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Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
111
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
Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
112
� 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= +
Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
113
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=
Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
114
Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
115
Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
116
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 +=
Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
117
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
Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
118
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’
Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
119
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.
Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
120
� 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
Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
121
� 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).
Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
122
Geotechnical Engineering
SNU Geotechnical and Geoenvironmental Engineering Lab.
123
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+>