Application of static loading tests to steel pipe piles with large diameters in Chinese offshore wind farms

Ocean Engineering 186 (2019) 106041

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Application of static loading tests to steel pipe piles with large diameters in Chinese offshore wind farms Xiaojuan Li a, b, Guoliang Dai a, b, *, Mingxing Zhu a, b, Weiming Gong a, b a b

School of Civil Engineering, Southeast University, China Key Laboratory of C&PC Structures, Ministry of Education, China

A R T I C L E I N F O

A B S T R A C T

Keywords: O-cell tests Steel pipe piles Bearing capacity Vertical piles Inclined piles

In this paper, three kinds of static loading test methods were introduced by referencing three offshore wind farms in China. In the first project, O-cell tests were performed on four steel pipe piles, namely, two vertical piles and two inclined piles, with diameters of 1.7 m and embedment depths from 67.24 to 69.70 m. Next, the equivalent load-settlements for top-down load and pull-out load tests at the pile heads were given by using a conversion method. Then, the conversion results were verified using different methods. Finally, the rationality of utilizing the coefficient factors of γ1 and γ2, which were 0.8 and 1.1 in the first project, were verified by studying three piles in two other offshore wind farm projects, where a top-down load test and pull-out load test were conducted on each pile. The results show that the O-cell test can successfully determine the axial compression capacity and uplift capacity by the proposed conversion method.

1. Introduction The use of wind energy is an important way to reduce air pollution and delay energy crises. Especially in China, offshore wind energy has been regarded as an important source of clear energy in recent years, generating 17% of renewable sources by 2030 and a predicted 150 GW by 2030 (http://www.cwea.org.cn,2016). Many offshore wind farms are currently under construction or have been built in coastal areas. Largediameter steel pipe piles (diameters greater than 1.5 m) are frequently used in such projects, and inclined piles are also used as a part of the pile group foundation. For steel pipe piles, the traditional static load test is a reliable method to determine the axial bearing load and pull-out load capacities (Russo, 2013). As a result, most of the data used in pile design are obtained from static load testing. However, for piles in marine environments with large diameters and high slenderness ratios, especially for inclined piles, the axial bearing load and pull-out load capacities are very difficult to determine by traditional static load tests, due to the following four main problems: (1) Traditional methods may be very expensive to execute in marine conditions because additional tension piles are needed to create the reaction systems.

(2) The construction of tension piles requires considerable time during the project, and sufficient time must be given for soil re­ covery between the top-down load test and pull-out test. (3) For piles with large diameters, load test reaction systems often do not have the capacity to reach pile plunging. (4) It is very difficult for the load system to apply an inclined load on top of inclined piles. Therefore, a more cost-effective alternative the O-cell test (also named the Osterberg load test or self-balance loading test) is introduced in this paper. As a kind of static load test, the O-cell test was first pro­ posed by Osterberg in 1984 (Osterberg, 1984); the load device is placed at the bottom of the pile to push the pile upward. As a result, both the shaft resistance and tip resistance of the pile can be measured at once. The usage of this method was then developed by removing the load cell in the pile body to adapt to more complex site conditions (Dai, Gong & Liu, 2013). However, the loading strategies used in the O-cell test will result in different soil reactions from those observed in the real condi­ tions of a pile foundation. Therefore, a conversion method is used to transform the results of O-cell tests into those of top-down load tests or pull-out load tests, and several hypotheses are considered in this con­ version. Fortunately, the accuracy of the conversion method and hy­ potheses have been verified by thousands of projects over the past

* Corresponding author. School of civil engineering, Southeast University, China. E-mail address: [email protected] (G. Dai). https://doi.org/10.1016/j.oceaneng.2019.05.023 Received 6 December 2018; Received in revised form 24 April 2019; Accepted 12 May 2019 Available online 24 June 2019 0029-8018/© 2019 Elsevier Ltd. All rights reserved.

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decades (Lee and Park, 2008; Osterberg, 1989; Kim and Mission, 2010; Misson and Kim, 2011; Mu et al., 2009; Russo, 2013). Now, this method is widely used to solve pile testing problems, especially with load test reaction systems that do not have the capacity to achieve pile plunging or site conditions that are too complex to replicate via applied load. In this paper, four steel pipe piles with diameters of 1.7 m and embedment depths of 67.24–69.7 m are evaluated by O-cell tests. The conversion results of the equivalent capacity and the shaft resistance at each soil layer are given for axial bearing loads and pull-out loads; the friction resistances are verified by different calculation methods, and the rationality of the conversion method was proven by considering two offshore wind farm projects. 2. Bidirectional loading test In an O-cell test, the test pile is loaded by simultaneous upward and downward displacement by a load cell. The equivalent load and displacement at the pile head can be expressed as follows (Dai et al., 2003; JTT738, 2009; JGJ/T403, 2017). For the equivalent Q-s curve in equivalent top-down load tests, Qupper W þ Qlower γ1

Q¼

� s¼

Qupper

W

� �� γ1 þ 2Qlower Lupper QL0 þ þ slower 2Ep Ap Ep Ap

(1) Fig. 1. γ1 and soil profile for the 132 test groups.

