Civil Engineering Reference
In-Depth Information
applied load and soil strength, such as whether mean or worst credible values have
been adopted.
22.4 Stress changes in foundations
The changes of stress and water content during undrained loading and subsequent
consolidation of a foundation are illustrated in Fig. 22.5. In Fig. 22.5(a) the total
stresses on a typical element below the foundation are
and the pore pressure is
illustrated by the rise of water in a standpipe. In Fig. 22.5(b) the total stress path A
τ
and
σ
→
B
corresponds to increases of
σ
and
τ
due to the loading of the foundation. The effective
stress path is A
B
which corresponds to undrained loading with constant water
content, as shown in Fig. 22.5(c). The exact effective stress path A
→
→
B
will depend
on the characteristics of the soil and its initial overconsolidation ratio, as discussed in
Chapter 11. Normally the long term steady state pore pressure
u
∞
is the same as the
intial pore pressure
u
0
.
As shown in Fig. 22.5(b), the pore pressure immediately after construction
u
i
is
greater t
ha
n the final steady state pore pressure
u
∞
and so the initial excess pore
pressure
u
i
is positive. As time passes the total stresses remain essentially unchanged
at B, since the foundation loading does not change, but the pore pressures drop. The
effective stress path is B
→
C
, which corresponds to compression and an increase in
the mean normal effective stress, as shown in Figs. 22.5(b) and (c).
Figure 22.5
Changes of
total and effective stress during loading and consolidation of a
foundation.
