Environmental Engineering Reference
In-Depth Information
4.0
1.5
1.0
road pavement
2A
4
2C
1
1
Figure 13.10.
Example of crest detail for an earthfill dam with a vertical filter drain.
13.3.3
Curvature of crest in plan
Some engineers have favoured curving the crest of earth and earth and rockfill dams in the
upstream direction. As discussed by Wilson (1973), the concept is that, when the water load
is imposed on a dam, the crest moves downstream as the water load is applied and, if the
dam crest is curved upstream in plan, this can result in compressive strains along the axis of
the dam. These compressive strains can counter tensile strains which are induced by settlement
of the dam and hence reduce the likelihood of hydraulic fracture and leakage through the
earthfill zones of the dam.
USBR (1977) suggest that for small dams the extra difficulty involved in constructing a
curved axis is not warranted. Sherard (1973) indicates that the additional cost is very
small, but the benefits of curvature are doubtful.
Sherard and Dunnigan (1985) consider that, with the greater confidence in the ability
of well designed filters to control erosion, curvature of the dam axis is not necessary, even
for high dams in steep valleys. The authors agree with this point of view.
13.4
EMBANKMENT DIMENSIONING AND TOLERANCES
13.4.1
Dimensioning
Embankment dimensioning is often done poorly by inexperienced engineers, because they
fail to consider properly how the dam will be set out and constructed.
Figure 13.11
shows
some of the common errors. They are:
- Dimensioning height of dam rather than the reduced level of the crest including camber
(RL b
camber). The height of dam varies across the valley and in any case is not
known at any section prior to construction because the general foundation level is not
known.
- Giving a reduced level at the base of the dam, RL t and/or depth of cutoff trench Z.
These vary across the valley and are also not known before construction at any section.
General foundation and cutoff foundation should be defined in geotechnical terms, not
as levels and depths below ground surface or general excavation.
- Setting out the cutoff trench from the contact of earthfill zone with general excavation
(i.e. point A) rather than as a fixed offset from the centreline. The position of A is not
