The objective of this study was to investigate the effect of different moisture contents of clay (13%, 15%, 17%, 19%, and 21%) and different coatings on the ability to reduce negative skin friction during a large-scale shear test. Four coating treatments of the concrete surface were investigated, i.e., no treatment, coating with a paraffin-oil mixture, coating with a polymer nanomaterial, and coating with paint. The results showed that when the moisture content of the clay was slightly larger than that of the plastic limit, the ability to reduce negative skin friction was the best, and the performance was similar for the paraffin-oil mixture, the polymer nanomaterial, and the paint. When the moisture content of the clay was lower than that of the plastic limit, the paraffin-oil mixture provided the best performance. The position of the neutral point can be determined by different methods, and the negative skin friction of piles should be reduced by applying coatings that are most suitable to different conditions.
With the development of geotechnical engineering, the role of piles is becoming increasingly important [
In essence, the negative skin friction of piles depends on the interface characteristics between the pile and soil. Uesugi and Kishida [
Different methods to mitigate the negative skin friction of piles include prepressure to reduce soil settlement when constructing a pile foundation, using a casing to avoid direct contact between the pile and soil, and using coatings such as bituminous materials to minimize the skin friction between the pile and soil [
Considering the above state and taking clay as the test soil, large-scale shear tests were carried out to study the reduction of negative skin friction by the paraffin-oil mixture, polymer nanomaterial and paint. The influences of the moisture content and different coatings on the ability to reduce the negative skin friction were investigated, and measures to reduce the negative skin friction of the pile under different conditions were proposed to provide guidance for engineering practice.
Due to the time and cost requirements of field tests, a large-scale shear test was designed to simulate a field test, and the ratio of the test model to real objects is 1 : 1. The test equipment of the large-scale shear test was provided by Chang’an University, which is located in Xi’an City, Shaanxi Province, China. The test equipment is shown in Figure
Test equipment of large-scale shear test.
Model dimensions.
According to the sectional shape, piles can be divided into square piles, round piles, heterogeneous piles, and so on. In the test design, the contact surface between the pile and soil is a plane, which can simulate square piles very well. However, in practical engineering, the contact between the pile and soil is always a curved surface. Since the frictional resistance is determined by the nature of the contact interface and the positive pressure and the shape does not have much effect, the results of this experiment can still provide reference for piles of other shapes.
The soil samples used in this study were clay derived from Xi’an, Shaanxi. The main physical properties were tested by ring knife tests (Figure
(a) Ring knife tests; (b) oedometer tests; (c) moisture content tests; (d) liquid limit and plastic limit tests; (e) direct shear tests.
Physical properties of clay.
Type | Clay |
---|---|
Density (g/cm3) | 1.7 |
Compressive modulus (MPa) | 11 |
Moisture content (%) | 13 |
Liquid limit (%) | 36 |
Plastic limit (%) | 18 |
Cohesion (kPa) | 22 |
Internal friction angle (°) | 25 |
Since moisture contents in the test area are mainly distributed between 13% and 21%, soil samples with moisture contents of 13%, 15%, 17%, 19%, and 21% were used. When the piles are built in the area with different soil or moisture contents, the design analysis can still be carried out according to the method of this experiment. The elastic modulus of the concrete was 30 GPa. Four concrete surface treatments were tested: no treatment (model 1), coating with a paraffin-oil mixture (model 2), coating with polymer nanomaterial (model 3), and coating with paint (model 4) (Figure
Four concrete surface treatments (models).
Model A was put on the pedestal; the prefabricated concrete blocks were placed on top of model A. Model B was placed on model A and the soil was placed on model B. Because the density of the soil sample was 1.7 g/cm3 and the capacity of model B was 35972 cm3, 61152.4 g of soil was compressed into model B. The moisture content of the soil sample was 13% and the amount of soil and water in the soil were 54117.2 g and 7035.2 g, respectively (see equations ( The pressure plate, roller, and sliding plate were placed on top of model B in turn. The vertical jack and sensor were placed on the sliding plates, and the horizontal jack and sensor were placed on the side of model B. The vertical load of this study was 10 kN, 20 kN, and 30 kN. Under each load level, a shear load was applied in the horizontal direction until shear failure occurred, at which time the horizontal force value was recorded. The concrete surface of model 2 was coated with the paraffin-oil mixture, the concrete surface of model 3 was coated with the polymer nanomaterial, and the concrete surface of model 4 was coated with the paint. The remaining steps were the same as above.
where
In the large-scale shear test, different vertical forces represent the horizontal force on the pile side at different depths of the pile. The principle of shear stress generation in the direct shear test is consistent with the principle of negative skin friction, which occurs due to the relative motion of the two interfaces; therefore, the friction strength represents the negative skin friction of the piles. The friction coefficient is the ratio of the horizontal force to the vertical force or the friction strength to the compressive stress. The test results are shown in Tables
Model 1.
