Cold Regions Science and Technology 146 (2018) 53–59
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Tensile strength of fiber reinforced soil under freeze-thaw condition a
Yan Li , Xianzhang Ling a b
a,b,⁎
b
a
a
, Lei Su , Lingshi An , Peng Li , Yingying Zhao
T
a
School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China School of Civil Engineering, Qingdao University of Technology, Qingdao 266033, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Freeze-thaw cycle Direct tensile strength Fiber reinforced soil 8-Shaped mold
Fiber reinforced soil technique has attracted the attention of numerous researchers. However, very limited information has been reported on its tensile behavior. Furthermore, the effect of fiber reinforcement on soil tensile property under freeze-thaw condition almost has not been investigated. To evaluate the factors influencing the tensile strength characteristics of fiber reinforced soil subjected to freeze-thaw cycles, direct tensile tests were performed using an 8-shaped compaction mold. To resolve the distributed randomness of fibers along the tensile failure plane in parallel specimens, the fibers were artificially and directionally arranged in the main tensile region of specimens. The experimental results indicate: (1) The addition of fibers to soil originates an increase in the stiffness, peak strength and residual strength, and a change in failure behavior from brittle to more ductile after 1st freeze-thaw cycle; (2) The tensile strength of reinforced soil subjected to freeze-thaw cycles significantly increases with the increasing of soil dry density and fiber content until optimum content; (3) With the increasing of water ratio, the tensile strength of fiber reinforced soil increases under the optimum water ratio and then decreases above it. The characteristic of tensile strength versus water ratio presents a typical unimodal pattern; (4) By increasing the number of freeze-thaw cycles to 9, the tensile strength decreases. Most of the strength reduction occurs at the first 5 freeze-thaw cycles, and the strength remains relatively constant for the 6th–9th cycles.
1. Introduction In cold regions, the fine-grained soil is normally recognized as frostsusceptible soil. However, they are still used as filling material in geotechnical engineering due to the expansion of construction project and lack of desirable coarse-grained soil. When such soils exposed to freezing and thawing, the physical and mechanical properties will change considerably due to volumetric changes caused by the interconversion of water and ice. This may result in the damage to geotechnical engineering structures. For instance, in Northeast of China, the soils will undergo the freeze-thaw cycles at least once a year, which has caused a large number of engineering problems such as crack, settlement and slope deformation in the road, railway, landfill and other engineering applications (Liu et al., 2010). In order to alleviate the geotechnical disease in engineering projects, some experimental studies have been undertaken through adding different additive materials in soil (e.g., cement, lime, fly ash and fibers) to improve the soil property. Fibers as an intensified material have attracted the attention of numerous researchers. A huge number of related experiments (e.g., unconfined compression tests, triaxial tests, CBR tests, direct shear tests and permeability tests) have been carried
⁎
out to explore the characteristics of fiber reinforced soil (Gray and Ohashi, 1983; Gray and Al-Refeai, 1986; Maher and Ho, 1994; Ranjan et al., 1996; Santoni and Webster, 2001; Prabakar and Sridhar, 2002; Yetimoglu et al., 2005; Cai et al., 2006; Ahmad et al., 2010; Tang et al., 2010; Ibraim et al., 2012; Tang et al., 2016b). Previous research findings showed that the inclusion of fibers can effectively reinforce the strength and ductility of soil as well as permeability. The soil can present significant tension property by the addition of fibers. However, traditional concept think that the soil tensile strength is almost zero, so very few studies focused on the tensile strength of fiber reinforced soil. This highlights the need for further understanding of tensile strength of fiber reinforced soil. (Maher and Ho, 1994; Consoli et al., 2002) conducted the splitting tensile tests and found that the tensile strength was significantly improved as fiber content increased, but the increasing of fiber length reduced the contribution to tensile strength. Considering the interaction between fibers and clayey soil, Miller and Rifai (2004) concluded that the crack reduction was more pronounced as the fiber content was 0.8%, and the maximum crack reduction approached 90% compared to the unreinforced samples. Similarly, Plé and Lê (2012) and Tang et al. (2012, 2016a, 2016b) also indicated that the fiber can be considered as a good reinforcement
Corresponding author at: School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China. E-mail address:
[email protected] (X. Ling).
https://doi.org/10.1016/j.coldregions.2017.11.010 Received 9 August 2016; Received in revised form 24 October 2017; Accepted 19 November 2017 Available online 21 November 2017 0165-232X/ © 2017 Elsevier B.V. All rights reserved.