(2)

for the ultimate capacity in equivalent top-down load tests, Qult ¼

f ¼

Qupper;ult γ1

W

þ Qlower;ult

fc γ1

(3) (4)

and for the equivalent Q-s curve in equivalent pull-out load tests, Qupper γ2

(5)

fc γ2

(6)

Q¼

f ¼

where Q is the equivalent load; Qupper is the load when the settlement equals the absolute value of slower on the load-supper curve (Dai et al., 2003); supper is the settlement of the upper plate; W is the weight of the upper part of the pile; Qlower is the load at the lower plate of the load cell; Lupper is the depth of the load cell; Ep is the elastic modulus of the pile; L0 is the pile length between the soil surface and the force point; Ap is the section area of the pile; slower is the settlement of the lower plate; Qupper, ult is the ultimate load at the upper plate of the load cell; Qlower,ult is the ultimate load at the lower plate of the load cell; f is the shaft resistance in the equivalent load test; and fc is the shaft resistance in the O-cell test. In Eq. (2), the shortening of the pile section above the soil ground is included. The most important factors in the above equations are the coefficient factors of the shaft resistance, γ 1 and γ 2, which can transform the shaft resistance of the pile in the O-cell test to that in the top-down load test or pull-out load test. However, the coefficient factor values are still controversial. In many countries, the coefficient factor of shaft resis­ tance, γ1, is assumed to be 1.0 (http://www.loadtest.com), which is also accepted by Kim (Kim and Mission, 2010) and Mission (Misson and Kim, 2011), while in China (Li et al., 2016a,b), this value is assumed to be less than 1.0. Our latest research suggests that γ1 ranges from 0.80 to 1.00 and that γ 2 is 1.1. This conclusion is based on the results of 132 groups of test piles; each group underwent O-cell testing and traditional top-down load testing at the same site. The results show that the value of γ 1 is

Fig. 2. The relationship between γ1 and .EpAp/Lupper

affected by the soil type, construction method and pile slenderness ratio. In Fig. 1, the abscissa is the ratio of the total soil thickness of the cohesive soil to Lupper, which is the thickness proportion of the cohesive soil along the pile above the load cell. The ordinate is the value of γ1 for each group, which increases with the total thickness of the cohesive soil. The value of γ 1 tends to be higher in pure clay than in sand. Therefore, we should take the proportion of thickness of cohesive soil into consideration when determining the value of γ 1. The relationship between γ1 and the pile compression stiffness EpAp/ Lupper was also studied. Fig. 2 illustrates that the value of γ1 is related to EpAp/Lupper. For cast-in-situ concrete piles, γ 1 can be regarded as 1 if EpAp/Lupper < 1200 kN/m, and γ 1 can be regarded as 0.8 if EpAp/Lupper > 400 kN/m, which is determined by the interpolation method if EpAp/ Lupper is 400–1200 kN/m. For the driven pipe pile, the boundary value may be 0.6–0.8. 2

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Table 1 Parameters of the pipe piles. No.

Pile Length (m)

Lupper (m)

L0

Pile Diameter (m)

Thickness of Pipe(m)

Test Method

40#ZZ1 40#XZ1 41#ZZ2 41#XZ2

85.2 85.9 85.2 85.9

67.0 67.0 65.24 65.24

16.2 16.2 17.96 17.96

1.7 1.7 1.7 1.7

0.022–0.030 0.025–0.030 0.022–0.030 0.025–0.030

O-cell O-cell O-cell O-cell

Fig. 3. Comparison of the O-cell test and pull-out test.

Figs. 1 and 2 show that γ1 of driven pipe piles tend to be lower than that of cast-in-situ piles. This is also an important factor in determining the value of γ 1 in the usage of O-cell tests for steel pipe piles of offshore wind farm projects. Comparison groups were also set to verify γ 2. Laboratory tests (Li et al., 1999), FEM (finite element method) analysis (Huang et al., 1999) and projects in China (JGJ/T403, 2017) have found that considering γ 2 ¼ 1.1 in the O-cell test is a reasonable assumption for the equivalent pull-out loading results at the pile head. The equivalent displacement at the pile head is a very important parameter in the verification of pull-out capacity. However, no equation has been determined to transform the equivalent displacement of the Ocell test to the pull-out load test. In this paper, assuming that, under the same load, the shortening of a pile in the O-cell test is equal to the stretch of the pile in the pull-out test (Kim and Mission, 2010) (Misson and Kim, 2011) (Fig. 3), the trans­ forming equation of settlement in an equivalent pull-out load test is given as s ¼ 2supper

st

Fig. 4. The geology and soil profile along the four piles.

