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Experimental study on direct shear behavior of frozen soil–concrete interface
Cold Regions Science and Technology 104–105 (2014) 1–6
Contents lists available at ScienceDirect
Cold Regions Science and Technology journal homepage: www.elsevier.com/locate/coldregions
Experimental study on direct shear behavior of frozen soil–concrete interface Jiankun Liu ⁎, Peng Lv, Yinghui Cui, Jingyu Liu Beijing Jiaotong University, School of Civil Engineering, Beijing 100044, China
a r t i c l e
i n f o
Article history: Received 26 November 2013 Accepted 30 April 2014 Keywords: Frozen soil Soil–concrete interface Direct shear Shear strength
a b s t r a c t In order to study the mechanical behavior of frozen soil–concrete interfaces, a series of direct shear tests were conducted on a large-scale temperature-controlled direct shear test system (TZJ-150). Synchronous variations were found in both the shear stress–displacement and vertical displacement–shear displacement curves. The whole shear process can be divided into five stages: the elastic deformation part, the plastic deformation part, the whole slide part, the strain hardening part, and the stable residual strength part. The influence rules of normal pressure, temperature, and water content on the shear strength of the interface were obtained by a series of sample tests with different parameters. Fitted equations between factors and shear strength were presented. The peak shear strength had linear relationships with the normal pressure and temperature, but a nonlinear relationship with the water content, which had a much greater effect on the peak shear strength. The residual shear strength also had linear relationships with the normal pressure and temperature, but was basically unrelated with the water content. A reasonable explanation about the changes of peak shear strength and residual shear strength was proposed. © 2014 Elsevier B.V. All rights reserved.
1. Introduction There are interfaces of soil and concrete in nearly all kinds of construction works. In foundations, tunnels, dams and embankments, the soil– concrete interface is a very important element of the safety of these projects. The mechanical properties of soil–concrete interface have been a frequent research focus; until now, direct shear tests and simple shear tests have been the most commonly methods used for experimental studies on such interfaces. Clough and Duncan (1971) performed some direct shear tests on soil–concrete interface and established a hyperbolic model of shear stress and shear displacement; Boulon and Nova (1990) presented an elastic-plastic model based on the shear stress–shear displacement curves; Desai et al. (1985) conducted many direct shear and simple shear tests on sand–structure interface and analyzed the main influencing factors. Yin et al. (1994) observed shear displacements at different positions on interfaces using a series of mini periscopes; they pointed out that the stress–displacement curve reflects the expanding failure area, and they presented a rigid-plastic model of this. Zhang and Zhang (2004) used steel plates with different roughnesses to replace the concrete in simple shear tests, and studied the movements of soil grains under different influencing factors. All past studies of soil–concrete interfaces were carried out at positive temperatures. However, for construction in cold regions, or artificial freezing works in unfrozen regions, it becomes very necessary to ⁎ Corresponding author. Tel./fax: +86 10 51684096. E-mail address: [email protected] (J. Liu).
http://dx.doi.org/10.1016/j.coldregions.2014.04.007 0165-232X/© 2014 Elsevier B.V. All rights reserved.
determine the mechanical properties of frozen soil–concrete interfaces. Current studies on frozen soil, current studies on the interfaces have mainly addressed pile-frozen soil working conditions and model tests. Chen (2011) developed a model pile test apparatus and carried out 54 freezing-strength tests on two layers of soil at an engineering field. In that study it was clear that the freezing strength and residual freezing strength linearly increased with the decrease of temperature. Also, there was a water content limit: when the water content was less than this limit, the freezing strength and residual freezing strength increased with the increment of water content, and vice versa. Results of basic direct shear or simple shear tests on frozen soil– concrete interfaces under negative temperatures have rarely been published. Fortunately, this situation is changing: Cui et al. (2013) developed a dynamic direct-shear system for frozen soil, and Lv et al. (2013) carried out some dynamic tests on frozen soil–concrete interfaces with that methodology. Zhao et al. (2013) designed a multi-functional direct-shear apparatus and operated a series of cyclic shear tests on frozen soil–structure interfaces. 2. Test apparatus and parameters We used the large-scale temperature-controlled direct shear test system (TZJ-150) in the Frozen Soil Laboratory at Beijing Jiaotong University for our tests (Fig. 1). This is a computerized, numericalcontrolled direct-shear apparatus with a built-in cooling bath circulation device. Vertical and horizontal pressures are driven by two stepper servo motors with maximum outputs of 100 kN and 150 kN (Fig. 2).
