A polymer material of polyurethane soil stabilizer (PSS) is used to reinforce the sand. To understand the permeability characteristics of PSS reinforced sand, a series of reinforcement layer form test, single-hole permeability test, and porous permeability test of sand reinforced with PSS have been performed. Reinforcement mechanism is discussed with scanning electron microscope images. The results indicated that the permeability resistance of sand reinforced with polyurethane soil stabilizer is improved through the formation of reinforcement layer on the sand surface. The thickness and complete degree of the reinforcement layer increase with the increasing of curing time and PSS concentration. The water flow rate decreases with the increasing of curing time or PSS concentration. The permeability coefficient decreases with the increasing of curing time and PSS concentration and increases with the increasing of depth in specimen. PSS fills up the voids of sand and adsorbs on the surface of sand particle to reduce or block the flowing channels of water to improve the permeability resistance of sand. The results can be applied as the reference for chemical reinforcement sandy soil engineering, especially for surface protection of embankment, slope, and landfill.
Natural sand material has certain deficiency in the permeability and stability of foundation, slope, and embankment for geotechnical engineering. The materials used in covering on the surface of sand or adding into sand body to reduce or prevent the water permeating into the sand body are mainly geomembrane, geotextile, concrete slab, plant, and so on [
At present, the polymer materials of polyvinyl acetate (PVA), polyurethane, polyacrylamide, and have been considered as the new soil stabilizers to reinforce the soil [
In this paper, the polyurethane soil stabilizer (PSS) is sprayed on sand surface to form a reinforcement layer, and the reinforcement layer form test, single-hole permeability test, and porous permeability test are performed in laboratory to analyze the effects of PSS reinforced on sand. Its reinforcement mechanism is discussed with images of scanning electron microscope (SEM). Based on the analyses of interaction between PSS and sand, the optimal curing time and PSS concentrations are suggested. The results can be applied as the reference for PSS reinforced sand engineering, especially for surface protection of embankment, slope, and landfill.
The sand selected from Nanjing City of Jiangsu Province, China, was used in this study. Its particle size distribution is shown in Figure
Grain size distribution of sand.
A new type of polyurethane soil stabilizer (PSS) was used in this study (Figure
Photo of polyurethane soil stabilizer (PSS).
PSS has the primary advantages: (a) it reacts with water to form elastic and viscous reinforced layer on soil surface with excellent mechanical property; (b) it is an environment-friendly product with no additional pollution, and it is a kind of biodegradable water-soluble polymer; (c) it is easy to produce and has a low cost.
PSS as a soil reinforcement material is sprayed on sand surface to form a reinforcement layer. The initial permeability time and the permeability coefficient of sand are considered as important factors to evaluate the permeability characteristics of sand reinforced with PSS. In this study, the laboratory tests of reinforcement layer form test, single-hole permeability test, and porous permeability test were performed to evaluate permeability characteristics of PSS reinforced sand.
The formation of reinforcement layer on sand surface is important to slope surface protection. In reinforcement layer form test, the reinforcement layer thickness of the specimen was measured. Sand specimens taken from slope surface were first oven dried and then filled in a container. The container has a diameter of 10 mm and height of 6 mm. The dry weight of each specimen was 500 g. Six groups of different concentrations of 0%, 1%, 3%, 5%, 7%, and 9% of PSS solution and 24 specimens of each group were proposed. The amount 1.6 L/m2 of each solution was sprayed on the specimen surface uniformly. After spraying, the reinforced specimens were kept in curing box with a temperature around 20°C, and the reinforcement layer thickness was tested with different curing time of 0 h, 0.1 h, 0.5 h, 1 h, 3 h, 6 h, 12 h, and 24 h. The block unit of quadrate area 3 × 3 cm was taken from the reinforcement layer on specimen surface and its thickness was measured by vernier caliper. Additionally, three specimens were measured for each test and their average values were used.
In single-hole permeability test, the permeability characteristic of PSS reinforced surface sand was tested by TST-70 permeameter (Figure
TST-70 permeameter used in single-hole permeability tests.
