoven geotextile layer in the middle of soil sample height reduces the freeze–thaw influence as the greatest strength
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
1800 1600
CT value
1400 1200 1000 800 600 400
UnRe Re
200 0 0
5
10
15
20
Number Of Sections Fig. 13. CT values of central points in unreinforced and reinforced samples.
-
-
-
-
-
reduction has been reduced from 43% to 14% after the 9th cycle by sample reinforcement. Resilient modulus of soil decreases by about 40% of unfrozen soil resilient modulus in unreinforced samples and 60% in the samples which are reinforced by a geotextile layer after 9 freeze–thaw cycles. The cohesion in the pure and reinforced soil decreases by increasing the number of freeze–thaw cycles, but by reinforcing the samples with a geotextile layer, the reduction trend of cohesion will reduce from 72.3% to 15.6%. Internal friction angle remains almost constant with increasing the number of freeze–thaw cycle. Based on the changes in strength and height of samples subjected to 1–9 freeze–thaw cycles, it can be said that most of the changes occur at 1st to 7th cycles. One of the most common effects of freeze–thaw cycles is the height increase after the cycles. The results of this study show that the height of both unreinforced and reinforced soil samples increases after the cycles, but most of the changes occur at 1st to 6th cycles and a little more for unreinforced samples compared with others. CT images and CT values of frozen pure and reinforced soils and also water content distribution results, show the effect of using the geotextile layer as a permeable material which collects water and leads it to deeper part of the soil and reduce the freeze–thaw effects in the shallow parts of ground.
Generally, based on the findings in this paper, it is recommended to use geotextile layer in cold climates where shallower soil is close to building foundations and road pavements and thus may be affected by freeze–thaw cycles. The existence of geotextile not only increases the peak strength of the soil but also decreases the negative effect of freeze–thaw cycles. As a result, the maintenance costs of structures or pavements will be reduced. References Alshibli, K., Batiste, A., Susan, N., 2000. Quantifying void radio variation in sand using computed tomogaphy. GeoDenver 2000 Specialty Conference, Geotechnical
29
Measurements ö Lab and Field, Reston,Virginia. American Society of Civil Engineers, Reston, VA, pp. 30–43. ASTM, 2007. Standard Test Method for Unconsolidated-Undrained Triaxial Compression Test on Cohesive Soils. American Society for Testing and Materials, Annual Book of ASTM Standards, ASTM Manual ASTM D2850-03a. Chamberlain, E.J., Iskander, I., Hunsiker, S.E., 1990. Effect of freeze-thaw on the permeability and macrostructure of soils. Proceedings of the International Symposium on Frozen Soil Impacts on Agriculture, Range, and Forest Lands. Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, U.S.A. Special Report 90–1, 145–155. Eigenbrod, K.D., 1996. Effects of cyclic freezing and thawing on volume changes and permeabilities of soft fine-grained soils. Canadian Geotechnical Journal 33, 529–537. Ghazavi, M., Roustaie, M., 2010. The influence of freeze–thaw cycles on the unconfined compressive strength of fiber-reinforced clay. Cold Regions Science and Technology 61, 125–131. Graham, J., Au, V.C.S., 1985. Effects of freeze–thaw and softening on a natural clay at low stresses. Canadian Geotechnical Journal 22 (1), 69–78. Hazirbaba, K., Gullu, H., 2010. California Bearing Ratio improvement and freeze–thaw performance of fine-grained soils treated with geofiber and synthetic fluid. Cold Regions Science and Technology 63, 50–60. Homem, M.R.P., Mascarenhas, N.D.A., Cruvinel, P.E., 1999. Method for nondestructive testing using multiple energy CT and statistical pattern classification. SPIE Proc., 3808. Society of Photo-optical Instrumentation Engineers, pp. 676–684. Kalkan, E., 2009. Effects of silica fume on the geotechnical properties of fine-grained soils exposed to freeze and thaw. Cold Regions Science and Technology 58, 130–135. Konrad, J.M., 1989. Physical processes during freeze–thaw cycles in clayey silts. Cold Regions Science and Technology 16, 291–303. Krishnaswamy, N.R., Srinivasula, R., 1988. Behavior of reinforced earth under triaxial compression. Proceedings of the First Indian Geotextiles Conference on Reinforced Soil and Geotextiles, Bombay, India. Lee, W., Bohra, N.C., Altschaeffl, A.G., White, T.D., 1995. Resilient modulus of cohesive soils and the effect of freeze–thaw. Canadian Geotechnical Journal 32, 559–568. Leroueil, S., Tardif, J., Roy, M., Konrad, J.-M., 1991. Effects of frost on the mechanical behavior of Champlain Sea clays. Canadian Geotechnical Journal 28 (5), 690–697. Liu, J., Wang, T., Tian, Y., 2010. Experimental study of the dynamic properties of cement- and lime-modified clay soils subjected to freeze–thaw cycles. Cold Regions Science and Technology 61, 29–33. Mirmoradi, S.H., Noorzad, R., 2010. Laboratory evaluation of the behavior of a geotextile reinforced clay. Geotextiles and Geomembranes 28, 386–392. Qi, J., Yuanming, L., Yibing, P., 2003. Experimental study on the micro-fabric of frozen sediment using triaxial deformation and computerized tomography. Annals of Glaciology 37, 105–107. Qi, J., Vermeer, P.A., Cheng, G., 2006. A review of the influence of freeze–thaw cycles on soil geotechnical properties. Permafrost and Periglacial Processes 17, 245–252. Siemens AG, 1991. Operator's manual: Somatom PLUS with SOMARIS/2.Version 11.91. English edition Siemens AG, München. Simonsen, E., Isacsson, U., 2001. Soil behavior during freezing and thawing using variable and constant confining pressure triaxial tests. Canadian Geotechnical Journal 38, 863–875. Srivastava, R.K., Jalota, A.V., Pankaj, K., 1988. Strength and deformation behavior of geotextile reinforced alluvial silt. Proceedings of the First Indian Geotextiles Conference on Reinforced Soil and Geotextiles, Bombay, India. Wang, D., Ma, W., Niu, Y.H., Chang, X., Wen, Z., 2007. Effects of cyclic freezing and thawing on mechanical properties of Qinghai–Tibet clay. State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute. Cold Regions Science and Technology 48, 34–43. Yarbesi, N., Kalkan, E., Akbulut, S., 2007. Modification of the geotechnical properties, as influenced by freeze–thaw, of granular soils with waste additives. Cold Regions Science and Technology 48, 44–54. Zaimoglu, A.S., 2010. Freezing–thawing behavior of fine-grained soils reinforced with polypropylene fibers. Cold Regions Science and Technology 60, 63–65.