2
J. Liu et al. / Cold Regions Science and Technology 104–105 (2014) 1–6 Table 1 Physical properties of silty clay.
Fig. 1. The large scale temperature-controlled direct shear test.
With the feedback control module on each motor, displacement or pressure in both directions can be precisely controlled. The box with a volume of 300 × 300 × 200 mm has a steel structure inside and a space for cold liquid outside with insulation layers. Two cooling bath circulators were connected to the upper and lower parts of the shear box, so the cooling process and temperature control could be carried out independently on both parts. A constant vertical temperature gradient could thus be maintained near the interface. The soil was silty clay taken from the subgrade filling of the Harbin– Dalian passenger railway line. Particle size analysis showed that 0.25–0.075-mm sand accounted for 17.5%, 0.075–0.005-mm silt for 41.5%, and clay b0.005 mm for 41%. The basic physical properties of this clay are shown in Table 1. In our soil test procedures, all of the soil was sifted through a 2-mm sieve, dried in an oven for 8 h, and mixed with water for the specified water content. The size of the concrete sample was 300 × 300 × 100 mm (Fig. 3). Before the test we placed the concrete into the lower part of the shear box and used the vertical loading module to compact the soil spread on the concrete, making sure that the soil achieved the required relative compaction under the pressure for each test. A series of PT100 temperature sensors were implanted in the shear box, inside the soil and right on the interface, to monitor the temperature. Then the cooling bath circulation
Fig. 2. Schematic of components ofTZJ-150.
Specific gravity of soil grain
Liquid limit (%)
Plastic limit (%)
Optimum water content (%)
Maximum dry density (g/cm3)
2.71
48.6
21.5
18.1
1.80
was started to ensure that the temperatures in the soil and on the interface were equal (this usually took 18–24 hours). Once the sample reached a thermal balance and the temperature field became stable, the vertical loading module was used to provide a steady normal pressure. The horizontal module was then activated to push the shear box at the speed of about 10 mm/h. A data collector automatically recorded the stresses and displacements in both directions and also the temperatures, with a sampling frequency of 50 Hz. During the whole shear process, feedback control kept the normal pressure unchanged, with an error of 2 kPa. The cooling bath circulation adjusted the temperature with a variation of b 1.0 °C. After each test, the cooling bath circulation was turned off to make the sample thaw. Three positions near the interface were selected to get an average water content. Different samples had different normal pressures, temperatures, and water contents, as shown in Table 2. 3. Test results and discussion 3.1. Features of the direct shear curves With high-precision, continuous data of displacement and stress, more details of the whole shear process could be observed. The shear displacements of failure were b 2 mm, and when the displacements reached 30 mm the tests ended.
Fig. 3. The concrete block used in this test and the interface after failure.
J. Liu et al. / Cold Regions Science and Technology 104–105 (2014) 1–6
3
Table 2 Sample with different factors. Vertical pressure (kPa)
Interface temperature (°C)
Water content (%)
Sample number
100 200 300 400 200 200 200 200 200 200 200
−10.1 −9.8 −9.5 −9.3 +1.1 −4.5 −9.8 −16.0 −8.1 −8.7 −8.4
17.4 17.6 16.7 17.1 18.1 17.8 17.6 18.3 14.9 18.3 22.5
2 3 4 5 10 11 3 9 21 19 22
The relationship between the average shear stress and shear displacement in a single shear process, called the τ–u curve, is shown in Fig. 4(a). The whole process can be divided into several stages: The a–b segment is a straight line, dominated by the elastic deformation of frozen soil; the b–c segment, with a reducing slope, is dominated by the plastic deformation of frozen soil; the c–d segment represents the whole slide, with a sudden decline of average shear stress and sometimes cracking sounds; the d–e segment is the strain-hardening stage with a rising shear stress, which was mainly due to the deformation of
Fig. 5. The shear stress–displacement curves under different factors.