The specimen was directly formed in the testing apparatus. The dry sand was divided into 10 equal parts and each part was put into the box of permeameter and compacted. Specimens for this test with a diameter 10 cm and a height 30 cm were prepared in ten layers of equal height to achieve the proposed density 1.44 g/cm3. After that, six concentrations of 0%, 1%, 3%, 5%, 7%, and 9% of PSS solution were sprayed on sand surface. The amount 1.6 L/m2 of each solution was sprayed on the specimen surface uniformly. After spraying, the specimens were kept in curing box with a temperature around 20°C, and the permeability test was carried out with different curing time of 0 h, 3 h, 6 h, 12 h, and 24 h. Firstly, the top inlet and bottom outlet tube were kept open; the water level of specimen surface was maintained. The initial time of water running from specimen surface to the bottom tube was recorded. The running water volume in every 3 minutes was recorded until the water volume difference for successive two times is less than 3%. The average water flow rate for each 3 minutes was given by the following equation [
In (
The permeability coefficient of specimens is obtained by
In (
The permeability resistance of reinforced specimens was evaluated using
In (
The permeability resistance of sand with different depth is an effective parameter to evaluate the reinforcement layer. The permeability coefficient of PSS reinforced surface sand with different depth was studied by porous permeability test. In this test, a self-manufactured permeameter shown in Figure
Porous permeameter used in porous permeability test.
The specimen was directly formed in the apparatus. All the tubes were closed at the beginning. The dry sand was divided into 9 equal parts and each part was put into the box of permeameter and compacted. Specimens for this test with a diameter of 20 cm and a height of 90 cm were prepared in 9 layers of equal height to achieve the proposed density 1.44 g/cm3. After that, six concentrations of 0%, 1%, 3%, and 5% of PSS solution were sprayed on sand surface. The amount 1.6 L/m2 of each solution was sprayed on the specimen surface uniformly. After spraying, the specimens were kept in curing box with a temperature around 20°C for 24 h to form the reinforcement layer. After the specimens were formed, the inlet tube and the outlet tubes numbers 0 and 9 were kept open, and the initial time of water running from specimen surface to the outlet tube number 9 and its running water volume in every 3 minutes were recorded. After that, the operation of close number 9 and open number 8 was performed and its running water volume of every 3 minutes with a number of 5 times was recorded. Then, the outlet tubes numbers 7 to 1 were open in sequence to record their running water volume. It should be noted that only one number of outlet tubes was open to record its running water volume every time. Finally, the initial time of water running from number 9 and the running water volume of number 9 to number 1 were taken to evaluate the permeability resistance of specimens. The data of each outlet tube was recorded until the water volume difference for successive two times is less than 3%. The average water flow rate (
The thickness of reinforcement layer measured with vernier caliper was presented in Figure
The reinforcement layer thickness of samples tested (cm).
Number | PSS concentrations (%) | Curing time | |||||||
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0 h | 0.1 h | 0.5 h | 1 h | 3 h | 6 h | 12 h | 24 h | ||
Group 1 | 0 | / | / | / | / | / | / | / | / |
Group 2 | 1 | / | / | / | / | / | / | / | / |
Group 3 | 3 | / | 0.28 | 0.44 | 0.95 | 1.87 | 2.09 | 2.13 | 2.12 |
Group 4 | 5 | / | 0.34 | 0.72 | 1.28 | 2.04 | 2.53 | 2.55 | 2.57 |
Group 5 | 7 | / | 0.45 | 1.15 | 1.56 | 2.54 | 3.00 | 3.02 | 3.05 |
Group 6 | 9 | / | 0.86 | 1.30 | 1.87 | 2.98 | 3.58 | 3.58 | 3.56 |
Photo of reinforcement layer measured with vernier caliper.
The reinforcement layer thickness of specimens with different curing times.
As seen in Table
In single-hole permeability test, the specimens reinforced with PSS 0%, 1%, 3%, 5%, 7%, and 9% at curing time of 0 h, 3 h, 6 h, 12 h, and 24 h were considered. The initial time of water running from the bottom tube and its running water volume of every 3 minutes with a number of 5 times were recorded. The test results are presented in Table
Results of single-hole permeability test.
