PILES

1
PILES

• there are two categories of piles according to
  the method of installation

• A. Driven Piles
• timber, steel, precast concrete, piles formed by
  driving tubes or shells and then casting concrete
• driven piles cause displacement and disturbance
  of adjacent soil
• Driving can be done by repeatedly raising and
  dropping a weight on the pile head or anvil or in
  favourable conditions vibration can be used

2
Driven Piles
Craig, 6th Ed.
a) timber or precast reinforced concrete, b)
steel H pile, c) precast concrete shell pile, d)
concrete pile cast as driven tube withdrawn (slip
form)
3

• B. Shaft Construction
• soil removed by boring or drilling to form a
  shaft
• concrete cast in a casing or without casing
  depends on soil conditions
• tip can be expanded by underreaming
• no disturbance to surrounding soil

4
Shaft Construction
Craig, 6th Ed.
e) bored pile (cast in-situ), f) underreamed
bored pile (cast in-situ)
5
Qf Base Resistance Shaft Resistance
Ultimate Load, Qf
area of shaft x frictional resistance of shaft
area of pile tip x pressure at tip
Qf qfAb fsAs
where qf ultimate bearing capacity of soil at
base tip of pile Ab cross-sectional area
of pile at base tip fs skin friction
between pile and soil As surface area of
pile shaft
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For Sands
(300N for non-plastic silts)
(use for bored piles)
(use for bored piles and H piles)
where N N-Value of soil at pile tip Db
length of pile (embedment) B diameter
(width) of pile average N-Value over
length of pile shaft
7
For Clays
where cu undrained shear strength of clay at
base tip of pile Nc Skemptons value of
Nc for average undrained shear
strength of undisturbed clay over the pile
length a adhesion factor (0.3 1.0)
depends on type of clay, installation method
and pile material
9
4
8
Negative Skin Friction

• a pile can be pulled down
• a) if the fill surrounding the pile is settling
• b) if fill is placed over a clay layer and
  causes
• it to consolidate

9
Pile Groups

• piles are rarely placed in isolation
• it is much more common to place an array of
  piles and place a pile cap over top

10
Pile Group Criteria

• piles in a pile group are assumed to have all the
  same dimensions and properties
• in order to act as a pile group the spacing
  between piles should be between 2B and 4B where B
  is the diameter of the piles

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Pile Group Shaft Area

• for a single pile, As is the perimeter of the
  pile multiplied by its length
• for a pile group, As is the perimeter of the pile
  group multiplied by the length

B
s
8s pB
s
pB
s
s
12
Pile Group Tip Area

• for a single pile, Ab is the area of the pile tip
• for a pile group, Ab is the area of the entire
  space enclosed by pile group perimeter

B
s
s
s
s
13
Pile Group Efficiency

• If the ultimate load of a pile group of n piles
  is Qfgroup and the ultimate load of a single pile
  of the same dimensions as those in the pile
  group has an ultimate load, Qfpile, then the pile
  group efficiency, EPG is the average ultimate
  load per pile of the pile group divided by the
  single pile ultimate load