01 Week 10 - Lecture Notes - A Slide Per Page -Gray

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CHAPTER 11:CHAPTER 11:

PILE FOUNDATIONSPILE FOUNDATIONS

PART A

BRAJA M. DAS BRAJA M. DAS



Contents…..

Introduce deep foundation

PP

Introduce various types of piles

Pile driving

How to estimate pile capacity?

End resistance in soil and rock, h a f t

Shaft resistance in soil

End End



Applications

Large Distributed Large Distributed Weight Weight

Very Large Concentrated Very Large Concentrated Weight Weight

ow ow Weight Weight

Soft to Soft to Firm Clay Firm Clay

Dense Sand

Stron Rock Stron Rock



Why do we require a pile?

a. Transfer load to a stronger strata: Top layers of soil are highly

compressible and too weak to support the load transmitted by thesuperstructure.

b. Transfer the structural load to the soil throu h frictional

resistance developed at soil-pile interface: The bedrock isnot encountered at reasonable depth below the ground surface

c. Carry horizontal loads: Lateral forces are relatively prominent

d. In presence of expansive and collapsible soils at the site

e. Foundations subjected to uplift forces: transmissiontowers, offshore platforms, basement mats below thewater table

f. To avoid the loss of bearing capacity due to erosion andscouring





Why do we require a pile?





Types of piles based on Materials -2► Concrete Piles:

Cast-in-situ piles Pre-cast piles

Advantages:

Usual length: 10 m – 45 mUsual Load: 7500 kN – 8500 kN


• Relatively cheap.• It can be easily combined with concrete superstructure• Corrosion resistant• It can bear hard driving

Disadvantages:

• Difficult to transport• Difficult to achieve desired cutoff



Types of piles based on Materials -3► Timber Piles:

Advantages:• Low const and renewable resource• Easy to handle• Can be joined together easily


•

Disadvantages:• Limited bearing capacity• Can easily be damaged during driving• The piles will rot above the water table

► Composite Piles:

combination of different materials in the same pile



Types of piles based on their function

► End bearing piles: Transfer their load to a firm stratum at the toe of the pile

► Friction piles: Transfer their load to the ground through skin friction

► Combination of friction and end bearing piles



Types of piles based on their installation

► Displacement piles: Driven concrete, Timber, or Steel piles

► Non-displacement piles: Bored Cast-in-situ piles

Displacement pile Non-displacement pile

Di l t ilDi l t il



Displacement pilesDisplacement piles In loose cohesionless soils

Densifies the soil upto a distance of 3.5 times the pile diameter (3.5D) whichincreases the soil’s resistance to shearing

The friction angle varies from the pile surface to the limit of compacted soil

In dense cohesionless soils

The dilatancy effect decreases the friction angle within the zone of influence of displacement pile ( 3.5D approx.).

Displacement piles are not effective in dense sands due to above reason.

In cohesive soils

Soil is remolded near the displacement piles (2.0 D approx.) leading to a.

Pore-pressure is generated during installation causing lower effective stress andconsequently lower shearing resistance.

Excess pore-pressure dissipates over the time and soil regains its strength.

Example: Driven concrete piles, Timber or Steel piles

NN di l t ildi l t il



NonNon--displacement pilesdisplacement piles

Due to no displacement during installation, there is no heave in theground.

Cast in-situ piles may be cased or uncased (by removing casing as .

if economical with their reduced diameter.

Enlarged bottom ends (three times pile diameter) may be providedin cohesive soils leading to much larger point bearing capacity.

Soil on the sides may soften due to contact with wet concrete orduring boring itself. This may lead to loss of its shear strength.

Concreting under water may be challenging and may resulting inwaisting or necking of concrete in squeezing ground.

