How to calculate the bearing capacity of pipe pile end

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2. 1 The bearing capacity of single pile can be estimated by CPT
the interaction between probe and soil in CPT is similar to that between pile and soil, so the bearing capacity of single pile can be estimated by the specific penetration resistance (single bridge) measured by CPT or the cone tip resistance and side wall friction resistance in double bridge probe. However, the end resistance and friction in CPT can not be directly used as the end resistance and friction of actual single pile, but must be corrected because the working performance of CPT is different from that of actual single pile
(1) if there is no local experience, the standard value of vertical ultimate bearing capacity of precast concrete single pile can be calculated according to the following formula: quk = qsk + qpk = u ∑ qsik · Li+ α· PSK · AP
where quk is the standard value of vertical ultimate bearing capacity of single pile
qsk standard value of total ultimate lateral resistance of single pile
qpk standard value of total ultimate end resistance of single pile
U -- circumference of pile body< Qsik -- the standard value of the ultimate lateral resistance of the i-layer soil around the pile estimated by the specific penetration resistance value of the static cone penetration test
Li -- the thickness of the pile passing through the i-layer soil< br /> α—— Correction coefficient of pile end resistance< br /> psk—— The standard value (average value) of CPT specific penetration resistance near the pile tip
AP -- pile tip area
(2) when determining the standard value of the vertical ultimate bearing capacity of a single precast concrete pile based on the static cone penetration test data of double bridge probes, if there is no local experience, it can be calculated as follows: quk = u ∑ Li β ifsi+ α Qcap
where FSI is the average lateral resistance of the probe in layer I soil
QC - probe resistance above and below the pile tip plane, the weighted average value of probe resistance within 4D above the pile tip plane (D is the diameter or side length of the pile) according to the thickness of soil layer is taken, and then the probe resistance within 1D below the pile tip plane is averaged< br /> α—— The correction coefficient of pile end resistance is 2 / 3 for cohesive soil and silt and 1 / 2 for saturated sand< br /> β I -- comprehensive correction coefficient of pile side resistance in the i-th layer< (2) when the bearing capacity of single pile is determined by the physical index method of soil, the standard value of vertical ultimate bearing capacity of single pile should be calculated by the following formula: quk = qsk + qpk = u ∑ qsikli + qpkap
where qsik is the standard value of ultimate lateral resistance of the i-layer soil on the pile side. If there is no local empirical value, the code can be consulted
qpk standard value of ultimate end resistance. If there is no local experience value, please refer to the table

(III) checking calculation of bearing capacity and settlement of pile group
when the pile center distance is less than or equal to 6 times of pile diameter and the number of piles exceeds 9 (including 9), the pile and soil can be taken as the imaginary solid foundation. At this time, the pile abutment, pile and soil between piles form a whole and sink together under the upper load, which is the effect of pile group. When checking the bearing capacity and settlement of this kind of pile foundation, it is considered as solid foundation
(1) checking calculation of pile group bearing capacity checking calculation of pile group bearing capacity refers to checking whether the foundation bearing capacity at the bottom of solid foundation (at the plane of pile end) meets the requirements. One of the common methods is to assume that the load is from the pile top of the outermost circle to the top of the pile φ 0 / 4 of the dip angle downward spread φ 0 is the average internal friction angle of each soil layer within the range of pile length), which should meet the following requirements: under the central load and eccentric load,
(2) when checking the settlement of pile group, the pile group is also taken as the solid foundation, and the calculated pile foundation deformation value should meet the provisions of the allowable value of building pile foundation deformation, If there is no local experience, the allowable value of building pile foundation deformation can be used according to the provisions in the table. For the allowable value of building pile foundation deformation not included in the table, it can be determined according to the adaptability of superstructure to pile foundation deformation and the requirements of use. The bottom dimension of solid foundation can be adjusted according to the φ The range of 0 / 4 diffusion can also be taken according to the range of pile group at the pile end. The calculation results of the two methods are slightly different. The settlement calculation of pile group can be carried out according to the settlement calculation steps of shallow foundation, that is, the settlement calculation method described above. It can also be calculated by the equivalent action layer summation method< (4) the negative friction force of the pile is e to the downward friction of the pile caused by the settlement of the soil layer around the pile. The main reasons for negative friction are as follows:

(1) self weight consolidation of under consolidated soft clay or new fill soil
(2) the soil layer around the pile sinks e to the large area of surcharge
(3) the groundwater level in normally consolidated soft clay area decreases, and the effective stress increases, which causes the soil layer to sink
(4) collapsible loess causes settlement. The effect of negative friction increases the axial load (additional load) on the pile, which should be paid attention to where the negative friction is obvious. The magnitude of negative friction is affected by many factors, such as the strength of the soil around the pile and the soil at the end of the pile, the consolidation history of the soil, the ground load, the type and setting method of the pile, the change of the groundwater level and the ration of the pile, etc. Therefore, the calculation of negative friction is a more complex problem. Most of them use semi empirical formula or empirical estimation, mainly based on the vertical effective stress, undrained shear strength of soil and mechanical properties of soil. In practice, it is generally estimated according to the effective stress, that is, the standard value of single pile negative friction is qnsi= ζ n σ′ Where qnsi is the standard value of negative friction on the pile side of the i-th soil layer; ζ N -- negative friction coefficient of soil around the pile, which can be found in the table; σ’ I-the average vertical effective stress of the i-th layer soil around the pile. The calculation of negative friction of pile is different when the stratum combination, groundwater condition and ground load condition are different. In the past, the problem of negative friction has not been considered in the coastal soft soil area of China, and accidents caused by negative friction are rarely found, because the negative friction may decrease or even disappear when the pile tip may continue to settle. However, when the pile passes through a thick soft soil layer of more than 15m, and the ground subsidence rate exceeds 2cm per year, or when the pile end is supported on a hard layer such as rock stratum and gravel, the negative friction may be large.

3.

According to 5.2.8 of pile foundation code, it is calculated as follows: quk = qsk + qpk

I Design data

1. Pile design parameters

pile forming process: Precast concrete pile

bearing capacity design parameter value: according to the building pile foundation specification table

orifice elevation 0.00 m

pile top elevation 0.50 m

pile body design diameter: D = 0.80 m

pile body length: l = 18.00 m

2. Geotechnical design parameters

layer number soil layer name layer thickness (m) layer bottom buried depth (m) rock Soil physical and mechanical index limit side resistance qsik (kPa) limit end resistance qpk (kPa)

1 fill 3.00 3.00 n = 5.00 17 -

2 red clay 3.00 6.00 α W = 0.70, IL = 0.50 26 -

3 red clay 3.009.00 α W = 0.70, IL = 0.50 29 -

4 red clay 3.00 12.00 α W = 0.70, IL = 0.50 32 -

5, red clay 3.00 15.00 α W = 0.70, IL = 0.50 33 -

6 red clay 3.00 18.00 α W = 0.70, IL = 0.50 34 2700

7, red clay 3.00 21.00 α W = 0.70, IL = 0.50 32 -

8, red clay 3.00 24.00 α W = 0.70, IL = 0.5032 -

3. Design basis

technical code for building pile foundation (JGJ 94-94), hereinafter referred to as pile foundation code

code for design of building foundation (GB 50007-2002), hereinafter referred to as foundation code

II Estimation of vertical compressive bearing capacity of single pile

1. Calculation parameter table

calculation thickness of soil layer Li (m) ultimate lateral resistance qsik (kPa) ultimate end resistance qpk (kPa)

13.00 17 0

23.00 26 0

33.00

53.00 33 0

62.50 34 2700

2; × 0.80 = 2.51 m

Ap =  × 802 / 4 = 0.50 M2

3. According to pile foundation code 5.2.8, the standard value of total ultimate lateral resistance of soil is calculated as follows:

qsk = u & # 61669; qsikli = 2.51 × ( seventeen × 3.00 + 26 × 3.00 + 29 × 3.00 + 32 × 3.00 + 33 × 3.00 + 34 × 50) = 1243 kn

the standard value of total ultimate end resistance is:

qpk = qpkap = 0.50 × 2700 = 1357 kn

the standard value of vertical compressive ultimate bearing capacity of single pile is:

quk = qsk + qpk = 1243 + 1357 = 2600 kn

the characteristic value Ra of vertical bearing capacity of single pile is calculated according to Article 7 of q.0.10 in Appendix Q of foundation code

RA = quk / 2 = 2600 / 2 = 1300 kn

extended data:

the rigidity of pile is small, The displacement of the top section of the pile is larger than that of the bottom, and the side friction at the top of the pile is usually larger; When the stiffness of the pile is large, the displacement of each section of the pile is close. Because the initial normal stress of the soil on the side of the lower part of the pile is large, the shear strength of the soil is also large, so that the side friction of the lower part of the pile is greater than that of the upper part of the pile

because the compression of the foundation soil at the bottom of the pile is completed graally, the load borne by the pile side friction will be transferred from the upper part of the pile to the lower part of the pile with time

in the process of pile foundation construction and after the completion of pile side soil properties, state will change in a certain range, affect the pile side friction, and often have time effect. Among the factors influencing the pile side friction, the type and property of soil are the main factors

When analyzing the bearing capacity of foundation piles, the influence of various factors on the size and distribution of pile side friction should be paid attention to. In the plastic state, pile driving in the viscous state will cause disturbance to the soil at the pile side, and the squeezing effect of pile driving will increase the pore water pressure in the soil around the pile, rece the shear strength of the soil, and rece the pile side friction

After a period of time, the excess pore water pressure graally dissipates, and the thixotropy of clay makes the shear strength around the pile not only recover, but also exceed its original strength, and the pile side friction is improved