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material for effectively mitigating potential cracking induced by settlement difference. Li et al. (2014) discussed the effect of fiber reinforcement on the soil tensile strength by an innovative 8-shaped tensile mold. Tang et al. (2007, 2014) analyzed the micromechanism of interficial interaction between fibers and soil particles by using scanning electron microscope and proposed a modified theory model to interpret the tensile strength of fiber reinforced soil. Moreover, most of the prior researches have been worked with fiber-reinforced soils without the consideration of freeze-thaw cycles, and very limited information about freeze-thaw behavior of fiber-reinforced soils has been obtained. Through adding the polypropylene fibers to fine-grained soil, Zaimoglu (2010) conducted a series of unconfined compression tests on this mixture after 12 freezing-thawing cycles, and concluded that the mass losses, evaluating the durability behavior of soil, was almost 50% lower than that in the unreinforced soil. Ghazavi and Roustaie (2010) analyzed the effect of freeze-thaw cycles on the unconfined compressive strength of fiber reinforced soil, and pointed out that the addition of 3% polypropylene fibers leads to 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%. Jafari and Esna-ashari (2012) studied the effect of fibers derived from waste tire cord reinforcement on compression strength of lime stabilized clayey soils under freeze-thaw cycles, the results indicated that the contribution of fibers in improving the strength of samples was enhanced as the freeze-thaw cycles increased and the optimal durability index increased by the inclusion of fibers. Gullu and Hazirbaba (2010), Hazirbaba and Gullu (2010) also confirmed the availability of geofiber reinforcing against freeze-thaw weakening in fine grained soils by CBR and UCS tests. To the best knowledge of the authors, the tensile strength behavior of fiber reinforced soil subjected to freeze-thaw cycles has not been well investigated. The main purpose of this study is to explore the effect of fiber reinforcement on the soil tensile strength under freeze-thaw condition. To achieve this purpose, a series of cyclic freeze-thaw and direct tensile tests were conducted on the mixture of soil and fiber by using a special 8-shape mold. The property of soil tensile strength after freeze-thaw cycles and the related interaction mechanism were detailed discussed according to the fiber content, water ratio, dry density and number of freezing and thawing cycles.
Table 1 Physical properties of soil. Parameter
Value
Specific gravity (g) Liquid limit (%) Plastic limit (%) Plasticity index (%) Optimum moisture content (%) Maximum dry density (g/cm3) Silt content (%) Clay content (%)
2.61 30 16 14 12.6 1.91 41 59
Table 2 Performance parameters of polypropylene fiber.