Contents lists available at SciVerse ScienceDirect
Cold Regions Science and Technology journal homepage: www.elsevier.com/locate/coldregions
Freeze–thaw performance of clayey soil reinforced with geotextile layer Mahmoud Ghazavi ⁎, Mahya Roustaei 1 Civil Engineering Department, K. N. Toosi University of Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 1 September 2012 Accepted 7 January 2013 Keywords: Freeze–thaw cycling Clay Geotextile layer UU triaxial test CT images
a b s t r a c t In cold climates, freeze–thaw cycling is an important issue in engineering. In freeze cycles, translocation of water and ice that can be caused by thermodynamic conditions at temperatures just below 0 °C, changes engineering properties of soils. In previous studies, changes in physical, chemical and mechanical properties of soils were investigated. In this study, UU Triaxial compressive tests have been performed to investigate the effect of freeze–thaw cycles on strength properties of soil reinforced with geotextile layer. A clayey soil, reinforced with a geotextile layer located at mid-height of the sample, was compacted in the laboratory and subjected to a maximum of 9 closed-system freeze–thaw cycles. Computerized tomography (CT) images have also been taken from samples. It was found that for the investigated soil, unconsolidated undrained triaxial compressive strength of unreinforced soil decreased with increasing the number of freeze–thaw cycles, whereas reinforced samples showed better performance and the strength reduction amount decreased from 43% to 14% by reinforcing the soil. CT images have shown that free water moved through the soil particles toward the lower part of the soil samples. In addition, it was found that sample reinforcement can reduce the effect of freeze–thaw cycles on changes of cohesion and resilient modulus of the soil. © 2013 Elsevier B.V. All rights reserved.
1. Introduction It has been recognized that freeze–thaw cycling is a weathering phenomenon which is normal in cold climates and considerably changes the structure of soils and thus has great influence on engineering properties of soils such as physical features including hydraulic permeability, densification, unfrozen water content, and mechanical features involving strength, compressibility, and bearing capacity. In the permafrost regions like Canada, it has been found that the embankment constructed on soil which has never experienced freeze–thaw cycles was damaged in just one year due to the loss of bearing capacity (Leroueil et al., 1991). Therefore, newly constructed highway embankments that are left unpaved for a few years may be subjected to possible damages by freeze–thaw cycles (Eigenbrod, 1996). Qi et al. (2006) reviewed the latest efforts which were done to investigate the influence of freeze–thaw cycles on soil properties. They summarized these influences in two parts: physical properties such as density and hydraulic permeability and mechanical properties such as ultimate strength, strain–stress behavior and resilient modulus. According to this research, loose soils tend to be densified and dense soils become looser after freeze–thaw cycles and both loose and dense soils may attain the same void ratio after a number of cycles (Konrad, 1989). Having increased the large pores that are left after the thaw of ice crystals, permeability will increase (Chamberlain et al., ⁎ Corresponding author. Tel.: +98 21 88779623; fax: +98 21 88779476. E-mail addresses: [email protected] (M. Ghazavi), [email protected] (M. Roustaei). 1 Tel.: +98 912 5604076; fax: +98 21 88779476. 0165-232X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.coldregions.2013.01.002
1990). These cycles reduce the ultimate strength of soils. All over consolidated soils exhibit a peak on the triaxial stress–strain curve that is reduced or may even disappear (Graham and Au, 1985). Resilient modulus is one of the most important factors in pavement designs that will be reduced significantly by even a small number of freeze–thaw cycles (Simonsen and Isacsson, 2001). In addition these cycles decrease the undrained shear strength considerably which is an important factor in engineering properties of fine-grained soils (Graham and Au, 1985). Along with many applications for soil improvement, there come several widely varied methods. Taking the influence of freeze–thaw cycles on soils into consideration, just a few researchers have contemporarily focused on using additives which can control the effects of these cycles. Yarbesi et al. (2007) stabilized two granular soils by silica fume– lime, fly ash–lime, and red mud–cement additive mixtures. Their experimental results show that stabilized samples with silica fume– lime, fly ash–lime, and red mud–cement additive mixtures have high freezing–thawing durability as compared to unstabilized samples. These additive mixtures have also improved the dynamic behavior of the soil samples. Therefore silica fume–lime, fly ash–lime, and red mud–cement additive mixtures, particularly silica fume–lime mixture, can be successfully used as an additive material to enhance the freeze–thaw durability of granular soils for road constructions and earthwork applications. Kalkan (2009) used a fine grained soil stabilized by adding silica fume which was generated during silicon metal production. The test results show that the stabilized fine-grained soil samples containing silica fume exhibit high resistance to the freezing and thawing effects
