Civil Engineering Reference
In-Depth Information
to be pumped. They will also be interested in the quantities of water leaking from
water storage dams.
It is common knowledge that landslides occur most frequently after periods of rain-
fall when pore pressures in the ground are highest. (Remember that this has nothing
to do with water lubricating soil because the critical state friction angle
φ
c
is the same
for dry and saturated soil as discussed in Chapter 9.) We have already seen that soil
strength and stiffness are governed by the effective stresses which depend on the pore
pressures as well as on the total stresses, so that calculation of pore pressures in soil with
steady state seepage will be an essential component of geotechnical design calculations.
Figure 14.2 illustrates two typical cases of steady state seepage in geotechnical
problems. In both cases water flows from regions of high water level to regions of
low water level along flowlines such as ABC: notice that in Fig. 14.2(a) the water flows
upwards fromB to C. In Fig. 14.2(a) the flow is confined because the top flowline PQRS
is confined by the impermeable concrete dam. In Fig. 14.2(b) the flow is unconfined
and there is a phreatic surface, which is also the top flowline TU. In both cases we will
be interested in calculating both the rates of leakage below or through the dams and
the distributions of pore pressures.
In Fig. 14.2(a) water flows upwards in the region of C, where the flowline emerges at
the downstream ground surface. If the seepage velocities are large, soil grains may be
disturbed and washed away. If this should happen the erosion would seriously jeopar-
dize the stability of the dam. The same thing might happen to the dam in Fig. 14.2(b)
if the downstream drain is inadequate so that the top flowline TU emerges from the
downstream face of the dam. After overtopping this is the most common cause of
failure of dams made by children at the seaside.
Figure 14.2
Problems in groundwater flow.