Example: Bored cast in-situ or pre-cast piles



Pile Driving, Then and NowPile Driving, Then and Now



Pile HammersPile Hammers



Pile HammersPile Hammers



TerminologyTerminology







Point Bearing Capacity, QPoint Bearing Capacity, Qpp►► For shallow foundation:For shallow foundation:

qquu = c’ N= c’ Ncc + q N+ q Nqq + 0.5+ 0.5 γ γ B NB Nγ γ -- stripstripqquu = 1.3 c’ N= 1.3 c’ Ncc + q N+ q Nqq + 0.4+ 0.4 γ γ B NB Nγ γ -- squaresquare

= 1.3 c’ N= 1.3 c’ N + N+ N + 0.3+ 0.3 B NB N -- circularcircular

►► Similarly, for pile tip:Similarly, for pile tip:qquu = c’ N= c’ Ncc* + q N* + q Nqq* +* + γ γ D ND Nγ γ **

►►

Since D of pile is small:Since D of pile is small:

qqpp = c’ N= c’ Ncc* + q N* + q Nqq** -- this isthis is pressurepressureQQpp = A = A pp qqpp = A = A pp (c’ N(c’ Ncc* + q’ N* + q’ Nqq*)*) – – this isthis is loadload

Note that q’ is effective vertical stress at the level of pile tipNote that q’ is effective vertical stress at the level of pile tipNNcc* and N* and Nqq* are the bearing capacity factors for piles* are the bearing capacity factors for piles

A A pp = area of pile tip (may include a soil plug)= area of pile tip (may include a soil plug)

FULL MOBILISATION OF QFULL MOBILISATION OF Qpp REQUIRES VERTICAL DISPLACEMENT OFREQUIRES VERTICAL DISPLACEMENT OF1010--25% x D25% x D



Frictional Resistance, QFrictional Resistance, Qss

►►QQss == ΣΣ (p(p ∆∆L f)L f)

p = per me er o p ep = per me er o p e(may include soil plug)(may include soil plug)

∆∆L = incremental pile lengthL = incremental pile length

f = unit friction resistancef = unit friction resistance

n h kP MPn h kP MP



Width, Diameter, Perimeter, AreaWidth, Diameter, Perimeter, Area



Point Resistance, QPoint Resistance, QPP

Meyerhof’s Method for QMeyerhof’s Method for Qpp

Vesic’s Vesic’s Method for QMethod for Qpp

Coyle & Costello’s Method for QCoyle & Costello’s Method for Qpp

QQpp of piles resting on rock of piles resting on rock

Meyerhof’s Method for QMeyerhof’s Method for Q



Meyerhof s Method for QMeyerhof s Method for QppFOR GRANULAR SOIL:FOR GRANULAR SOIL:

►► c’ = 0c’ = 0►► QQpp = A = A pp qqpp = A = A pp q’ Nq’ Nqq**

►►

Limit:Limit: pp ≤≤ A A pp qqll►► qql = 0.5 p= 0.5 paa NNqq* tan* tan φφ’ ’

qqll = 0.4 p= 0.4 paa NN6060 L/DL/D

≤≤ 4 p4 paa NN6060

ppaa = 100 kPa (air pressure)= 100 kPa (air pressure)

WHEREFROM ???

φφφφφφφφ’ ’ ==Friction angleFriction angle, A , A pp == Pile tip areaPile tip area,, NNqq* =* = Bearing capacity factorBearing capacity factor

qqpp = Ultimate tip resistance,= Ultimate tip resistance, q’ q’ = Effective vertical stress at the pile tip= Effective vertical stress at the pile tip

DD = Width or diameter of the pile= Width or diameter of the pile

NN6060 = average SPT (between 10D above and 4D below the= average SPT (between 10D above and 4D below thepile point)pile point)



Depth for SPT CalculationDepth for SPT Calculation



44 NN = 4 . 100 . 30= 4 . 100 . 30 ≈≈ 1200012000

ql

= 0.4 pa

N60

L D ≤ 4 pa

N60



The mobilization of shear strength also depends on theextent of the pile’s penetration into the bearing layer. Ingranular soils, the depth ratio at which the maximumstrength is mobilized is called the critical depth ratio(Lb /D)cr , of about 15.