When the pile is driven in the sand, the change of pile side friction is related to the initial density of sand. If there is dilatancy in the dense sand, the peak value of pile side friction will decrease

the size and distribution of pile side friction determine the change and value of pile axial force with depth, so it is important to master and understand the distribution law of pile side friction for studying and analyzing the working state of pile

because the relative displacement between pile and soil, the lateral stress in soil and the distribution and behavior of soil mass change with depth, it is more complex to accurately describe the distribution of pile side friction with physical and mechanical equations

4. The details are as follows:
1. The most reliable method is to determine the static load test, that is, to drive a number of test piles on the site to test the ultimate bearing capacity, and then to obtain the characteristic value of the bearing capacity according to the code formula (in fact, dividing the ultimate bearing capacity by 2). Of course, to meet the necessary probability distribution
2. If no experiment is done, the ultimate bearing capacity can be estimated according to the friction and end bearing capacity of each layer of soil given in the geological report, and then the characteristic value can be obtained
3. The pile foundation with high requirements must be determined by experiment.

5. Calculation method tensile force, namely tensile value MPa =[ The nominal cross-sectional area of prestressed steel bar multiplied by the number of steel bars multiplied by the standard value of tensile strength of steel bar [1420] multiplied by 70%] / the area of tensioning machine piston. 70% is the control stress of prestressed steel bar in 0.7, which is 70% according to the provisions of gb13476-2009 and 10g409.

6. The characteristic value of the vertical bearing capacity of a single pile generally refers to the limit value of the vertical bearing capacity of a single pile divided by a safety factor (generally 2), The other is the serviceability limit state. The characteristic value of vertical bearing capacity of single pile is probably the bearing capacity considering certain deformation factors.

characteristic value of single pile bearing capacity of precast pile:

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7.

According to the outer diameter, pipe piles are divided into specifications of 300 mm, 350 mm, 400 mm, 450 mm, 500 mm, 550 mm, 600 mm, 800 mm and 1000 mm. The actual pipe diameters are mainly 300 mm, 400 mm, 500 mm and 600 mm. At present, the diameter of our company is mainly 400 and 600, the pipe piles are all factory proction, and the commonly used section length is 8-12m

pipe piles can be divided into type A, type AB and type B according to the anti cracking moment of pile body. The effective prestress of type A is about 3.5-4.2mpa, that of type AB is 5.0mpa, that of type B is about 5.5-6.0mpa

According to concrete strength grade and wall thickness, pipe piles are divided into PC pipe pile, PTC pipe pile and PHC pipe pile. The concrete strength of PC pile shall not be lower than C50 concrete, the strength grade of PTC pipe pile shall not be lower than C60, and the concrete strength grade of PHC pile shall not be lower than C80

PC pile and PTC pile are generally cured by normal pressure steam, which generally takes 28 days to drive. The PHC pile, after demoulding, needs to be autoclaved and cured. After 10 atmospheres and 180 degrees autoclave curing, the concrete strength grade reaches C80. The shortest time from forming to using is only three or four days

extended data:

there are three kinds of prestressed pipe piles, PHC, PC and PTC, which is a very mature construction technology, widely used in instrial plants and civil buildings in Yangtze River Delta, Pearl River Delta, Bohai Bay, with simple technology and easy construction quality control

There are two kinds of pile sinking technology: static pressure and hammering. At present, static pressure pile is the main method in Yangtze River Delta. As far as Shanghai is concerned, the vertical bearing capacity of single phc-500 (100) pipe pile can reach more than 2000kN, the price of phc-a500 (100) pipe pile is 105 yuan / m, and the construction cost is about 15 yuan / m

When PHC pile is used, the selection of pile hammer and pile frame should be done well before driving, the age of pipe pile should be determined, and corresponding technical measures should be taken for pile insertion, hammering, pile connection and pile delivery. In this project, PHC pile has the characteristics of high single pile bearing capacity, good hammer resistance, strong penetration and low cost

8. MPA represents the strength value of pipe pile, while kn represents the bearing capacity. These two kinds of construction technologies have different concepts
the bearing capacity can be calculated only after the construction is completed and tested together with the foundation.

9. The basis of single pile bearing capacity of PHC pile is: bearing capacity needs to consider two factors:
first, the bearing capacity of pipe pile itself
Second, the bearing capacity of foundation soil
according to article 8.5.5-3 of code for design of building foundation (GB 50007-2002), RA = QPA * AP + up ∑ qsia * LIF, where AP is the sectional area of the pile, up is the perimeter of the pile section, LIF is the length of the pile, QPA is the bearing capacity of the pile end soil, and qsia is the friction resistance of the soil around the pile. That is to say, the bearing capacity of single pile is determined by pile length, pile diameter, bearing capacity of pile tip soil and friction of soil around pile. The latter two parameters are related to soil type and pile forming technology
therefore, the characteristic values of single pile bearing capacity range from 100kN to 10000kN, and 1000KN to 3000kN are common.

10. Generally, different hammers are selected according to different pipe piles, but the selection of hammers is not the main reason affecting the driving depth. Geology is the factor affecting the depth of pipe piles. After driving into the strongly weathered layer, you can't drive in with any hammers. According to the penetration of different types of pipe piles, you can basically know whether the design bearing capacity is reached.