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Application of Double-row Piles Retaining Structure in Electric Power Engineering

DING Hai-rong1,2

1 college of water resource HYDROPOWER, Sichuan University , chengdu 610065 ,China

; 2 center of scholarly journal , chengdu University of technology , chengdu 610059, China ; Email: Hrding1222@sina.com

Abstract: Advantage and analysis models of double-row piles retaining structure are described in this paper. Based on project practice of double-row piles retaining structure in Huaneng Fuzhou power plant, multi-schemes of supporting and retaining structure have been contrasted and the optimal scheme is recommended. To deal with the soil loss between piles, scheme of small interval and small diameter pipes shall be employed and jet grouting shall be used to improve the soil between piles, so that the soil shall be self-stable on the front surface. The success of the project ensures the operation of the existing power plant and construction of the building plant, which is one avail application case for reference of similar project. Keywords: Double-row piles; elastic foundation beam model;Jet grouting pile

RAO Jun-yong3

3 Southwest Electric Power Design Institute CPECC Email: raojunyong@sina.com

structure in construction process, so the system has more widely application scope than single-row pile supporting system [2]. 2 CALCULATING MODEL OF DOUBLE-ROW PILE RETAINING SYSTEM There are lots of calculating models for double-row pile retaining system [3-6] , which can be divided according to the suppositions and simplification about the soil pressure and mechanical action of pile. Elastic foundation beam is one reasonable model in which the main presumptions and simplifications are as following: Soil pressure is acted as

L0

H

L

Depth of excavation

1 INTRODUCTION Single-row pile is one common supporting and retaining structure system to deal with deep foundation excavation and slope support, which consists of single-row reinforced concrete piles with some interval. This system has the character of easy construction, low price and short time consumption when the pit or slope free surface suspension is not very deep. But with the increasing of pit depth, the displacement at top of the piles and deformation of the structure shall become larger and larger till it is not proper to be used. Although the economic frontal surface suspension of single-row piles retaining structure depends on the soil properties and other factors, from common view, the reasonable pit depth for sing-row pile to support should be within 8m, which is very shallow and the application scope of the pile supporting system is limited. In this case, the common improving methods are to add internal bracing, tension anchor, double-row pile and combination of those. So the double-row pile is one kind of supporting system to settle deep foundation pit and high slope support [1]. The double-row pile retaining structure is one cantilever-type composite space system which consists of 2 rows of reinforced concrete piles, crest beams to connect piles at the top in the same row and binding beams connecting the piles in different rows together. The system has the character of large horizontal stiffness, small deformation and strong integration, which is proper to be permanent supporting system to shield back construction or temporary supporting

Equivalent excavation

Frontal Pile

Figure 1 Calculation model Rankine active soil pressure, which shall be distributed to frontal pile and rear piles according to special ratio [3]. The soil reaction below the bottom of pit shall be simulated by elastic foundation beam, and the passive soil pressure is calculated according to elastic reaction, which means the reaction of soil shall be simulated by some independent springs, and when the structure is deformed under load of soil pressure, the reaction of springs shall be soil pressure acting on pile. The down end of piles shall be simplified as hinge support. The detail of calculating model refers to figure 1. 2.1 Presumption about Soil Pressure Because the double-row pile structure is 3D space model in fact while the elastic foundation beam model is the planar