Number | PSS concentrations (%) | Curing time (h) | Initial time (s) | Water volume of 0–3 min (ml) | Water volume of 3–6 min (ml) | Water volume of 6–9 min (ml) | Water volume of 9–12 min (ml) | Water volume of 12–15 min (ml) |
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S-1 | 0 | 0 | 2 | 215.35 | 283.34 | 274.67 | 282.57 | 278.85 |
S-2 | 1 | 0 | 2 | 216.84 | 274.57 | 272.41 | 275.11 | 277.33 |
S-3 | 3 | 0 | 4 | 115.18 | 120.31 | 110.93 | 147.83 | 147.62 |
S-4 | 5 | 0 | 5 | 103.92 | 105.97 | 98.54 | 105.41 | 108.54 |
S-5 | 7 | 0 | 6 | 59.38 | 78.64 | 77 | 80.11 | 83.93 |
S-6 | 9 | 0 | 8 | 46.63 | 67.8 | 53.01 | 51.51 | 64.59 |
S-7 | 0 | 3 | 2 | 213.46 | 280.24 | 270.78 | 274.70 | 281.36 |
S-8 | 1 | 3 | 8 | 145.06 | 161.43 | 168.5 | 160.77 | 160.61 |
S-9 | 3 | 3 | 82 | 14.35 | 43.69 | 45.17 | 43.96 | 43.57 |
S-10 | 5 | 3 | 135 | 2.71 | 3.85 | 4.24 | 4.21 | 4.31 |
S-11 | 7 | 3 | 2230 | 2.28 | 2.39 | 2.22 | 2.14 | 1.91 |
S-12 | 9 | 3 | 4000 | 0.91 | 1.04 | 1.15 | 1.08 | 1.22 |
S-13 | 0 | 6 | 3 | 212.16 | 276.84 | 274.45 | 282.42 | 278.58 |
S-14 | 1 | 6 | 25 | 107.12 | 139.05 | 136.68 | 137.36 | 138.87 |
S-15 | 3 | 6 | 143 | 11.94 | 38.03 | 41.6 | 40 | 37.48 |
S-16 | 5 | 6 | 302 | 2.96 | 3.8 | 3.94 | 3.8 | 3.96 |
S-17 | 7 | 6 | ∞ | 0 | 0 | 0 | 0 | 0 |
S-18 | 9 | 6 | ∞ | 0 | 0 | 0 | 0 | 0 |
S-19 | 0 | 12 | 3 | 215.46 | 281.25 | 275.28 | 281.30 | 278.37 |
S-20 | 1 | 12 | 27 | 104.32 | 122.08 | 134.74 | 135.66 | 137.07 |
S-21 | 3 | 12 | 180 | 9.55 | 30.22 | 30.7 | 28.95 | 27.24 |
S-22 | 5 | 12 | 366 | 2.16 | 3.15 | 3.94 | 3.83 | 3.9 |
S-23 | 7 | 12 | ∞ | 0 | 0 | 0 | 0 | 0 |
S-24 | 9 | 12 | ∞ | 0 | 0 | 0 | 0 | 0 |
S-25 | 0 | 24 | 3 | 213.27 | 284.56 | 280.35 | 283.35 | 279.47 |
S-26 | 1 | 24 | 5 | 70.51 | 129.58 | 125.17 | 126.6 | 123.52 |
S-27 | 3 | 24 | 248 | 7.64 | 13.91 | 13.58 | 14.37 | 13.51 |
S-28 | 5 | 24 | 1810 | 2.41 | 3 | 3.35 | 3.25 | 3.38 |
S-29 | 7 | 24 | ∞ | 0 | 0 | 0 | 0 | 0 |
S-30 | 9 | 24 | ∞ | 0 | 0 | 0 | 0 | 0 |
The initial times of running water from bottom tube that is the time of water flowing from top to bottom of specimen are given in Figure
Initial time of running water from bottom outlet.