ρc =
Parameter
Value
Fiber type Unit weight (Mg/m3) Diameter (mm) Length (mm) Breaking tensile strength (MPa) Elongation at break (%) Modulus of elasticity (MPa) Fusion point (°C) Burning point (°C)
Single fiber 0.91 0.031 7 330–370 30 3500 165 590
Wf Ws
× 100(%)
(1)
where Wf is the dry weight of fiber, and Ws is the dry weight of soil. 2.3. Test method and device Soil tensile strength is an important parameter in the design of geotechnical engineering, where many failures are clearly attributed to tensile fracture. However, the traditional methods used for sample preparation of classical tensile tests (uniaxial direct tensile test, splitting test, flexure test, indirect Brazilian test and so on) are not suitable for preparing of fiber reinforced samples. This is because the geometrical morphology and quantity of fiber distribution are discrepant along with the tensile fracture surface during tensile tests of each sample, and the randomness of fiber distribution leads to the nonrepeatability of tensile strength of parallel specimens. To evaluate the tensile strength of fine-grained soils, Tamrakar et al. (2005) designed a new tension method similar to that developed by Li et al. (2014). In their experiment, the tensile strength of compacted clay was measured by using an 8-shaped mold which consists of two separated “C” structures. The 8-shaped mold simplifies the measurement on tensile behavior of soil by limiting the tension failure plane at the middle of specimen, and the potential tension failure plane of samples caused by conventional compaction method can be avoided by changing the compacted direction of sample from parallel to tensile loading direction to perpendicular to that. In this study, in order to qualitatively obtain the tensile strength of fiber reinforced soil and simplify the randomness of fiber distribution in main tensile region, a modified 8-shaped mold proposed by Li et al. (2014) and a new sample preparation method of artificial directional arrangement of fiber in main tensile region were employed to prepare specimens and carry out tensile tests. The length of 8-shaped specimen is 80 mm and its height is 24 mm. The width of this specimen is 25 mm at the middle and 50 mm at two ends. And the sample was composed of two parts: the main tension zone (part A) and the secondary tension zone (part B), shown in Fig. 1. The length of part A was set as 8 mm according to single fiber length. During the preparing process of part A, soil and fiber were arranged hierarchically and alternately, five layers of fiber were evenly placed between six layers of soil, and the fibers in
2. Materials and experimental procedure 2.1. Silty clay The soil employed in this study was obtained from the subgrade of ShenHua Heavy Haul Railway of Ordos, located in Southwestern Inner Mongolia Autonomous Region of China. In this region, the lowest temperature in winter is nearly − 30 °C and snow is always remained on the ground for 5 months. Summer here lasts only for 2 months and the highest temperature is close to 35 °C. Therefore, soil in this area are exposed to freezing-thawing cycles every year. According to Code for Design on Subgrade of Railway of China (TB10001-2005/J447-2005), the soil can be classified as low liquid-limit silty clay, the physical properties of the silty clay are summarized in Table 1.
2.2. Fiber The polypropylene fiber was chosen as the reinforcement material in this study, considering its high tensile strength, low cost, easy to mix with soil and non-pollution for environment. The performance parameters of polypropylene fiber given by the manufacturer are listed in Table 2. There are six percentages of fiber content (0%, 0.1%, 0.15%, 0.20%, 0.25%, 0.3%) adopted in this study and the fiber content, ρc, is defined as follows: 54
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8-shaped tensile mold
Tensile load
B
A
Fig. 1. Sketch drawing of the compacted 8-shaped soil specimen and tensile mold.
B 8-shaped specimen
(a) Front view of tensile mold and compacted specimen
(b) Compaction and tensile mold
(c) Compaction plate
each layer were artificially and equidistant arranged to perpendicular to tensile fracture surface (the cross section at the middle of 8-shaped mold). For part B, the mixture of soil and fiber was directly used as compaction material and the fibers were randomly distributed. The tensile testing equipment used in this experiment is CMT-2102 with a load capacity of 100 N and a resolution of 0.01 N, and a displacement capacity of 100 mm with a resolution of 0.001 mm, manufactured by SUST company in China. During the testing, the 8-shaped mold contained specimen was installed between two stationary clamps of testing instrument and the axial tensile load was applied as the movable beam moved upwards, seen in Fig. 2. In this study, the tensile load was applied with a speed of 0.1 mm/min of tensile strain. The test was manually forced to stop as the macroscopic cracks with 0.4–0.5 mm width were appeared at the main tensile region. This means that soil particles separated thoroughly along with the tensile failure plane. The test data that contained tensile load and associated displacement were obtained by the automatic data acquisition system. What is noteworthy here is that the half weight of tensile mold and sample had already subtracted in advance before testing start based on a preset option of system. Then, the tensile strength (σt) was calculated as follows:
σt =
Tmax A
(2)
where Tmax is the maximum tensile load during the test, and A is the cross-sectional area at the middle of 8-shaped mold. 2.4. Specimen preparation The specimens used in this experiment were consisted of silty clay, water and fiber. Each of the specimens was formed in an 8-shaped mold. These considerations of six fiber contents, five water ratios, five dry densities and six cycles of freezing-thawing were chosen as the factors which may significantly affect the tensile strength behavior of fiber reinforced samples. The detailed descriptions of samples based on different considerations are listed in Table 3 and Table 4. All the silty clay used herein was dried in an oven with a temperature of 110 ± 5 °C for 24 h, the required quality of soil, fiber and water of each sample were calculated before mixing. Fibers are assumed to be distributed uniform in specimens, and the weight of fibers respectively used for part A and part B can be calculated according to the volume of specimen. Firstly, the dry silty clay was manually mixed with the polypropylene fibers, and considerable care was taken to avoid the flocculation of fibers until the mixture of soil and fiber was uniform. Table 3 Parameters of each test group without freeze-thaw cycles.
