1 Axial Capacity

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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering

Indian Institute of Technology Madras-36

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Pile Foundation Concepts and Design

Main / Skirt PilePile Group ArrangementVertical Axial Soil CapacityStructural Capacity

CONTENTS



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Pile Simulation for an Offshore Jacket

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Pile group arrangements for 4 legged platform



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Ultimate Bearing Capacity The ultimate bearing capacity of pile, including

belled piles, Qd should be determined by the equation.

Qd= Qf+Qp=fAs+qAp

WhereQf=skin friction resistance, Ib (kN)Qp= total end bearing, Ib (kN),f = unit skin friction capacity, Ib/ft2 (kPa)As = side surface area of pile, ft2 (m2),Ap = gross end area of pile, ft2 (m2),



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Where α = a dimensionless factor,c = un-drained shear strength of the soil at the point in

questionThe factor, α, can be computed by the equations:α = 0.5 Ψ-0.5 Ψ≤1.0α = 0.5 Ψ-0.25 Ψ >1.0With the constraint that, α ≤ 1.0,Where Ψ = c/p’0 for the point in question c/p’0 effective overburden pressure at the point in question Ib/ft2 (kPa)

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For piles end bearing in cohesive soils, the unit end bearing q.in Ibs/ft2

(kPa), may be computed by the equation

q = 9c

Cohesive Soil



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Cohesionless SoilsFor pipe piles in cohesionless soils, the shaft friction, f. in Ib/ft2 (kPa) may be calculated by the equation:

f = Kpao tan δWhere K = coefficient of lateral earth pressure (ratio of

horizontal to vertical normal effective stress)po = effective overburden pressure Ib/ft2 (kPa) at the point

in questionδ = friction angle between the soil and pile wall

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For pipe piles in cohesionless oils, the unit shaft friction at a given depth, f, may be calculated by the equation

f = βp´0

β = dimensionless shaft friction factorp´0 = effective overburden pressure at

the depth in question



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For piles end bearing in cohesionlesssoils, the unit and bearing q may be computed by the equation

q=Nqp´0

WhereNq=dimensionless bearing capacity

factorp´0 = effective overburden pressure at

the depth in question

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Pile Foundation Concepts and Designold

API 2A

Recommended Practice



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Pile Foundation Concepts and DesignNew

API 2A

Recommended Practice

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t-z curve for deformation of a pile under vertical axial loading



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Where

z = local pile deflection, in. (mm),

D = pile diameter, in. (mm).

t = mobilized soil pile adhesion, Ib/ft2 (kPa)

tmax = maximum soil pile adhesion or unit skin friction capacity computed accoringto Section 6.4. Ib/ft2 (kPa)

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Pile Foundation Concepts and DesignTypical Axial Pile Load Transfer-Displacement (t-z) Curves



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Tip-load – Displacement Curve

Where

z = axial tip deflection, in. (mm),

D = pile diameter, in. (mm).

Q = mobilized end bearing capacity, Ib(KN)

Qp = total end bearing, Ib(KN), computed according to section 6.4

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Pile tip load – Displacement (Q-z) curve



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STRUCTURAL DESIGN ISSUES

Design for structural strength to resistAxial forceLateral shear forceBending moment

Make allowance for corrosion / durabilityConsider possibility of buckling

Only likely to be of concern for slender piles in very soft clay with unsupported length

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PILE BUCKLINGReplace pile by

equivalent cantilever

CRITICAL LOAD is

k) increasing(linearly )J4(SIE.

P

k)constant ()J4(S

IE. P

22RR

pp2

cr

2RR

pp2

cr

R+=

+=

π

π



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PILE BUCKLING

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DURABILITY ISSUESCOUNTER MEASURES FOR STEEL PILES(COASTAL STRUCTURES)

Corrosion protection paintPolyethylene cover (steel pipes)Zinc coating Electro – chemical (cathodic) protection Cement or concrete cover

1 Axial Capacity

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