Environmental Engineering Reference
In-Depth Information
- the cost of filters in a long, low earth and rockfill dam is relatively high because the fil-
ters constitute a high percentage of the total volume.
They describe the use of CFRD for a 2000 m long 20 m high dam in Venezuela.
Varty et al. (1985) indicate that the Hydro-Electric Commission of Tasmania used CFRD
for several smaller dams, ranging in height from 20 m to 44 m. They list faceplate construc-
tion techniques which are different from their normal practice for these smaller dams, includ-
ing the elimination of starter bays, a lighter slip-form and a mobile crane on the dam crest.
These examples show that smaller dams can be economically constructed as CFRD, but
probably where the economy of scale applies, such as a long crest, or a requirement to
construct several dams, to offset the cost of setting up for slip-forming.
Sherard and Cooke (1987) and Cooke (2000) argued that CFRD should be considered for
the very highest dams. Sherard and Cooke (1987) argue that CFRD have fundamental advan-
tage over earth and rockfill dams in that there is no possibility of piping erosion of the earth
core and over arch dams in that they rely on gravity for stability, not high strength abutments,
and that the CFRD supports high abutments, rather than stressing them. They argue that
there is no reason CFRD up to 300 m high could not be built and suggest that the jump in
precedent from current heights is not excessive and is similar to practice in earth and rockfill
dams. They suggest that conservative perimetric joint details would have to be used for very
high CFRD, and wider, more conservatively designed, processed material used for Zones 2D
and 2E to ensure homogeneity, lack of segregation and low permeability.
By 2000 (Cooke, 2000) two 190 m high dams had been constructed (Aquamilpa and
Tianshengqiao 1) and seven CFRD dams are proposed with heights of up to 230 m.
Fitzpatrick et al. (1985) and Mori (1999) suggest that because of the high modulus obtained
with compacted gravel fills, they are preferable construction material for very high dams.
They counsel caution in the use of rockfill for dam heights greater than have so far been built.
15.2
ROCKFILL ZONES AND THEIR PROPERTIES
15.2.1
Zone 2D - Transition rockfill
As the design of concrete face rockfill dams has developed and higher dams have been con-
structed, more emphasis has been placed on the grading and placement of the rockfill zone
immediately below the concrete face slab. In CFRDs constructed in the 1960s the function
of Zone 2D was seen as providing uniform support for the face slab. Specifications
required that the Zone 2D was screened to remove all materials less than 25 or 50 mm.
This was to ensure that, in the event of a leak in the face slab, there would be no fines to
be washed away which might lead to loss of support of the face slab. However with this
grading Zone 2D was very permeable and, rocks were easily dislodged during construc-
tion, and a rough, porous surface led to excess concrete being required for the face slab.
Beginning with the 110 m high Cethana Dam in 1971 (Wilkins et al., 1973) there was
a change in design approach which resulted in Zone 2D being specified as “crusher run”
or “quarry run” (after passing through a grizzly) rockfill passing 150 to 225 mm (up to
300 mm). This resulted in a material with a lower permeability and a smoother and more
stable surface on which to construct the face slab. Many dams have been successfully con-
structed in this way and this was the established practice up till about the mid 1980s.
Gradations for Cethana, Alto Anchicaya and Foz de Areia are shown in
Table 15.2
.
Later designs have specified a finer grading with Zone 2D not only meeting filter require-
ments with Zone 2E, but also having a specified requirement for a silty fines content to
reduce the permeability, and hence leakage, in the event of a face slab joint opening or the
slab cracking.
