Civil Engineering Reference
In-Depth Information
σ
∆
′
= −
15
285
=
v
C
=
1 0 0 5
.
−
.
1
0 5
.
×
0 97
.
1
p
Assume that C
2
=
1.0, then:
ρ
=
. . . mm
Total settlement of centre of foundation
=
25 mm
0 97 285 0 09
×
×
=
24 9
The plate loading test
The results from a plate loading test (see Section
6.6.5
) can be used to predict the average settlement of
a proposed foundation on granular soil. The test should be carried out at the proposed foundation level
and the soil tested must be relatively homogeneous for some depth.
If
ρ
1
is the settlement of the test footing under a certain value of bearing pressure, then the average
settlement of the foundation,
ρ
, under the same value of bearing pressure can be obtained by the empiri-
cal relationship proposed by Terzaghi and Peck (
1948)
:
2
2B
B B
ρ
=
ρ
1
+
1
where
B
1
=
width or diameter of test footing
B
=
width or diameter of proposed foundation.
One aspect of using the results from a plate loading test for settlement predictions is that it is important
to know the position of the groundwater level.
It may be that the bulb of pressure from the test footing is partly or completely above the groundwater
level whereas, when the foundation is constructed, the groundwater level will be significantly within the
bulb of pressure. Such a situation could lead to actual settlement values as much as twice the values
predicted by the formula.
11.3 Consolidation settlement
This effect occurs in clays where the value of permeability prevents the initial excess pore water pressures
from draining away immediately. The design loading used to calculate consolidation settlement must be
consistent with this effect.
A large wheel load rolling along a roadway resting on a clay will cause an immediate settlement that is
in theory completely recoverable once the wheel has passed, but if the same load is applied permanently
there will in addition be consolidation. Judgement is necessary in deciding what portion of the superim-
posed loading carried by a structure will be sustained long enough to cause consolidation, and this involves
a quite different procedure from that used in a bearing capacity analysis, which must allow for total dead
and superimposed loadings.
11.3.1 One-dimensional consolidation
The pore water in a saturated clay will commence to drain away soon after immediate settlement has
taken place: the removal of this water leading to the volume change is known as consolidation (Fig.
11.1b
).