inclined piles; the inclination is 5.5%. The parameters are listed in Table 1. The geology and soil profile along the four piles in this area are shown in Fig. 4. The available site investigation results of the clay and sand are shown in Table 2 and Table 3. 3.2. Testing approach In this project, a high load capacity is needed to ensure that these test piles are fully plunged. Additionally, is impossible to use the top-down load test or pull-out load test on inclined piles. Therefore, O-cell tests were applied to measure the compressive bearing capacity and uplift capacity of the piles. The load cell assembly was welded to the steel pipe pile. The upper and lower plates of the load cell are separated during the loading stages. The location of the load cell along a pipe pile is shown in Fig. 4. When the load cell applies upward and downward loads at this depth, the potential soil resistance can be fully mobilized. The distance between the load cell and pile base is 2 m in 40#ZZ1, 2.7 m in 40#XZ1, 2 m in 41#ZZ2 and 2.7 m in 41#XZ2. In each test, the data required to determine the upward and down­ ward load-movement curves of the load cell plates were recorded. The displacement at the pile head was also measured so that the overall movement of the pile above the upward cell plate can be determined. The strain between the top of the pile and the top of the cell plate was measured by sliding micrometers at different depths so that the axial force of the upward pile can be obtained, as well as the shaft resistance.

(7)

where st is the displacement at the pile head. Eq. (7) depends on the behavior of the pile-soil reaction in the O-cell test and pull-out loading test, as shown in Fig. 3. The failure surface is confident with the pile-soil interface. Unlike the top-down load test, the direction of shaft resistance in the pull-out load test is the same as that in the O-cell test. Therefore, the test pile is compressed by the soil resis­ tance and the load cell, and the resultant shortening is the difference between supper and st. However, in the pull-out load test, the test pile will be stretched by the soil resistance and pull-out load at the pile head, and the extension is s. Under the same load level, it is assumed that the ul­ timate shaft resistance of the O-cell test equals that of the pull-out test in terms of both its amplitude and direction. The contraction of the pile in the O-cell test equals the extension in the pull-out test. 3. Application of bidirectional loading test in offshore wind farm projects

3.3. Test results

3.1. Donghai Bridge offshore wind farm

Fig. 6 shows the recorded upward (supper) and downward (slower) load-movement curves of the load cell plates for the four test piles. Fig. 6 also gives the movements at the head (st) of each pile. In Fig. 6(a), the Qupper-supper and Qt-st curves present the overall

The test site is located in the offshore area of Shanghai, near the Donghai Bridge. The water table is 10 m above the seabed. In Fig. 5, 40#ZZ1 and 41#ZZ2 are vertical piles, and 40#XZ1 and 41#XZ2 are 3

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Table 2 Soil parameters of the clay. Soil Types

Mud

Mud Clay

Clay

Gray and Green Silty Clay

Dark Green Silty Clay

Water Content (%) Gravity of Soil (kN/m3) Specific Gravity Soil Saturation (%) Void Ratio Liquid Limit WL% Plastic Limit Wp % Plastic Limit Index Ip Liquid Limit Index IL Consolidated Cohesive Quick Shear Strength (kPa) Internal Friction Angle (� ) Unconsolidated Cohesive Undrained Strength (kPa)

50.4 16.8 2.74 98 1.409 40.1 22.2 17.9 1.56 12

59.4 16.8 2.75 98 1.68 46.9 25.3 21.6 1.58 12

43.5 17.3 3.74 97 1.239 39.4 21.7 17.7 1.27 13

24.3 19.4 2.72 93 0.708 31.2 17.5 13.7 0.48 26

25.2 19.3 2.73 94 0.733 34.6 19.5 15.1 0.36 43

17

9

14

22

17

0

17

22

27

118

Table 3 Soil Parameters of the S and. Soil Type

Fine Sand with Silt and Clay

Fine Sand with Silt

Fine Sand with Silt

Water Content (%) Gravity of Soil (kN/m3) Specific Gravity Soil Saturation (%) Void Ratio Particle Analysis 10–5 (mm) 5–2 2–0.5 0.5–0.25 0.25–0.075 0.075–0.005 <0.005 Consolidated Cohesive Quick Shear Strength (kPa) Internal Friction Angle (� )

