Analysis of PHC Performance Applied in Levees from Ou River Levee

Analysis of PHC performance applied in levees from Ou River Levee

Yanming Chen, Wei Wu Zhejiang University of Water Resources and Electric Power, Hangzhou 310018， China e-mail: [email protected]

Tangdai Xia Research Center of Coastal and Urban Geotechnical Engineering, Zhejiang University, Hangzhou, 310058, P. R. C.

ABSTRACT

In cities along rivers, while a sudden flood can cause huge loss in economy, life and so on, the construction of flood control projects is an important issue of public construction. High strength prestress concrete pipe pile (PHC) has apparent advantage in capacity, deformation and construction, and recently been applied to flood dam. This paper discussed the theoretical superiority for applying PHC piles in levees, and verified that from a project in Wenzhou, in ways of situ test and ABAQUS finite element software simulation. Finally we proved the conclusion that PHC levee foundation can reduce sedimentation effectively and improve levee overall stability, additionally put reasonable proposals for application.

KEYWORDS: PHC pile, project, ABAQUS, settlement calculation

INTRODUCTION All over world, most cities are developed along river basin, because river offers great conditions for their development, but also poses serious threats to their security, that is flood. In ancient times, there are lots of problems from flood in human social life, mainly due to the randomness in time and space, and its paroxysm. Recently, PHC was applied widely in China, which is a new type of precast pile with high strength [1][2].

Compared with the general levee foundation design, applying PHC has many distinctive characteristics[3][4]: with industrialized technology, good pile quality, short curing time and

- 5171 -

Vol. 20 [2015], Bund. 13 5172 working period, adaptable engineering geological conditions, convenient environmental conditions, which is approved by varieties of basis projects. Compared precast piles with normal piles or others, PHC overcomes the shortcomings that sludge pile in soft soil area are prone to frequent bring quality problems during bearing in unsmooth surface.

And then, levee design requires a lot, such as security, application, appearance, even consistent with the surrounding environment, the most economic, reasonable type to make full use of local building materials, engineering area. Compared with commonly used levees in traditional design, as dug pile and bored pile, PHC pile owns certain advantages, and researches about calculation of PHC bearing capacity and settlement are mature.

FEATURES OF PHC

Load transfer

About pile load transfer theory in vertical, many scholars have detailed analysis[5]，which can be divided into load transfer method, shear displacement method, elastic theory and numerical analysis. Load transfer method is one of the most widely used simplified method. The basic idea is to segment pile into a number of elastic elements, assuming that at any point of pile displacement only relates to pile lateral friction, and between every unit and soil is contacted by nonlinear spring, which simulates the relationship between pile and soil. As pile displacement and load decreasing with depth, pile lateral friction gradually play a role from top to bottom. And pile friction related to displacement of the pile, as shown in Figure 1.

Basic differential equations calculating pile-soil system load of transferring AE d2sz () qz()=p ⋅ (1) s Uzd 2

where A is Pile cross-sectional area; Ep is Pile modulus of elasticity; U is Pile perimeter.

Plug Effect

Soil plug phenomenon is the most significant feature of PHC, which is the key point of the opening pipe pile differencing from a solid pile in bearing capacity mechanism. In the course of sinking prestressed hollow tube pile, due to the squeeze effect, there will be a part of the soil pile tip into the pile to form a soil plug, and the rest were pressed against the pile. As sinking continuing, the soil section into the lumen gradually increased, and when the height reaches a certain level, due Vol. 20 [2015], Bund. 13 5173 to the friction resistance pile wall and the soil plug squeeze, resulting in the closure effect, whereby soil plug effects generated, such as Figure 2.

Q S O Q O

S O z Q(z) S z q s Q z S z dz ds Q(z)+dQ(z) S(z) q s(z) Q(z)

q s

S b S b Q b Q b Q z

Figure 1: Diagram of load transfer

Figure 2: Schematic of soil plug phenomenon

Kindel[6], Paikowsky[7], Paik and Lee etc.[8] studied of the relationship of the soil plug effect with height, soil properties and buried deep by means of field trials, as well as pipe pile bearing capacity calculation has been derived considering the effect of soil plug. Vol. 20 [2015], Bund. 13 5174

Squeezing effect

Soil squeezing phenomenon is due to pile soil around the pile structure is disturbed, the stress state of the soil changes. There are plenty of researchers study the influences caused by squeezing effect to soil pore water change[9].

