deep foundations

Deep Foundations

Axial Load Capacity based on Analytical Methods (Chapter 14)Downdrag Loads (Chapter 18)

CV3301 - LEC (2008) Lecture 6 2

Hard Stratum

Deep Foundation Load TransferP

Pt’

Ps

P

Pt’

Ps

CV3301 - LEC (2008) Lecture 6 3

Toe Bearing Resistanceγγ++= BN5.0qNcNq qcult

CV3301 - LEC (2008) Lecture 6 4

Toe Bearing Resistance – Sands (Driven Piles)

*qzD

*t N'NB'q σ+γ= γ

When D/B > 5, first term is negligible.

( )( )'tan'12EI

:I index,rigidity a define toNeedgoverns. failureshear of modes any three asility compressib

andstrength shear both on depends N and N

zDr

r

*q

*

φσν+=

γ

CV3301 - LEC (2008) Lecture 6 5

Toe Bearing Resistance – Sands (Driven Piles) (cont’d)

CPT. and SPT fromor analysisn Schmertman of E as estimated becan E

shear punchingor local important, isility compressib soil I Lowfailureshear general IHigh

.400I 10ypically T

s

r

r

r

⇒⇒

<<

CV3301 - LEC (2008) Lecture 6 6

Toe Bearing Resistance – Sands (Driven Piles) (cont’d)

CV3301 - LEC (2008) Lecture 6 7

Example 14.1 – A 400-mm square prestressed concrete pile is to be driven 19 m into the soil profile shown in Figure 14.6. Compute the net ultimate toe-bearing capacity. The water table is at a depth of 3m below the ground surface.

( )( ) ( )( ) ( )( )

( ) ( )( )

( )

( )( )

( )( )( ) ( )( )( )( ) kN22614.014131'P

kPa14131758.1878.138.92.180.4

N'NB'q

75N and 8.13N figures, From

9936tan8.1870.312

35000

'tan'12EI

kPa3500025120015000

7.17) Eq. (See NOCRE

kPa 187.8 168.9162.1838.17uH'

2t

*qzD

*t

*q

*

o

zDr

6010s

zD

==

=+−=

σ+γ=

==

=+

=

φσν+=

=+=

β+β=

=∑ −+=−γ=σ

γ

γ

16 m

CV3301 - LEC (2008) Lecture 6 8

Toe Bearing Resistance – Sands (Drilled Shaft)

settlement mm 25 within mobilized be toassumed is q' diameter base is B

toebelow 2Bdepth and ebetween to N SPTmean is N where kPa2900N5.57q'

,50 Nor F

t

b

b60

60t

60

≤=<

tb

trtb 'qB

1200q' toq' reduce mm, 1200 BFor =>

CV3301 - LEC (2008) Lecture 6 9

toebelow 2B and ebetween tostrength shear undrained is skPa100sfor .09N

kPa50sfor .08N

kPa25sfor 6.5N:where

1999) Reese and Neill(O' Ns'q

,kPa250sFor

bu

u*c

u*c

u*c

*cut

u

≥=

==

==

=

<

Toe Bearing Resistance – Clays

CV3301 - LEC (2008) Lecture 6 10

Toe Bearing Resistance – Clays (cont’d)

( )

( )kPain s s065.0

BD083.0B28.0

min B 0.15.2B

5.2F

'qFq' toq' Reducemm, 1900 BFor

uu2

bb1

b2b1

r

trtrt

b

=ψ

⎟⎟⎠

⎞⎜⎜⎝

⎛+=ψ

≤ψ+ψ

=

=>

CV3301 - LEC (2008) Lecture 6 11

A driven pile in clay is shown in the figure below. The pile has a diameter of 406mm. Determine the net toe bearing capacity.