30.1 18.6 2.69 96 0.843 0 0 0 2.5 79.4 14.4 3.7 2

27.4 19.0 2.69 96 0.769 0 0 0 0.4 78.1 16.9 4.6 0

24.9 19.5 2.68 97 0.668 0 0.2 2.6 12.9 67.9 12.7 3.7 0

30.5

32

32

the interface above the load cell is triggered during the last loading stage. In Fig. 7, the distributions of the axial load of the main soil layers for each loading step are presented so that the shaft resistance in the main soil layers can be deduced by the equilibrium of forces. By using Eqs. (1)–(7), the results of the O-cell test can be transformed to those of top-down load and pull-out load tests. The values of γ 1 and γ2 were given by the standards mentioned in this paper. Because the proportion of the thickness of the clay layer for 40#ZZ1 is 0.37 and EpAp/Lupper > 1200 kN/m, the pile type is a driven pipe pile, so the value of γ 1 can be taken as 0.8, while γ 2 is 1.1. The values of γ1 and γ 2 for the four test piles are given in Table 4. By using Eqs. (1) and (2), the results of the O-cell tests can be transformed to the equivalent load-displacement curves at the pile head. Eqs. (5) and (7) can also be used to transform the results of the O-cell tests to those of pull-out results. The translation results are shown in Fig. 8 and Fig. 9. For the inclined pile, the loading direction is perpen­ dicular to the pile section, and the axial force shown in Fig. 7 is not vertical, but the conversion method used is the same as that for vertical piles because the inclination is rather small. In Fig. 8, for the compression capacity of the piles, compared with the load-displacement at the pile head in the real loading state, there are

Fig. 5. (a) Vertical and inclined piles and (b) a load cell with a pipe pile at the Shanghai Donghai Bridge offshore wind farm.

movement of the pile section above the load cell. Although the typical point of ultimate capacity was not observed, the ultimate state was still available. The displacement at the last load stage is large enough to motivate the ultimate shear resistance of the pile above the load cell. This conclusion can be made for all tested piles. Therefore, the ultimate shaft resistance can be obtained during the last load stage. The overall shaft capacity in Fig. 6(a) is larger than that in Fig. 6(b), as well as the overall tip resistance. Therefore, the soil resistance around a vertical pile is larger than that around an inclined pile under the same soil conditions, even when the incline angle is very small. Therefore, the inclined angle has a negative effect on the bearing capacity of a pile. Compared with the load-displacement curves in Fig. 6(a) and (c), the overall shaft capacity illustrated by the Qupper-supper curves are similar, while the overall tip resistance is not the same, as 40#ZZ1 has a lower tip resistance than that of 41#ZZ2. On the other hand, compared with the load-displacement curves in Fig. 6(b) and (d), the overall shaft resistance and tip resistance of 41#XZ2 and 41#XZ2 are similar. The values of supper and st of each pile suggest that complete sliding at 4

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Fig. 6. Load movements of (a) 40#ZZ1, (b) 40#XZ1, (c) 41#ZZ2 and (d) 41#XZ2.

several differences in the equivalent top-down load test. First, the soil resistance above the load cell is transformed by the coefficient factor γ1. Second, during the initial loading period, the distribution of shaft resistance is different from that in Fig. 6. In the O-cell test, the shaft resistance is mobilized from the area near the load cell to the ground surface, while in the top-down load test, the shaft resistance first developed in the opposite direction. In conclusion, from these obser­ vations, combined with the data in Fig. 6 and Eq. (3), for 40#ZZ1 and 41#ZZ2, Qupper,ult ¼ 12600 kN, Qlower,ult ¼ 11900 kN, and the ultimate bearing capacity is 25914 kN; for 40#XZ1 and 41#XZ2, Qupper, ult ¼ 10500 kN, Qlower,ult ¼ 112000 kN, and the ultimate bearing capac­ ity is 22573 kN. In Fig. 9, regarding the uplift capacity of the piles, there are several differences between the equivalent results and the capacity in the real state. First, the equivalent results are from the pile section above the load cell, and the soil resistance below the cell is not considered. Thus, the equivalent result is more conventional than the capacity determined in the real state. Second, during the initial loading period, the distri­ bution of the shaft resistance is different from that in the real state. In Ocell test, the shaft resistance mobilized from the area near the load cell to the ground surface, while in the real state of the pull-out load test, the shaft resistance developed toward the opposite direction. However, this situation does not exist when the static friction state ends, and kinetic friction arises along the whole pile. Therefore, the equivalent results can reveal the real state of the uplift capacity of steel pipe piles. In conclu­ sion, the ultimate uplift capacity of 40#ZZ1 and 41#ZZ2 is 15750 kN, while that of 40#XZ1 and 41#XZ2 is 14000 kN. The ultimate shaft resistances of 40#ZZ1 and 40#XZ1 are shown in Table 5, and those of 41#ZZ2 and 41#XZ2 are shown in Table 6.