Fig. 4. The shear stress–displacement curve and vertical displacement–shear displacement curve of a whole shear process.
frozen soil; and in the e–f segment the residual shear strength declines and finally stabilizes. Similar stages can also be found in the v–u curve, the vertical displacement versus the shear displacement, in Fig. 4(b): a–c is the elastic and plastic stage before failure; c is the overall failure point; c–e is the segment of increasing residual shear strength; and e–f is the stage when the shear strength finally becomes stable. No significant positions for b and d were found because of their small magnitudes.
4
J. Liu et al. / Cold Regions Science and Technology 104–105 (2014) 1–6
Fig. 6. The fitting of peak strength on different factors.
3.2. Variations of the shear strength Peak shear strength and residual shear strength are used to describe the strength of the frozen soil–concrete interface. Peak shear strength is the maximum shear stress before break, and it can be found at the highest point before shear stress release in the τ–u curve. If no obvious
shear stress release is found, 0.5% strain (displacement of 1.5 mm for a scale of 300 mm) is considered as the peak shear strength point in this paper. Residual shear strength is the final stable shear stress after break. It should be at the end of the e–f segment in Fig. 4(a). There are several factors that can influence the shear strength of the frozen soil–concrete interface: normal pressure, interface temperature,
J. Liu et al. / Cold Regions Science and Technology 104–105 (2014) 1–6
water content, roughness of the concrete surface, and soil properties. The first three factors were tested using the same soil and the same block of concrete (Table 2). The temperature changed with time and water content were not exactly the same in the different samples. Therefore, the temperature at peak shear strength was taken as the standard, and the moisture content was measured at three different positions near the interface; the average value was taken as the moisture content of the sample. In this case, the temperatures with differences of b 1 °C and water contents with differences of b1% were considered as the same. The τ–u curves of different normal pressures in Fig. 5(a) have quite similar shapes. The larger the normal pressure, the higher the peak shear strength of the interface and also the residual shear strength. As seen in Fig. 6(a,d), these parameters have a positive linear relationship. Linear fitting shows that when the average temperature T ¼ −9:6 C and the average water content w ¼ 17:2%, within the test pressure range: τ f ¼ 0:977N þ 25:945
ð1Þ
τr ¼ 0:978N þ 59:67
ð2Þ
where τf is the peak shear strength and τr is the residual shear strength. Both of these parameters had almost the same slopes under normal pressure. Residual shear strength was always slightly higher than peak shear strength, and as the normal pressure increased, the point of the peak shear strength moved progressively to the right. This change indicates that the sample needed a larger shear displacement to reach the peak shear strength at a higher normal pressure. The curves of different interface temperatures in Fig. 5(b) have similar shapes. The lower the temperature, the higher the peak shear strength and the residual shear strength. Because the water in soil freezes at 0 °C, the relationship between shear strength and temperature only exists below 0 °C. The shear strength should remain unchanged above the freezing point. Under this hypothesis, the curve under 1.1 °C was taken as the one under 0 °C, and a linear fitting was made (Fig. 6(b,e)). The result shows that when the normal pressure N = 200 kPa, the average water content w ¼ 18:0%, and the temperature was within 0 to ~−16 °C: τ f ¼ −11:801T þ 124:885
ð3Þ
τr ¼ −11:688T þ 152:405
ð4Þ
The same slopes on temperature could also be found in these two formulas. Additionally, the − 16 °C τ–u curve had an obvious larger shear stress release after the peak shear strength point than did the other three. The curves of different water contents in Fig. 5(c) are significantly different. The 14.9% curve has a monotonic increase and a stable final value but no obvious extremum, showing typical plastic failure features. The 18.3% curve has a higher rising start, a long decline, and a stable end. It has a maximum point which represents the peak shear strength. The 22.5% curve has a straight rising segment and achieves the highest peak shear strength of all the tests, 580.22 kPa. The shear stress release in this curve is as large as the residual shear strength and it shows obvious brittle failure features. These three curves are approximately coincident within the elastic deformation stage. They also have basically equal residual strengths after failure. However, their shapes change from plastic failure to brittle failure. Fig. 6(c) shows that the peak shear strength increases rapidly with the water content; they are not linearly dependent. Fig. 6(f) shows that the residual shear strength stays almost unchanged. If an exponential function was used to fit the points. When the pressure