Moisture content (%) | Vertical force (kN) | Compressive stress (kPa) | Horizontal force (kN) | Friction strength (kPa) | Friction coefficient | Average friction coefficient |
---|---|---|---|---|---|---|
21 | 10 | 47.26 | 3.09 | 14.60 | 0.310 | 0.31 |
20 | 94.52 | 6.22 | 29.40 | 0.311 | ||
30 | 141.78 | 9.34 | 44.14 | 0.311 | ||
19 | 10 | 47.26 | 3.32 | 15.69 | 0.332 | 0.35 |
20 | 94.52 | 7.53 | 35.59 | 0.376 | ||
30 | 141.78 | 9.93 | 46.93 | 0.331 | ||
17 | 10 | 47.26 | 4.88 | 23.06 | 0.488 | 0.45 |
20 | 94.52 | 8.39 | 39.65 | 0.420 | ||
30 | 141.78 | 13.49 | 63.75 | 0.450 | ||
15 | 10 | 47.26 | 6.75 | 31.90 | 0.675 | 0.64 |
20 | 94.52 | 13.12 | 62.00 | 0.656 | ||
30 | 141.78 | 17.98 | 84.97 | 0.599 | ||
13 | 10 | 47.26 | 6.81 | 32.18 | 0.681 | 0.70 |
20 | 94.52 | 15.52 | 73.35 | 0.776 | ||
30 | 141.78 | 18.91 | 89.37 | 0.630 |
Model 2.
Moisture content (%) | Vertical force (kN) | Compressive stress (kPa) | Horizontal force (kN) | Friction strength (kPa) | Friction coefficient | Average friction coefficient |
---|---|---|---|---|---|---|
21 | 10 | 47.26 | 1.49 | 7.04 | 0.149 | 0.15 |
20 | 94.52 | 2.80 | 13.23 | 0.140 | ||
30 | 141.78 | 4.42 | 20.89 | 0.147 | ||
19 | 10 | 47.26 | 2.58 | 12.19 | 0.258 | 0.23 |
20 | 94.52 | 4.44 | 20.98 | 0.222 | ||
30 | 141.78 | 6.03 | 28.50 | 0.201 | ||
17 | 10 | 47.26 | 2.83 | 13.37 | 0.283 | 0.27 |
20 | 94.52 | 5.84 | 27.60 | 0.292 | ||
30 | 141.78 | 7.13 | 33.70 | 0.238 | ||
15 | 10 | 47.26 | 2.93 | 13.85 | 0.293 | 0.29 |
20 | 94.52 | 5.79 | 27.36 | 0.290 | ||
30 | 141.78 | 8.72 | 41.21 | 0.291 | ||
13 | 10 | 47.26 | 2.60 | 12.29 | 0.260 | 0.23 |
20 | 94.52 | 4.35 | 20.56 | 0.217 | ||
30 | 141.78 | 6.15 | 29.06 | 0.205 |
Model 3.
Moisture content (%) | Vertical force (kN) | Compressive stress (kPa) | Horizontal force (kN) | Friction strength (kPa) | Friction coefficient | Average friction coefficient |
---|---|---|---|---|---|---|
21 | 10 | 47.26 | 1.17 | 5.53 | 0.117 | 0.13 |
20 | 94.52 | 2.84 | 13.42 | 0.142 | ||
30 | 141.78 | 4.28 | 20.23 | 0.143 | ||
19 | 10 | 47.26 | 2.71 | 12.81 | 0.271 | 0.25 |
20 | 94.52 | 4.73 | 22.35 | 0.237 | ||
30 | 141.78 | 6.85 | 32.37 | 0.228 | ||
17 | 10 | 47.26 | 4.09 | 19.33 | 0.409 | 0.39 |
20 | 94.52 | 7.72 | 36.48 | 0.386 | ||
30 | 141.78 | 11.12 | 52.55 | 0.371 | ||
15 | 10 | 47.26 | 5.06 | 23.91 | 0.506 | 0.51 |
20 | 94.52 | 10.40 | 49.15 | 0.520 | ||
30 | 141.78 | 15.02 | 70.98 | 0.501 | ||
13 | 10 | 47.26 | 5.30 | 25.05 | 0.530 | 0.53 |
20 | 94.52 | 10.57 | 49.95 | 0.529 | ||
30 | 141.78 | 15.76 | 74.48 | 0.525 |
Model 4.
