Environmental Engineering Reference
In-Depth Information
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In some areas, often but not always tropical, the rock is deeply weathered and some-
times with a lateritic profile which may lead to a high permeability, soil strength foun-
dation, favouring embankments with flatter slopes and good under drainage, e.g.
earthfill with vertical and horizontal drain;
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Embankments constructed on deep soils, e.g. deep alluvium in or adjacent to the river
bed, may be subject to a large amount of settlement, leading to differential movement
and cracking. In such dams it is particularly important to provide good filters to con-
trol seepage and prevent internal erosion.
8.4.3
Climate
It is difficult (and in many cases impossible) to construct earthfill embankments during
wet weather, or in freezing temperatures. This is particularly critical when the rain is rela-
tively continuous without high evaporation (and not so critical when the rain is in short
storms, followed by hot sunshine).
In these circumstances, it is often advantageous to adopt concrete face rockfill or sloping
upstream core construction, so that the rockfill can continue to be placed in the wet
weather, and the face slab or core constructed when the weather is favourable.
In very arid areas there may be a shortage of water for construction, thus favouring
concrete face rockfill rather than earthfill.
8.4.4
Topography and relation to other structures
The selection of embankment type and overall economics of a project is determined with
consideration of all components of the project, i.e. embankment, spillway, river diversion
outlet works etc. These components are interrelated, e.g.:
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The diversion tunnel will be longer for an earthfill dam (with relatively flat side slopes)
than for a rockfill dam;
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The spillway may generate rockfill (and possibly earthfill and random fill) for use in the
embankments. The size of spillway can be varied by storing more floodwater in the
reservoir, necessitating a higher embankment but smaller spillway so the optimisation
of total project cost may influence embankment zoning and size;
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It is common practice in Australia to allow large floods during construction to pass
over the embankment, rather than providing a larger diversion tunnel and coffer dam.
This necessitates incorporation of some rockfill in the downstream toe of the dam with
steel mesh reinforcement (see Chapter 13).
The topography of the site, i.e. valley cross section or slope, curve of the river in plan or
the presence of “saddles” in the abutments, can have a significant effect on embankment
selection, e.g.:
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In narrow steep sided valleys there is restricted room for construction vehicles and haul
roads, favouring embankments with simple zoning, e.g. concrete face rockfill;
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The curve of the river in plan, and changes in valley cross section, may favour adoption
of an upstream sloping core rather than central core (or vice versa) to reduce the quanti-
ties of earthfill;
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Local changes in slope of the abutments may lead to differential settlement and cracking,
necessitating more extensive filter drains, or favouring concrete face rockfill construction.
8.4.5
Saddle dam
Directly related to the question of topography is a group of dams called saddle dams.
