Civil Engineering Reference
In-Depth Information
necessity for them is illustrated in Example
9.2,
which demonstrates the effect on q
u
of a small variation
in the value of
φ
′
.
The net ultimate bearing capacity is the increase in vertical pressure, above that of the original over-
burden pressure, that the soil can just carry before shear failure occurs.
The original overburden pressure is
γ
z and this term should be subtracted from the bearing capacity
equations, i.e. for a strip footing:
q
=
cN
+
γ
z N
(
− +
1
)
0 5
.
γ
BN
u net
c
q
γ
The safe bearing capacity is therefore the above expression divided by F plus the term
γ
z:
cN
+
γ
z N
(
− +
1
)
0 5
.
γ
BN
c
q
γ
Safe bearing capacity
=
+
γ
z
F
In the case of a footing founded in undrained clay, where
φ
u
=
0°, the net ultimate bearing capacity is,
of course, c
u
N
c
.
The safe bearing capacity notion is not used during design to Eurocode 7 where, as will be demonstrated
in Section
9.7,
conformity of the bearing resistance limit state is achieved by ensuring that the design
effect of the actions does not exceed the design bearing resistance.
9.5 The effect of groundwater on bearing capacity
9.5.1 Water table below the foundation level
If the water table is at a depth of not less than B below the foundation, the expression for net ultimate
bearing capacity is the one given above, but when the water table rises to a depth of less than B below
the foundation the expression becomes:
′
q
=
cN
+
γ
z N
(
− +
1
)
0 5
.
γ
BN
u net
c
q
γ
where
γ
=
unit weight of soil above groundwater level
γ
′
=
effective unit weight.
For cohesive soils
φ
′
is small and the term 0.5
γ
′
BN
γ
is of little account, and the value of the bearing
capacity is virtually unaffected by groundwater. With sands, however, the term cN
c
is zero and the term
0.5
γ
′
BN
γ
is about one half of 0.5
γ
BN
γ
, so that groundwater has a significant effect.
9.5.2 Water table above the foundation level
For this case Terzaghi's expressions are best written in the form:
q
=
cN
+ ′
σ
(
N
− +
1
)
0 5
.
γ
′
BN
u net
c
v
q
γ
where
σ
v
effective overburden pressure removed
.
From the expression it is seen that, in these circumstances, the bearing capacity of a cohesive soil can
be affected by groundwater.
When designing foundations to Eurocode 7, unless an adequate drainage system and maintenance plan
are ensured, the ground water table should be taken as the maximum possible level. This could of course
be the ground surface.
′ =