3.4. Comparison of shaft resistance for vertical piles with different methods To prove that the transformed results of shaft resistance are correct, here, we decided to compare the transformed results with results from other calculation methods. Because complete sliding at the interface occurred above the load cell in all four piles, the ultimate shaft resistance can be computed by Coulomb strength theory, and the relationship of the ultimate friction resistance at the pile-soil interface and soil conditions are given as fult ¼ αca þ σ n tanδinter

(8)

where fult is the ultimate shaft resistance, ca is the adhesive force of the soil, and σ n is the normal stress around the pile section. For marine clay, tanδinter ¼ 0; thus, α can be computed by the following equations (API, 1993): 8 < 0:5ψ 0:5 ψ � 1:0 α¼ (9) : 0:5ψ 0:25 ψ > 1:0 where ψ ¼ ca/σ0 n and p’0 is the effective overburden pressure at the point in question (kPa). For cohesionless soil, σ n ¼ K σ0 n, where K is 0.8 for open-ended pipe piles driven unplugged for both tensile and compressive loading; tanδinter ¼ 0.25 according to the API (API, 1993), NAVFAC (Navy, U.S., 1971) and Zhu et al. (2018). According to Flaate and Selnes (Flaate and Selnes, 1977) the shaft friction at any point of a pile is presented by 5

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Fig. 7. The distribution of axial force with soil depth for (a) 40#ZZ1, (b) 40#XZ1, (c) 41#ZZ2 and (d) 41#XZ2.

Table 4 The Values of γ1 and γ2 for the Four Test Piles.

γ1 γ2

40#ZZ1

40#ZZ2

41#ZZ1

41#ZZ2

0.8 1.1

0.8 1.1

0.8 1.1

0.8 1.1

fult ¼ μL σ ’n ⋅tan ϕ’c ⋅Dm þ xσ ’c

fult

�

For vertical piles, Eq. (11) can be simplified as pffiffiffiffiffiffiffiffiffiffi � 0:4μL OCRσ ’v

(10)

(11)

where μL in this paper is 0.565 for 40#ZZ1 and 40#XZ1 and 0.566 for 41#ZZ2 and 41#XZ2; OCR ¼ 1. The results from the conversion method, Eqs. (8), (9) and (11) and the Chinese criteria (JTS167-4, 2012) are compared in Table 7. Table 7 shows that the shaft friction at every soil layer is lower than those in the API and Chinese criteria except in the fourth layer, but the value is still within the range of 24–40 kPa. These results are more reasonable than those from Flaate and Selnes (Flaate& Selnes, 1977), proving that the conversion method of shaft resistance at each layer in 40#ZZ1 is safe and reasonable. Table 8 also suggests that the shaft friction at every soil layer is lower than those in the API and Chinese criteria and is more reasonable than that from Flaate and Selnes (Flaate& Selnes, 1977). This also proves that the conversion method of shaft resistance at each layer in 41#ZZ2 is safe and reasonable.

Fig. 8. The equivalent load-settlement at the pile head in the top-down load test.

6

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The value of γ for the four piles in this project is 0.73, which is in the range of 0.6–0.8; therefore, if the transforming shaft resistance in the equivalent top-down load test is reasonable, the transformed results for the pull-out load test are also correct. 4. Discussion on coefficient factors To prove the rationality of γ 1 and γ 2 proposed in the above offshore wind farm project, the results of the axial top-down load test and pullout load test in two additional offshore wind projects were introduced in this section. The main information on the test piles used in these projects is presented in Table 9. In the above projects, the top-down load test and pull-out load test Table 7 Comparison of shaft resistance (40#ZZ1). Soil Type

Fig. 9. The equivalent load-displacement at the pile head in the pull-out load test.

Mud Mud Clay Clay Clay Green Silty Clay Fine Sand with Silt and Clay Fine Sand Fine Sand

The ultimate pile pull-out capacity is equal to or less than the total skin friction resistance. In the API criteria, the calculation methods are the same as those in the top-down load test, while in the Chinese criteria, the shaft resistance in the pull-out test is 0.6–0.8 times that in the topdown load test, which is described by the conversion factor γ (the ratio of shaft resistance in the O-cell test to that in the top-down test at the same soil layer). The relationship of γ, γ1 and γ 2 is given by γ¼

γ1 γ2

(12)

The Thickness of Soil Layers (m)

Shaft Resistance of 40#ZZ1 in Top-down Load Test (kPa) Transforming Method

API (1993)

Chinese Criterion

Flaate and Selnes (1977)

1.4 6.8 4.9 9.7 2.9

0 10.1 15.0 32.6 33.8

0.0 15.4 22.4 29.9 35.8

14–20 22–30 24–40 24–40 70–86

2.2 12.6 20.7 36.7 42.8

7.3

36.5

63.1

48–66

57.0

19

58.6

81.3

66–88

95.7

15.0

67.5

81.3

66–88

127.9

Table 5 Ultimate shaft resistances of 40#ZZ1 and 40#XZ1. Soil Type

The Thickness of Soil Layers (m)

