Civil Engineering Reference
In-Depth Information
Fig. 11.2
Immediate settlement of thin clay layer.
The total immediate settlement at the corner of a rectangular foundation on an infinite layer is
(
)
pB
1
−
ν
2
I
p
ρ
i
=
E
The values of the coefficient I
p
(when
ν
=
0.5) are given in Fig.
11.2c
. To determine the settlement of a
point beneath the foundation the area is divided into rectangles that meet over the point (the same
procedure used when determining vertical stress increments by Steinbrenner's method). The summation
of the settlements of the corners of the rectangles gives the total settlement of the point considered.
This method can be extended to determine the immediate settlement of a clay layer which is at some
depth below the foundation. In Fig.
11.2b
the settlement of the lower layer (of thickness H
2
−
H
1
) is obtained
by first determining the settlement of a layer extending from below the foundation that is of thickness H
2
(using E
2
); from this value is subtracted the imaginary settlement of the layer H
1
(again using E
2
).
It should be noted that the settlement values obtained by this method are for a perfectly flexible foun-
dation. Usually the value of settlement at the centre of the foundation is evaluated and reduced by a
rigidity factor (generally taken as 0.8) to give a mean value of settlement that applies over the whole
foundation.
Example 11.1:
Rigid foundation
A reinforced concrete foundation, of dimensions 20 m
×
40 m exerts a uniform pressure
of 200 kPa on a semi-infinite saturated soil layer (E
=
50 MPa).
Determine the value of immediate settlement under the foundation using Table
11.2.
Solution:
L
B
=
40
20
=
2 0
.
(
)
pB
1
2
I
−
ν
200 20 0 75 1 0
50000
× ×
.
×
.
p
ρ
=
=
=
0 06
.
m
=
60
mm
i
E