5
was N = 200 kPa, the average temperature was T ¼ −8:4BC, and the water content was 14.9% b w b 22.5%, the results were: 0:367w
τ f ¼ 0:087e
þ 244:376
τr ¼ 1:301w þ 250:804
ð5Þ
ð6Þ
3.3. Mechanisms of shear strength The peak shear strength is actually the freezing strength of the frozen soil–concrete interface. The freezing strength consists of the freezing strength of ice-concrete and the cohesion among soil grains, unfrozen water, and the concrete surface. The freezing strength of iceconcrete has a close relationship with the temperature and disappears as the interface slides, which can be observed in the c–d segment of the τ–u curve. This shear stress release stays constant in Fig. 5(a), increases as the temperature falls in Fig. 5(b), and enlarges as the water content increases in Fig. 5(c). The cohesion increases as the negative temperature falls, but does not disappear after the interface slides. The residual shear strength is composed of the cohesion of the interface and the friction force. The former is affected by the negative temperature, as mentioned above, and the latter is affected by the normal pressure. Thus, the residual shear strength appears to be related with the normal pressure in Fig. 5(a), shows changes with temperature in Fig. 5(b), but stays constant in Fig. 5(c) when the normal pressure and temperature are fixed. 4. Conclusions In this paper, a series of direct shear tests for soil–concrete interface were conducted on a large-scale temperature-controlled direct shear test system, the observed phenomenon in the shear process was discussed and analyzed. The following conclusions can be drawn: (1) To study the properties of frozen soil–concrete interfaces, a simple method of placing a concrete block in a shear box is described in this paper. The shear displacements of failure were b2 mm and the horizontal size of the concrete block was 300 mm, so the change of interface area can be ignored. This method proved to be simple and effective. (2) Obvious features could be observed in the τ–u and v–u curves. The whole shearing process can be divided into five stages: The a–b segment represents the elastic deformation part, the b–c segment the plastic deformation part, the c–d segment the whole slide part, the d–e segment the strain hardening part, and the e–f segment the stable residual strength part. (3) The shear strength of the interface was affected by normal pressure, temperature, and water content. Within the factor ranges in these tests, the peak shear strength and residual shear strength had a positive linear dependence on the normal pressure but they had a negative linear dependence on the negative temperature. However, as the water content rose, the peak shear strength increased rapidly while the residual shear strength stayed stable. (4) A reasonable explanation for the changes in these factor curves is: The peak shear strength consisted of the freezing strength of ice-concrete and the cohesion among soil grains, unfrozen water, and the concrete surface. The residual shear strength was composed of the cohesion of the interface and the friction force. The freezing strength of ice-concrete was affected by temperature and water content, and the cohesion was affected by negative temperature but had no relationship with water content (frozen). The friction force was only affected by normal pressure and had no dependence on temperature and water content (frozen).
6
J. Liu et al. / Cold Regions Science and Technology 104–105 (2014) 1–6
Acknowledgments This work was supported by the National Basic Research Program of China (973 Program, Grant No. 2012CB026104) and the National Natural Science Foundation of China (Grant Nos. 41171064 and 1378057). References Boulon, M., Nova, R., 1990. Modeling of soil structure interface behavior: a comparison between elastic and rate-type laws. Comput. Geotech. 9 (1–2), 21–46. Chen, W.H., 2011. Experimental research on freezing strength and negative friction of pile side in permafrost region. (Master's thesis) , Harbin Institute of Technology, Harbin, Heilongjiang, China.