L = Lb for homogeneous soils (L = Embedment length of thepile)



Critical Depth: A Fallacy?Critical Depth: A Fallacy?

►► Many texts suggest the existence of a critical depth, belowMany texts suggest the existence of a critical depth, belowwhich the shaft and toe resistances would be constant.which the shaft and toe resistances would be constant.

►► Fellenius & Altaee (1995) showed that the aboveFellenius & Altaee (1995) showed that the aboveassumption is wrong and may be based on incorrectassumption is wrong and may be based on incorrectinterpretation of field data.interpretation of field data.

►►

However, along very long offshore piles, the phenomenonHowever, along very long offshore piles, the phenomenonmay occur.may occur.

,,exists. But please be aware that you mayexists. But please be aware that you mayhave to discard this assumption in thehave to discard this assumption in the

real world!real world!

Meyerhof’s Method for QMeyerhof’s Method for Qpp



Meyerhof s Method for QMeyerhof s Method for QppFOR clay SOILFOR clay SOIL (φ(φ’ =0)’ =0)::

►► QQpp = A = A pp ccuu NNcc*=9c*=9cuu A A pp

A A pp == Pile tip areaPile tip area,, NNcc* =* = Bearing capacity factorBearing capacity factor

CCuu = undrained cohesion of the soil below the tip of the pile= undrained cohesion of the soil below the tip of the pile



ExampleExample --11

►► A concrete pile is 14 m long and 400 mm x A concrete pile is 14 m long and 400 mm x400 mm in cross section. The pile is driven400 mm in cross section. The pile is driven

..φφ’= 30’= 30°°. Calculate Q. Calculate Qp.p. using Meyorhofusing Meyorhofmethodmethod

Using Meyerhof’s methodUsing Meyerhof’s method



►►φφ’ = 30’ = 30°°,,

►►NNqq* =57* =57►►D=400 mm=0.4 m,D=400 mm=0.4 m, A App =0.4x0.4 ==0.4x0.4 =0.16 m0.16 m22

►►L=14 m ’ =?L=14 m ’ =?

γ γγ γγ γγ γ =1.76 t/m=1.76 t/m33 =1.76 x 1000 x9.8/1000 kN/m=1.76 x 1000 x9.8/1000 kN/m33

=17.25 kN/m=17.25 kN/m33

q’q’ = 17.25 x 14 == 17.25 x 14 =241.5 kN/m241.5 kN/m22

►►QQpp

= A = A pp

qqpp

= A = A pp

q’ Nq’ Nqq**

== 2202.5 kN2202.5 kN

►►Limit: QLimit: Q ≤≤ A A qq

►►qql = 0.5 p= 0.5 paa NNqq* tan* tan φφ’ ’

= 0.5x100x57xtan30= 0.5x100x57xtan30

=1645.5 kN/m=1645.5 kN/m22

►►QQpp max = 0.16 x 1645.5 =max = 0.16 x 1645.5 =263.3 kN263.3 kN

==




Rigidity Index (Rigidity Index (IIrr))

►► Rigidity Index =Rigidity Index = IIrr = G= Gss / (c’+ q’ tan / (c’+ q’ tan φφ’)’)c’ = effective cohesion, q’ = effective vertical stress at the pilec’ = effective cohesion, q’ = effective vertical stress at the pile

’’

QQpp

,,

►► GGss = E= Ess / 2 (1 + / 2 (1 + µµss)) = soil shear modulus= soil shear modulus

Poisson’s ratio of soil,Poisson’s ratio of soil, µµss = 0.1 + 0.3(= 0.1 + 0.3(φφ’’ – – 25)/2025)/20 forfor ((2525

00

<<φφ’ ,45’ ,45

00

))

