Civil Engineering Reference
In-Depth Information
strength for this is the critical state strength and the factors applied are factors of safety.
Normally engineers do not worry about relatively small ground movements near safe
slopes. Foundations, however, are designed to a serviceability limit state in which set-
tlements are smaller than those which might lead to damage of the supported structure.
It is still necessary of course to check that the foundation loads do not come close to the
ultimate bearing capacity but normally settlement criteria control the design of a foun-
dation. The loads are limited to the allowable bearing capacity shown in Fig. 22.3(b)
which cause allowable settlements
a
.
Two methods for design for serviceability limit states were described in Chapter 18.
In one method a load factor is applied to a the bearing capacity to bring the design
to a point where settlements are small. In the other method settlements are related to
bearing pressures through soil stiffness and this will be covered later in this chapter.
The allowable net bearing pressure
q
a
is related to the net bearing capacity
q
c
by
ρ
q
a
=
L
f
q
c
(22.4)
where
L
f
is a load factor. (Notice that the range of a load factor is from 0 to 1.0 while a
factor of safety is
≥
1.0.) The question is which strength, drained or undrained, peak,
critical state or residual should be used to calculate the net bearing capacity.
Changes of stress and pore pressure in soil below a foundation as it is loaded drained
or undrained are considered in Sec. 22.4. These analyses show that the state in the
soil approaches failure more rapidly during undrained loading than during drained
loading and, after undrained loading pore pressures during subsequent consolidation
fall, effective stresses rise so the soil becomes stronger. So, for foundations on fine
grained soils the bearing capacity should be found from the undrained strength. In the
case of a foundation on a coarse grained soil the soil would be drained throughout
loading and the bearing capacity should be found from the effective stress strength.
As discussed in Sec. 18.6 movements in soil below foundations are too small to
develop residual strengths and use of the critical state strength is illogical because then
you would design the same foundation on dense and loose sand. The peak strength
should be used to calculate the value of the net bearing capacity for Eq. (22.4). This
is because, as discussed in Sec. 18.6, all samples of the same soil will reach their peak
states at about the same strain so stiffness is related to peak strength.
If the width of a typical foundation is 10 m and the allowable settlement is 10 mm
the value of
a
/
B
is 0.1% and this is representative of the strains in the ground beneath
the foundation. If
ρ
ε
=
1%, then
ε
≈
10
ε
a
and if the curves in Fig. 18.5(a) can be
p
p
approximated by parabolas then
q
a
q
c
≈
1
3
L
f
=
(22.5)
In practice many shallow foundations are designed with load factors of 1/4 to 1/3 and
they usually have small settlements.
It is important to understand that the load factor defined above is not a factor of
safety: it is a factor to reduce the bearing pressure from the bearing capacity to a
point at which settlements will be small. You may want to apply additional factors,
particularly to the soil strength, to take account of uncertainties in your values of