Moisture content (%) | Vertical force (kN) | Compressive stress (kPa) | Horizontal force (kN) | Friction strength (kPa) | Friction coefficient | Average friction coefficient |
---|---|---|---|---|---|---|
21 | 10 | 47.26 | 2.12 | 10.02 | 0.212 | 0.22 |
20 | 94.52 | 4.94 | 23.35 | 0.247 | ||
30 | 141.78 | 5.88 | 27.79 | 0.196 | ||
19 | 10 | 47.26 | 2.65 | 12.52 | 0.265 | 0.25 |
20 | 94.52 | 4.85 | 22.92 | 0.243 | ||
30 | 141.78 | 7.07 | 33.41 | 0.236 | ||
17 | 10 | 47.26 | 3.17 | 14.98 | 0.317 | 0.32 |
20 | 94.52 | 6.56 | 31.00 | 0.328 | ||
30 | 141.78 | 9.72 | 45.94 | 0.324 | ||
15 | 10 | 47.26 | 5.42 | 25.61 | 0.542 | 0.55 |
20 | 94.52 | 10.56 | 49.91 | 0.528 | ||
30 | 141.78 | 17.16 | 81.10 | 0.572 | ||
13 | 10 | 47.26 | 6.26 | 29.58 | 0.626 | 0.61 |
20 | 94.52 | 10.98 | 51.89 | 0.549 | ||
30 | 141.78 | 19.50 | 92.16 | 0.650 |
Due to manual measurement and equipment errors, the friction coefficient varies under different vertical forces. Therefore, the average friction coefficient was used to evaluate the ability of different materials to reduce the negative skin friction between the piles and soil. The friction strength and friction coefficient of clay and concrete with different surface treatments and different moisture content are analyzed. Subsequently, the influences of the moisture content and coatings on the ability to reduce the negative skin friction of the piles are evaluated.
The frictional strength of the clay with different moisture contents and the untreated concrete is shown in Figure
Friction strength of concrete and soil for different moisture contents.
The results in Figure
The frictional strength between the clay for different moisture contents and the concrete coated with the paraffin-oil mixture is shown in Figure
Friction strength of the paraffin-oil mixture on the concrete surface and soil for different moisture contents.
When the vertical pressure is small, the frictional strength is barely affected by the change in the moisture content. The friction strength is the largest at a moisture content of 15% and vertical forces of 10 kN and 30 kN; the reason is that the clay and the paraffin-oil mixture have the largest frictional resistance at this moisture content. At a moisture content of less than 15%, the friction strength increases with the increase in the moisture content. When the moisture content is greater than 15%, the friction strength decreases with the increase in the moisture content for the vertical forces of 10 kN and 30 kN. At a vertical force of 20 kN, the trend of the frictional strength is slightly different at a moisture content of about 17%, which may be due to a measurement error. These results indicate that the ability to reduce the negative skin friction of the piles is greatest when the moisture content of the clay is small or near the plastic limit when a paraffin-oil mixture is applied to the pile.
The frictional strength between the clay with different moisture contents and the concrete coated with the polymer nanomaterial is shown in Figure
Friction strength of the polymer nanomaterial on the concrete surface and soil for different moisture contents.
When the moisture content is less than 15%, the friction strength does not change with increasing moisture content. At a moisture content of more than 15%, the friction strength decreases rapidly with the increase in the moisture content. These results demonstrate that the ability to reduce the negative skin friction of the piles is greatest when the moisture content of the clay exceeds the plastic limit when the polymer nanomaterial is applied to the pile.
The frictional strength between the clay with different moisture contents and the concrete coated with paint is shown in Figure
Friction strength of paint on the concrete surface and soil for different moisture contents.