Young’s modulus of soil, Young’s modulus of soil, EEss = m P= m Paa

PPaa = atmospheric pressure = 100 kN/m= atmospheric pressure = 100 kN/m22

m = 100 to 200 (loose soil)m = 100 to 200 (loose soil)= 200 to 500 (medium dense soil)= 200 to 500 (medium dense soil)= 500 to 1000 (dense soil)= 500 to 1000 (dense soil)




Rigidity Index (Rigidity Index (IIrr))

►► Reduced rigidity index,Reduced rigidity index, IIrrrr = I= Irr / (1 + I/ (1 + Irr ∆∆∆∆∆∆∆∆))

withwith ∆∆ = average volumetric strain in the plastic zone below the= average volumetric strain in the plastic zone below theile ti .ile ti .

QQpp

►► For dense sand and saturated clayFor dense sand and saturated clay ∆∆ ≈≈ 00

►► ∆∆ = 0.005(1= 0.005(1-- ((φφ’’ – – 25)/20)q’/P25)/20)q’/Paa

q’ = effective vertical pressure at the pile tip,q’ = effective vertical pressure at the pile tip, φφ’ = effective’ = effective

friction angle, Pfriction angle, Paa = atmospheric pressure = 100 kpa= atmospheric pressure = 100 kpa

►► Typical values of ITypical values of Irr

SandSand 7070--150150Silts & Clays (drained)Silts & Clays (drained) 5050--100100Clays (undrained)Clays (undrained) 100100--200200

►► Prediction using CPT’s friction ratio FPrediction using CPT’s friction ratio Frr = f = f cc /q /qcc

IIrr = 300 / F= 300 / Frr (%)(%) – – mechanical conemechanical coneII = 170 F= 170 F %% – – electric coneelectric cone

Friction ratioFriction ratio



CPT dataCPT data

Friction ratio Fr =Friction ratio Fr = f f cc /q /qtt

Vesic’sVesic’s Method for QMethod for Qpp



Vesic s Vesic s Method for QMethod for Qpp

►►

QQpp = A = A pp qqpp = A = A pp (c’ N(c’ Ncc* + q’ N* + q’ Nqq*)*)= A = A pp (c’ N(c’ Ncc* +* + σσoo’ N’ Nσσ*)*)

’’oo

point location =point location = (1 + 2 K (1 + 2 K oo) q’/3) q’/3

►►K K oo = earth pressure coeff. at rest == earth pressure coeff. at rest =11 –– sinsin φφ’ ’ ►►ThusThus NNσσ* = 3 N* = 3 Nqq*/ (1 + 2 K */ (1 + 2 K oo))

►►NNcc* and N* and Nσσ* for various values of reduced* for various values of reducedrigidity index Irigidity index Irrrr andand φφ’ ’ and given in Tableand given in Table

11.711.7

Vesic’sVesic’s Method for QMethod for Qpp



Vesic s Vesic s Method for QMethod for Qpp




pp

FOR SATURATED CLAY SOILFOR SATURATED CLAY SOIL (φ(φ(φ(φ(φ(φ(φ(φ’ =0)’ =0)::::::::

QQpp = A= App qqpp = A= App ccuu NNcc**

A A pp == Pile tip areaPile tip area,,NNcc* =* = Bearing capacity factorBearing capacity factor

CCuu = undrained cohesion of the soil= undrained cohesion of the soilbelow the tip of the pilebelow the tip of the pile

12)1(ln3

*

+++= rr c I N

The variation of NThe variation of Ncc* with* with

IIrrrr forfor φφφφφφφφ = 0 condition= 0 conditionare given in Table 11.8are given in Table 11.8

C l & C t ll ’ M th d f QC l & C t ll ’ M th d f Q



Coyle & Costello’s Method for QCoyle & Costello’s Method for Qpp

►► FOR SAND ONLY FOR SAND ONLY::

pp == pp qqpp == pp qq qq

A p

= Pile tip area,Nq* = Bearing capacity factorq’ = Effective vertical stress at the pile tipD = Pile diameterL = Pile embedment de th

