single pile
DESCRIPTION
design of pilesTRANSCRIPT
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8. Axial Capacityof Single Piles
8. Axial Capacityof Single Piles
CIV4249
©1998 Dr. J.P. Seidel
Modified by J.K. Kodikara, 2001
CIV4249
©1998 Dr. J.P. Seidel
Modified by J.K. Kodikara, 2001
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MethodsMethods
• Pile driving formulae• Static load test• Dynamic or Statnamic load test• Static formulae
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Pile driving formulaePile driving formulae
• e.g. Hiley formula (Energy balance)
Q = e.W.h .
F (set + tc / 2)• Ru= working load, W=weight of the hammer,
h= height of the hammer drop (stroke), F=factor of safety
• tc= elastic (temporary) compression• = efficiency
F
D
s
tc
Ru
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Static Load TestStatic Load Test
Plunging failure
Load to specifiedcontract requirement
What is thefailure load?
Davisson’s MethodButler and HoyChin’s MethodBrinch Hansonetc. etc.
What is the distributionof resistance?
Approximate methodsInstrumentation
Load
Deflection
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Dynamic and StatnamicTesting Methods
Dynamic and StatnamicTesting Methods
• Rapid alternatives to static testing• Cheaper• Separate dynamic resistance• Correlation
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Axial CapacityAxial Capacity
W
Pu
Qs
Qb
Pu = Qb + Qs - W
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Base ResistanceBase ResistanceQb = Ab [cbNc + P’ob(Nq-1) + 0.5gBNg + Pob]
minus weight of pile, Wp
but Wp » Ab.Pob
and as L >>B, 0.5gBN g << Wp
Qb = Ab [cbNc + P’obNq]
and for f > 0, Nq - 1 » Nq
Qb
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Shaft ResistanceShaft Resistance
Due to cohesion or friction
Cohesive component : Qsc = As . a . cs
Frictional component : Qsf = As .K P’ostan d
P’os
K.P’os
Qs = Qsc + Qsf = As [ a .cs + K P’ostan d ]
As
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Total Pile ResistanceTotal Pile Resistance
Qu = Qb + Qs
Qu = Ab [cbNc+P’obNq] + As [ a .cs+K P’otan d ]
How do we compute Qu when shaft resistance along the pile is varying?
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MobilizationMobilization
Shaft
2 - 5mm
Base
10 - 20% diam
Total
Settlement
Lo
ad
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Piles in ClayPiles in Clay
Qu = Ab [cbNc+P’obNq] + As [ a .cs+K P’otan d ]
Qu = Ab [cbNc+P’obNq] + As [ a .cs+K P’otan d ]
Qu = AbcbNc + Asa .cs
Qu = Ab [cbNc+P’obNq] + As [ a .cs+K P’otan d ]
Qu = Ab P’obNq + AsK P’otan d
Qu = AbcbNc + Asa .cs
Qu = Ab P’obNq + AsK P’ostan d
Undrained
Drained / Effective
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Driven Piles in ClayDriven Piles in Clay
2.0
1.5
1.0
0.5
0 10 20 30 40 50 60ra
u
vo
Average curve for sensitiveamarine clay
Average curve for clays oflow-medium sensitivity
2.0
1.5
1.0
0.5
0 10 20 30 40 50 60ra
u
vo
Average curve for sensitiveamarine clay
Average curve for clays oflow-medium sensitivity
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Driven Piles in ClayDriven Piles in Clay
300
250
200
150
100
50
01 5 10 50 100 500 1000
Time after driving in days
Bea
ring
cap
acity
in kN 200 x 215mm conrete
(Gothenberg)
300 x 150mm tapered timber (Drammen)
150mm (8 in) steel tube (San Francisco)300 x 125mm I-Beam
(Gothenberg)
30
25
20
15
10
5
Bea
ring
cap
acity
in to
ns
300
250
200
150
100
50
01 5 10 50 100 500 1000
Time after driving in days
Bea
ring
cap
acity
in kN 200 x 215mm conrete
(Gothenberg)
300 x 150mm tapered timber (Drammen)
150mm (8 in) steel tube (San Francisco)300 x 125mm I-Beam
(Gothenberg)
30
25
20
15
10
5
Bea
ring
cap
acity
in to
ns
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Nc ParameterNc Parameter
Nc
Compare Skempton’s Nc for shallow foundations
Nc= 5(1+0.2B/L)(1+0.2D/ B)
10
9
8
7
6
50 1 2 3 4 5
L /d B
Ben
ding
cap
acity
fac
tor
Nc
10
9
8
7
6
50 1 2 3 4 5
L /d B
Ben
ding
cap
acity
fac
tor
Nc
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Adhesion Factor, Adhesion Factor,
50 100 150 200 250
1000 2000 3000 4000 5000
2.0
1.5
1.0
0.5
0
Figures denote penetration ratio =Depth of penetration in clay
Pile diameter Key:Steel tube pilesPrecast concrete
pilesDesign curve forpenetration ratio >
49 4949 56
13 1517 27
33
4010 5815
3833273944
44 39
1917
19
13
35 44Adh
esio
n fa
ctor
Undrained shear strength (c ) lb/ft2u
Undrained shear strength (c ) kN/m2u
20
50 100 150 200 250
1000 2000 3000 4000 5000
2.0
1.5
1.0
0.5
0
Figures denote penetration ratio =Depth of penetration in clay
Pile diameter Key:Steel tube pilesPrecast concrete
pilesDesign curve forpenetration ratio >
49 4949 56
13 1517 27
33
4010 5815
3833273944
44 39
1917
19
13
35 44Adh
esio
n fa
ctor
Undrained shear strength (c ) lb/ft2u
Undrained shear strength (c ) kN/m2u
20
1.0
0.8
0.6
0.4
0.2
0 100 200
Average Undrained Shear Strength, c , kPau
Red
uctio
n F
acto
r ,
1.0
0.8
0.6
0.4
0.2
0 100 200
Average Undrained Shear Strength, c , kPau
Red
uctio
n F
acto
r ,
Aust. Piling Code, AS159 (1978)
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Bored Piles in ClayBored Piles in Clay
• Skempton’s recommendations for side resistance– =0.45 for cu <215 kPa
– cu =100 kPa for cu>215 kPa
– Nc is limited to 9.