Clough, G.W., Duncan, J.M., 1971. Finite element analysis of retaining wall behavior [J]. J. Soil Mech. Found. Div. ASCE 97 (SM12), 1657–1672. Cui, Y.H., Liu, J.K., Lv, P., 2013. Research on the permafrost dynamic load direct shear apparatus. Rock Soil Mech. 34 (S2), 486–490. Desai, C.S., Drumm, E.C., Zaman, M.M., 1985. Cyclic testing and modeling of interfaces [J]. J. Geotech. Eng. 111 (6), 793–815. Lv, P., Liu, J.K., Cui, Y.H., 2013. A study on dynamic shear strength of frozen soil–concrete contact interface. Rock Soil Mech. 34 (S2), 180–183. Yin, Z.Z., Zhu, H., Xu, G.H., 1994. Numerical simulation of the deformation in the interface between soil and structural material. Chin. J. Geotech. Eng. 16 (3), 14–22. Zhang, G., Zhang, J.M., 2004. Experimental study on monotonic behavior of interface between soil and structure. Chin. J. Geotech. Eng. 26 (1), 21–25. Zhao, L.Z., Yang, P., Wang, H.B., 2013. Development and application of large-scale multifunctional frozen soil–structure interface cycle-shearing system. Chin. J. Geotech. Eng. 35 (4), 707–713.
Contents lists available at ScienceDirect
Cold Regions Science and Technology journal homepage: www.elsevier.com/locate/coldregions
Experimental study on direct shear behavior of frozen soil–concrete interface Jiankun Liu ⁎, Peng Lv, Yinghui Cui, Jingyu Liu Beijing Jiaotong University, School of Civil Engineering, Beijing 100044, China
a r t i c l e
i n f o
Article history: Received 26 November 2013 Accepted 30 April 2014 Keywords: Frozen soil Soil–concrete interface Direct shear Shear strength
a b s t r a c t In order to study the mechanical behavior of frozen soil–concrete interfaces, a series of direct shear tests were conducted on a large-scale temperature-controlled direct shear test system (TZJ-150). Synchronous variations were found in both the shear stress–displacement and vertical displacement–shear displacement curves. The whole shear process can be divided into five stages: the elastic deformation part, the plastic deformation part, the whole slide part, the strain hardening part, and the stable residual strength part. The influence rules of normal pressure, temperature, and water content on the shear strength of the interface were obtained by a series of sample tests with different parameters. Fitted equations between factors and shear strength were presented. The peak shear strength had linear relationships with the normal pressure and temperature, but a nonlinear relationship with the water content, which had a much greater effect on the peak shear strength. The residual shear strength also had linear relationships with the normal pressure and temperature, but was basically unrelated with the water content. A reasonable explanation about the changes of peak shear strength and residual shear strength was proposed. © 2014 Elsevier B.V. All rights reserved.
1. Introduction There are interfaces of soil and concrete in nearly all kinds of construction works. In foundations, tunnels, dams and embankments, the soil– concrete interface is a very important element of the safety of these projects. The mechanical properties of soil–concrete interface have been a frequent research focus; until now, direct shear tests and simple shear tests have been the most commonly methods used for experimental studies on such interfaces. Clough and Duncan (1971) performed some direct shear tests on soil–concrete interface and established a hyperbolic model of shear stress and shear displacement; Boulon and Nova (1990) presented an elastic-plastic model based on the shear stress–shear displacement curves; Desai et al. (1985) conducted many direct shear and simple shear tests on sand–structure interface and analyzed the main influencing factors. Yin et al. (1994) observed shear displacements at different positions on interfaces using a series of mini periscopes; they pointed out that the stress–displacement curve reflects the expanding failure area, and they presented a rigid-plastic model of this. Zhang and Zhang (2004) used steel plates with different roughnesses to replace the concrete in simple shear tests, and studied the movements of soil grains under different influencing factors. All past studies of soil–concrete interfaces were carried out at positive temperatures. However, for construction in cold regions, or artificial freezing works in unfrozen regions, it becomes very necessary to ⁎ Corresponding author. Tel./fax: +86 10 51684096. E-mail address: [email protected] (J. Liu).
http://dx.doi.org/10.1016/j.coldregions.2014.04.007 0165-232X/© 2014 Elsevier B.V. All rights reserved.