978-1-4577-0290-7/11/$26.00 ?2011 IEEE

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R

Rea Pile

il

Depth

45

2

H1

2D model, the traditional method is to divide the double-row pile retaining structure into series of units, which consists of one front pile, one back pile and binding beam in the width of one pile interval. In the unit, the soil pressure shall be distributed to front pile and rear pile according to special ratio depending on pile plan arrangement [3]. In the vertical direction, distribution of soil pressure shall be presumed as following [4]: the soil pressure acting on frontal pile above the bottom of pit shall be triangle distributing with proportion to depth of soil, whose maximum is pa and can be calculated according to formula (2) and (3), while the soil pressure acting on frontal pile under bottom of pit shall be rectangle distributing. The active soil pressure on rear pile from the earth behind the rear pile is triangle distributing above the equivalent excavation, whose maximum is pb and can be calculated according to formula (4), while the soil pressure acting on rear pile under equivalent excavation shall be rectangle distributing. The soil resistance on frontal pile from the soil in the pit shall be simulated by soil spring, and that acting on rear pile from the soil between frontal and rear pile shall be also simulated by soil spring. The front and rear piles shall be considered as vertical elastic foundation beam, whose value of foundation coefficient can be determined by m method. K mZ 1 Maximum soil pressure on frontal pile pa ea 2 For rectangle plan arrangement pa 1 ea 3 For triangle plan arrangement Maximum soil pressure on rear pile pb 1 ea 4 Hereinto, K denotes the horizontal soil reaction factor, m is the scale factor between horizontal soil reaction factor and depth, Z shall be depth of the calculated point, ea is the horizontal active soil force. 2.2 Equivalent Excavation Depth In the elastic foundation beam model of double-row pile retaining structure, only the active soil pressure should be calculated which is proportion to effective cover depth. The effective depth for frontal pile can be determined by excavation slope but that for rear pile shall be calculated according to equivalent excavation depth. The angle between vertical and failure sliding slope shall be estimated according to Rankine theory, which is 450

2

When L0

L

H1

0

3 APPLICATION CASE The double-row pile retaining system is applied in the 3rd extension project of Fuzhou thermal power plant of China Huaneng Group, the most famous electric power developing company in China. There are 4 units with capacity of 300MW respectively in the existing plant and 2 600MW units shall be installed in this extension project. When it is complete, the thermal power plant shall be equipped with capacity of 2400MW, which shall be the key power plant in Southeast China. The project is located at the riverside of the Minjiang river estuary and the water intake structure consists of the reinforced concrete water intake pier and cooling water intake pipe (hereafter CW pipe for short, it is steel pipe). The water intake pier and CW pipe shall be installed by underwater excavation, which means the construction sequence shall be: firstly, excavating pit underwater for water intake pier and trench for CW pipes, then floating transport and installing them underwater, and then back filling around the water

Figure 2 Section of Double-row Pile Retaining Structure in Huaneng Fuzhou Power Plant

, so the distance from intake pier and cover protection above the CW pipe by block stone. Because the bottom level of pipe trench is about -11.0m in the river, while the elevation of natural ground is about 2.0m on the bank during construction (that shall be filled to level of 5.0m after the construction), which refers to figure 2, so the free surface with suspension of 13m must be supported during construction period. Because there are several import riverside buildings near the free surface which cannot be affected during construction period, otherwise, it shall do harm to the operation of the existing plant, so the supporting and retaining structures must be built to protect the free surface from sliding and prevent the existing plant form damage.

failure sliding surface on the ground to frontal pile is

L0 H tan(450 2 ) , and the depth of failure sliding slope

at rear pile is the equivalent excavation depth. From fig 1, the equivalent excavation depth can be calculated according to following formula. When L0

L

H1

H

L tg (450 2 )

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3.1 Geotechnical Condition According to geotechnical report, the soil layers and

parameters of project are as table 1.

Table 1 Physical and mechanics characteristic parameters

Soil layer No. Back filling(1) Back filling(2)

1Mucky

Thickness of stratum m 4.50 0.70

Weight kN/m3 19 22 15.5

Compress model Es (MPa) 6 8 1.7

Cohesion (kPa) 8 15 11 13 12 1 2 3 4 16 / / /

Inner friction angle ° 10 28 1.4 2.8 1 15 13 17 2

Characteristic value of foundation bearing capacity fak (kPa) 90 150 50

Permeability coefficient ×10 5 cm/s 100 200 10 200 300 20

clay(soft)

2

Mucky clay

19.43

16.3

2.7 19 15 3 4 21 / / /

70

10

20

Silty clay(hard)

