Tính toán và bố trí cọc váy CTB

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4.0 DESIGN CRITERIA
4.1 Pile Penetration
The ultimate pile axial capacities are determined based on the soil criteria using soil friction along the pile cylindrical surface and soil bearing resistance at pile bottom end as presented in the Attachment 3.1.
In accordance with API RP 2A recommendations, factors of safety for extreme storm load and operating storm loads shall be 1.50 and 2.0 respectively. For this project, the minimum pile group factor of safety considered for the extreme storm conditions will be of 1.5 and that for operating storm conditions will be 2.0. No factor of safety will be considered for the pile self weight.
The pile head loads for individual pile and group pile can be obtained from the results of in-place analysis. The ratio of the pile head axial capacity to the pile axial load shall be required larger than 1.5 and 2.0 for extreme and operating storm condition. The required ultimate soil capacities are thus determined and the design pile penetrations are obtained from the ultimate soil capacity curves.
4.2 Pile Stresses and Deflections
The pile stresses and displacements along the entire pile length are depicted from the analyses results for each design conditions.
The design stresses due to axial loads and bending moments will be determined at all critical sections along the pile and be checked in accordance with the requirements of API RP 2A. An one-third increase for basic allowable stresses is permitted for the load cases subject to extreme storm condition.
4.3 Pile Group Effect
4.3.1 Pile Group Efficiency
A group efficiency factor, h is used to compare the ultimate axial capacity of a pile group comprising “n” piles (or Equivalent Pier) and the ultimate capacity of “n” single piles (“n” x Isolated Pile). Depending on the pile group configuration, embedment depth and the soil strata, the group capacity may not be the same as a single isolated pile capacity multiplied by the number of piles in the group.
The efficiency factor (h) of a pile group as per Poulos and Davis method (1980) is defined as
1/h[SUP]2[/SUP] = 1 + n[SUP]2[/SUP]Qu[SUP]2[/SUP] /Qg[SUP]2 [/SUP](Eq. 4.3.1)
Where :
Qg = Min. axial capacity of the pile group (Equivalent Pier)
Qu = Ultimate axial pile capacity of individual pile
n = Number of piles in group
This efficiency factor is applied as reduction factor to each ultimate axial pile capacity of individual single pile in the group pile.
4.3.2 Pile Group Deformation
The load deformation of the pile group is determined by using the combined Poulos , H.G and Davis E.H. [7], for t-z data for the settlement of pile group. The elastic method suggested by Poulos [7] is used to predict the cumulative group deformation since this effect is due primarily to the superposition of small stress increments that will remain in the elastic range, even for a large loading on a group.
The mathematical formulation as shown below followed closely that proposed in “Pile Foundation Analysis and Design” by Poulos, HG and Davis, EH (1980).
Pk = pj SumPjakj + Pk, j#k (Eq. 4.3.2)
r[SUB]f[/SUB] = r[SUB]k[/SUB][SUP]0 [/SUP]/ P[SUB]k[/SUB]
Where :
r[SUB]k[/SUB] = Axial displacement of pile ‘k’
r[SUB]k[/SUB][SUP]0[/SUP] = Axial displacement (settlement) of individual single pile ‘k’
r[SUB]f[/SUB] = Axial displacement of single pile ‘k’ under unit axial load
P[SUB]j [/SUB] = Axial load on pile ‘j’
a[SUB]kj[/SUB] = Dimensionless interaction factor for axial displacement of pile ‘k’ due to pile ’j’
P[SUB]k[/SUB] = Axial load on pile ‘k'
Equation (4.3.2) can be modified as:
Formula
r[SUB]k[/SUB] = r[SUB]k[/SUB][SUP]0[/SUP] (Eq. 4.3.3)
The ratio of pile settlement with group effect, r[SUB]k [/SUB]to the settlement of individual single pile, r[SUB]k[/SUB][SUP]0 [/SUP]is considered as the multiplier factor.
C[SUB]k[/SUB] = r[SUB]k[/SUB][SUP]0 [/SUP]/ P[SUB]k [/SUB]= (Eq. 4.3.4)
The above multiplier factor C[SUB]k[/SUB] is applied to the ‘z’ value of t-z curve for the pile soil interaction analysis.
Similar method and procedure are applied in the determination of ’y’ factor in p-y data for lateral load deformation on pile group.
Calculation of pile group efficiency and pile group deformation are shown in Attachment 3.5.
Dimensionless interaction factor is determined such as follow:
α = α[SUB]F[/SUB] –F[SUB]E[/SUB] (α[SUB]F[/SUB] – α[SUB]E[/SUB])
Where :
α[SUB]F [/SUB] = interaction factor based on Figure 6.2, 6.3, 6.4, 6.5 (in Attachment 3.5)
α[SUB]E [/SUB]= interaction factor based on Figure 6.10, 6.11, 6.12, 6.13 (in Attachment 3.5)
F[SUB]E [/SUB]= interaction factor based on Figure 6.14a and 6.14b (in Attachment 3.5)
The above multiplier factor α is applied to the ‘y’ value of p-y curve for the pile soil interaction analysis.
4.4 Pile Factor of Safety
Pile penetration of 100m below seabed is selected to yield the desired soil capacity computed based on the method recommended in API RP 2A 21[SUP]st[/SUP] Ed. [1]. The same penetration depth of 57m below seabed is applied to all conductors.
The required minimum factors of safety for pile and conductor axial loads are summarized below:
Table 4.1 Piles Minimum Safety of Factors
Type of piles: Skirt pile
Factor of Safety
for Operating: 2.0
for Storm&Seismic: 1.5

Hình 1: Bản vẽ minh họa cọc váy

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