movable beam upper displacement transducer
Sample no.
Fiber content (%)
Water ratio (%)
Dry density (g/cm3)
Tensile strength (kPa)
SN-1 SN-2 SN-3 SN-4 SN-5 SN-6 SN-7 SN-8 SN-9 SN-10 SN-11 SN-12 SN-13 SN-12
0 0.1 0.15 0.2 0.25 0.3 0 0 0 0 0 0 0 0
12.6 12.6 12.6 12.6 12.6 12.6 8.6 10.6 14.6 16.6 12.6 12.6 12.6 12.6
1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.51 1.61 1.71 1.81
70.09 91.30 122.30 152.48 156.47 141.59 41.80 54.96 58.13 41.50 25.00 37.66 48.69 57.30
lower displacement transducer load transducer
tensile area (not to scale) stationary fixture
Fig. 2. Schematic drawing of tensile test device.
55
0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
88.97 116.70 111.60 90.49 42.71 78.90 104.80 134.50
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Table 4 Parameters of each test group subjected to freeze-thaw cycles. Sample no.
Fiber content (%)
Water ratio (%)
Dry density (g/cm3)
Freeze-thaw cycles
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18
0 0.1 0.15 0.2 0.25 0.3 0 0 0 0 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
12.6 12.6 12.6 12.6 12.6 12.6 8.6 10.6 14.6 16.6 8.6 8.6 8.6 8.6 12.6 12.6 12.6 12.6
1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.91 1.51 1.61 1.71 1.81 1.51 1.51 1.51 1.51
1 1 1 1 1 1 1 1 1 1 1 1 1 1 3 5 7 9
0.15 0.15 0.15 0.15
0.2 0.2 0.2 0.2
0.3 0.3 0.3 0.3
10.6 10.6 10.6 10.6
12.6 12.6 12.6 12.6
16.6 16.6 16.6 16.6
1.61 1.61 1.61 1.61
1.81 1.81 1.81 1.81
3 5 5 3 5 5
5
5
Tensile strength (kPa) 7 9 9 7 9 9
9
9
47.66 69.70 96.32 111.74 120.48 103.80 30.86 38.37 30.50 23.34 21.97 35.19 58.48 69.50 24.26 20.13 19.27 18.66
36.79 45.43 55.40 86.62 81.36 66.23 74.16 83.85 60.68 29.19 27.20 40.60 60.43 79.16 39.21 30.37 27.14 26.88
27.81
67.68
78.30 94.40 78.55 38.91 35.14 47.76 77.02 93.76
23.92 40.80 53.87 66.62 77.49 65.46 73.77 86.15 46.71 31.14 23.23 36.80 50.90 68.34 54.30 46.76 44.39 39.20
23.18
60.96
80
Secondly, the required amount of water was sprayed into the dry soilfiber mixture and a homogeneous distribution of soil-fiber-water mixture used for part B of specimen was attained by continually mixing. Meanwhile, the soil-water mixture used for part A of specimen was also prepared by mixing the soil directly with water. Then, two kinds of mixtures were conserved into a sealed container for at least 12 h before the subsequent compaction. Thirdly, the calculated weight of soil-water mixture and fibers used for part A was divided into six portions, and same partition was done on the soil-fiber-water mixture used for part B. Before the first compaction of specimen, the weight of one of six portions of two mixtures were fetched and laid down within the part A and part B, then, compacting into a 4 mm layer by using the 8-shaped compaction plate. After the first compaction, the weight of one piece of six portion of fibers used for part A was artificially and equidistant arranged in a row one by one to perpendicular to tensile fracture surface. Repeating the above compaction steps to achieve the wholely preparation of one specimen. Finally, the 8-shaped mold contained compacted specimen was placed into an air-proof bag and then moved into a freezing container, maintaining a constant temperature of − 20 °C for 24 h. After the freezing process, the specimen with 8-shaped mold was placed into another container with a temperature of 25 °C for 12 h for thawing. This process was called 1 freeze-thaw cycle, repeating this process to achieve more cycles.
