Environmental Engineering Reference
In-Depth Information
the spillway retaining wall. In
Figure 13.22(a)
the foundation slopes away at the base of
the wall. In Figure 13.22(b) there is a change in slope of the back of the wall. Both situa-
tions are best avoided.
It should be noted that if a fine abutment contact soil is used against the wall (or a con-
duit in the previous section) the critical fine filter should be designed to suit this material
as well as the general core material.
13.7
FLOOD CONTROL STRUCTURES
Flood control structures, e.g. flood storage basins and levee banks and dykes, have par-
ticular features which require consideration. These include:
- The embankments stand without water adjacent to them for long periods;
-Water levels rise quickly and usually fall within days or even hours;
- The consequences of failure may not be as critical as for other dams;
- The embankments may serve other purposes, e.g. as a parking area;
- The structures often have culverts or pipes passing through them;
- The foundations are usually soil and often these have sandy layers in them, so “heave”
or “blow-up” and piping through fissures in the soil is an issue.
From a design viewpoint this has the following implications:
(a) The embankments may crack due to desiccation if built of clay soils;
(b) The rapid rate of rise of the water makes it more likely erosion will begin before
cracks close by swelling of the soils adjacent to the cracks;
(c)
There are potential points of weakness around the conduits as described in Section 13.5.
The design of such structures, particularly levee banks and dykes which are long, low
structures, built with limited funds, is often a compromise between cost and the likelihood
of failure. Many levees are built only to withstand a 1 in 100 AEP flood, so it is probably
inappropriate to provide fully intercepting filters at great cost.
Figure 13.23 shows a possible detail for a levee bank where the consequences of failure
are low and there is no “heave” or “blow-up” risk. The section is homogeneous earthfill.
Cracking is controlled by providing a 50-100 mm thick sand layer which acts as a vapour
barrier. A road surface on the crest would also assist in this. The cutoff is taken below the
base of cracked and fissured soil.
Figure 13.23.
An example of the design for a levee bank or dyke in a low consequence of failure
situation.