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
as compared to natural fine-grained soil samples. The silica fume decreases the effects of freeze–thaw cycles on unconfined compressive strength and permeability. Hazirbaba and Gullu (2010) performed CBR tests on a fine-grained soil to investigate the influence of freeze–thaw conditions and also no freeze–thaw conditions on soil samples which were treated with the inclusion of geofiber and synthetic fluid in soaked and unsoaked conditions. The results indicate that the addition of geofiber together with synthetic fluid is generally successful in providing resistance against freeze–thaw weakening, and that the addition of synthetic fluid alone isn't very effective against the detrimental impact of freeze–thaw. The results from soaked samples subjected to a freeze–thaw cycle showed poor CBR performance for treatments involving synthetic fluid while samples improved with geofibers alone generally offer better performance. Liu et al. (2010) conducted dynamic triaxial tests on cement and lime-modified soils with different blend ratios in freeze–thaw cycles. The results show that after repeated freeze–thaw cycles, the modified soils exhibit better performance than before modification, the cement-modified clay is superior to the lime modified clay, and all soil mechanical properties are visibly improved. Zaimoglu (2010) investigated the effect of randomly distributed polypropylene fibers on strength and durability behavior of a fine-grained soil subjected to freezing–thawing cycles. The content of polypropylene fiber was varied between 0.25% and 2% by dry weight of soil in the tests. It was observed that the mass loss in reinforced soils was almost 50% lower than that in unreinforced soil. It was also found that the unconfined compressive strength of specimens subjected to freezing–thawing cycles generally increases with increasing fiber content. In addition, the results indicated that the initial stiffness of the stress–strain curves is not affected significantly by the fiber reinforcement in the unconfined compression tests. Ghazavi and Roustaie (2010) reinforced a caolinite clay with steel and polypropylene fibers and exposed the soil samples to a maximum of 10 closed-system freeze–thaw cycles. They found that increasing the number of freeze–thaw cycles results in the decrease of unconfined compressive strength of clay samples by 20–25%. Moreover, the inclusion of fiber in clay samples increases the unconfined compressive strength of soil and decreases the frost heave. Furthermore, the results of the study indicate that the addition of 3% polypropylene fibers results in the increase of unconfined compressive strength of the soil before and after applying freeze–thaw cycles by 60% to 160% and decrease of frost heave by 70%. The strength of cohesive soils may be enhanced using geosynthetics. Most laboratory and theoretical investigations have been conducted on granular soils reinforced with geosynthetic material, while limited studies have concentrated on geosynthetic-reinforced cohesive soils. Krishnaswamy and Srinivasula (1988) reported the influence of the distance between the reinforced materials as well as moisture content of samples tested in undrained condition by triaxial apparatus. The soil was silty clay and reinforced with geotextile. Srivastava et al. (1988) studied the behavior of silty soil reinforced with geotextiles by using unconfined and triaxial tests. By analyzing the confining pressure, the number of reinforcing layers and the ratio of height to the diameter of the sample were evaluated. To evaluate the behavior of cohesive soil reinforced with a geotextile, Mirmoradi and Noorzad (2010) examined soil samples with different moisture contents, relative compaction, soil type, confining pressure, type and number of geotextile layers. They found that as the moisture content increases, the peak strength of both the reinforced and unreinforced samples decreases and the axial strain at failure increases. Moreover, by increasing relative compaction, the peak strength of the sample and the axial strain at failure increase, whereas the peak strength ratio decreases. The peak strength ratio is the ratio of the peak strength of the reinforced samples to that of the unreinforced samples. For soils with low plasticity, it was found that
23
