Civil Engineering Reference
In-Depth Information
Figure 21.7
Steady state seepage towards excavations.
We can consider the general case where flow lines meet the surface from different
directions. If the slope is submerged as in Fig. 21.7(b) the pore pressures are governed
by the whole flownet, but if the slope is not submerged as in Fig. 21.7(a) the pore
pressures are governed by the local conditions.
The point to remember is that water will rise to the same height in standpipes whose
tips are on the same equipotential and where the equipotential meets the phreatic
surface the pore pressure is zero. Figure 21.8 shows
h
w
for a standpipe whose tip
is at a depth
z
for various directions of seepage flow near a slope. In Fig. 21.8(a)
the flow is vertically downwards. This would correspond to heavy rain on soil which
was drained from below. The equipotentials are horizontal so
u
0 everywhere.
In Fig. 21.8(b) the seepage is down the slope, the flowlines are parallel with the
slope and the equipotentials are normal to the slope. From the geometry of the
figure
=
w
z
cos
2
i
u
=
γ
w
h
w
=
γ
(21.3)
where
i
is the slope angle. In Fig. 21.8(c) the seepage is horizontal and the equipotentials
are vertical. From the geometry of the figure
u
=
γ
w
h
w
=
γ
w
z
(21.4)