When the moisture content of the experimental clay exceeds the plastic limit, the frictional strength is not affected by the change in the moisture content. When the moisture content is less than that of the plastic limit, the friction strength increases rapidly with the decrease in the moisture content. At a moisture content of less than 15%, the friction strength does not substantially increase as the moisture content decreases. The results show that the ability to reduce the negative skin friction of the piles is greatest when the moisture content of the clay is near the plastic limit when paint is applied to the pile.
As is shown in Tables
Friction coefficients of concrete with various coatings and soil for different moisture contents.
For concrete with coating, when the moisture content is 13%–15%, the reduction effect of the paraffin-oil mixture is the best, and the reduction effect of the paint is the worst; When the moisture content is 15%–19%, the reduction effect of the paraffin-oil mixture is the best, and the reduction effect of the polymer nanomaterial is mostly the worst; when the moisture content is 19%–21%, the paraffin-oil mixture and polymer nanomaterial have better reduction effects, and the reduction effect of the paint is the worst.
It is observed that the friction coefficient can be reduced by applying the paraffin-oil mixture, polymer nanomaterial, or paint on the surface of the concrete. This indicates that these treatments on the concrete surface reduce the negative skin friction of the piles.
The most effective treatment to reduce the negative skin friction of the piles is to coat the concrete surface with the paraffin-oil mixture. In addition, a paraffin-oil mixture is relatively common. It should be the first choice for reducing the negative skin friction of the piles in engineering practice.
The reduction in the negative skin friction for the polymer nanomaterial coating on the concrete surface is greater at a moisture content of less than 15% or when the plastic limit has been exceeded.
The friction coefficient of the concrete coated with paint shows changes that are similar to that of the untreated concrete. When the moisture content is near the plastic limit, the friction coefficient changes abruptly and there are only small changes when the moisture content is far from the plastic limit. Therefore, when the surface of the concrete pile is coated with paint, the moisture content should be slightly larger than that of the plastic limit to reduce the negative skin friction of the piles.
A comparison of the effects of the three kinds of coatings and the different moisture contents shows that when the moisture content is slightly larger than that of the plastic limit, the three coatings exhibit similar performance for reducing the negative skin friction. Overall, the paraffin-oil mixture results in the best performance.
As shown in Table
Reduction rate of average friction coefficient.
Moisture content (%) | Reduction rate of average friction coefficient |
||
---|---|---|---|
Paraffin-oil mixture | Polymer nanomaterial | Paint | |
13 | 67.14 | 24.29 | 12.86 |
15 | 58.57 | 27.14 | 21.43 |
17 | 61.43 | 44.29 | 54.29 |
19 | 67.14 | 64.29 | 64.29 |
21 | 78.57 | 81.43 | 68.57 |
The location of the neutral point is important for the reduction in the negative skin friction of the piles with different coatings. The position of the neutral point can be determined by theoretical calculation methods and empirical methods. The most common method is the empirical method [
Neutral point depth.
Force layer properties | Clay, powder soil | Sand above medium density | Self-weight collapsible loess | Gravel, pebble | Bedrock |
---|---|---|---|---|---|
Depth ratio of neutral point |
0.5–0.6 | 0.7–0.8 | 0.77–0.8 | 0.9 | 1.0 |
The coating should be applied to the pile above the location of the neutral point, as shown in the schematic of the construction process in Figure
Schematic of the construction process.
In this study, the effects of different coatings on the ability to reduce negative skin friction on piles during a large-scale shear test were examined. The following conclusions were drawn: When the moisture content of the clay was slightly larger than that of the plastic limit, the negative skin friction of the piles could be reduced considerably. Different coatings exhibited different degrees of reduction of the negative skin friction of piles; the performance of the paraffin-oil mixture was the best when the moisture content of the clay was lower than that of the plastic limit. When the moisture content of the clay was near the plastic limit, the performances of the paraffin-oil mixture, polymer nanomaterial, or paint were similar and all coatings reduced the negative skin friction. The paraffin-oil mixture is the best choice when the moisture content of the clay is lower than that of the plastic limit.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare no conflicts of interest.
This research was funded by the National Key Research and Development Program of China (no. 2018YFC1504801); Traffic Science and Technology Projects in Guangdong Province (2013-02-010 and 2011-01-001); Key Transportation Science and Technology Research Projects in Qinghai Province (2014-07); and Traffic Science and Technology Projects in Hainan Province (HNZXY2015-045R).