Analysis of PHC Performance Applied in Levees from Ou River Levee
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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 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 [10-12]. In the formula for solving vertical ultimate bearing capacity of prestressed pipe, generally in the form of： Quk=+= Q s Q bξξ si uh1 q sik + p Aq 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.