Environmental Engineering Reference
In-Depth Information
Figure 8.13.
Thomson Dam - section through the saddle dam. (Courtesy of Melbourne Water)
underlain by sandstone, which weathers to give only a shallow cover of sandy soil, it will
normally be appropriate to construct a rockfill dam with concrete (or other) impervious
membrane, or a concrete gravity dam.
The uniformity of the available earthfill will also influence design and the method
of construction. If the borrow areas produce two different types of earthfill, the earth-
fill may be zoned into two parts, e.g. for an earthfill dam with vertical and horizontal
drains, the earthfill with lower proportion of fines and more variable properties would
be best placed downstream of the vertical drain. Alternatively the earthfill zone may be
separated into Zones 1a and 1b with the coarser soil in Zone 1b adjacent the Zone 2A
filter.
Alluvial clayey soils are often more variable than residual soils derived from weather-
ing of underlying rocks and, hence, it may be necessary to provide additional zoning as
described above. Alternatively, the soils may be mixed by borrowing from a vertical face
with a shovel and truck operation (rather than use of scrapers). Blending of soils on the
dam embankment is generally avoided as it leads to increased cost and difficulty in qual-
ity control.
If cobbles and boulders are present in clayey soil deposits, these will have to be removed
prior to compaction either by passing the earthfill through a “grizzly” or by grader or
hand labour on the embankment. This is necessary to prevent the oversize particles affect-
ing compaction.
Relatively permeable soils can be used for earthfill in many dam projects. The perme-
ability of most dam foundations is between 1 Lugeon and 10 Lugeons (10
7
m/sec to
10
6
m/sec) so for most dams an earthfill core with permeability, say 10
9
m/sec, seepage
through the foundations will far exceed that through the dam. Even if silty sand is used for
the earthfill, a permeability of 10
6
m/sec should be achieved, i.e. not higher than the
foundation permeability, and from a seepage viewpoint such a high permeability soil
would be acceptable for many dams. Weathered or even relatively unweathered siltstone
and sandstone with a clayey matrix, may break down sufficiently when compacted in thin
layers to achieve a satisfactory core material. This will usually require field trials to
observe the actual properties. MacKenzie and McDonald (1985) and ANCOLD (1985)
describe trials and actual performance of compacted siltstone and sandstone, during con-
struction of Mangrove Creek concrete face rockfill dam.
8.4.1.2
Rockfill
Over the last 30 years the unit rate for construction of rockfill has reduced compared to that
for earthfill and as usually less rockfill is required (because the side slopes can be steeper),
