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10) Laterally Loaded Vertical Piles

� Sources of Resistance

① Passive resistance against the side of pile cap. (Scouring or artificial

excavation can eliminate its effect.)

② Shearing resistance along bottom of pile cap and soil interface.

(Settlement of soil beneath cap may eliminate its effect.)

③ Moment and shear resistance of pile itself.

→ Use ③ for determining lateral capacity.

� 5 Potential Failure Modes

� Free headed piles

Lateral resistance of

the soil is exceeded.

Maximum moment

resistance of pile

is exceeded.

Plastic

Hinge

forming

No.1

No.2

No.3

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� Fixed(restrained) pile head

i) Ultimate Soil Capacity

�

⇒ Brom’s method

a) Cohesive soils

- Simplified distribution of the ultimate static resistance )( ultP

(Fig 17.8 & 17.9)

- Free head piles

min

( 1.5 0.5 )1.5

2.25 u

FV e B fD B f

Bs

+ += + +

9 u

FVf

Bs=

( ≡F Factor of safety=3.0, e=M/V)

- Restrained head piles

min 1.59 u

FVD B

Bs= +

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b) Cohesionless soils

'3ult v PP K Dσ= ⇒ triangular distribution in a uniform soil (Fig 17.10 &

17.11)

- Free head piles

)0.3()(

'5.0

min

3

min≈

+=

DeV

KBDF

pγ

- Restrained head piles

pBK

FVD

'5.1min

γ=

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ii) Ultimate structural capacity

⇒ Brom’s method

� Computing Mmax using the ultimate soil resistance distribution

a) Cohesive soils

- Free head piles

)5.05.1(max fBeVM ++=

- Restrained head piles

(Computed the moment immediately below the pile cap and at a depth of

fB +5.1 )

2

1 9 (1.5 0.5 ) 2.25 0u uM Bs f B f Bs g= + − ≥

fBDg −−= 5.1min

)5.05.1(2

12 fBVM +=

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b) Cohesionless soils

- Free head piles

M max )67.0( feV += pKB

FV82.0f

γ=⇐

- Restrained head piles

05.0 3

1 ≥−= pKBDVDM γ

VDM 67.02 =

1M : Just below the pile cap

2M : At a depth “f” below the ground surface

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11) Group Piles

i) Bearing capacity of group piles

� Differences in behavior between group piles & single pile

① Block failure potential (Ideally but not practically)

② Overlapping of displacement or stress fields of soils adjacent to piles

⇒ This may reduce or increase the load-bearing capacity of piles

(2.5~3.5)D

A A’

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③ The overlapping effect of pile installation (In particular, driven piles)

⇒

⇒

� Pile spacing in group piles

� Practically : Dd 5.2≥

Optimal spacing : DdD 5.30.3 ≤≤

� The group efficiency )/( )( ∑= individualgroupu QQη depends on several factors;

- The number, length, diameter, arrangement and spacing of the piles.

- The load transfer mode (skin friction vs. end bearing).

- The construction methods used to install the piles.

- The sequence of installation of the piles.

- The soil type.

- The elapsed time since the piles were driven.

- The interaction, if any, between the pile cap and soil.

- The direction of the applied load.

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� Bearing Capacity of Pile Groups

→ Conservative (based on ∑≤ individualgroup QQ )

a) True end bearing pile

b) All other cases

① Point bearing capacity

)(21)( individualpgroupp QnnQ =

② Frictional resistance

LPfQ gruopavgroups =)( ……………..(1)

)(2 gggroup BLP +=

or

pLfnnQ avgroups 21)( = ………….(2)

⇒ Take minimum from (1) and (2).

pileindividualgroup QnnQ 21=

(No interaction between piles)

Bg

Lg

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� Alternative way to get )(groupuQ for friction piles

0.1)(

)(≤=

∑ individualu

groupu

Q

Qη

Notes

1) Clay : a)

b)

c)

2) Sand :

3) Pile cap resting on soil contributes to the load bearing capacity, but its effect

is neglected for design purpose. → (Fig 11.46)

4) Conclusively, the method to determine bearing capacity of pile groups is not

well defined and conservative design rule is employed (group efficiency ≤

1.0).

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Fig 11.46 Variation of group efficiency with d/D (after Brand et al., 1972)

For pile design purpose, following comments can be drawn.

1. For driven group piles in sand with Dd 3≥ , )(ugQ may be taken to be uQΣ ,

which includes the frictional and the point bearing capacities of individual piles.

2. For bored group piles in sand at conventional spacings )3( Dd ≈ , )(ugQ may

be taken (2/3~3/4) times uQΣ (frictional and point bearing capacities of

individual piles).

* Piles in rock

Point bearing pile ⇒ ∑= individualgroup QQ in case of

center to center spacing mmD 300+>