Modified Calculation

In the formula for solving vertical ultimate bearing capacity of prestressed pipe, generally in the form of：

Q=+= Q Qξξ uh q + Aq uk s b si1 sik p pk (2)

where Quk is PHC pile ultimate bearing capacity; Qs is ultimate bearing capacity of the pile side; u Qb is ultimate bearing capacity of the pile tip; A is pile tip area; is pile perimeter; h1 represents each soil thickness; qsik is ultimate side resistance standard value layers of soil; qpk is ultimate end resistance standard value; ξsi is pile side resistance correction factor; ξ p is pile tip resistance correction factor. The first part of the formula calculates lateral friction, and the second part is the calculation of end resistance. From the previous section of this chapter analyzes the value we can get, in large diameter pile length long PHC pile, and considering the length of the pile to play the inside pipe pile skin friction only part a small at the end, while the pile lateral friction resistance the play is no longer play the ultimate lateral resistance of pile length standard value within the whole paragraph, to which we improved the first part of the formula to be:

Qs =() a + b uh1 qsik (3)

a = the outside skin friction function coefficient along the pile length; b =the inside skin friction function coefficient along the pile length.

Two coefficient values could refer side resistance and end resistance correction factor of tubular pile of Professor Xie presented, as shown in the following table.1.

Vol. 20 [2015], Bund. 13 5175

Table 1: Correction factor of tubular pile side resistance and end resistance

Soil Correction factor

Side End

Clay 0.95~1.05 1.20~1.30

Silt, silty sand 0.95~1.05 1.15~1.30

Sand 0.95~1.05 1.00~1.15

Gravelly sand，gravel, Pebble 0.90~1.00 0.90~1.00

Granite residual soil 0.80~0.90 1.05~1.25

Strongly weathered rock 1.00~1.10 1.10~1.35

PROJECT EXAMPLE

Engineering Profile

Wenzhou is located in southern Zhejiang coast, the south bank of the Ou River estuary downstream. The project is located downstream of the Ou River watershed, the confluence of Ou River and the largest tributary of the Nanxi River in downstream. Magnanimity measured by Wei Ren station of one, three and seven days, combined with historical magnanimity a statistical analysis results the values in Table 2.

Table 2: Design flood flow from Weiren Station

Design Frequent (%) Project Unit 0.5 1 2 5 10 20 50

Flood peak m3/s 28277 25506 22641 18915 15953 12897 8407

One day 104m3 181245 164475 147705 125130 107070 87720 58050

Three days 104m3 341546 312866 282883 241168 208578 173380 119932

Seven days 104m3 497843 459855 419868 363885 317900 267916 187941

Vol. 20 [2015], Bund. 13 5176

From engineering geological section, a list of physical and mechanical properties of the various soil layers, we analysis their engineering properties, get: layer1 is miscellaneous fill; layer 2 is silty clay; layer 3 is fine sand silt folder; layer 4 is silt folder sand; layer 5 is silt: layer 6 is silty clay; layer 7 is pebbles, of medium dense and low compression; layer 8 is all weathered tuff, of sallow and the original structure; layer 9 is strong weathered tuff.

Situ Static load test

From vertical static load test results, we compiled the load Q and S settlement relations. Q-s curve of pile S3 and S6 are shown in Figure 4.

The lgt-s curve of Settlement S versus time is shown in Figure 5. Q (kN) Q (kN) 2000 4000 6000 8000 1000 2000 3000 4000 0 0

5 20 10 40 15 60 20 s(mm) s(mm) 25 80

30 100 35 120 a.S3 b.S6 Figure 4: Q-s curve of test pile S3 and S6

Numerical Analysis

Firstly, we build a three-dimensional numerical model by ABAQUS to simulating the capacity status of PHC. The numerical simulation analysis using displacement PHC for penetration, maximum displacement value of penetration is 50mm, and the loading process is divided into 10 steps, each time you load 5mm displacement, to reach the load set value or make pile piercing damage or not able to reach loaded position. The Q-s curve is as follow: Vol. 20 [2015], Bund. 13 5177

5 50 95 10 100 0 t (min) 1500kN t (min) 5 0 1500kN 2250kN 2250kN 3000kN 10 20 3750kN