the main cause of increase in the strength is the increase in the cohesion of the reinforced sample. However, in soils with higher plasticity index, as the number of geotextile layers increases, the internal friction angle of the reinforced samples increases as well. The aim of the present study is to investigate the influence of reinforcement with geotextile layer on reducing the freeze–thaw cycles effects. For this purpose, the physical and mechanical behavior of cohesive soils reinforced with geotextile have been evaluated before and after applying freeze–thaw cycles in triaxial tests. 2. Materials In this study, laboratory tests were carried out on a clayey soil classified as CL in the Unified Soil Classification System. It is noted that the effects of freeze–thaw cycles are more considerable in fine-grain soils in comparison with sand or gravel (Qi et al., 2006). The soil properties are presented in Table 1 and its grain size distribution is shown in Fig. 1. Standard Proctor Compaction tests were performed on the soil, and a maximum dry mass density of approximately 1.78 g/cm 3 at optimum moisture content (OMC) of approximately 17.4% was obtained. The specimens were reinforced with one layer of geotextile which was placed in the middle of sample height. The physical and mechanical properties of geotextile reported by the manufacturer are presented in Table 2. 3. Testing procedure The scope of this investigation is to study the effects of application of geotextile layer on the strength changes of highly compressible clay compacted at maximum dry density with the optimum moisture content and subjected to 0–9 freeze–thaw cycles. To evaluate the soil changes in these cycles, four main following steps should be taken for each sample. Sufficient amount of extra samples (app. thirty percent of the whole) was prepared and tested in order to examine the repeatability of test results. 3.1. Specimen preparation All specimens having 50 mm diameter and 100 mm height were prepared with the maximum dry unit weight and optimum water content. For sample preparation, initially, the necessary OMC was determined and mixed with the soil and the mixture was then placed within double layered plastic bags and sealed for 24 h to achieve uniform water content within the soil mass. The moisture content was then checked before sample compaction. During the soil compaction procedure in the mold, when half of the sample was compacted, one geotextile layer was placed in the middle height of the sample. After removal of each sample from the mold, it was immediately covered with a plastic layer to protect its moisture content. 3.2. Freeze–thaw cycles To prepare the samples for the closed system freezing and thawing cycles, specimens were placed in a digital refrigerator at −20 °C for 6 h and then at +20 °C for thawing phase for 6 h. These temperatures had been previously used in some research works (Qi et al., 2006). Six hours is a proportional period after which the alteration of specimens' Table 1 Properties of soil. Gs
2.657
Plastic limit Liquid limit Plastic index
36% 20% 16%
24
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
Fig. 1. Grain size distribution of clay.
height would become constant. This means that the height increase in freeze phase and the height decrease in thaw phase stop. The cycles were continued up to 9 cycles. This number of cycles was chosen since most soil strength reduction would occur in primary cycles and after 5–10 cycles a new equilibrium condition would become predominant on samples (Ghazavi and Roustaie, 2010).
As mentioned in the previous section, several geotextilereinforced and unreinforced samples were tested to study the strength change of reinforced soil subjected to freeze–thaw cycles. The variations of stress–strain response of samples are named UnRe-NC for unreinforced samples under N freeze–thaw cycles and Re-NC for geotextile-reinforced samples subjected to N freeze–thaw cycles.
3.3. Strength testing 4.1. Effect of the freeze–thaw cycles on stress strain behavior Strength features of soil were measured in unconsolidated– undrained (UU) triaxial compression tests after 0, 1, 3, 6, and 9 cycles. The strain rate was constant at 1 mm per minute throughout the testing program and according to ASTM D2850-03a. Three different confining pressures of 30, 60, and 90 kPa had been selected for triaxial tests in order to model the real condition of freeze and thaw cycles which occur practically in the ground surface. 3.4. CT images For more investigation, the soil samples were scanned with computerized tomography (CT) apparatus from various sections. In recent years, CT has been widely used in geo-engineering studies (Alshibli et al., 2000; Homem et al., 1999; Qi et al., 2003). CT has two main advantages over the microscope as a means of describing the microstructure. Firstly, the information on the internal fabric of specimens can be obtained quantitatively and dynamically without damaging the specimen during testing procedure. Secondly, three-dimensional images can be obtained with no overlap. In this study, two reinforced and unreinforced frozen samples after 9 freeze and 8 thaw phases were placed in the CT apparatus and have been scanned from 20 sections each of which was 5 mm thick. 4. Results In order to learn the detailed changes of mechanical behavior influenced by freeze–thaw cycles, triaxial compression tests have been conducted on unfrozen clay and thawed clay under three different confining pressures after the soil was subjected to 1, 3, 6, and 9 cycles of freeze–thaw.