Based on the data of water volumes in Table
Water flow rate (
0 h
3 h
6 h
12 h
24 h
The variations of water flow rate (
The evaluation parameters of specimens with different curing times in single-hole permeability tests.
PSS |
( |
( |
( |
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0 h | 3 h | 6 h | 12 h | 24 h | 0 h | 3 h | 6 h | 12 h | 24 h | 0 h | 3 h | 6 h | 12 h | 24 h | |
0 | 92.95 | 93.79 | 92.86 | 92.79 | 93.16 |
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1 | 1 | 1 | 1 | 1 |
1 | 92.44 | 53.54 | 46.29 | 45.69 | 41.17 |
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1.01 | 1.75 | 2.01 | 2.03 | 2.26 |
3 | 49.21 | 14.52 | 12.49 | 9.08 | 4.50 |
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1.89 | 6.46 | 7.43 | 10.22 | 20.70 |
5 | 36.18 | 1.44 | 1.32 | 1.30 | 1.13 |
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2.57 | 65.13 | 70.35 | 71.38 | 82.44 |
7 | 27.98 | 0.64 | 0 | 0 | 0 |
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0 | 0 | 0 | 3.32 | 146.55 |
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9 | 21.53 | 0.41 | 0 | 0 | 0 |
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0 | 0 | 0 | 4.32 | 228.76 |
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Variations of water flow rate (
The relative permeability resistance
The relative permeability resistance
Photos of reinforcement layers taken from the specimens with curing time of 24 h after single-hole permeability tests.
In porous permeability test, the specimens reinforced with PSS 0%, 1%, 3%, and 5% at curing time of 24 hours were considered to study the influence depth of sand surface reinforcement. The initial times of water running from the outlet tube number 9 and the running water volume of outlet tubes numbers 1 to 9 of every 3 minutes with a number of 5 times were recorded. The test results are presented in Table
Results of porous permeability test.
Number | PSS concentrations |
Sand depth |
Initial time (s) | Water volume of 0–3 min (ml) | Water volume of 3–6 min (ml) | Water volume of 6–9 min (ml) | Water volume of 9–12 min (ml) | Water volume of 12–15 min (ml) |
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P-1 | 0% | 90 | 108 | 163.58 | 1152.47 | 1158.19 | 1164.67 | 1152.22 |
0% | 80 | / | 1113.69 | 1119.76 | 1110.97 | 1115.56 | 1122.39 | |
0% | 70 | / | 1018.21 | 1081.89 | 1037.63 | 1047.71 | 1059.72 | |
0% | 60 | / | 858.27 | 871.75 | 864.56 | 853.86 | 854.19 | |
0% | 50 | / | 724.82 | 733.72 | 718.24 | 720.44 | 786.06 | |
0% | 40 | / | 699.01 | 710.60 | 677.15 | 702.00 | 697.38 | |
0% | 30 | / | 698.38 | 676.47 | 692.47 | 691.87 | 695.30 | |
0% | 20 | / | 501.95 | 508.12 | 486.43 | 469.03 | 489.07 | |
0% | 10 | / | 365.28 | 419.73 | 392.43 | 383.27 | 388.21 | |
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P-2 | 1% | 90 | 305 | 147.07 | 277.42 | 989.39 | 1062.42 | 1041.25 |
1% | 80 | / | 885.82 | 865.74 | 866.86 | 880.41 | 881.39 | |
1% | 70 | / | 753.78 | 762.15 | 755.60 | 777.70 | 773.99 | |
1% | 60 | / | 749.71 | 754.97 | 747.34 | 758.36 | 758.73 | |
1% | 50 | / | 743.83 | 734.2 | 726.61 | 737.77 | 707.91 | |
1% | 40 | / | 570.61 | 569.03 | 562.11 | 553.26 | 554.50 | |
1% | 30 | / | 568.32 | 574.71 | 584.79 | 544.77 | 549.69 | |
