Civil Engineering Reference
In-Depth Information
Figure 15.4
Dissipation of excess pore pressure during consolidation.
the soil. At relatively small times, such as
t
n
in Fig. 15.4, consolidation is limited to
the upper levels only and below a depth
n
the excess pore pressures have not fallen.
At large times, such as
t
m
, consolidation is occurring throughout the layer. There is
a critical time
t
c
when excess pore pressures at the base first start to dissipate; the
isochrone for
t
c
is shown in Fig. 15.4(a). Figure 15.4(b) illustrates the dissipation of
excess pore pressure at the different depths indicated in Fig. 15.4(a). Near the surface,
at a depth
z
1
, the excess pore pressures dissi
pa
te very rapidly but near the base, at a
depth
z
3
, the excess pore pressures remain at
u
i
until the critical time
t
c
.
The gradient of an isochrone is related to the hydraulic gradient by
∂
u
=−
γ
w
i
(15.11)
∂
z
and from Darcy's law the seepage velocity is
k
γ
w
∂
u
V
=−
(15.12)
∂
z
By inspection of isochrones in Fig. 15.4(a) the gradients of the isochrones, and hence
the seepage velocities, increase towards the surface. At the base of an isochrone there