Ultimate Shaft Resistances (kN/m2) 40#ZZ1

Mud Mud Clay Clay Clay Gray and Green Silty Clay Fine Sand with Silt and Clay Fine Sand Fine Sand

1.4 6.8 4.9 9.7 2.9 7.3 19 15.0

40#XZ1

O-cell

Top-down Loading

Pull-out Loading

O-cell

Top-down Loading

Pull-out Loading

0 8.1 12 26.1 27 29.2 46.9 54

0 10.1 15.0 32.6 33.8 36.5 58.6 67.5

0 7.4 10.9 23.7 24.5 26.5 42.6 49.1

0 8.4 11.3 21.5 25.0 27.6 38.8 51.0

0 10.5 14.1 26.9 31.2 34.5 48.5 63.8

0 7.6 10.3 19.5 22.7 25.1 35.3 46.4

Table 6 Ultimate shaft resistances of 41#ZZ2 and 41#XZ2. Soil Type

Thickness of Soil Layers (m)

Ultimate Shaft Resistances (kN/m2) 41#ZZ2

Mud Clay Clay Dark Green Silty Clay Fine Sand with Silt and Clay Fine Sand Fine Sand

5.9 4.1 3.2 19.8 17.3 15.0

41#XZ2

O-cell

Top-down Loading

Pull-out Loading

O-cell

Top-down Loading

Pull-out Loading

9.9 14.1 28.1 35.4 45.6 53.2

12.4 17.6 35.2 44.2 57.0 66.5

9.0 12.8 25.6 32.2 41.5 48.4

11.7 12.9 26.2 29.0 37.8 44.4

14.6 16.1 32.8 36.2 47.2 55.5

10.6 11.7 23.8 26.4 34.4 40.4

7

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Table 8 Comparison of shaft resistance (41#ZZ2). Soil Type

Mud Clay Clay Dark Green Silty Clay Fine Sand with Silt and Clay Fine Sand Fine Sand

The Thickness of Soil Layers (m)

Shaft Resistance of 41#ZZ2 in Top-down Load Test (kPa) Transforming Method

API (1993)

Chinese Criterions

Flaate and Selnes (1977)

5.9

12.4

13.1

22–30

9.1

4.1 3.2

17.6 35.2

19.6 56.6

24–40 70–86

15.8 22.6

19.8

44.2

67.0

48–66

61.0

17.3

57.0

81.3

66–88

96.2

15

66.5

81.3

66–88

128.4

Table 9 Parameters information of test piles in two projects. Project Name

Pile ID

Pile Length (m)

Pile Diameters (m)

L/D

Test Method

Project 1

ZK01 ZK28 1–9

71.5 77.5 46.6

2.0 2.0 1.8

35.8 38.8 25.9

Top-down Load Test and Pull-out Load Test

Project 2

were conducted on the same piles. These 3 steel pipe piles have di­ ameters of 1.8 m–2.0 m and lengths of 46.6 m–77.5 m, and displacement measurements were taken to record the behavior of these piles throughout the loading test. Fiber optic sensors were used along the pile length to measure changes in the strain, so the ultimate shaft resistance of the piles can be given in terms of the top-down load test and pull-out load test at each soil layer. The vertical capacities from the two kinds of loading tests of each pile were compared, and the shaft resistance from the top-down load test and that from the pull-out load test were given to determine the conversion factor γ (the ratio of shaft resistance in the topdown test to that in the pull-out test at the same soil layer) of different soil layers for each pile. 4.1. Project 1

Fig. 10. ZK01: (a) the downward load-displacement of the pile top from the top-down load test and (b) the uplift load-displacement of the pile top from the pull-out load test.

This wind farm project is located in the coastal area of Jiangsu Province, and the soil profile consists of marine clay and sand. The relative parameters are shown in Table 9. The vertical capacity results are shown in Fig. 10 and Fig. 11; the shaft resistance of each layer is shown in Table 10 and Table 11, respectively. Figs. 10 and 11 show that the displacement of the piles is enough to induce ultimate friction resistance along the whole pile, and the ultimate bearing capacity was reached at the last loading stage in both tests. The compression capacity of ZK01 is 32800 kN, its total shaft resistance is 31076 kN, and its uplift capacity is 22800 kN; the vertical compression capacity of ZK28 is 34850 kN, its total shaft resistance is 33260 kN, and its uplift capacity is 22000 kN. The corresponding total shaft resistance ratios from the tests are 0.73 and 0.66. Tables 10 and 11 clearly show that the ultimate resistance from the pull-out load tests is lower than that from the top-down load tests. The ratios of the resultant shaft resistances range from 0.28 to 0.83.

