PILES

there are two categories of piles according to
the method of installation:
A. Driven Piles
1. timber, steel, precast concrete, piles formed by
driving tubes or shells and then casting concrete
2. driven piles cause displacement and disturbance
of adjacent soil
3. 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
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as driven tube withdrawn (slip form)
B. Shaft Construction
1. soil removed by boring or drilling to form a
shaft
2. concrete cast in a casing or without casing
– depends on soil conditions
3. tip can be expanded by underreaming
4. no disturbance to surrounding soil
Shaft Construction
Craig, 6th Ed.
e) bored pile (cast in-situ), f) underreamed bored pile
(cast in-situ)
Ultimate Load, Qf
Qf = Base Resistance + Shaft Resistance
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
For Sands
 Db 
q f  40N 
  400N (300N for non-plastic silts)
 B 
(use 13 q f for bored piles)
f s  2 N (use
1
2
f s 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
N = average N-Value over length of pile shaft
For Clays
q f  cu N c
f s  a cu
where cu = undrained shear strength of clay at base tip of pile
D
49
Nc = Skempton’s value of Nc for
B
9
4
c u = 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
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
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
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
Pile Group Shaft Area
8s + pB
s
B
s
 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
pB
s
s
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
s
2
πB
4s2  4sB 
4
s
B
πB2
4
s
s
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
E PG
  Q fgroup
 
n



 Q fpile



 
   100%


