Environmental Engineering Reference
In-Depth Information
1500
Filter 2: 0.25-100mm
Filter 1: 0.5-70mm
Calculated
1000
Le300D 5F
O E D 95B
500
B
Le 50D 5F
O E D 85B
A
0
80
85
90
95
100
BASE MASS FINER THAN EFFECTIVE OPENING SIZE O E %
Figure 9.28.
Depth of penetration of base soil into filter material (Le) versus effective opening size (O E )
(Adapted from Witt, 1993).
The theoretical minimum width or thickness for filters designed according to no-erosion
criteria, is very small, and does not control the dimensions of filters. Witt (1986, 1993) car-
ried out some calculations and experiments which demonstrated that the depth of penetra-
tion of the base soil into the filter is small, even if the filter is somewhat coarser than required
by the design criteria described above.
Figure 9.28 shows the depth of penetration versus effective opening size O E
The graph shows:
At A,
O E
D 85B ,
Depth Penetration
50D 5F
and at B,
O E
D 95B ,
Depth Penetration
300D 5F
Given that O E
D 15F /9 (Sherard et al. 1984a) this means that for D 15F
9D 85B , Depth
penetration
50D 5F .
Sherard and Dunnigan (1985, 1989) design criteria range from D 15F
4D 85B for
Group 3 soils, to D 15F
9D 85B for Group 1 soils, so the depth of penetration will be less
than 50D 5F for filters designed according to Sherard and Dunnigan criteria. Commonly,
Zone 2A filters would have a D 5F less than 0.5 mm, so the depth of penetration would be
less than 25 mm.
Larger depths of penetrations, and hence larger minimum thicknesses would be calcu-
lated using Witt (1986, 1993) approach for Zone 2A filters penetrating Zone 2B, but it is
not clear if his work would apply to this case.
For assessing the filters in existing dams, the degree of compliance with these require-
ments would be taken into account.
9.6.1.2 Placement methods
Given that the width needed to achieve a proper filter against the protected soil is small,
the width of filter zones is often determined by construction requirements.
The selection of the economic dam zoning should account for filter placement require-
ments and the cost of manufacture of the filter materials. In most dam projects the cost of
 
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