Table 2 Properties of geotextile. Geotextile
Bontec-NW8-nonwoven
Tensile strength (kN/m) Water permeability (m/s) Opening size (μm) Thickness (mm)
8 0.125 117 1
Separation of soil aggregates which is caused by ice lenses, made of soil pure water at temperatures just below 0 °C, disrupts the interlocking of soil grains and changes the mechanical properties of soil. In this study, UU triaxial tests were conducted to investigate the changes of soil mechanical features after freeze–thaw cycles. Figs. 2 to 4 show the test results for samples subjected to 0, 1, 3, 6, and 9 freeze–thaw cycles in three different confining pressures. As seen, in all reinforced and unreinforced samples, by increasing freeze–thaw cycles, the soil strength decreases. However, the strength decrease is more pronounced in unreinforced samples than in geotextile-reinforced ones. The stress– strain variation of thawed clay tends to vary from strain-softening type to strain-hardening type when reinforced with geotextile. The strength reduction for unreinforced soil subjected to freeze–thaw cycles was also observed in previous studies (Ghazavi and Roustaie, 2010; Graham and Au, 1985; Wang et al. 2007). 4.2. Effect of the freeze–thaw cycles on resilient modulus Resilient modulus is a fundamental material property used to characterize unbound pavement materials. It is a measure of material stiffness and provides a mean to analyze stiffness of materials under different circumstances, such as moisture content, density, and stress level. It is also a required input parameter to mechanistic-empirical pavement design method. 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 E¼
Δσ σ 1:0% −σ 0 ¼ Δε ε1:0% −ε0
ð1Þ
where Δσ is the increment of deviator stress, Δε is the increment of axial strain; σ1.0% is the deviator stress corresponding to the axial strain of 1.0% (ε1.0%); and σ0 and ε0 are the initial stress and strain, respectively (Wang et al., 2007). Lee et al. (1995) investigated the resilient properties of cohesive soils and found that cohesive soils with resilient modulus lower than 55 kPa would exhibit negligible freeze–thaw effects. In contrast,
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
1.4
0C 1C
a
0.7
Deviator Stress (MPa)
Deviator Stress (MPa)
0.8
3C
0.6
6C
0.5
9C
0.4 0.3 UnRe-0C UnRe-3C UnRe-9C
0.2 0.1 0 0
5
UnRe-1C UnRe-6C
1C
a
1.2
3C 0C
1 0.8
15
0.4
UnRe-0C
UnRe-1C
0.2
UnRe-3C
UnRe-6C
UnRe-9C 0
20
5
b
0.7
1C
0C
3C
6C
0.6
9C
0.5 0.4 0.3 Re-0C Re-3C Re-9C
0.2 0.1 0 0
5
Re-1C Re-6C
10
15
20
Strain (%)
20
soils with resilient modulus higher than 103 kPa would exhibit a decrease of over 50% in this parameter due to freeze–thaw. Simonsen and Isacsson (2001) conducted tests on various coarse and finegrained 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 results 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–
1.2
a
1C
b
1
0C
0.8
9C
3C
6C
0.6 0.4 0.2
Re-0C
Re-1C
Re-3C
Re-6C
Re-9C
0 0
5
10
15
20
Strain (%)
Fig. 2. Stress–strain variation of: a) unreinforced and b) reinforced samples during the freeze–thaw cycles under 30 kPa confining pressure.
1
Deviator Stress (MPa)
Deviator Stress (MPa)
15
1.2
0.8
Deviator Stress (MPa)
10
Strain (%)
Strain (%)
1C 0C
3C
0.8
6C 0.6
Fig. 4. Stress–strain variation of: a) unreinforced and b) reinforced samples during the freeze–thaw cycles under 90 kPa confining pressure.
thaw, making the soil structure slightly looser than it was prior to freezing. Wang et al. (2007) also reported that the magnitude of the resilient modulus decreases by 18–27% of unfrozen soil depending on the confining pressure in triaxial compression tests. The resilient modulus of unreinforced and reinforced samples subjected to each freeze–thaw cycle can be calculated by Eq. (1), shown in Fig. 5. As seen, by increasing the number of cycles, the resilient modulus of both reinforced and unreinforced samples decrease and minimum values are obtained after 6 cycles. When the number of freeze–thaw cycle exceeds seven, the resilient modulus will reach a certain value and remains constant upon applying further freeze– thaw cycles (Wang et al., 2007). Fig. 5 also indicates that the magnitude of resilient modulus decreases by about 40% of unfrozen unreinforced samples and 60% in geotextile-reinforced samples.
9C
0.4 UnRe-0C UnRe-3C UnRe-9C
0.2 0 0
5
10
UnRe-1C UnRe-6C 15
20
Strain (%) 1
Deviator Stress (MPa)
9C
6C
0.6
0
10
25
1C
b
0.8
3C 0C 6C
0.6
9C
0.4 Re-0C Re-3C Re-9C
0.2
Re-1C Re-6C
0 0
5
10
15
20
Strain (%) Fig. 3. Stress–strain curves of a) unreinforced and b) reinforced samples during the freeze–thaw cycles under 60 kPa confining pressure.
4.3. Effect of the freeze–thaw cycles on failure strength Fig. 6 shows the triaxial strength ratio of reinforced and unreinforced samples versus the number of freeze–thaw cycles for various confining pressures. This ratio is defined as the strength of a reinforced or unreinforced sample at a given cycle divided by that of the same sample which is not subjected to freeze–thaw cycles. The strength values are denoted by qu–N and qu–0, respectively. It is obvious from Fig. 6 that by increasing the number of freeze–thaw cycles, the strength of both reinforced and unreinforced samples decreases. In addition, it is obvious that by increasing the confining pressure, the strength reduction decreases. Therefore, freeze–thaw cycles are more destructive on surface of the ground which is in contact with structure foundations or road pavement. According to these figures, using one geotextile layer in the soil affects the strength reduction caused by freeze–thaw cycles. As seen in Table 3, the compression strength of unreinforced samples decreases by 14–43% due to the application of 9 freeze–thaw cycles while this reduction is about 7–14% for geotextile-reinforced soil. The greatest
26
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
1.2 UnRe -30 kPa
25
1
UnRe -60 kPa
20
UnRe -90 kPa
qu-N/qu-0
Resilient modulus (MPa)
30
15 10 5
a
0.8 0.6 0.4
0
2
4
6
8
10
Re
0
Number Of Cycle
0
2
4
6
8
10
Number Of Cycle
35 Re-30 kPa
1.4
30 Re-60 kPa 25
1.2
Re-90 kPa 1
20
qu-N/qu-0
Resilient modulus (MPa)
UnRe
σ3 = 30 kPa
0.2
0
15 10 5
0.6 0.4
b 2
4
6
8
10
UnRe
σ3 = 60 kPa
0.2
0 0
0.8
Re
0
Number Of Cycle
0
2
4
6
8
10
Number Of Cycle
Fig. 5. Variation of resilient modulus of: a) unreinforced and b) reinforced samples versus freeze–thaw cycles.