QQpp from SPT and CPTfrom SPT and CPT



pp

►►For homogeneous granular soilFor homogeneous granular soil

►►QQpp = A = A pp qqpp

qq = (0.4 P= (0.4 Paa NN6060) L/D) L/D ≤≤ 4 P4 Paa NN6060 (kPa)(kPa)

NN6060 is the weighted SPT average in the vicinity of the pile tipis the weighted SPT average in the vicinity of the pile tip(about 10D above and 4D below the pile point)(about 10D above and 4D below the pile point)

►►For nonplastic silt:For nonplastic silt:qqpp = 300 N (kPa)= 300 N (kPa)

►►For granular soil (Meyerhof, 1956):For granular soil (Meyerhof, 1956):

qqpp

= q= qcc

qqcc = cone penetration resistance at the pile tip= cone penetration resistance at the pile tip

QQpp of piles resting on rock of piles resting on rock



pp

►►QQpp = A = A pp qqpp

qqpp = q= quu (N(Nφφ +1)+1)

NNφφ = tan= tan22

(45 +(45 + φφ’ ’ /2) /2)qquu = unconfined compressive strength of rock = unconfined compressive strength of rock

φφ’ = Effective friction angle’ = Effective friction angle

To take theTo take the scale effect, slip linesscale effect, slip lines of rock specimens tested inof rock specimens tested inthe laboratory for unconfined compressive strength:the laboratory for unconfined compressive strength:

qquu = q= qu(lab)u(lab) /5/5



Shaft Resistance, QShaft Resistance, Qss

ss

BROM’S METHODBROM’S METHOD

COYLE & COSTELLO’S METHODCOYLE & COSTELLO’S METHOD

QQss from SPTfrom SPT

QQss from CPTfrom CPT

ss ALPHA METHOD ALPHA METHOD

BETA METHODBETA METHOD

LAMBDA METHODLAMBDA METHOD

Shaft Resistance (QShaft Resistance (Qss) in Sand) in Sand



BROM’S METHODBROM’S METHOD

►► QQss == ΣΣΣΣΣΣΣΣ (p(p ∆∆∆∆∆∆∆∆L f)L f)

f = Kf = K σσσσσσσσoo’ tan’ tan δδδδδδδδ’’ from ground surface to L’ from ground surface to L’

’’ ==

Bored or jetted piles:Bored or jetted piles: KK ≅≅ K K oo = 1= 1 – – sinsin φφ’ ’

Low displacement piles:Low displacement piles: K = K K = K oo to 1.4 K to 1.4 K ooLarge displacement piles:Large displacement piles: K = K K = K oo to 1.8 K to 1.8 K oo

δδ’ = 0.5’ = 0.5 φφ’ to 0.8’ to 0.8 φφ’ ’

(can use 0.67(can use 0.67 φφ’)’)

MAX

ionconsideratunderdepthat the

stressverticalEffective'

anglefrictioninterfacepile-soil'

resistanceFrictional

incrementlenghtPileLpiletheof Perimeter

0 =

=

=

=∆

=

σ

δ

f

p



ExampleExample 22Qs =Qs = ΣΣΣΣΣΣΣΣ (p(p ∆∆∆∆∆∆∆∆L f)L f)



ExampleExample --22Qs =Qs = ΣΣΣΣΣΣΣΣ (p(p ∆∆∆∆∆∆∆∆L f)L f)

f = Kf = K σσσσσσσσo’ tano’ tan δδδδδδδδ’’ from ground surface to L’ , f maximum at L’ = 15 Dfrom ground surface to L’ , f maximum at L’ = 15 D

