Environmental Engineering Reference
In-Depth Information
AUSTRALIAN
STANDARD SIEVES
8A
100
0
8B
10
90
80
20
ADJUSTED
PSD
70
30
3
7B
7C
40
60
50
50
2A
60
40
70
30
1
20
80
6A
5
90
10
7A
6B
100
0
CLAY
S I L T
S A N D
G R A V E L
Cobbles
Fine
Medium
Coarse
Fine
Medium
Coarse
Fine
Medium
Coarse
0.001
0.002
0.006
0.01
0.02
0.06
0.1
0.2
0.6
2
6
20
60
200
1
10
100
PARTICLE SIZE (mm)
Notes: Step 1 - Plot particle size distributions (PSD); Step 3 - Adjust PSD for gravelly soils;
Step 4 - Select base soil category for finest soil - Type 2; Step 5 - Determine D
15Fmax
from
0.1 mm,
6(A), less than 2% passing 0.075 mm (6B); Step 7 - D
XFmax
/D
XFmin
less than 5 e.g. D
15Fmax
0.7 mm; D
15Fmin
0.7 mm; Step 6 - Determine permeability criterion; D
15Fmin
0.72
(7B); D
10Fmax
0.45, D
60Fmax
less than 5D
10Fmax
2.7mm (7C); Step 8A - D
100F
less than
50 mm or 75 mm, say 50 mm (8A); Step 8 B - D
90Fmax
0.14mm (7A); D
60
/D
10
less than 6, say D
10Fmin
0.12; D
60Fmin
Figure 9.26.
Example of design of a Zone 2A filter.
Most experienced dam designers will design the Zone 2A filter for the finest soil to be
used in the dam, or which has been identified by investigations of an existing dam. It is not,
in the view of the authors, defensible to design on an average grading - the filters are
required to stop all soils in the dam core or foundation eroding, not just half of them!
If one can be confident that finer soils in the proposed borrow area for a new dam will
be mixed with coarser soils by the loading, dumping and spreading of the soil, the Zone 2A
filter may be designed for other than the finest soil. However this is difficult to control.
In this example the D
85B
is less than 0.075 mm, and must be obtained from hydrometer
particle size analysis. This should be done using the standard approach, in which disper-
sant is added to the soil as part of the test procedure.
For Zone 2B, a maximum particle size of 75 mm has been adopted. Larger size particles
may be used in Zone 2B filters but at the risk of segregation in stock piles and during
placement and the authors would not recommend this be done.
9.5.2
Durability
Filters are required to be constructed of sand and gravel which is sufficiently durable not
to break down excessively during the mechanical action of placement in the dam, under
the chemical action of seepage water or under wetting and drying within the dam.