1.6 1.4
amount of strength reduction obtained after the 9th cycle is 43% for unreinforced soil and 14% for reinforced soil.
qu-N/qu-0
1.2
4.4. Effect of the freeze–thaw cycles on the shear strength parameters of clay To investigate the effect of freeze–thaw phenomenon on the soil shear strength parameters, the variations of p–q are considered where q ¼ ðσ 1 −σ 3 Þ 2 and p ¼ ðσ 1 þσ 3 Þ 2 . Here σ1 and σ3 are failure normal and lateral stresses, respectively. If test specimens are compacted when they are partially saturated, consolidation may occur when the confining pressure and deviatoric stress are applied even drainage is not permitted. Therefore, if several partially saturated specimens of the same material are tested at different confining stresses, they will not have the same undrained shear strength. Thus, the Mohr– Coulomb failure envelope for unconsolidated undrained triaxial tests on partially saturated soils is usually curved and for the soil investigated in this study a small friction angle will be obtained from the curves (ASTM, 2007). The influence of the number of freeze–thaw cycles on the cohesion of unreinforced and reinforced samples is illustrated in Fig. 7. As seen, the soil cohesion decreases with increasing the number of freeze–thaw cycles although there are some points in which the soil cohesion increases. This suggests that voids between clay particles may increase due to forming ice lenses and thus the volume increases. These results are in agreement with the findings of Wang et al. (2007) who performed tests on unreinforced soil. According to Fig. 7, trendlines of reinforced and unreinforced samples show that by reinforcing samples with a geotextile layer, the reduction trend for cohesion decreases to 15.6% compared with unreinforced samples which is 72.3%, after the 9th cycle. Fig. 8 shows the internal friction angle changes of samples during freeze–thaw cycles. As observed, although unreinforced samples experience a negligible increase in the friction angle during freeze–
1 0.8 0.6 0.4
UnRe
σ3 = 90 kPa
0.2
Re
0 0
2
4
6
8
10
Number Of Cycle Fig. 6. Variation of strength ratio of unreinforced and reinforced samples versus freeze–thaw cycles under confining pressure of 30, 60, and 90 kPa.
thaw cycles, the values of friction angle of reinforced samples remain relatively constant. 4.5. Effect of the freeze–thaw cycles on the height of the samples During freeze–thaw cycles, the heights of the specimens have been measured several times. Fig. 9 shows the variation of unreinforced sample height during freeze–thaw cycles. In this figure, ΔH represents the height change for every freeze–thaw cycle and H is the sample initial height. When environment temperature drops below 0 °C, the soil moisture starts to freeze. As a result, ice crystals Table 3 Strength reduction amounts during the freeze–thaw cycles. Confining pressure
Unreinforced samples
Reinforced samples
30 kPa 60 kPa 90 kPa
43% 27% 14%
14% 7% 8%
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
200
27
2.0%
UnRe Re
1.5% 120
Re
80
ΔH/H
Cohesion(kPa)
160
1.0%
UnRe
40
0.5%
UnRe
UnRe Re
Re 0 0
2
4
6
8
0.0%
10
0
2
4
Number Of Cycle
6
8
10
Number Of Cycle
Fig. 7. Variation of soil cohesion of unreinforced and reinforced samples versus freeze– thaw cycles.
Fig. 9. Height changes of reinforced and unreinforced samples during freeze–thaw cycles.
are formed in a freezing procedure and the sample is subjected to volumetric change. The crystals' volume would increase up to 9 times and applies a considerable stress on soil aggregates, resulting in the change of soil characteristic in micro and macro scales (Konrad, 1989). A common influence of freeze–thaw cycles on soil is the frost heave. When samples are frozen, their heights increase and then, in the thawing phase, they decrease. However, the decrease and increase values are not the same and the specimens will not reach their initial height. By cohesion decrease, reinforced sample in thaw condition cannot return to the first position as it was before freezing and this phenomenon leads to increase in height. It is obvious from Fig. 10 that the greatest height increase occurs in the first 6–7 cycles and for unreinforced samples, they are slightly more than those of reinforced samples.
determination of water contents in several parts of reinforced sample after 9 freeze–thaw cycles. Fig. 10 shows water content ratio of reinforced and unreinforced samples versus the sections from which this ratio is determined. This ratio is defined as the water content of a reinforced or unreinforced sample at the 9th cycle divided by that of the same sample which is not subjected to freeze–thaw cycles. The water content values are denoted by w(N) and w(0), respectively. This figure shows that the lower part of soil samples has more moisture content than the upper part. However, this trend is more obvious for reinforced sample because geotextile layer can drain water from the upper sample part and water freely flows down toward geotextile layer. Therefore, by increasing the number of freeze–thaw cycles and the number of largened pores left after the thaw of ice crystals, the soil strength decreases. This can be the reason of reduction of freeze–thaw cycle influence on decreasing the strength of reinforced soil in which the upper part of the sample is more resistant during the cycles because of lower water content. As mentioned in the previous section, computerized tomography (CT) images were used in order to describe the microstructure changes during the cycles. Figs. 11 and 12 contain 7 of 20 scanned sections from top to the bottom of the samples. It is worth mentioning that in the reinforced samples, section D is the closest section to geotextile layer. As seen in Fig. 12, on the left column related to unreinforced sample, the appearance of sample sections remained almost identical which shows the homogeneousity of ice distribution, whereas on the right column related to reinforced sample, from top down, the visibility of the soil particles due to greater amount of ice is harder. Thus, the upper part of reinforced sample with lower water content has consequently less ice lenses and offers more resistance to freeze–thaw cycles. This is interesting since the upper part of the soil is close to foundations and road pavements.