( )

( )( )( ) kN6.11691001295.0

NsA'qA'P

m1295.0406.04

D4

A

*utttt

222t

c

==

==

=π

=π

=

CV3301 - LEC (2008) Lecture 6 12

Toe Bearing Resistance – Intermediate Geomaterials

O’Neill and Reese (1999):

Hard soils, soft rocks: N60 > 50250 kPa < su < 2500 kPa

Rocks: su ≥ 2500 kPa (qu ≥ 5000 kPa)

CV3301 - LEC (2008) Lecture 6 13

Toe Bearing Resistance – Cohesive Intermediate Geomaterial and Rock

( )

( )[ ] u5.05.00.5

t

51.0ut

u

ut

qtmtt q'

:evaluated becan condition joints' where andn orientatio random having joints with jointed is material If

q4830 q'

kPa, 500 q and )horizontalnearly and closed joints (all 100%RQD70%For q5.2q'

1.5, D/B and 100% RQDFor :(1999) Reese and NeillO'

++=

=

><<=

≥=

CV3301 - LEC (2008) Lecture 6 14


CV3301 - LEC (2008) Lecture 6 15


CV3301 - LEC (2008) Lecture 6 16

Toe Bearing Resistance – Noncohesive Intermediate Geomaterial

( )[ ]

tb

trtb

zD601

zD8.0

601t

'qB

mm 1200q' toq' reduce mm, 1200 BFor

'kPa 100N N where

'N59.0q'

:(1999) Reese and NeillO'

=>

σ=

σ=

CV3301 - LEC (2008) Lecture 6 17

A steel H-pile (At = 0.52m2) is driven to a layer of sandstone. The unconfined compressive strength of the sandstone is qu = 17 MPa. Estimate the net toe bearing capacity.

( )[ ]( )( )( )

kN36098 170004830052.0

q4830A'qA'P51.0

51.0utttt

==

==

CV3301 - LEC (2008) Lecture 6 18

Contact Areas At and As

In open-ended piles, the soil plug may be considered rigidly embedded if

D/B > 10 to 20 for claysD/B > 25 to 35 for sands

CV3301 - LEC (2008) Lecture 6 19

Review

What is the difference in bearing capacity of shallow foundations and toe bearing resistance of pile?How do you compute the toe bearing resistance for a pile in different soil types?

CV3301 - LEC (2008) Lecture 6 20

Side Friction

Effective Stress Analysis – β MethodTotal Stress Analysis – α Method

CV3301 - LEC (2008) Lecture 6 21

Side Friction – Effective Stress Analysis

⎥⎦

⎤⎢⎣

⎡φ⎟⎟

⎠

⎞⎜⎜⎝

⎛φφ

⎟⎟⎠

⎞⎜⎜⎝

⎛=β

ββσ=

⎥⎦

⎤⎢⎣

⎡φ⎟⎟

⎠

⎞⎜⎜⎝

⎛φφ

⎟⎟⎠

⎞⎜⎜⎝

⎛σ=

σσ

=φσ=

''

tanKKK i.e.

Method - '

''

tanKK'Kf

''K , tan'f

f

oo

z

f

ozos

z

xfxs

fs

σ’x

P

P+ΔP

CV3301 - LEC (2008) Lecture 6 22

Side Friction – Effective Stress Analysis (cont’d)

CV3301 - LEC (2008) Lecture 6 23


CV3301 - LEC (2008) Lecture 6 24

Displacement piles

Large-displacement piles include all solidpiles such as precast concrete piles, andsteel or concrete tubes closed at the lowerend by a driving shoe or a plug, i.e. cast-in-place piles.Small-displacement piles include rolled steelsections such as H-piles and open-endedtubular piles. However, these piles willeffectively become large displacement piles ifa soil plug forms.