1

0.90

17.7 19

15 25 45 /

180 300 450 1000

2 1 1 /

4 3 2

fully 1.00 30.00

weathered Bedr ock

2

strongly moderate

weathered

3

22 26

weathered

Notes: A Soil layer with labe1 (1) of back filling denotes the soil layer mainly includes clay and sand while that with label (2) means the soil layer mainly includes crushed stone, block stone and sand. B Value of c and with label (1), (2), (3) and (4) of 2 silty clay are corresponding with test value of quick direct shear test, consolidated quick direct shear test, consolidated undrained triaxial test and consolidated drained triaxial test respectively. 3.2 Hydrological Condition can be divided into 3 types of I II III. Double-row pile According to hydrological report, the low tide of 100-year retaining system shall be deployed for type I and II because of return period is -3.02m, the low tide with 20-year return the high suspension depth of free surface while single-row period is -2.7m, the underground water level is o.om during retaining structure is proper to type III because of the relative construction. narrow suspension. Because calculation method of different 3.3 Scheme Comparison type is similar, type I support structure shall be calculated and From condition above, it is very difficult to design the calculation of type II and III shall be suppressed. retaining structure because the soil is very soft (most soil According to conclusion of reference [5] and [6], the layers are muck and mucky clay with low inner friction angle) bending moment value of frontal and rear piles shall be and the free surface suspension is somewhat high (it is 13m), similar if the row distance is about 3d, in which d is the furthermore, the stability of support structure is affected by diameter of pile. The larger the distance of row, the larger the seepage of underground water and water level in Minjiang negative moment of frontal pile and positive moment of the river, and the free surface must be excavated underwater, rear pile, which shall lead to re-bar area increase. But with the which means the support structure cannot be built by dry decrease of row distance, the horizontal displacement and construction and lowering underwater. Based on the site bending moments of piles shall increase, which shall lead to condition, the possible schemes for support structure are cost increase too. So the row distance of all types of retaining single-row pile, double-row pile and soil anchor. The support structures shall be constant while the pile interval in the same structure scheme of single-row cannot meet the stability row shall be determined by calculation. requirement except the piles with huge diameter are adapted, Table 2 Table of Scheme Contrast which means high price and long construction period. There is Scheme No. 1 2 3 no enough space to arrange soil anchor because there are Pile diameter(m) 1 1.2 1.5 several buildings with deep foundations near the project Row distance(m) 3 3.6 4.5 location. So the double-row pile support system is the most Pile interval in the same 1.5 1.8 2.5 reasonable scheme. row (m) According to excavation depth, the support structures

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RC volume of binding 32 37.44 45 beam(m3) Unit price of RC binding 1000 1000 1000 beam(Yuan/ m3) Price of binding beam 32000 37440 45000 Yuan 3 RC volume of piles (m ) 753.984 882.161 1060.29 Unit price of 1200 1200 1200 pile(Yuan/( m3) Price of pile(Yuan) 904780.8 1058594 1272348 Total(Yuan 936780.8 1096034 1317348 Scheme comparison shall be done as table 2 based on the type I retaining structure with different pile diameter and pile interval in the same row so as to determine the most reasonable scheme. The common parts, for example, crest beam, block stone protection, jet grouting shall be excluded because the cost of those are same in the different scheme. According to Table 2, the cost of scheme 1 is the lowest. In fact, there are 2 more advantages of that scheme: Firstly, the pile interval is the smallest, which means the free space between piles is the smallest and soil between piles is easy to be self-stable. That shall benefit the protection of soil between piles. Secondly, the construction equipments to build piles with diameter of 1.0m are widespread and cheap while the equipments to build larger piles are few and expensive. So the scheme 1 is the most reasonable scheme and shall be recommended. 3.4 Calculation Condition There are 2 calculation conditions: CW pipe construction condition and operation condition. In CW pipe construction condition, the soil top level is -11.0m and the river water level is -2.7m(low water level with 20-year return period) in front of piles (at river side), while the soil level is 2.0m and the underground water level is 0.0m behind the piles(at bank side). Because the condition is corresponding to construction of CW pipe, so the grade of support system shall be grade 3 and the importance coefficient shall be 0.9[4]. In operation condition, the CW pipe shall have been installed and the trench shall have been filled, so the soil top level is -1.0m and the river water level is -3.02m ( low water level with 100-year return period) in front of piles (at river side), while the soil level is 5.0m and the underground water level is 2.0m behind the piles (at bank side). Because the condition is the long-term existing, so the grade of support system shall be grade 2 and the importance factor shall be 1.0[4]. 3.5 Calculation According to reference [4], the calculation shall include seepage, sliding stability, overturning stability, embedded depth, inner force and reinforcement of piles and beams, displacement at top of pile. The results refer to figure 3.

