CALCULATION OF PILE
CAPACITY - EXAMPLE
Data:
i) A
Bore log furnishing the properties of the soil is given in Table-4
ii)
The pile founding level is considered at 20.0m depth
from the ground level.
iii)
The pile cut off level is at 3.5m below the ground
level.
iv)
The diameter of the pile is 550 mm.
v)
Concrete grade used : M20
11.1 PILE CAPACITY BASED ON SOIL SUPPORT
The
properties of the soil at depths are as under :
Depth in 'm'
|
'C' in kg/cm² |
'f'
in degree
|
3.5 to 5.0
|
0.50
|
28
|
5.0 to 10.0
|
0.50
|
31
|
10.0 to 16.0
|
0.0
|
34
|
16.0 to 19.0
|
0.5
|
29
|
19.0 to 25.0
|
0.0
|
36
|
Since the pile founding level is at
20.0m depth, f value of 36° is
considered to calculate the point resistance at the pile tip.
From the formula 4.1
Ppu = Ap (PD Nq + 0.5 g D
Ng)
For
f = 36°, Nq = 60
from fig.1 and Ng = 56.3 from Table-1
D = 0.55 m
L = 16.5 m
g
= 1.0 t/m3
For
calculation of maximum effective overburden pressure PD, fifteen
times diameter is considered from pile cut off level as all the soil layers are
silty soil with angle of internal friction (f)
more than 280.
PD = 15 x D x g = 15 x 0.55 x 1.0 = 8.25 t/m²
Ppu = p
x 0.552/4 [8.25 x 60 + ½ x 0.55 x 1.0 x 56.3] = 120.0 t
The frictional resistance of the
soil is calculated using the formula 4.10
K = 0.70, d
= 3/4 f
Between depths 3.5 and 5.0 m
Psu1 = [0.70
x (4.25 -3.5) x tan 21.0 + 0.5 x 5.0] p x
0.55 x 1.5 = 7.0 t
Between depths 5.0 and 10.0 m
Psu2 = [0.70
x (7.5 -3.5) x tan 23.25 + 0.5 x 5.0] p
x 0.55 x 5.0 = 32.0 t
Between
depths 10.0 and 16.0 m
Psu3 = [0.70
x 8.25 x tan 25.5 + 0] p x
0.55 x 6.0 = 28.6 t
Between
depths 16.0 and 19.0 m
Psu4 = [0.70
x 8.25 x tan 21.75 + 0.5 x 5.0] p x
0.55 x 3.0 = 24.9 t
Between
depths 19.0 and 20.0 m
Psu5 = [0.70
x 8.25 x tan 27] p x 0.55 x 1.0 = 5.1 t
Psu = Psu1 + Psu2 + Psu3 + Psu4 + Psu5
=
7.0 + 32.0 + 28.6 + 24.9 + 5.1 =
97.6 t
Wp
= p/4
x 0.552 x 16.5 x 2.5 =
9.8 t
Ultimate
Pile capacity Pu = Ppu + Psu - Wp =
120.0 + 97.6 - 9.8 = 207.8 t
Safe load on pile Pall = 207.8/2.5 = 83.1 t
11.2 STRUCTURAL CAPACITY OF PILE
Considering
concreting is done using tremie,
Allowable
compressive stress in concrete sc=
0.33 x 20 = 6.6 N/mm²
As
the pile is wholely embedded in good soil, pile is designed as short column.
Allowable
load on pile = Ap sc -Wp = p/4 x 5502 x
6.6 x10-4 - 9.8 = 147.0 t
11.3 The safe load on the
pile is least of the values obtained in 11.1 and 11.2 which is 83.1 t.