fiber content 0.1%
fiber content 0.15%
fiber content 0.2%
fiber content 0.25%
60
Tensile load (N)
fiber content 0%
40
20
0 0
0.1
0.2
0.3
0.4
Displacement (mm) Fig. 3. Tensile load-displacement curves for specimens with different fiber content after 1st freeze-thaw cycle.
deformation of 0.04 mm occurred, presenting a brittle failure pattern, but the reinforced specimens continued to undertake a relatively lower tensile loads to elongate deformation in a pliable manner. The tensile curves of fiber reinforced soil also indicate that the inclusion of 0.1% fiber is sufficient to change the brittle behavior of unreinforced sample into ductile. However, the high percent of fibers illustrates more excellent behavior. As such, the peak and residual strength of fiber reinforced soil increase with fiber content increase. These tensile characteristics of fiber reinforced soil had already been confirmed by Li et al. (2014) and Tang et al. (2015). As expected, the fiber inclusion plays a significant effect in changing soil failure pattern. This is because the fiber inclusion served as a connection in the soil/fiber mixtures after the tensile cracks formed and the interaction between fibers and soil particles provided the tensile resistance.
3. Results and discussions Experimental results on unreinforced and fiber reinforced soil subjected to freeze-thaw cycles were presented in Table 3 and Table 4 from direct tensile tests. In the following sections, the effects of fiber content, dry density, water ratio and freeze-thaw cycles on the tensile strength will be investigated. For abbreviating, the test specimen properties are presented by some symbols and numbers in the following. Such as, the specimen 0.25F-12.6W-1.91D-1C has 0.25% fiber and 12.6% water and a dry density of 1.91 and exposed to 1 cycle of freeze-thaw.
3.2. Effect of fiber content 3.1. Tensile curves Effect of fiber content on tensile strength of reinforced specimens after freeze-thaw cycles is presented in Fig. 4. It's clear that, no matter how many freeze-thaw cycles that specimens exposed, the samples display more tensile strength due to fiber reinforcing, and the strength increased monotonically with the increase of fiber content until 0.25%, after which the peak tensile strength decreased. This indicates that there is an optimum fiber content for the reinforced specimen. The observation of tensile strength increased with fiber content, which is
Fig. 3 represents some typical tensile load-displacement curves for fiber reinforced samples after applying 1 freeze-thaw cycle. From Fig. 3, it can be seen that the tensile load increases monotonically with displacement to the peak load for all samples, and higher slope was observed in fiber reinforced specimens indicating more stiffness and ductile behavior. After that, the tensile load of specimen without fiber reinforcement drops to zero abruptly while a very small failure 56
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160
120 12.6W-1.91D-0C 12.6W-1.91D-1C
140
100
12.6W-1.91D-5C
Tensile strength (kPa)
Tensile strength (kPa)
12.6W-1.91D-9C
120 100 80 60
80
60
40
40 20 0
0.05
0.1
0.15
0.2
0.25
0.2F-8.6W-1C
0.2F-10.6W-1C
0.2F-12.6W-1C
0.2F-14.6W-1C
20
0.3
1.5
Fiber content (%) Fig. 4. Effect of fiber content on tensile strength of reinforced specimens after freeze-thaw cycles.
1.6
1.7 Dry density (g/cm3 )
1.8
1.9
Fig. 5. Effect of dry density on tensile strength of reinforced specimens after 1st freezethaw cycle.