Figure 3 Calculation Result

3.6 Improvement of Soil between Piles According to calculation result, the pile diameter of type I retaining structure is 1.0m, the row interval is 3m and the pile interval is 1.5m, which means the frontal surface is not complete protected by piles and the soil in 0.5m-wide space between piles shall be direct touch with river water. Because the free surface suspension is 13m and the soft soil of 2 mucky clay between pile is 19.43m-thick, which has the character of strong fluidness, it is necessary to settle the problem, otherwise, the soil between piles shall be eroded by river water so as to led to soil loss, ground settlement and damage of support structure[6], which is serious because in this case, the buildings on the riverside shall be damaged and the operation of the existing plant and the construction of the extension engineering shall be affected. But because most of the retaining structure is underwater and no baffle plate can be installed, so the problem must be settled by other method. In order to deal with possible soil loss between piles, besides the scheme of small diameter pile and small interval is chosen so as to decrease the free space of the surface, jet grouting method is used to improve the soil between pile, which means before excavation, the jet grouting pile foundation treatment shall be adapted to improve the soil between piles. When the improved soil solidified, the CW trench can be excavated and the soil between piles can be self-stable because of improved soil property and small free interval. 4 CONCLUDING REMARKS The difficult of large suspension height of retaining structure with soft soil and narrow construction space is settled by double-row pile support structure in Huneng Fuzhou power plant. The supporting and retaining project had been completed before December of 2009 with sound evaluation. Following conclusion can be acquired. (1)Double-row piles support system is the ideal scheme to meet the requirement of inner force and deformation of the project. In case of narrow construction space and strict deformation standard, double-row support system has outstanding advantage. (2)The elastic foundation beam model of double-row pile support system includes the interaction of soil and piles. But the distribution of soil pressure is assumed, which cannot meet the deformation compatibility and cannot completely simulate interaction of soil and piles. So the distribution of soil pressure should be further studied. (3)The retaining structure system is calculated according to planar model while in fact, it is space structure, besides the heterogeneous stratum, so the calculation result is not exact. In this case, monitoring system is recommended, whose datum shall provide the offer to testify the design, optimize design and guide construction. It is a pity there is no monitoring system in the project. (4)The design and construction of baffle plate to prevent soil between piles from loss is very import, otherwise, it is possible for soil to lose, which shall led to structure damage. But in some cases, the baffle plate cannot be installed, which can be settled by improvement of soil between piles by jet grouting pile. In this project, the jet grouting pile is only acting as improvement of soil so as to prevent soil loss, its improvement effect on mechanical character of retaining structure in ignored, which should be further studied.

REFERENCES [1] Zhi-cheng Yu,Wen-hua Shi, Design and Construction of Deep Foundation Pit, Beijing: China Building Industry Press,1996,pp.61-67. [2] Peng LIN,Yan-feng WANG,Zhi-xiong FAN,Di-yu HUANG,”Application and analysis of retaining structure with double-row piles in soft ground

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excavation engineering,” Chinese Journal of Geotechnical Engineering, vol. S2, 2010,pp. 331-334. [3] Zhi-yan Cheng,Wei-lun Qiu,Ke-neng Zhang,Ming-yu Huo,” Design calculation method on double-row retaining structure,”Geology and Prospecting. Vol. 2,2001, pp. 88-93. [4] Jun WANG,Lei WANG,,Zhao-ran XIAO, “Analysis of the interval between double-row piles by finite element method,”Chinese Journal of

Underground Space and Engineering,vol.7, 2005, pp.1096-1099. [5]Xin-shan ZHUANG,,Ying QIU,Cui-ying WANG, “Behavior analysis of the factors of deep Foundation Pit Excavation with Double-row pile,” Journal of Hubei University of Technology, vol.43,pp.93-96. [6]Peng TANG, Xue-song HAN, Li-mei FENG, ”Analysis on the drop of soil in double-row pile retaining structures,” Engineering Design of Ground,vol. 35, 2008,pp. 68-70

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