4.6. Effect of using geotextile layer on the physical and mechanical characteristics of the soil after freeze–thaw cycles As stated in previous sections, the use of a geotextile layer can reduce the effect of freeze–thaw cycles on the mechanical characteristics of the soil by decreasing the triaxial strength reduction amount from 43% for unreinforced samples to 14% for geotextile reinforced samples. This change in mechanical part of the soil is the result of some changes in its physical part during the cycles. According to properties presented in Table 2, it is obvious that due to the permeability of geotextile layer, water can freely move toward and through it from the soil mass. During freezing phase, ice crystals are formed and then, during thawing phase, these crystals start to melt and free water appears in the sample. The free water moves to lower parts of the sample due to gravity force and geotextile permeability makes water movement easier. This phenomenon can be demonstrated by
1.02 1
60
UnRe
w(N)/w(0)
Friction angle(Degree)
80
Re
40
a
20
0.98
Re
0.96 0.94
UnRe Re-9C
0.92
UnRe Re
UnRe-9C
0.9 0
0 0
2
4
6
8
10
Number Of Cycle Fig. 8. Variation of friction angle of unreinforced and reinforced samples versus freeze– thaw cycles, a) actual changes, and b) trendlines of changes.
1
2
3
4
5
6
7
8
Section number from top to the bottom of the sample Fig. 10. Water content ratio of unreinforced and reinforced samples after the 9th cycle from top to the bottom of the samples.
28
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
Fig. 11. Sections of CT images from frozen unreinforced and reinforced samples after the 8th cycle.
Besides soil particles, the fabric of frozen soil is related to internal ice and unfrozen water. The average density, which characterizes the distribution of particles, ice and unfrozen water across a specimen section, is described by the CT value defined as (Siemens AG, 1991): CT value ¼ 1000
μ i −μ H2 o μ H2 o
ð2Þ
where μi denotes the absorption index of the target, and μ H2 o denotes the absorption index of pure water. The unit of CT value is HU (Hounsfield unit). The CT values of a central point in all sections versus the number of sections were plotted in Fig. 13. The section number starts from 1 at the top and ends with 19. It is clear that by approaching the bottom of the reinforced sample, CT values and average densities increase. This means that the distribution of ice and unfrozen water in reinforced sample is less than those in unreinforced sample at the top of geotextile layer and more than the unreinforced one at the lower part of it. These results are in agreement with what are illustrated from water content determination tests in Fig. 10. 5. Conclusions Laboratory tests were performed to demonstrate the influence of freeze–thaw cycles on compressive strength of a reinforced clayey soil. The results show that: - By increasing the number of freeze–thaw cycles, the strength of all reinforced and unreinforced samples decreases. The effect of freeze– thaw cycles is more pronounced in unreinforced samples compared with the reinforced ones.
Fig. 12. CT images of frozen unreinforced and reinforced samples after 8th cycle.
- By increasing the confining pressure, the strength reduction during freeze–thaw cycles decreases. Therefore, freeze–thaw cycles are more destructive on surface part of the ground which is in contact with structure foundations or road pavement. - The stress–strain variation of thawed clay tends to vary from strain-softening type to strain-hardening type when reinforced with geotextile. - Existence of a nonwoven geotextile layer in the middle of soil sample height reduces the freeze–thaw influence as the greatest strength
M. Ghazavi, M. Roustaei / Cold Regions Science and Technology 89 (2013) 22–29
1800 1600
CT value
1400 1200 1000 800 600 400
UnRe Re
200 0 0
5
10
15
20
Number Of Sections Fig. 13. CT values of central points in unreinforced and reinforced samples.
-
-
-
-
-
reduction has been reduced from 43% to 14% after the 9th cycle by sample reinforcement. Resilient modulus of soil decreases by about 40% of unfrozen soil resilient modulus in unreinforced samples and 60% in the samples which are reinforced by a geotextile layer after 9 freeze–thaw cycles. The cohesion in the pure and reinforced soil decreases by increasing the number of freeze–thaw cycles, but by reinforcing the samples with a geotextile layer, the reduction trend of cohesion will reduce from 72.3% to 15.6%. Internal friction angle remains almost constant with increasing the number of freeze–thaw cycle. Based on the changes in strength and height of samples subjected to 1–9 freeze–thaw cycles, it can be said that most of the changes occur at 1st to 7th cycles. One of the most common effects of freeze–thaw cycles is the height increase after the cycles. The results of this study show that the height of both unreinforced and reinforced soil samples increases after the cycles, but most of the changes occur at 1st to 6th cycles and a little more for unreinforced samples compared with others. CT images and CT values of frozen pure and reinforced soils and also water content distribution results, show the effect of using the geotextile layer as a permeable material which collects water and leads it to deeper part of the soil and reduce the freeze–thaw effects in the shallow parts of ground.