Tensile strength (kPa)
consistent with that of Tang et al. (2016a, 2016b). However, the decreasing strength after peak strength was not appeared in their research due to the narrow range of fiber content. Under the condition of 1 freeze-thaw cycle, the maximum tensile strength was obtained at 0.25% fiber content and the strength increased by 152.8%, from 47.66 kPa to 120.48 kPa, as the fiber content increased from 0% to 0.25%. The increase in tensile strength might be due to the cooperation between fiber and soil particles which can efficiently prevent and delay the development of tensile failure plane and deformation of soils. Tang et al. (2010) analyzed the interfacial interaction mechanism between fibers and soils by single fiber pull-out tests and SEM. Experimental records confirmed that the single fiber is capable to bear tensile load by interfacial friction and bonding force between fiber surface and soil particles, which resisted the relative movement of fiber in soil matrix. Accordingly, the more percentages of fiber content in soil matrix lead to greater interfacial friction and bonding force, and therefore increase the tensile strength. However, excessive fiber content may give rise to fiber agglomerating which reduces the contact area between fibers and soil particles, thus, leading to a decrease of tensile strength. From Fig. 4, it is noticed that the tensile strength of specimens decreased with the increasing of freeze-thaw cycles, and the strength tended towards stability after 5 cycles of freeze-thaw.
150
0.2F-12.6W-0C
0.2F-12.6W-1C
130
0F-12.6W-0C
0F-12.6W-1C
110 90 70 50 30 10
Dry density (g/cm3) Fig. 6. Variation of tensile strength of unreinforced and reinforced specimens with dry density.
thaw cycle than that of unreinforced soil. These results are mainly attributed to the following reasons. i) As the specimens are exposed to freeze-thaw condition, the soil structure can be changed due to the formation of ice lenses during the freezing process. Such process results in the increase of pore volume and rearrangement of soil particles, decreasing the contact area between soil particles and fibers. However, though the specimens are subjected to freeze-thaw cycles, the increment of pore volume caused by the increase of ice crystals volume is smaller than that induced by the decrease of dry density of specimens. Therefore, the higher dry density specimens still keep more interfacial contact area between soil matrix and fibers, and the high bonding force and interfacial friction along fiber longitudinal direction is accordingly remained in higher dry density specimens. ii) During the compaction process, the higher dry density is, the more compaction work is needed. The interlock force may be developed at high compaction load due to the hard soil particles partially stabbed into the bodies of fibers, which further helps to enhance the interfacial mechanical behavior of fiber reinforced soil.
3.3. Effect of dry density Fig. 5 shows the effect of dry density on tensile strength of fiber reinforced specimens after 1 cycle of freeze-thaw. As can be seen from Fig. 5, the dry density has a significant influence on providing high tensile strength for reinforced specimens, and all the tensile strength of specimens with different water ratio increased with increase of soil dry density. Under the condition of 12.6% water ratio, the tensile strength increased by 134%, from 47.76 kPa to 111.74 kPa, as the dry density increased from 1.61 g/cm3 to 1.91 g/cm3. This trend is well with the previous study performed by Li et al. (2014), and their results presented the tensile strength of fiber reinforced soil without freeze-thaw cycles increased by 275.2%, when dry density increased from 1.4 g/cm3 to 1.7 g/cm3. Fig. 6 shows the variation of tensile strength of unreinforced and reinforced specimens with dry density under the condition of 0–1 freeze-thaw cycle. It was found that the beneficial effect of dry density on soil tensile strength is never affected by the way of reinforcement and freeze-thaw cycles. Under the dry density of 1.91 g/cm3, after 1 freeze-thaw cycle, the tensile strength of 0.2% fiber reinforced specimen decreased by 26.7%, from 152.48 to 111.74 kPa, and the unreinforced specimen decreased by 32%, from 70.09 to 47.66 kPa. Samples with reinforcement performed better behavior after 1 freeze-
3.4. Effect of water ratio Fig. 7 represents the effect of water ratio on tensile strength of fiber reinforced specimens after 1 cycle of freeze-thaw. It can be observed from the Fig. 7 that the unimodal curves of tensile strength versus tensile displacement were found regardless of fiber content. Under the 57
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result, the soil particles and fibers will be separated from each other, causing the pore volume increases due to water volume expansion. However, as the water content is within the dry side of optimum water ratio, the water volume expansion cannot be fully filled with pore volume, leading to no detrimental to soil structure. Moreover, the increase of lower water content before optimum water ratio resulted in high bonding force between particles and fibers, contributing to soil tensile strength. As the water content beyond optimum water ratio, the more soil water is, the more volume expansion is. During the thawing phase, the ice starts to melt and large pores remain in specimens because the increased pore volume developed in freezing phase cannot be fully recovered after soil thawing. Thus, the original connection between adjacent particles as well as particles and fibers are weakened after freeze-thaw cycles, directly leading to a decrease in bonding strength and interfacial friction. The more initial water content before freezing is, the greater decrease in interfacial interaction of particles and fibers after thawing is. Secondly, in the direct tensile process, water plays a role of lubricant agent on the interface of fibers and soil particles. The interfacial friction decreases with the increases of water ratio. Thus, the interfacial resistance to tensile load is weakened and fibers can also be easily extracted from soil matrix.
