Environmental Engineering Reference
In-Depth Information
Table 15.2.
Zone 2D gradation specifications (Sherard 1985b).
Dam, Year (% finer)
Cethana,
Alto Anchicaya,
Foz do Areia,
Size (mm)
1971
1974
1980
300
100
225
100
150
80-100
100
100
75
51-100
50-100
55-100
40
20
15-63
25-60
18-65
5
0-40
0-30
5-22
0.5
0-17
0-5
0-8
0.075
0-5
0-1
Sherard (1985b) points out that quite large leakage had occurred at some dams con-
structed with crusher-run Zone 2D including 1800 litres/second at the Alto Anchicaya Dam
on first filling. Sherard notes that, for such wide graded materials, with a small percentage
of sand size material, segregation was a problem, and this could in part explain why such
large leakage had occurred. He suggests that about 40% of sand size (passing 4.76 mm) is
required to avoid the segregation. Sherard also shows that leakage through the dam result-
ing from a crack in the face slab, is controlled more by the permeability of Zone 2D, than
by the crack aperture. Sherard concludes that Zone 2D should be a 4 m to 5 m wide zone.
The properties of the Zone 2D material placed during the construction of six major
CFRD in Brazil (Sobrinho et al., 2000) are summarized in
Table 15.3
.
These dams have
transition zones that range in width from 10 m at the bottom to 4 m at the crest.
Current practice follows the recommendations of Sherard (1985b) and reported in
ICOLD (1989a) and Amaya and Marulanda (2000) as shown in
Table 15.4
.
Sherard (1985b) indicates that a grading for Zone 2D as shown in Table 15.4 will be
“stable”, i.e. internally stable, not susceptible to washing out of the fines, and that a per-
meability of 10
6
m/sec would be achieved. Earlier Zone 2D are not internally stable, and
will be susceptible to the finer particles washing from the zone, leading to a higher per-
meability and possibly some settlement of the face slab as the fines are eroded.
ICOLD (1989a) recommend a Zone 2D grading as shown in Table 15.4, i.e. virtually the
same as that suggested by Sherard. They indicate that a maximum of 10% to 12%, passing
0.075 mm is desirable, while giving 15% as the upper limit. ICOLD (1989a) indicates that
Zone 2D should be 4 m to 5 m wide, possibly wider for dams higher than 150 m. At Antamina
Dam in Colombia (Amaya and Marulanda, 2000) where strict control of seepage is required,
a Zone 2D width of 8 m has been adopted.
Sherard (1985b) acknowledges that the required Zone 2D grading may require pro-
cessing and/or blending of materials, but points out that the incremental cost is small. Mori
(1999) suggests the use of compacted alluvial gravel in Zone 2D to reduce deformation and
cracking of the face slab.
Fitzpatrick et al. (1985) and Sherard (1985b) and Cooke (2000) point out that an added
advantage of having Zone 2D graded as shown in Table 15.4, is that, if a crack forms in
the face slab, it can be sealed by spreading silty sand over the crack and having the silty
sand wash into it, with further erosion controlled by Zone 2D acting as a filter. This
method has been used to reduce leakage on several dams including Shiroro Dam in Nigeria
(Bodtman and Wyatt, 1985), Khao Laem Dam in Thailand (Watakeekul et al., 1985) and
Bailey (Cooke, 2000).