15 4500kN 40 3000kN 20 5250kN 60 s(mm) s(mm) 25 6000kN 80 30 7500kN

35 100

40 120 3750kN

a.S3 b.S6 Figure 5: s-logt curve of test pile S3 and S6

Load (kN) 0 2000 4000 6000 8000 -5 0 5 10 15

s(mm) 20 25 30 35

Figure 6: PHC settlement - displacement curve of simulation

Working on part results of pile lateral friction and resistance, we get the curve of lateral friction and end resistance versus pile length, as shown in Figure 7. Vol. 20 [2015], Bund. 13 5178

Friction stress (kPa) Side friction(kN) 0 50 100 -5 0 2000 4000 0 skin friction coefficient 0 friction outside pile 5 outside pile skin friction coefficient friction inside pile 10 inside pile 10 15 20 20 25 length(m) 30 length(m) 30 35 40 40 45 50 50

a. Side friction coefficient b. Side friction Figure 7: The contrast of pile outside and inside skin friction refer the pile length

From Figure b of 4.7, we get the case of the inner and outer lateral friction of piles, and analyzed: Piles friction resistance outside the pile are functioning all along the pile, but play different percent; the upper part of the pile take a small part, and the lower part of the pile body, especially near the end of the pile, skin friction take more percentage, playing a major role in carrying; the pile inside skin friction can also be classified into the category of the pile tip resistance. From the figure, the pile side friction resistance only function in the a 2-3m range end of pile, while the relative displacement of pile and soil, and inside frictional resistance is larger, so this part cannot be ignored.

Here described the status of bearing capacity in the ring end of pile. Due to underlying layer of soil is better, the carrying capacity of pile or a larger play space, and with the increase of the settlement, the carrying capacity of piles will be further play a roll, which the carrying capacity of the pile end cyclic moiety of 2400kN when the pile tip displacement reaches 45mm.

In this numerical model, pile side friction and end resistance has a charge ratio of about 2: 1, which proved that this pile is the end bearing pile, and consistent with the engineering design requirements instance, thus also proved the correctness of the simulation value and their credibility.

Vol. 20 [2015], Bund. 13 5179

Capacity (kN) 1500 2000 2500 -5 0 5 10 15 20 25 s(mm) 30 35 40 45 50 Figure 8: Pile end bearing capacity

Using the modified formula to calculate PHC pile bearing capacity,

Quk =× 1.00[ (1.25 +×××+ 0.2) 38 25 2.512 (1.25 +××× 0.2) 2 90 2.512]

+×××1.25 3.14 0.42 5500 = 7569kN

Comparing results from the model with equations, their difference is slight. Results calculated by formula are closer to the theoretical value of 7569kN in test, thus modified formula play a guiding role for the design of the bearing capacity of pile construction PHC pile calculations. However, due to the complexity and pile end bearing behavior of the uncertainty of soil properties, bearing capacity calculation formulas and numerical simulation exist some errors, so in the actual project there should be a good combination of both two, in order to achieve the correct estimation of the pile the role of bearing capacity.

Settlement Calculation

Within the scope of piles 0+876、1+176、1+488、1+632、1+812 and 1+950 the levees choose the form of a standard cross-section structure, using frame to PHC pile as basis, and through the bottom of the levees is empty style, combined with the upper green landscape layout. On pile cap foundation, each station set A, B two observation points. A comparison of the results of the analysis in the following table:

Vol. 20 [2015], Bund. 13 5180

Table 3: Comparison of results in different calculation method

Calculation Method Value(cm)

Recommended method in technical 1.55 specifications (JGJ94-94)

Equivalent pier method 2.62

Average of the measured value 1.75

Analysis by Finite Element

In two-dimensional plane model, soil is applied Mohr - Coulomb constitutive model, where bulk density is 16.8kN/m3, cohesion is 11.6kPa, friction angle is 11°. Considering the impact of cohesion strength reduction, friction angle in the model is set, and the field variables is also set as a strength reduction factor; along with load and when loads changes, in order to prevent damage in the beginning stages, Fr can be set smaller than number 1 in the initial, which magnified intensity[13]. Part of results is shown in follows:

a. without pipe pile enforcing b. with new pipe pile enforcing Figure 9: Deformation of embankment simulated by ABAQUS

The settlement of embankment without pile enforcement after load is shown as Figure 5.10. In the embankment without pile reinforcement, the figure shows that, under the deadweight, landslide is likely to occur in shallow depth, which is close to the calculating sliding surface by slice method division. Meanwhile landslide deformation reached the maximum of 0.12m , which means a security risk. Vol. 20 [2015], Bund. 13 5181

Figure 9 shows that, due to the presence of pile foundation, the next slide for sliding trend is blocked, and the growth potential sliding surface to be deeper than the no pile reinforcement embankment, and the maximum displacement of the landslide soil only about 1.5cm, far less than the value of the non-pile embankment landslide displacement under the same conditions, and thus may explain the overall security and stability of pile foundation embankment is much larger than the original.

