Civil Engineering Reference
In-Depth Information
The method is based on two main assumptions:
(i) the greatest vertical strain in the soil beneath the centre of a loaded foundation of width B occurs at
depth B/2 below a square foundation and at depth of B below a long foundation;
(ii) significant stresses caused by the foundation loading can be regarded as insignificant at depths greater
than z
=
2.0B for a square footing and
=
4.0B for a strip footing.
The method involves the use of a vertical strain influence factor, I
z
, whose value varies with depth. Values
of I
z
, for a net foundation pressure increase,
Δ
p, equal to the effective overburden pressure at depth B/2,
are shown in Fig.
11.4.
The procedure consists of dividing the sand below the footing into n layers, of thicknesses
Δ
z
1
,
Δ
z
2
,
Δ
z3
. . .
Δ
z
n
. If soil conditions permit it is simpler if the layers can be made of equal thickness,
Δ
z. The
vertical strain of a layer is taken as equal to the increase in vertical stress at the centre of the layer, i.e.
Δ
p multiplied by I
z
, which is then divided by the product of C
r
and a factor x. Hence:
n
I
xC
∑
z
ρ
=
C C p
∆
∆
z
1
2
1
r
1
where
x
=
2.5 for a square footing and 3.5 for a long footing
I
z
=
the strain influence factor, valued for each layer at its centre, and obtained from a diagram similar
to Fig.
11.4
but redrawn to correspond to the foundation loading
C
=
a correction factor for the depth of the foundation
1
σ
v
p
′
=
.
−
1 0
0 5
.
(
=
1 0
.
for a surface footing
)
∆
C
2
=
a correction factor for creep
=
1
+
0.2 log
10
10t (t
=
time in years after the application of foundation loading for which the settle-
ment value is required).