determine the mechanical properties of frozen soil–concrete interfaces. Current studies on frozen soil, current studies on the interfaces have mainly addressed pile-frozen soil working conditions and model tests. Chen (2011) developed a model pile test apparatus and carried out 54 freezing-strength tests on two layers of soil at an engineering field. In that study it was clear that the freezing strength and residual freezing strength linearly increased with the decrease of temperature. Also, there was a water content limit: when the water content was less than this limit, the freezing strength and residual freezing strength increased with the increment of water content, and vice versa. Results of basic direct shear or simple shear tests on frozen soil– concrete interfaces under negative temperatures have rarely been published. Fortunately, this situation is changing: Cui et al. (2013) developed a dynamic direct-shear system for frozen soil, and Lv et al. (2013) carried out some dynamic tests on frozen soil–concrete interfaces with that methodology. Zhao et al. (2013) designed a multi-functional direct-shear apparatus and operated a series of cyclic shear tests on frozen soil–structure interfaces. 2. Test apparatus and parameters We used the large-scale temperature-controlled direct shear test system (TZJ-150) in the Frozen Soil Laboratory at Beijing Jiaotong University for our tests (Fig. 1). This is a computerized, numericalcontrolled direct-shear apparatus with a built-in cooling bath circulation device. Vertical and horizontal pressures are driven by two stepper servo motors with maximum outputs of 100 kN and 150 kN (Fig. 2).
2
J. Liu et al. / Cold Regions Science and Technology 104–105 (2014) 1–6 Table 1 Physical properties of silty clay.
Fig. 1. The large scale temperature-controlled direct shear test.
With the feedback control module on each motor, displacement or pressure in both directions can be precisely controlled. The box with a volume of 300 × 300 × 200 mm has a steel structure inside and a space for cold liquid outside with insulation layers. Two cooling bath circulators were connected to the upper and lower parts of the shear box, so the cooling process and temperature control could be carried out independently on both parts. A constant vertical temperature gradient could thus be maintained near the interface. The soil was silty clay taken from the subgrade filling of the Harbin– Dalian passenger railway line. Particle size analysis showed that 0.25–0.075-mm sand accounted for 17.5%, 0.075–0.005-mm silt for 41.5%, and clay b0.005 mm for 41%. The basic physical properties of this clay are shown in Table 1. In our soil test procedures, all of the soil was sifted through a 2-mm sieve, dried in an oven for 8 h, and mixed with water for the specified water content. The size of the concrete sample was 300 × 300 × 100 mm (Fig. 3). Before the test we placed the concrete into the lower part of the shear box and used the vertical loading module to compact the soil spread on the concrete, making sure that the soil achieved the required relative compaction under the pressure for each test. A series of PT100 temperature sensors were implanted in the shear box, inside the soil and right on the interface, to monitor the temperature. Then the cooling bath circulation
Fig. 2. Schematic of components ofTZJ-150.
Specific gravity of soil grain
Liquid limit (%)
Plastic limit (%)
Optimum water content (%)
Maximum dry density (g/cm3)
2.71
48.6
21.5
18.1
1.80
was started to ensure that the temperatures in the soil and on the interface were equal (this usually took 18–24 hours). Once the sample reached a thermal balance and the temperature field became stable, the vertical loading module was used to provide a steady normal pressure. The horizontal module was then activated to push the shear box at the speed of about 10 mm/h. A data collector automatically recorded the stresses and displacements in both directions and also the temperatures, with a sampling frequency of 50 Hz. During the whole shear process, feedback control kept the normal pressure unchanged, with an error of 2 kPa. The cooling bath circulation adjusted the temperature with a variation of b 1.0 °C. After each test, the cooling bath circulation was turned off to make the sample thaw. Three positions near the interface were selected to get an average water content. Different samples had different normal pressures, temperatures, and water contents, as shown in Table 2. 3. Test results and discussion 3.1. Features of the direct shear curves With high-precision, continuous data of displacement and stress, more details of the whole shear process could be observed. The shear displacements of failure were b 2 mm, and when the displacements reached 30 mm the tests ended.
Fig. 3. The concrete block used in this test and the interface after failure.
