Environmental Engineering Reference
In-Depth Information
AUSTRALIAN
STANDARD SIEVES
100
0
D85B
D85B
90
10
80
20
70
30
60
40
D50B
50
50
40
60
30
70
Ty pical zone 2A filter
D15B
20
80
D15F
10
90
0
100
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
PA RTICLE SIZE (mm)
Figure 9.3.
Filter design notation and adjustment of the particle size distribution for gravelly base soils.
D
85B
Particle size of the base material for which 85% by weight is finer.
pp% 0.075 mm Percent finer than a particle size of 0.075 mm.
Fines content pp% 0.075 mm.
Figure 9.3 shows particle size distributions for two base soils, A and B, and a Zone 2A
filter.
For broadly graded soils, such as soil B, which have some medium and coarse gravel,
the USBR (1977 and 1987), USA-SCS (1994), Sherard and Dunnigan (1989) and other
methods use an adjusted particle size distribution. As shown in Figure 9.3, this involves
taking the particle size distribution and adjusting it to what it would be if only the frac-
tion passing the 4.75 mm sieve were used.
9.1.4.2
Filtering concepts
Figures 9.4(a)
and (b) show the interface between a filter and base soil. The basic concept
of filter design is to design the particle size distribution of the filter so that the voids in the
filter are sufficiently small to prevent erosion of the base soil.
The void size in the filter is controlled by the finer particles and, for design purposes,
the D
15F
is usually used to define this void size. Sherard et al. (1984a) showed that for
granular soils the void size between the soil particles, known as the opening size, is given
by O
E
A further basic concept, inherent in filter design, is that the base soil will generally pro-
vide a degree of “self-filtering”. Hence in Figure 9.4(a), in a well graded base soil, the
coarser particles in the base soil are prevented from eroding into the filter and they in turn
prevent the medium sized particles in the base soil from eroding and the medium sized
particles in the base soil prevent the fine particles in the base soil from eroding.
If the base soil is gap-graded or graded concave upwards (
Figure 9.5
), there is a defi-
ciency of medium sized particles, as shown in Figure 9.4(b). The self filtering does not