120 0F-1.91D-1C 0.15F-1.91D-1C
Tensile strength (kPa)
100
0.2F-1.91D-1C 0.3F-1.91D-1C
80
60
40
20 8
10
12
14
16
Water ratio (%) Fig. 7. Effect of water ratio on tensile strength of reinforced specimens after 1st freezethaw cycle.
fiber content of 0.2%, the tensile strength increased by 30%, from 78.3 to 111.74 kPa as the water ratio raised from 10.6 to 12.6%. However, the strength decreased by 65.2% as the water ratio raised from 12.6 to 16.6%. It indicated that water ratio plays an important role in controlling soil tensilt strength. Tang et al. (2014) proved the tensile strength characteristic curve of clayey soil exhibits one peak value occurring at optimum water ratio, and pointed out that with increase of water ratio, the strength increased at the dry side of optimum water ratio, and then strength decreased strongly at the wet side of optimum water ratio. Fig. 8 shows the variation of tensile strength of unreinforced and reinforced specimens with water ratio under the condition of 0–1 freeze-thaw cycle. From the figure, we saw that the specific characteristic of the unimodal curve of tensile strength versus water ratio of soil sample was not affected by the way of fiber reinforcement and freeze-thaw cycle. Under the higher water ratio condition of 16.6%, after 1 freeze-thaw cycle, the tensile strength of 0.2% fiber reinforced specimen decreased by 57%, and the unreinforced specimen decreased by 43.7%. Under the lower water ration of 8.6%, after 1 freeze-thaw cycle, the sample with and without freeze-thaw condition separately decreased by 12% and 26.17%. This indicates that, for both specimens with and without fiber reinforcement subjected to freeze-thaw cycles, the more water content is the more reduction of soil tensile strength. This phenomenon probably owes to such reasons: firstly, as the specimen is exposed to freezing, the soil water is changed into ice. As a
160
In order to explore the effect of freeze-thaw cycles on soil tensile strength, several specimens were prepared and exposed to 0–9 freezethaw cycles before testing, and the obtained tensile strength was shown in Fig. 9. It is apparent that all the tensile strength of specimen with different dry densities decreases as the number of freeze-thaw cycles increases. Maximum loss of strength occurs after the first five freezethaw cycles, and the strength remains roughly stable for the subsequent cycles. The tensile strength of specimen, that did not experience freezethaw cycles, was 152.48 kPa. After 1, 3, and 5 freeze-thaw cycles, the strength of specimens were 111.74 kPa, 86.62 kPa and 67.68 kPa, respectively. The tensile strength was decreased by 55.6% during the first five freeze-thaw cycles. Between the 5th and 9th freeze-thaw cycles, the tensile strength of tested samples was almost unchanged and maintained more or less constant around 66 kPa. The totally strength loss after 9 freeze-thaw cycles was 60%, and the decreased rate was very large within five cycles. It is obvious that freeze-thaw cycles dramatically reduce the tensile strength. Fig. 10 presents the comparison of tensile strength of unreinforced and reinforced specimen subjected to freeze thaw cycles. It is obviously that the fiber reinforcement was very efficient in enhancing the tensile behavior of specimen subjected to freeze-thaw cycles. The freeze-thaw cycle is such process that samples develop from an unstable state to a dynamic stable state (Lee et al., 1995). During freeze-
0F-1.91D-0C 160
0F-1.91D-1C
140
0.2F-1.91D-0C
140
0.2F-12.6W-1.91D
120
0.2F-12.6W-1.81D
0.2F-1.91D-1C
120
Tensile strength (kPa)
Tensile strength (kPa)
3.5. Effect of freeze-thaw cycles
100 80 60
0.2F-12.6W-1.61D 100 0.2F-12.6W-1.51D 80 60 40
40 20
20 8
10
12
14
16
0
Water ratio (%)
0
1
2
3
4
5
6
7
8
9
Number of freeze-thaw cycles
Fig. 8. Variation of tensile strength of unreinforced and reinforced specimens with water ratio.