CV3301 - LEC (2008) Lecture 6 25

Construction of Piles – Jetted piles

From www.fhwa.dot.gov

Jetting is the process of forcing water under pressure around and under the pile to lubricate and/or to displace the surrounding soil

CV3301 - LEC (2008) Lecture 6 26

Construction of Piles – Driven piles

CV3301 - LEC (2008) Lecture 6 27

Construction of Piles – Driven piles

CV3301 - LEC (2008) Lecture 6 28

Construction of Piles – Drilled Shaft

CV3301 - LEC (2008) Lecture 6 29

Construction of Piles – Drilled Shaft

CV3301 - LEC (2008) Lecture 6 30


Typical β values:

For sands: 0.25 ≤ β ≤ 1.20For gravels (> 50% gravel size): 0.25 ≤ β ≤ 3.00For gravelly sands ( 25 to 50% gravel size): 0.25 ≤ β ≤ 1.80For silts: 0.27 ≤ β ≤ 0.50For clays: 0.25 ≤ β ≤ 0.35

CV3301 - LEC (2008) Lecture 6 31

Example: A concrete pile is 25 m long and 305 mm x 305 mm in cross section. The pile is fully embedded in sand, for which γ = 17.5 kN/m3 and φ’ = 35o. The groundwater table is at the surface. Calculate the total side resistance of the pile for K = 1.3 and φf = 0.8φ’.

[ ]

( )( )( )

( )( )( ) kN2028kPa5.66m25m305.04 BLf4fA resistance side Total

kPa5.66kPa25.96691.0'f

kPa25.962m258.9m/kN5.17z'' Average

691.0)35)(8.0(tan3.1tanK''

tanKKK

sss

zs

3centrez

of

f

oo

====

==βσ=

=⎟⎠⎞

⎜⎝⎛−=γ=σ

==φ=⎥⎦

⎤⎢⎣

⎡φ⎟⎟

⎠

⎞⎜⎜⎝

⎛φφ

⎟⎟⎠

⎞⎜⎜⎝

⎛=β

CV3301 - LEC (2008) Lecture 6 32

Side Friction – Total Stress Analysis (α Method for insensitive clays, St < 4)

us sf α=

CV3301 - LEC (2008) Lecture 6 33

Side Friction – Total Stress Analysis (α Method) (cont’d)

0.5 :kPa 75 sFor kPa50

kPa25s0.5-1.0 :kPa 75 s kPa 25For

1.0 :kPa 25 sFor :(1974) API from is usedcommonly Most

u

uu

u

=α>

⎟⎟⎠

⎞⎜⎜⎝

⎛ −=α<<

=α<α

CV3301 - LEC (2008) Lecture 6 34

Side Friction – Total Stress Analysis (α Method) (cont’d)

For drilled shafts

CV3301 - LEC (2008) Lecture 6 35

Side Friction – Total Stress Analysis (α Method) (cont’d)For drilled shafts(O’Neill and Reese 1999): maximum fs = 260 kPa

See Example 14.4

For compression load

CV3301 - LEC (2008) Lecture 6 36

Underreamed Pile

CV3301 - LEC (2008) Lecture 6 37

Example: A driven pile in clay is shown in the figure below. The pile has a diameter of 406mm. Determine the total side resistance by the α method using API formulas.

( )( )( )( )

( )( )( )( )

( )( )( )( )

kN1.16395.12758.1818.181 PPPP

kN5.1275kPa1005.0m20406.0 sDLfAP

5.0kN8.181kPa3095.0m5406.0

sDLfAP

95.0kPa50

kPa25s5.01

kN8.181kPa3095.0m5406.0 sDLfAP

95.0kPa50

kPa25s5.01

3s2s1ss

u333s3s3s

3

u222s2s2s

u2

u111s1s1s

u1

=++=++=

=π=απ==

=α=π=

απ==

=⎟⎠⎞

⎜⎝⎛ −

−=α

=π=απ==

=⎟⎠⎞

⎜⎝⎛ −

−=α

CV3301 - LEC (2008) Lecture 6 38

Upward Load CapacityWhen a deep foundation is subjected to downward loads, it experience some elastic compression and, due to Poisson effect, a small increase in diameter. The opposite occurs when the pile is subjected to upward loading. For this reason, fsmay be conservatively reduced by 25% for design i.e.

Alternatively, a higher factor of safety is usually used when computing the upward loading capacity of the pile.