J. Liu et al. / Cold Regions Science and Technology 104–105 (2014) 1–6
3
Table 2 Sample with different factors. Vertical pressure (kPa)
Interface temperature (°C)
Water content (%)
Sample number
100 200 300 400 200 200 200 200 200 200 200
−10.1 −9.8 −9.5 −9.3 +1.1 −4.5 −9.8 −16.0 −8.1 −8.7 −8.4
17.4 17.6 16.7 17.1 18.1 17.8 17.6 18.3 14.9 18.3 22.5
2 3 4 5 10 11 3 9 21 19 22
The relationship between the average shear stress and shear displacement in a single shear process, called the τ–u curve, is shown in Fig. 4(a). The whole process can be divided into several stages: The a–b segment is a straight line, dominated by the elastic deformation of frozen soil; the b–c segment, with a reducing slope, is dominated by the plastic deformation of frozen soil; the c–d segment represents the whole slide, with a sudden decline of average shear stress and sometimes cracking sounds; the d–e segment is the strain-hardening stage with a rising shear stress, which was mainly due to the deformation of
Fig. 5. The shear stress–displacement curves under different factors.
Fig. 4. The shear stress–displacement curve and vertical displacement–shear displacement curve of a whole shear process.
frozen soil; and in the e–f segment the residual shear strength declines and finally stabilizes. Similar stages can also be found in the v–u curve, the vertical displacement versus the shear displacement, in Fig. 4(b): a–c is the elastic and plastic stage before failure; c is the overall failure point; c–e is the segment of increasing residual shear strength; and e–f is the stage when the shear strength finally becomes stable. No significant positions for b and d were found because of their small magnitudes.
4
J. Liu et al. / Cold Regions Science and Technology 104–105 (2014) 1–6
Fig. 6. The fitting of peak strength on different factors.
3.2. Variations of the shear strength Peak shear strength and residual shear strength are used to describe the strength of the frozen soil–concrete interface. Peak shear strength is the maximum shear stress before break, and it can be found at the highest point before shear stress release in the τ–u curve. If no obvious
shear stress release is found, 0.5% strain (displacement of 1.5 mm for a scale of 300 mm) is considered as the peak shear strength point in this paper. Residual shear strength is the final stable shear stress after break. It should be at the end of the e–f segment in Fig. 4(a). There are several factors that can influence the shear strength of the frozen soil–concrete interface: normal pressure, interface temperature,
J. Liu et al. / Cold Regions Science and Technology 104–105 (2014) 1–6
water content, roughness of the concrete surface, and soil properties. The first three factors were tested using the same soil and the same block of concrete (Table 2). The temperature changed with time and water content were not exactly the same in the different samples. Therefore, the temperature at peak shear strength was taken as the standard, and the moisture content was measured at three different positions near the interface; the average value was taken as the moisture content of the sample. In this case, the temperatures with differences of b 1 °C and water contents with differences of b1% were considered as the same. The τ–u curves of different normal pressures in Fig. 5(a) have quite similar shapes. The larger the normal pressure, the higher the peak shear strength of the interface and also the residual shear strength. As seen in Fig. 6(a,d), these parameters have a positive linear relationship. Linear fitting shows that when the average temperature T ¼ −9:6 C and the average water content w ¼ 17:2%, within the test pressure range: τ f ¼ 0:977N þ 25:945
ð1Þ
τr ¼ 0:978N þ 59:67
ð2Þ
where τf is the peak shear strength and τr is the residual shear strength. Both of these parameters had almost the same slopes under normal pressure. Residual shear strength was always slightly higher than peak shear strength, and as the normal pressure increased, the point of the peak shear strength moved progressively to the right. This change indicates that the sample needed a larger shear displacement to reach the peak shear strength at a higher normal pressure. The curves of different interface temperatures in Fig. 5(b) have similar shapes. The lower the temperature, the higher the peak shear strength and the residual shear strength. Because the water in soil freezes at 0 °C, the relationship between shear strength and temperature only exists below 0 °C. The shear strength should remain unchanged above the freezing point. Under this hypothesis, the curve under 1.1 °C was taken as the one under 0 °C, and a linear fitting was made (Fig. 6(b,e)). The result shows that when the normal pressure N = 200 kPa, the average water content w ¼ 18:0%, and the temperature was within 0 to ~−16 °C: τ f ¼ −11:801T þ 124:885
ð3Þ
τr ¼ −11:688T þ 152:405
ð4Þ
