10) laterally loaded vertical pilesocw.snu.ac.kr/sites/default/files/note/466.pdf10) laterally...
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Geotechnical Engineering
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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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Geotechnical Engineering
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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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Geotechnical Engineering
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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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Geotechnical Engineering
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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+>