Generally, based on the findings in this paper, it is recommended to use geotextile layer in cold climates where shallower soil is close to building foundations and road pavements and thus may be affected by freeze–thaw cycles. The existence of geotextile not only increases the peak strength of the soil but also decreases the negative effect of freeze–thaw cycles. As a result, the maintenance costs of structures or pavements will be reduced. References Alshibli, K., Batiste, A., Susan, N., 2000. Quantifying void radio variation in sand using computed tomogaphy. GeoDenver 2000 Specialty Conference, Geotechnical
29
Measurements ö Lab and Field, Reston,Virginia. American Society of Civil Engineers, Reston, VA, pp. 30–43. ASTM, 2007. Standard Test Method for Unconsolidated-Undrained Triaxial Compression Test on Cohesive Soils. American Society for Testing and Materials, Annual Book of ASTM Standards, ASTM Manual ASTM D2850-03a. Chamberlain, E.J., Iskander, I., Hunsiker, S.E., 1990. Effect of freeze-thaw on the permeability and macrostructure of soils. Proceedings of the International Symposium on Frozen Soil Impacts on Agriculture, Range, and Forest Lands. Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, U.S.A. Special Report 90–1, 145–155. Eigenbrod, K.D., 1996. Effects of cyclic freezing and thawing on volume changes and permeabilities of soft fine-grained soils. Canadian Geotechnical Journal 33, 529–537. Ghazavi, M., Roustaie, M., 2010. The influence of freeze–thaw cycles on the unconfined compressive strength of fiber-reinforced clay. Cold Regions Science and Technology 61, 125–131. Graham, J., Au, V.C.S., 1985. Effects of freeze–thaw and softening on a natural clay at low stresses. Canadian Geotechnical Journal 22 (1), 69–78. Hazirbaba, K., Gullu, H., 2010. California Bearing Ratio improvement and freeze–thaw performance of fine-grained soils treated with geofiber and synthetic fluid. Cold Regions Science and Technology 63, 50–60. Homem, M.R.P., Mascarenhas, N.D.A., Cruvinel, P.E., 1999. Method for nondestructive testing using multiple energy CT and statistical pattern classification. SPIE Proc., 3808. Society of Photo-optical Instrumentation Engineers, pp. 676–684. Kalkan, E., 2009. Effects of silica fume on the geotechnical properties of fine-grained soils exposed to freeze and thaw. Cold Regions Science and Technology 58, 130–135. Konrad, J.M., 1989. Physical processes during freeze–thaw cycles in clayey silts. Cold Regions Science and Technology 16, 291–303. Krishnaswamy, N.R., Srinivasula, R., 1988. Behavior of reinforced earth under triaxial compression. Proceedings of the First Indian Geotextiles Conference on Reinforced Soil and Geotextiles, Bombay, India. Lee, W., Bohra, N.C., Altschaeffl, A.G., White, T.D., 1995. Resilient modulus of cohesive soils and the effect of freeze–thaw. Canadian Geotechnical Journal 32, 559–568. Leroueil, S., Tardif, J., Roy, M., Konrad, J.-M., 1991. Effects of frost on the mechanical behavior of Champlain Sea clays. Canadian Geotechnical Journal 28 (5), 690–697. Liu, J., Wang, T., Tian, Y., 2010. Experimental study of the dynamic properties of cement- and lime-modified clay soils subjected to freeze–thaw cycles. Cold Regions Science and Technology 61, 29–33. Mirmoradi, S.H., Noorzad, R., 2010. Laboratory evaluation of the behavior of a geotextile reinforced clay. Geotextiles and Geomembranes 28, 386–392. Qi, J., Yuanming, L., Yibing, P., 2003. Experimental study on the micro-fabric of frozen sediment using triaxial deformation and computerized tomography. Annals of Glaciology 37, 105–107. Qi, J., Vermeer, P.A., Cheng, G., 2006. A review of the influence of freeze–thaw cycles on soil geotechnical properties. Permafrost and Periglacial Processes 17, 245–252. Siemens AG, 1991. Operator's manual: Somatom PLUS with SOMARIS/2.Version 11.91. English edition Siemens AG, München. Simonsen, E., Isacsson, U., 2001. Soil behavior during freezing and thawing using variable and constant confining pressure triaxial tests. Canadian Geotechnical Journal 38, 863–875. Srivastava, R.K., Jalota, A.V., Pankaj, K., 1988. Strength and deformation behavior of geotextile reinforced alluvial silt. Proceedings of the First Indian Geotextiles Conference on Reinforced Soil and Geotextiles, Bombay, India. Wang, D., Ma, W., Niu, Y.H., Chang, X., Wen, Z., 2007. Effects of cyclic freezing and thawing on mechanical properties of Qinghai–Tibet clay. State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute. Cold Regions Science and Technology 48, 34–43. Yarbesi, N., Kalkan, E., Akbulut, S., 2007. Modification of the geotechnical properties, as influenced by freeze–thaw, of granular soils with waste additives. Cold Regions Science and Technology 48, 44–54. Zaimoglu, A.S., 2010. Freezing–thawing behavior of fine-grained soils reinforced with polypropylene fibers. Cold Regions Science and Technology 60, 63–65.