The displacement of new pile foundation levees is shown in Figure 5.11. The figure shows that, due to the presence of pile foundation, the next slide for sliding trend is blocked, and the growth potential sliding surface to be deeper than the no pile reinforcement embankment, and the maximum displacement of the landslide soil only about 1.5cm, far less than the value of the non- pile embankment landslide displacement under the same conditions, and thus may explain why the overall security and stability of pile foundation embankment is much larger than the original.

CONCLUSION

PHC performances applied in levees is analyzed, conclusions can be drawn as follows:

(1) PHC pile reduce the overall settlement of levees foundation in a certain degree, and there are two ways to estimate the value: one is a method recommended by "Pile Technical Specifications" (JGJ94-94), and the other is equivalent pier method, which calculation results in the former is smaller than actual value due to the thickness measure of deforming soil layer is not conservative. So a model based on equivalent pier method is recommended to estimate, which should consider the diffusion angle of 1/4.

(2) From the stability examine by Swedish slice method and numerical simulation analysis by ABAQUS, we proved that levee of pile foundation is safe enough, and the location of potential sliding surface is deeper. And under deadweight, structure displacement is far smaller than that without pile foundation reinforcement. Levees applied PHC in foundation is of high security in general.

REFERENCES

1. Jardine, R. J., and F. C. Chow. New design methods for offshore piles. Marine Technology Directorate, 1996. 2. Chow, Fiona, Robert Overy, and Jamie Standing. ICP design methods for driven piles in sands and clays. London: Thomas Telford, 2005. 3. Poulos, Harry George, and Edward Hughesdon Davis. Pile foundation analysis and design. No. Monograph. 1980. Vol. 20 [2015], Bund. 13 5182

4. Randolph, Mark Felton, J. P. Carter, and C. P. Wroth. "Driven piles in clay—the effects of installation and subsequent consolidation." Geotechnique 29.4 (1979): 361-393. 5. O’Neill, Michael W., Richard A. Hawkins, and Larry J. Mahar. "Load transfer mechanisms in piles and pile groups." Journal of the Geotechnical Engineering Division 108.12 (1982): 1605-1623. 6. Kindel, C. E. "Mechanism of soil resistance for driven pipe piles." Ports'77. 4 th annual symposium of the American Society of Civil Engineers, Waterway, Port, Coastal and Ocean Division, Long Beach, California, v. 2. 1977. 7. Paikowsky, Samuel G., and Robert V. Whitman. "The effects of plugging on pile performance and design." Canadian Geotechnical Journal 27.4 (1990): 429-440. 8. Smith, Trevor, and Clifton E. Deal. "Cracking studies at Sand H Basin by the finite element method." (1988). 9. Chen, Wu. "Treatment method and instrumentation system." U.S. Patent No. 4,505,275. 19 Mar. 1985. 10. Pestana, Juan M., Christopher E. Hunt, and Jonathan D. Bray. "Soil deformation and excess pore pressure field around a closed-ended pile." Journal of geotechnical and geoenvironmental engineering 128.1 (2002): 1-12. 11. Jianyong Shi and Peng Jie. "REVIEW OF COMPACTION EFFECT OF JACKED PILE [J]." Dam Observation and Geotechnical Tests 3 (2001): 001. (in Chinese) 12. Qingyong W. Analysis on Ultimate Capacity of Open-end Pile and the Simplified Calculation [J]. Journal of Geotechnical Investigation & Surveying, 2006, 4. (in Chinese) 13. ZHOU J, CHEN X, ZHOU K, Model Test and Numerical Simulation of Driving Process of Open-Ended Jacked Pipe Piles [J]. Chinese Journal of Rock Mechanics and Engineering, 2010: S2. (in Chinese)