( ) supwards f75.0f =

CV3301 - LEC (2008) Lecture 6 39

Upward Load Capacity (cont’d)

( )( ) ( )

9BD7.0N clays, fissuredFor

9BD5.3N clays, unfissuredFor

F

BB4

NsP

:1999) Reese and Neill(O' as claysfor estimated bemay base enlarged toduecapacity Additional

b

bu

b

bu

2s

2bzDuu

aupward

≤=

≤=

−⎟⎠⎞

⎜⎝⎛ π

σ+=

Important: Neglect fs from bottom to 2Bbfrom bottom

See Tutorial Q11.1

α = 0

α = 02Bb

1.5m

CV3301 - LEC (2008) Lecture 6 40

Pile Groups

A single pile usually does not have enough capacityPiles are located with low degree of precision and can easily be 150 mm or more from the desired location. The eccentricity would generate unwanted moments and deflections in both pile and column.Multiple piles provide redundancy, and thus can support the structure even if one pile is defective.The lateral compression during pile driving is greater and therefore side friction capacity can be greater than that of a single pile.

CV3301 - LEC (2008) Lecture 6 41

Typical configurations of pile caps

CV3301 - LEC (2008) Lecture 6 42

Pile supported or pile-enhanced mats

CV3301 - LEC (2008) Lecture 6 43

CV3301 - LEC (2008) Lecture 6 44

Pile Group

CV3301 - LEC (2008) Lecture 6 45

Group Effects

Piles are usually installed in groups of three or more. If the piles are too close (< 2 to 2.5 B or 600 mm), there may not be enough room for positioning and alignment errors.

If the pile spacing, s, is too wide, pile cap will be large and expensive.

Typically, 2.5 < s/B < 3.0

CV3301 - LEC (2008) Lecture 6 46

Group Effects (cont’d)

In a pile group, there are interactions between the piles and the adjacent soil and

Pultg ≠ NPult

Therefore,

factor efficiency group where

NPP aag

=η

η=

CV3301 - LEC (2008) Lecture 6 47


η depends on several factors, including:The number, length, diameter, arrangement, and spacing of pilesThe load transfer mode (side friction vs end bearing)The construction procedures used to install the pilesThe sequence of pile installationThe soil typeThe elapsed time since the piles were drivenThe interaction, if any, between the pile cap and the soilThe direction of applied load

CV3301 - LEC (2008) Lecture 6 48


( ) ( )

piles of spacingcenter -to-center s diameter pile B

degrees)(in sB tan

rowper piles ofnumber n piles of rows ofnumber m where

nm90n1mm1-n-1

:1941)(Bolin Formula Labarre-Converse geometry, group pile gConsiderin

1-

==

⎟⎠⎞

⎜⎝⎛=θ

==

−+θ=η

CV3301 - LEC (2008) Lecture 6 49


( ) 1Bnm

B42nms2:failure group and failure pile individual gConsiderin

≤π

+−+=η

Bs)1n( +−

Bs)1m( +−

CV3301 - LEC (2008) Lecture 6 50

Pile Group Failure

Qs

Qt

1NP

QQ:failureBlock

glesin

st ≤+

=η

CV3301 - LEC (2008) Lecture 6 51


In loose sands, η ≥ 1 and reaches a peak at s/B ≈ 2. It also seems to increase with no. of piles in the group.In dense sands with 2 ≤ s/B ≤ 4, η is usually slightly greater than 1 so long as pile was installed without predrilling or jetting.Piles installed by predrilling or jetting, and drilled shafts have lower η, as low as 0.7.

Tests in sands suggest (O’Neill 1973):

CV3301 - LEC (2008) Lecture 6 52


Generally, η < 1 and decreases with no. of piles in the group.η can be as low as 0.5.η increases with time.

Tests in clays suggest :

CV3301 - LEC (2008) Lecture 6 53

Settlement

Most piles designed as covered so far will not have settlement greater than 25 mm –acceptable for nearly all structures.Therefore settlement computations are usually not needed.

CV3301 - LEC (2008) Lecture 6 54

Settlement (cont’d)

The structure is especially sensitive to settlements.The foundation has a large B and a large portion of the allowable capacity is due to toe bearing.One or more compressible strata are present, especially if these strata are below the toe.Downdrag loads might develop during the life of the structure.