For concrete pile:δ’ = 0.75 φ’ = 0.75 x 300 = 22.50 ) / ( 2mkN f

L’ = 15D = 15 x 0.4 = 6.0 m

K = (1.0 + 2.0)/2 = 1.5

γ = 1.76 x 1000 x 9.8/1000 =17.25 kN/m3

At 6 m depth,f max =1.5 x (17.25 x 6) x tan 22.50 = 64.31 kN/m2

2

max / 31.64 mkN f =

Perimeter of the pile, p = (0.4x4) = 1.6 m

Qs =Qs = ΣΣΣΣΣΣΣΣ (p(p ∆∆∆∆∆∆∆∆L f) =L f) = p [p [ΣΣΣΣΣΣΣΣ ∆∆∆∆L f] = 1.6[(0.5 x 6.0 x 64.31) + (8.0 x 64.31)] = 1131.9 kN

Qp =Qp = 263.3263.3 kNkNFrom Example -1

ExampleExample --22



ExampleExample --22

QsQs = 1131.9 kN Qp =Qp = 263.3263.3 kNkN

s pu

... ===

kN FS

QQ u

all 8.3484

2.1395===

COYLE & COSTELLO’S METHODCOYLE & COSTELLO’S METHODShaft Resistance (QShaft Resistance (Qss) in Sand) in Sand



COYLE & COSTELLO S METHODCOYLE & COSTELLO S METHOD



QQ from CPTfrom CPT



QQss from CPTfrom CPT

►►Schmertmann’s method for sandSchmertmann’s method for sand

►►

ss == pp►►f =f = αα’ f ’ f cc►►αα’ varies with depth depending on pile types’ varies with depth depending on pile types

and cone typesand cone types

For electrical cone

testCPTfrom

friction)(sleeveresistanceFrictional


incrementlenghtPileL

piletheof Perimeter

=

=

=∆

=

c f

f

p





Shaft Resistance (QShaft Resistance (Q ) in Clay) in Clay



Shaft Resistance (QShaft Resistance (Qss) in Clay) in Clay

ALPHA METHOD ALPHA METHOD►► f =f = αα cc

αα = adhesion factor= adhesion factor= reduction factor= reduction factor

►►QQss == ΣΣ (f. p .(f. p . ∆∆L)L)

== ΣΣ ((αα ccu.u.. p .. p . ∆∆L)L)

Note:Note:Sladen (1992) suggestedSladen (1992) suggestedαα = C (= C (σσoo’/c’/cuu))

0.450.45

C = 0.4 to 0.5C = 0.4 to 0.5 – – boredboredC > 0.5C > 0.5 -- drivendriven depthover the

stressverticaleffectiveAverage'

strengthshearUndrained


piletheof Perimeter

0 =

=

=∆

=

σ

uc

p




BETA METHODBETA METHOD

’ ’


==oo

►►ββ = K tan= K tan φφR R ’ ’

►►K = 1K = 1 – – sinsin φφR R ’ for NCC’ for NCC

►►K = 1K = 1 – – sinsin φφR R ’’ √√(OCR)(OCR)

►► == ΣΣΣΣΣΣΣΣ ∆∆∆∆∆∆∆∆L f L f

ionconsideratof depthat the

stressverticalEffective'

clayremouldedof glefrictionanDrained'

tcoefficienpressureEarthK


piletheof Perimeter

0 =

=

=

=∆

=

σ

φ R

p





►► LAMBDA METHODLAMBDA METHOD

avav uu

►► = mean effective vertical stress for the= mean effective vertical stress for theembedded pile lengthembedded pile length

►►CCuu

= mean undrained shear strength over the pile= mean undrained shear strength over the pilelengthlength

►► λλ is from Table 11.9 or this chartis from Table 11.9 or this chart

►►QQss == p L f p L f avav

Lambda Method in Stratified SoilLambda Method in Stratified Soil








= (A1 + A2 + A3 + …..)/L

QQss of Clay from CPTof Clay from CPT



QQss yy

►►Schmertmann methodSchmertmann method

►► == αα cc

►►QQss == ΣΣ (p(p ∆∆L f)L f)

piletheof Perimeter

=

= p

testCPTfrom

friction)(sleeveresistanceFrictional


=

=

c f

f

ENDEND



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01 Week 10 - Lecture Notes - A Slide Per Page -Gray