- A nominal hydraulic gradient through the core of the order of
1.5–2.5 is satisfactory; a value greater than 3.5–4 is undesirable. - Core geometry is not critical provided that the upstream core slope is
not such as to control overall slope stability. It is preferable that the
core be approximately central and it can ease placing of the down-
stream drain and transition/filter zones if that face of the core is kept
vertical. - The dimensioning of structural zones or intermediate transition
zones is governed by stability and deformation considerations. It is
sometimes assisted by data from construction of special test fills. - A compromise must be made between sophistication of design and
zoning and ease of construction. Internal zoning and associated
specification requirements should be kept as simple as possible.
(b) Spillway location
Geotechnical and hydraulic design considerations require that to minimize
the risk of damage to the dam under flood conditions the spillway and dis-
charge channel are kept clear of the embankment. Spillways are therefore
generally built on natural ground with the channel bypassing the flank of the
dam and discharging to a stilling basin clear of the downstream toe. The
alternative is to use a dropshaft-type spillway located within the reservoir and
discharging via an outlet tunnel or culvert. In the latter case it is preferable to
tunnel the outlet through the natural ground of the abutment wherever pos-
sible. The alternative is a concrete culvert if founded on incompressible rock.
The hydraulic aspects of spillways are discussed in Chapter 4.
In a number of instances auxiliary or emergency spillways have been
constructed on embankment crests using suitable proprietary grass rein-
forcement techniques (see Section 4.4).
(c) Freeboard
The provision necessary for long-term settlement within the overall
minimum freeboard is determined by the height of dam and the depth of
compressible foundation at any section. It is therefore customary to con-
struct the crest of the dam to a longitudinal camber to accommodate the
predicted consolidation settlement.
A proportion of the design freeboard is sometimes provided by con-
struction of a continuous concrete wavewall along the upstream edge of
the dam crest. This can also be done when it is necessary to uprate the
freeboard of older dams following reassessment of the design flood. The
overall minimum freeboard from spillway sill to dam crest (or top of a
structural wavewall) should be at least 1.0 m on the smallest reservoir
embankment, and it will be very much greater for larger embankments
and/or reservoirs (see Section 4.4).