1% | 20 | / | 413.01 | 395.92 | 398.83 | 392.69 | 393.34 | |
1% | 10 | / | 282.60 | 293.46 | 307.17 | 286.12 | 289.92 | |
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P-3 | 3% | 90 | 1505 | 72.13 | 329.33 | 360.59 | 360.39 | 363.18 |
3% | 80 | / | 245.77 | 334.64 | 350.96 | 357.55 | 359.86 | |
3% | 70 | / | 188.36 | 292.52 | 324.62 | 348.77 | 350.26 | |
3% | 60 | / | 217.59 | 303.63 | 328.66 | 346.53 | 337.17 | |
3% | 50 | / | 219.46 | 296.48 | 326.05 | 338.52 | 323.58 | |
3% | 40 | / | 269.97 | 301.23 | 319.64 | 313.52 | 321.66 | |
3% | 30 | / | 234.20 | 267.63 | 278.76 | 292.08 | 293.12 | |
3% | 20 | / | 197.11 | 213.06 | 216.26 | 213.55 | 214.65 | |
3% | 10 | / | 128.55 | 129.40 | 148.07 | 144.43 | 141.15 | |
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P-4 | 5% | 90 | 7200 | 17.19 | 35.50 | 33.24 | 33.46 | 34.26 |
5% | 80 | / | 16.75 | 30.07 | 31.74 | 32.34 | 33.75 | |
5% | 70 | / | 15.72 | 30.72 | 32.75 | 32.76 | 32.96 | |
5% | 60 | / | 14.36 | 29.61 | 30.48 | 30.19 | 32.41 | |
5% | 50 | / | 14.61 | 29.69 | 29.13 | 28.58 | 28.33 | |
5% | 40 | / | 12.88 | 28.64 | 28.25 | 27.58 | 27.78 | |
5% | 30 | / | 10.89 | 24.42 | 26.34 | 26.49 | 25.97 | |
5% | 20 | / | 10.36 | 21.02 | 23.04 | 22.27 | 22.83 | |
5% | 10 | / | 8.62 | 19.16 | 18.53 | 18.43 | 18.93 |
Initial time of running water from outlet number 9 of specimens with different PSS concentrations.
Based on the data of water volumes in Table
The water flow rate (
PSS 0%
PSS 1%
PSS 3%
PSS 5%
The variations of water flow rate (
The water flow rate at time of 12–15 min of specimens with different depth (ml/min).
Number | PSS concentrations |
Depth (cm) | ||||||||
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90 | 80 | 70 | 60 | 50 | 40 | 30 | 20 | 10 | ||
G-1 | 0 | 384.07 | 374.13 | 353.24 | 284.73 | 262.02 | 232.46 | 231.77 | 163.02 | 129.4 |
G-2 | 1 | 347.08 | 293.8 | 258.00 | 252.91 | 245.97 | 184.83 | 183.23 | 131.11 | 96.64 |
G-3 | 3 | 121.06 | 119.95 | 116.75 | 112.39 | 107.86 | 107.22 | 97.71 | 71.55 | 47.05 |
G-4 | 5 | 11.42 | 11.25 | 11.00 | 10.80 | 9.44 | 9.26 | 8.66 | 7.61 | 6.31 |
The permeability coefficient of specimens with different depth (cm/s).
Number | PSS concentrations |
Depth (cm) | ||||||||
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90 | 80 | 70 | 60 | 50 | 40 | 30 | 20 | 10 | ||
G-1 | 0 |
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G-2 | 1 |
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G-3 | 3 |
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G-4 | 5 |
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Variations of water flow rate (
The relative permeability resistance
The relative permeability resistance of specimens with different depth.
Number | PSS concentrations |
Depth (cm) | ||||||||
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90 | 80 | 70 | 60 | 50 | 40 | 30 | 20 | 10 | ||
G-1 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
G-2 | 1 | 1.11 | 1.27 | 1.37 | 1.13 | 1.07 | 1.26 | 1.26 | 1.24 | 1.34 |
G-3 | 3 | 3.17 | 3.12 | 3.03 | 2.53 | 2.43 | 2.17 | 2.37 | 2.28 | 2.75 |
G-4 | 5 | 33.63 | 33.26 | 32.11 | 26.36 | 27.76 | 25.10 | 26.76 | 21.42 | 20.51 |
Photos of reinforcement layers taken from the specimens with curing time of 24 h after the porous permeability tests.