4.2. Project 2 This offshore wind farm is located in the coastal area of southern China; one pile (1–9 in Table 9) is tested with both the top-down load test and the pull-out load test. The vertical capacity is shown in Fig. 12, and the shaft resistance results from the two loading methods are shown in Table 12. Fig. 12 shows that the displacement in pile 1–9 is sufficient to fully mobilize the ultimate shaft resistance along whole pile in the last test stage. The vertical compression capacity of 1–9 is more than 22000 kN, the total shaft resistance is 17902 kN, and the uplift capacity of 1–9 is 11400 kN. Therefore, the ratio of total shaft resistance in piles 1–9 is 0.64. 8

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Ocean Engineering 186 (2019) 106041

Table 10 The shaft resistance of ZK01 from the top-down load test and pull-out load test. Thickness of Layer (m)

Soil Type

Shaft Resistance (kPa) Top-down Load Test

Shaft Resistance (kPa) Pull-out Load Test

γ

2.62 7.90

Mud Silt clay with mud Silt sand Silt sand Silt clay with mud Silt clay Silt clay Silt sand

23 54

11 20

0.48 0.37

67 107 105

36 69 81

0.54 0.64 0.77

94 95 128

77 79 101

0.82 0.83 0.79

3.00 8.20 9.80 16.30 1.70 4.08

Table 11 The shaft resistance of ZK28 from the top-down load test and pull-out load test. Thickness of Layer (m)

Soil Type

Shaft Resistance (kPa) Top-down Load Test

Shaft Resistance (kPa) Pull-out Load Test

γ

1.37 7.90

Silt Sand Silt Clay with Mud Silt Sand Silt Sand Silt Clay with Mud Silt Sand Silt Clay Silt Sand Silt Clay Silt

32 40

9 12

0.28 0.30

63 77 80

31 44 53

0.49 0.57 0.66

103 111 135 97 114

66 81 93 64 77

0.64 0.73 0.69 0.66 0.68

3.30 7.00 10.10 12.00 4.60 7.40 1.90 3.53

that the method to determine the values of γ1 and γ 2 is correctly applied in the first project. 5. Conclusion This paper presented a method to solve the loading problem of conducting a static load test in an offshore wind farm, and it also gave the results of equivalent compression capacity and pull-out capacity of four piles by using a conversion method. The values of the conversion factors were discussed, as well as the friction resistance of vertical piles and inclined piles. The conversion results of friction resistance were verified by different calculated methods. The rationality of utilizing conversion factors was also proven by case studies of three piles in two other offshore wind farm projects. Based on the observations from this study, the following conclusions can be drawn:

Fig. 11. ZK28 (a) the downward load-displacement of the pile top from the topdown load test and (b) the uplift load-displacement of the pile top from the pullout load test.

Table 12 clearly shows that the ultimate resistance from the pull-out load test is smaller than that from the top-down load test. The ratio of their shaft resistance ranges from 0.53 to 0.71. From the results of the above projects, the value of γ in different soil layers of each pile ranges from 0.28 to 0.83, which is within the values suggested by related criteria (JTGD63, 2007; JTS147-1-2010, 2010; Shi, 2012) for onshore projects. The values of the conversion factor in this paper can be used in offshore wind farm projects, providing a good reference for engineering practice. On the other hand, the value of γ of the four piles in the first project is 0.73, while those in the other two projects range from 0.28 to 0.83; therefore, the value of γ 1/γ2 is within the range of γ, which agrees with the traditional load tests performed on the same pile. This can also prove

(1) The value of γ1 ranges from 0.8 to 1.0 and is affected by soil type, pile construction method and the ratio of compression stiffness and embedment depth EpAp/Lupper; the value of γ2 is 1.1, which is conservative for steel pipe piles with large diameters. (2) In the first offshore wind farm project studied, γ1 is 0.8, and γ 2 is 1.1. The conversion method of equivalent axial capacity under a compressive load and pull-out load is reasonable; the accuracy of the shaft resistance determined for vertical piles under an equivalent compressive load were confirmed by three other methods.

