Environmental Engineering Reference
In-Depth Information
40 mm
30
Range of DF15 for dams with
poor filter performance
25
Average DF15 for dams with
poor filter performance
Contours of
Erosion Loss
from Filter
Tests
20
Range of DF15 for dams with
good filter performance
15
1.0 g/cm 2
0.5 g/cm 2
0.25 g/cm
No Erosion
Boundary
for Soil Group 2
DF15=0.7 mm
10
Hyttejuvet
Hills Creek
0.1 g/cm 2
5
Brodhead
So n ga
0
0
5
10
15
20
25
30
35
40
45
50
55
60
Core material % fine - medium sand (0.075 - 1.18 mm)
Figure 9.23. Comparison of erosion losses measured in filter tests to dams with poor and good filter
performance.
downstream gravel zone; the filter opening size of this coarsest gravel zone material was
1.0/9
0.1 mm and the D 95B of the finest core grading is 0.075 mm.
The other dams with poor filter performance generally have broadly graded core mate-
rials which fall into Soil Groups 2 and 4 (fines content 15-85%) and have D 95B
2 mm,
which places them in the soil types where a continuing erosion boundary could not be
identified by the CEF tests. Figure 9.23 shows the range of D 15F of the filter plotted
against the average percentage of fine-medium sand sizes (% 0.075 mm to 1.18 mm) of
the core material for the dams which have had poor and good filter performance. The
contours of equal erosion losses from the CEF tests and the no erosion boundary for Soil
Group 2 soils (D 15F
0.7mm) are shown on the plot. The percentage fine-medium sand
has been taken off the grading curves of the core materials after adjustment to a maximum
particle size of 4.75 mm.
Dams with good filter performance generally have filters with an average D 15F
0.5 mm,
which is finer than the Sherard and Dunnigan design criterion for Soil Group 2 base soils
(D 15F
0.7 mm). The coarsest gradings are only slightly coarser than this (D 15F up to
1.5 mm).
Dams with poor filter performance have filters with an average D 15F
1.0 mm and
generally with D 15F greater than or equal to about the 0.25 g/cm 2 contour of erosion loss.
Where a range of filter gradations is given, the coarsest grading is significantly coarser
than the design criteria. Balderhead Dam has the finest coarse limit grading of the filter
(D 15F
7 mm) and this is 10 times coarser than the recommended design criteria.
The D 15F for the coarsest gradings is typically greater than or equal to about the 1.0 g/cm 2
contour.
One notable exception is Songa Dam, which has a range of D 15F of 0.4-1.5 mm. This is
considerably lower than the other dams with poor filter performance. However, the grad-
ings of the filter for Songa Dam ( Figure 9.22 ) have a wide grading and low proportion of
sand sizes which, as discussed later on, would have made the filters particularly suscepti-
ble to segregation during placement. Therefore it is likely that the actual gradings of the
filter in this dam are probably locally much coarser than that shown.
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