Fig. 9. Effect of freeze-thaw cycle on tensile strength of reinforced specimens.
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strength was very large within five cycles. For the 5th to 9th cycles, the tensile strength remained approximately stable since a new dynamic balance was achieved in samples.
160 140
Tensile strength (kPa)
0.2F-12.6W-1.91D 120
Acknowledgments
0F-12.6W-1.91D 100
The authors are grateful to the Key Laboratory of Frozen Soils Engineering of Hydraulic Research Institute of Heilongjiang Province in China for the test support. The work obtained supports from the follow agents: the National Natural Science Foundation of China (Grant No. 51174261), China Shenhua Energy Company Limited (Grant No. CSIE12021243) and the Opening Fund for Innovation Platform of China (Grant No. 2016YJ004).
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Fig. 10. Comparison of tensile strength of unreinforced and reinforced specimen subjected to freeze thaw cycles.
thaw process, the effective interfacial contact area between fibers and soil matrix is mainly affected by soil particles rearrangement due to the formation and melting of ice crystals, and a relative steady interfacial contact area will be attained until a new equilibrium condition becomes predominant on soil particles. Usually, the internal fabric of finegrained soil will reach to a new dynamic balance when the soil has been experienced for 6–10 cycles of freeze-thaw (Wang et al., 2007). Xie et al. (2015) investigated the effects of freeze-thaw cycles on soil physical and mechanical properties, and stated that the volume strain and porosity of specimen increased greatly in the first five freeze-thaw cycles, causing a decrease in uniaxial compression strength. Between the 6th and 15th freeze-thaw cycles, the strength of tested specimen was relatively unchanged due to stabilizing of the volume strain and porosity. 4. Conclusions In this study, direct tensile tests are conducted to investigate the tensile strength of fiber reinforced soil subjected to freeze-thaw cycles using an 8-shaped mold. The main experimental results can be summarized as follows: 1) The tensile strength of samples subjected to 1 freeze-thaw cycle increased with the increasing fiber content until optimum fiber content, after that the peak tensile strength decreased. By addition of 0.25% polypropylene fibers, the tensile strength is increased by 152.8% compared to untreated specimen. Moreover, the fiber reinforcement changes the soil behavior from brittle to more ductile. Comparing to unreinforced samples, the addition of fibers reduces the loss of tensile strength after peak, exhibiting a non-negligible residual strength. 2) After 1st freeze-thaw cycle, the tensile strength of fiber reinforced soil with special water content increased with increasing dry density. This is mainly because the increment of void volume caused by water freezing and thawing is smaller than that caused by dry density decreasing during the freeze-thaw process. Accordingly, the reinforced specimens with higher dry density possess more interfacial interaction between soil particles and fibers, and more contacts between soil particles. 3) With the increasing of water ratio, the tensile strength of fiber reinforced soil increases at the dry side of optimum water ratio and then decreases at the wet side of it. The characteristic of tensile strength versus water ratio presents a typical unimodal pattern. 4) Based on the variation in tensile strength of fiber reinforced samples subjected to 1–9 freeze-thaw cycles, it can be said that most of the changes occur at 1st to 5th cycles, and the decreased rate of tensile 59