The same slopes on temperature could also be found in these two formulas. Additionally, the − 16 °C τ–u curve had an obvious larger shear stress release after the peak shear strength point than did the other three. The curves of different water contents in Fig. 5(c) are significantly different. The 14.9% curve has a monotonic increase and a stable final value but no obvious extremum, showing typical plastic failure features. The 18.3% curve has a higher rising start, a long decline, and a stable end. It has a maximum point which represents the peak shear strength. The 22.5% curve has a straight rising segment and achieves the highest peak shear strength of all the tests, 580.22 kPa. The shear stress release in this curve is as large as the residual shear strength and it shows obvious brittle failure features. These three curves are approximately coincident within the elastic deformation stage. They also have basically equal residual strengths after failure. However, their shapes change from plastic failure to brittle failure. Fig. 6(c) shows that the peak shear strength increases rapidly with the water content; they are not linearly dependent. Fig. 6(f) shows that the residual shear strength stays almost unchanged. If an exponential function was used to fit the points. When the pressure
5
was N = 200 kPa, the average temperature was T ¼ −8:4BC, and the water content was 14.9% b w b 22.5%, the results were: 0:367w
τ f ¼ 0:087e
þ 244:376
τr ¼ 1:301w þ 250:804
ð5Þ
ð6Þ
3.3. Mechanisms of shear strength The peak shear strength is actually the freezing strength of the frozen soil–concrete interface. The freezing strength consists of the freezing strength of ice-concrete and the cohesion among soil grains, unfrozen water, and the concrete surface. The freezing strength of iceconcrete has a close relationship with the temperature and disappears as the interface slides, which can be observed in the c–d segment of the τ–u curve. This shear stress release stays constant in Fig. 5(a), increases as the temperature falls in Fig. 5(b), and enlarges as the water content increases in Fig. 5(c). The cohesion increases as the negative temperature falls, but does not disappear after the interface slides. The residual shear strength is composed of the cohesion of the interface and the friction force. The former is affected by the negative temperature, as mentioned above, and the latter is affected by the normal pressure. Thus, the residual shear strength appears to be related with the normal pressure in Fig. 5(a), shows changes with temperature in Fig. 5(b), but stays constant in Fig. 5(c) when the normal pressure and temperature are fixed. 4. Conclusions In this paper, a series of direct shear tests for soil–concrete interface were conducted on a large-scale temperature-controlled direct shear test system, the observed phenomenon in the shear process was discussed and analyzed. The following conclusions can be drawn: (1) To study the properties of frozen soil–concrete interfaces, a simple method of placing a concrete block in a shear box is described in this paper. The shear displacements of failure were b2 mm and the horizontal size of the concrete block was 300 mm, so the change of interface area can be ignored. This method proved to be simple and effective. (2) Obvious features could be observed in the τ–u and v–u curves. The whole shearing process can be divided into five stages: The a–b segment represents the elastic deformation part, the b–c segment the plastic deformation part, the c–d segment the whole slide part, the d–e segment the strain hardening part, and the e–f segment the stable residual strength part. (3) The shear strength of the interface was affected by normal pressure, temperature, and water content. Within the factor ranges in these tests, the peak shear strength and residual shear strength had a positive linear dependence on the normal pressure but they had a negative linear dependence on the negative temperature. However, as the water content rose, the peak shear strength increased rapidly while the residual shear strength stayed stable. (4) A reasonable explanation for the changes in these factor curves is: The peak shear strength consisted of the freezing strength of ice-concrete and the cohesion among soil grains, unfrozen water, and the concrete surface. The residual shear strength was composed of the cohesion of the interface and the friction force. The freezing strength of ice-concrete was affected by temperature and water content, and the cohesion was affected by negative temperature but had no relationship with water content (frozen). The friction force was only affected by normal pressure and had no dependence on temperature and water content (frozen).
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Acknowledgments This work was supported by the National Basic Research Program of China (973 Program, Grant No. 2012CB026104) and the National Natural Science Foundation of China (Grant Nos. 41171064 and 1378057). References Boulon, M., Nova, R., 1990. Modeling of soil structure interface behavior: a comparison between elastic and rate-type laws. Comput. Geotech. 9 (1–2), 21–46. Chen, W.H., 2011. Experimental research on freezing strength and negative friction of pile side in permafrost region. (Master's thesis) , Harbin Institute of Technology, Harbin, Heilongjiang, China.
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