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Ocean Engineering 186 (2019) 106041

Table 12 The shaft resistance of 1–9 from the top-down load test and pull-out load test. Thickness of Layer (m)

Soil Type

7.8 3.6 5.4 1.0

Fine Sand Medium Sand Clay Medium Coarse Sand Fully Weathered Granite Strongly Weathered Granite

4.2 16.3

Shaft Resistance (kPa)

γ

Top-down Load Test

Pull-out Load Test

71.2 77.5 63.1 92.9

48.1 53.4 44.8 62.1

0.68 0.69 0.71 0.67

122.5

72.3

0.59

145

77.4

0.53

the same pile. Acknowledgments The authors thank all those involved in the organization of OFW13. This work was supported by the China Scholarship Council, National Key Research Program of China (2017YFC0703408), and National Natural Science Foundation of China (51878160) and (51808112), Jiangsu Fundamental Research Program (Natural Fund Project) (BK20180155). References API, D., 1993. Recommended practice for planning, designing, and constructing fixed offshore platforms-working stress design. API RP A 2. Dai, G., Gong, W., 2012. Application of bi-directional static loading test to deep foundations. J Rock Mech Geotech Eng 4 (3), 269–275. Dai, G.L., Gong, W.M., Liu, X.L., 2003. Experimental study of pile-soil load transfer behavior of self-balanced pile. Rock Soil Mech. 24 (89), 1065–1069. Flaate, K., Selnes, P., 1977. Side friction of piles in clay. In: Proceedings of the 9th International Conference on Soil Mechanics and Foundation Engineering, vol. 1, pp. 517–522. Huang, F., Li, G.X., Zheng, J.Q., 1999. Study on the shaft friction of single pile incompressive tensile loading. Eng. Mech. 16 (6), 97–101. JGJ/T403, 2017. Technical Specification for Static Loading Test of Self-Balanced Method of Bulding Foundation Piles. China Architecture & Building Press, Beijing. JTGD63, 2007. Code for Design of Ground Base and Foundation of Highway Bridges and Culverts. China Communications Press, Beijing. JTS147-1, 2010. Code for Soil Foundations of Port Engineering. China Communications Press. JTS167-4, 2012. Code for Pile Foundation of Harbor Engineering. China Communications Press, Beijing. JTT738, 2009. Static Loading Test of Foundation Pile-Self-Balanced Method. China Communications Press, Beijing, China. Kim, H.J., Mission, J.L.C., 2010. Improved evaluation of equivalent top-down loaddisplacement curve from a bottom-up pile load test. J. Geotech. Geoenviron. Eng. 137 (6), 568–578. Lee, J.S., Park, Y.H., 2008. Equivalent pile load–head settlement curve using a bidirectional pile load test. Comput. Geotech. 35 (2), 124–133. Li, X., Chen, X., Dai, G., 2016. The research of conversion factor of cast-in-situ pile at clay area in self-balanced loading test. Rocks and Mechanicals 37 (Suppl. 1), 226–232. Li, X., Dai, G., Gong, W., 2016. The research on conversion factor of self-balanced loading test in sandy area. Rocks and Mechanicals 37 (Suppl. 1), 659–667. Li, G., Huang, F., Shuai, Z., 1999. Test study on influence of loading ways on friction of pile. Ind. Constr. 29 (12), 19–21. Mission, J.L.C., Kim, H.J., 2011. Design charts for elastic pile shortening in the equivalent top–down load–settlement curve from a bidirectional load test. Comput. Geotech. 38 (2), 167–177. Mu, B.G., Gong, W., Gao, F., Ban, X., 2009. Research on the in-situ tests between the selfbalanced method and the anchor pile method. J Transp Sci Eng 25 (3), 40–46. Osterberg, J.O., 1984. A new simplified method for loading testing drilled shafts. ADSC 9–11. Osterberg, J.O., 1989. Breakthrough in load testing methodology. ADSC 28 (8), 13. Russo, G., 2013. Experimental investigations and analysis on different pile load testing procedures. Acta Geotechnica 8 (1), 17–31. Shi, P., 2012. Pile and Pile Foundation Handbook. China Communication Press, Beijing. Navy, U.S., 1971. Soil Mechanics, Foundations, and Earth Structures, NAVFAC Design Manual DM-7. US Navy, Waghington, DC. Zhu, M., Lu, H., Gong, W., Wan, Z., 2018. Effect of slope angle on stabilizing piles in C-φ soil. In: Proceedings of China-Europe Conference on Geotechnical Engineering. Springer, Cham, pp. 1583–1587.

Fig. 12. Pile 1–9: (a) the downward load-displacement of the pile top from the top-down load test and (b) the uplift load-displacement of the pile top from the pull-out load test.

(3) Based on the axial capacity and shaft resistance results from two different offshore wind farm projects, the range of γ in the topdown load test and pull-out load test is from 0.28 to 0.83, which agree with the values suggested by related criteria for onshore projects, proving that the values of γ1 and γ 2 in the offshore wind farm are reasonable. In conclusion, compared with traditional static load tests, the O-cell test approach can be used to test axial compression capacity and uplift capacity, as demonstrated in this paper. Moreover, the inner force of the pile and the ultimate shaft resistance can also be obtained if the strain in the pile is measured. This approach is cost-effective and more conve­ nient than conducting top-down load testing and pull-out load testing at

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