Environmental Engineering Reference
In-Depth Information
Figure 9.6.
USSCS filter test apparatus details for fine to coarse sand base soils (Sherard et al., 1984a,
reproduced with permission of ASCE).
occur and the fine particles in the base soil will erode through the coarse particles, a
process called suffusion, or internal instability. In these situations, for the filter to be suc-
cessful in controlling erosion, it must be able to control the erosion of these finer particles.
In most filter design methods, the base soil particle size is characterised by the D
85B
size,
although some use D
95B
and D
50B
. For the USBR (1977, 1987), Sherard and Dunnigan
(1989) and USDA-SCS (1994) methods, and as recommended by the authors, the D
85B
for
gravelly base soils is based on the particle size distribution adjusted to what it would be if
all the gravel and cobble sized particles greater than 4.75 mm size were removed as shown
in
Figure 9.3
.
9.1.4.3
Laboratory test equipment
The particle size distribution of critical filters is determined using methods which are
based on laboratory testing, where base soils are placed against a filter and water is passed
through the sample under pressure. Figures 9.6 and
9.7
show the test apparatus used by
the US Soil Conservation Service which is reported in Sherard et al. (1984a and b) and
Sherard and Dunnigan (1985).
