Environmental Engineering Reference
In-Depth Information
Table 9.2.
Summary of statistical analysis of the results of no-erosion filter tests, and proposed no-
erosion filter criteria (Foster, 1999; Foster and Fell, 1999a, 2001).
Base soil
Fines
Design criteria of Sherard
Range of DF15 for no
Criteria for no
group
content (a)
and Dunnigan (1989)
erosion boundary D
85B
erosion boundary
1
85%
D
15F
9D
85B
6.4-13.5D
85B
D
15F
9D
85B
(b)
2A
35-85%
D
15F
0.7 mm
0.7-1.7 mm
D
15F
0.7 mm (b)
3
15%
D
15F
4D
85B
6.8-10D
85B
D
15F
7D
85B
4A
15-35%
D
15F
(40-pp%
1.6 D
15F
-2.5D
15F
of Sherard
D
15F
1.6D
15F
d,
0.075 mm)
(4D
85B
-0.7)/
and Dunnigan design criteria
where D
15F
d
25
0.7mm
(35-pp% 0.075 mm)
(4D
85B
-0.7)/20
0.7 mm
Notes: (a) The subdivision for Soil Group 2 and 4 was modified from 40% passing 0.075 mm, as
recommended by Sherard and Dunnigan (1989), to 35% based on the analysis of the filter test data.
The modified soil groups are termed Group 2A and 4A. The fines content is the % finer than 0.075 mm
after the base soil is adjusted to a maximum particle size of 4.75 mm.
(b) For highly dispersive soils (Pinhole classification D1 or D2 or Emerson Class 1 or 2), it is
recommended to use a lower D
15F
for the no erosion boundary:
For Soil Group 1, use the lower limit of the experimental boundary, i.e. D
15F
6.4D
85B
.
For Soil Group 2A, use D
15F
0.5 mm.
Table 9.2 summarizes the results. The proposed “criteria for no-erosion boundary” are
intended for applying to the assessment of filter performance of existing dams and do not
include any margin of safety.
This work showed that the division between Group 2 and 4 soils is better defined on a
fines content of 35% than on the 40% used by Sherard et al. (1984a, b) and that the
Sherard and Dunnigan (1989) criteria generally have a margin of safety but they are not
sufficiently conservative to define no-erosion condition for dispersive soils.
9.2.2
Review of other factors affecting filter design and performance
9.2.2.1
Criteria to assess internal instability or suffusion
Numerous criteria exist for the assessment of suffusion including those by Sherard (1979),
De Mello (1975), Kenney and Lau (1985) and Burenkova (1993). These criteria are based
on the analysis of the grain size distribution of the soil. Schuler (1993) critically reviews
these criteria and recommends the following:
- For gap-graded soils, use the method of splitting the grain size distribution at the point
of inflection in the gap,
Figure 9.14
(Sherard, 1979; De Mello, 1975);
The soil is considered to be self-filtering if:
D
D
15coarse
85fine
(9.3)
5
D
85
of the fine fraction;
- For soils with smooth gradations with a tail of finer sizes use the criterion of Burenkova
(1993), shown in
Figure 9.15
,
or Kenney and Lau (1985, 1986) in
Figure 9.16
and
where D
15coarse
D
15
of the coarse fraction, and D
85 fine