The polyurethane soil stabilizer (PSS) contains a significant proportion of the long-chain macromolecule of polyurethane resin and enormous amount of isocyanate group (–NCO). The structural formula of PSS is represented by formula (
SEM images of specimen reinforced with PSS 5%: (a) 50 times magnification, (b) 100 times magnification, (c) 150 times magnification, and (d) 200 times magnification.
The specimen surface is sprayed by PSS dilution to form the reinforcement layer. The curing time and dilution concentration are two important factors to the formation of reinforcement layer on specimen surface. This forming process requires a certain amount of curing time. This is the reason why the thickness of reinforcement layer reaches a stable value when the curing time is larger than 3 hours (see Figure
The flowing water rate of reinforcement layer on specimen surface reduces with the decreasing of its void radio. This decrement of void radio is improved by the curing time or PSS concentration. The flowing water rate of tested part of sand is mainly controlled by the sand layer with lowest void radio. So the flowing water rate decreases with the increase in curing time or PSS concentration (see Figures
As seen in Figure
PSS is a kind of waterborne polyurethane polymer and a form of oil liquid. When polymer as a reinforcing material is used in practical engineering, the environmental concerns must be taken seriously. The potential environmental concerns are mainly because of volatile organic compounds (VOC) and residual PSS solution, which will cause air pollution and water resource pollution. In order to reduce or avoid the environmental concerns, the VOC of PSS must be strictly controlled according to international regulations; the residual PSS solution must be recycled with effective methods. The sunny day should be selected for spraying PSS in field to ensure sufficient reaction of polymer and sand particle to reduce the polymer loss and water pollution.
The aging degradation of polymer is mainly affected by temperature, ultraviolet ray, water, and chemical mediator. Reactions (
In order to evaluate the permeability characteristics of PSS reinforced sand, the laboratory tests of reinforcement layer form test, single-hole permeability test, and porous permeability test were considered. The test results and reinforcement mechanism were analyzed. Based on the results of the tests presented herein, the main conclusions can be summarized as follows: The permeability resistance of sand reinforced with polyurethane soil stabilizer (PSS) is improved through the formation of reinforcement layer on the sand surface. The thickness and complete degree of the reinforcement layer increase with the increasing of curing time and PSS concentration. The reinforcement layer thicknesses of specimens modified by concentrations of 3%, 5%, 7%, and 9% with curing time of 24 h were approximately 2.12, 2.57, 3.05, and 3.56 cm, respectively. Laboratory tests of single-hole permeability test and porous permeability test indicated that the permeability time of reinforcement layer increases with the PSS concentration. The flowing water rate decreases with the increasing of curing time or PSS concentration. The permeability coefficient decreases with the increasing of curing time and PSS concentration and increases with the increasing of depth in specimen. The relative permeability resistances of specimens were mainly improved with the curing time larger than 3 hours and PSS concentrations higher than 3%. The water-resisting layer on sand surface is formed with the curing time and PSS concentration larger than 6 hours and 7%, respectively. The PSS contains a significant proportion of the long-chain macromolecule of polyurethane resin and enormous amount of isocyanate group (–NCO). Microscale SEM observations show that the PSS enwrap the sand particle and interlink them to form a reinforcement layer on the sand surface. PSS fills up the voids of sand and adsorbs on the surface of sand particle to reduce or block the flowing channels of water to improve the permeability resistance of sand. The results can be applied as the reference for PSS reinforced sand engineering, especially for surface protection of embankment, slope, and landfill. The reinforcement method, curing time, and optimal concentration should be adjusted to meet the different engineering requirements in practical applications.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This research was financially supported by the National Natural Science Foundation of China (Grants nos. 41472241 and 41202208) and Natural Science Foundation of Jiangsu Province, China (Grant no. BK20141415).