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

MethodsMethods

• Pile driving formulae• Static load test• Dynamic or Statnamic load test• Static formulae

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

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

Dynamic and StatnamicTesting Methods

Dynamic and StatnamicTesting Methods

• Rapid alternatives to static testing• Cheaper• Separate dynamic resistance• Correlation

Axial CapacityAxial Capacity

W

Pu

Qs

Qb

Pu = Qb + Qs - W

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

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

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?

MobilizationMobilization

Shaft

2 - 5mm

Base

10 - 20% diam

Total

Settlement

Lo

ad

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

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

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

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

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)

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)

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

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

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 ]

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

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

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)

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

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

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

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

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

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

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

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

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

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)