However certain conditions can produce excessive settlement and warrants evaluation:

CV3301 - LEC (2008) Lecture 6 55

Load-Settlement Response( )

( )

( )( )

0.5 - 0.02 h (sand) 1.0 - (clay) 0.5 gfriction sidefor mm 10

bearing for toe B/10 resistance ultimate mobilize torequired settlement

resistancefriction -sideunit fresistance bearing unit toenet mobilizedq'

:where

1f

f

q'q'

:1999) Fellenius from (adapted follows as edapproximat bemay response settlement load The

u

ms

mt

h

us

ms

g

ut

mt

=====δ

=

=

≤⎟⎟⎠

⎞⎜⎜⎝

⎛δδ

=

⎟⎟⎠

⎞⎜⎜⎝

⎛δδ

=

CV3301 - LEC (2008) Lecture 6 56

Load-Settlement Response (cont’d)

concretefor MPa f'4700

steelfor GPa 200 foundation of elasticity of modulus E

foundation single a of area sectional-cross A 0.75D) (typically resistance soil of centroid depth to z

foundation ofn compressio elastic todue settlement :where

AEPz

:settlement apparent"" of sourceanother iswhich n,compressioelasticexperiencealso sfoundation Deep

c

c

e

ce

=

=====δ

=δ

See Example 14.7 in book

CV3301 - LEC (2008) Lecture 6 57

Load-Settlement Response for Drilled Shafts – O’Neill and Reese (1999) Method

CV3301 - LEC (2008) Lecture 6 58

Load-Settlement Response –O’Neill and Reese (1999) Method (cont’d)

See Example 14.8

CV3301 - LEC (2008) Lecture 6 59

Load-Settlement Response –Pile Group

Imaginary footing method(Equivalent raft method)

(See Tutorial Q11.3)

CV3301 - LEC (2008) Lecture 6 60

Downdrag Loads (Negative Skin Friction)

CV3301 - LEC (2008) Lecture 6 61

Downdrag Loads (cont’d)

Both β and αmethods may be used to compute negative skin friction

∑= snsnsn AfPload,friction Negative

CV3301 - LEC (2008) Lecture 6 62

Downdrag Loads – Design by CP4 (2003)

( )

stratum.clay lecompressibhighly in pilescapacity low involving cases specialfor used bemay 1.0although 0.67, typicallyon,mobilizati of degree theis where

PP)4(F

A'fP

capacity, structural Allowable

pile bearing-endfor L 1.0 pilefriction for L 0.6 plane neutral Depth to

purposes,design For

snc

sta

s

s

ζ

ζ+≥=

=

==

CV3301 - LEC (2008) Lecture 6 63

Downdrag Loads – Design by CP4 (2003) (cont’d)

2.5. to2.0between as taken bemay F where

PPF

P'PP

capacity, algeotechnic Allowable

snspt

a ζ+≥+

=

CV3301 - LEC (2008) Lecture 6 64

Downdrag Reduction Techniques

Coat the pile with bitumen. This method is very effective so long as the pile is not driven through an abrasive soil, such as sand, that might scrape off the bitumen coating.Drive the piles before placing the fill, wrap the exposed portions with lubricated polyethylene sheets or some other low-friction material and place the fill around the piles.Use a large diameter predrill hole, possibly filled with bentonite, thus reducing K.Use a pile tip larger in diameter than the pile, thus making a larger hole as the pile is driven.Drive an open-end steel pipe pile through the consolidating soils, remove the soil plug, then drive a smaller diameter load-bearing pile through the pipe and into the lower bearing strata. This isolates the inner pile from the downdrag loads.Accelerate the settlement using surcharge fills or other techniques, and then install the foundations after the settlement is complete.

CV3301 - LEC (2008) Lecture 6 65

Review

How do you compute side resistance using the αand β methods?How do you compute the load settlement behaviour of pile?How do you compute the uplift resistance of a pile?What is negative skin friction?How do you compute negative skin resistance?What methods can be used for reducing negative skin resistance?

deep foundations

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