– A reduction factor is applied to account for likely fissuring (I.e., Qb = Ab cb Nc)
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Soil disturbanceSoil disturbance
• sampling attempts to establish in-situ strength values
• soil is failed/remoulded by driving or drilling
• pile installation causes substantial disturbance– bored piles : potential loosening– driven piles : probable densification
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Scale effectsScale effects
• Laboratory samples or in-situ tests involve small volumes of soil
• Failure of soil around piles involves much larger soil volumes
• If soil is fissured, the sample may not be representative
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Qu = Ab [cbNc+P’obNq] + As [ a .cs+K P’ostan d ]
Piles in SandPiles in Sand
Qu = Ab [cbNc+P’obNq] + As [ a .cs+K P’ostan d ]
Qu = Ab P’obNq] + AsK P’ostan d ]
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Overburden Stress P’obOverburden Stress P’ob
Qu = Ab P’obNq] + AsK P’ostan d ]
Meyerhof Method : P’ob = g’z
Vesic Method : critical depth, zc
for z < zc : P’ob = g’zfor z > zc : P’ob = g’zc
zc/d is a function of f after installation - see graph p. 24
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Critical Depth (zc)Critical Depth (zc)
L
zc
vc
W.T.
d
L
zc
vc
W.T.
d
20
15
10
5
028 33 38 43
z /
dc
20
15
10
5
028 33 38 43
z /
dc
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Bearing Factor, NqBearing Factor, Nq
Nq is a function of : friction angle, fNq is a function of :
Qu = Ab P’obNq] + AsK P’ostan d ]
What affects f ? • In-situ density• Particle properties• Installation procedure
Nq determined from graphs appropriateto each particular method
Total end bearing may also be limited:
Meyerhof : Qb < Ab50Nqtanf
Beware if f is pre- or post-installation:
Layered soils :Nq may be reduced if penetrationinsufficient. e.g. Meyerhof (p 21)
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Nq factor (Berezantzev’s Method)Nq factor (Berezantzev’s Method)
1000
100
1025 30 35 40 45
Nq
1000
100
1025 30 35 40 45
Nq
If D/B <4
reduce proportionately to Terzaghi and Peck values
For driven piles : 10+' 75.0=' 1For bored piles : 1 3
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Overburden Stress P’osOverburden Stress P’os
Qu = Ab P’obNq] + AsK P’ostan d ]
Meyerhof Method : P’os = g’zmid
Vesic Method : critical depth, zc
for zmid < zc : P’ob = g’zfor zmid > zc : P’ob = g’zc
zc/d is a function of f after installation - see graph p. 24
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Lateral stress parameter, KLateral stress parameter, K
• A function of Ko
– normally consolidated or overconsolidated - see Kulhawy properties manual
– see recommendations by Das, Kulhawy (p26)
• A function of installation– driven piles (full, partial displacement)– bored piles– augercast piles– screwed piles
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Das (1990) recommends the following values for K / Ko:
Pile Type K / Ko
Bored or Jetted piles 1
Low-displacement, driven piles 1 to 1.4
High-displacement, driven piles 1 to 1.8
Kulhawy (1984) makes the following similar recommendations:
Pile Type K / Ko
Jetted piles 1/2 to 2/3
Drilled shaft, cast-in-place 2/3 to 1
Driven pile, small displacement 3/4 to 5/4
Driven pile, large displacement 1 to 2
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K.tandK.tand
• The K and tand values are often combined into a single function
• see p 28 for Vesic values from Poulos and Davis
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Pile-soil friction angle, dPile-soil friction angle, d
• A function of f• See values by Broms and Kulhawy (p26)• A function of pile material
– steel, concrete, timber
• A function of pile roughness– precast concrete– Cast-in-place concrete
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Pile-soil friction anglePile-soil friction angleBroms (1966) suggests the following
Pile Material / '
Steel
Concrete 0.75
Timber 0.66
Kulhawy (1984)
Pile Material / ' Typical analogy
Rough concrete 1.0 Cast-in-place
Smooth concrete 0.8 to 1.0 Precast
Rough steel 0.7 to 0.9 Corrugated
Smooth steel 0.5 to 0.7 Coated
Timber 0.8 to 0.9 Pressure-treated
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ExampleExample
• Driven precast concrete pile• 350mm square• Uniform dense sand ( f = 40o ; g = 21kN/m3)• Water table at 1m• Pile length 15m• Check end bearing with Vesic and Meyerhof Methods• Pile is driven on 2m further into a very dense layer• f = 44o ; g = 21.7 kN/m3
• Compute modified capacity using Meyerhof
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ExampleExample
• Bored pile• 900mm diameter• Uniform medium dense sand ( f = 35o ; g = 19.5kN/m3)• Water table at 1m• Pile length 20m• Check shaft capacity with Vesic and Meyerhof Methods• By comparsion, check capacity of 550mm diameter
screwed pile
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Lateral load on single pileLateral load on single pile
• Calculation of ultimate lateral resistance (refer website/handouts for details)
• Lateral pile deflection (use use subgrade reaction method, p-y analysis)
• Rock socketed pile (use rocket, Carter et al. 1992 method)