Environmental Engineering Reference
In-Depth Information
fines content and nature such as that required by the USBR (1977) seems to be implied i.e.
filters should contain not more than 5% fines passing 0.075 mm, and the fines should be
“cohesionless”.
It should be noted that for Soil Group 1 hydrometer particle size analysis will be
required to define the particle size below 0.075 mm.
The Sherard and Dunnigan (1985, 1989) method has become widely adopted including
by USBR (1987), USDA-SCS (1994) and throughout Australia.
9.2.1.3
Kenney and Lau
Kenney and co-workers at the University of Toronto, Canada, have carried out extensive
testing on filters. Their work is reported in Kenney et al. (1985) and Kenney and Lau
(1985).
(a)
Filters for cohesionless base soils
. Kenney et al. (1985) developed the concept of a
controlling constriction size D
*
which is a size characteristic of the void network in a
granular filter and is equal to the diameter of the largest particle that can possibly be
transported through the filter by seepage. They show that:
D . 5D
*
(9.1a)
5F
and
D . 0D
C
*
(9.1b)
15F
They found that D
*
is primarily dependent on the size of the small particles in the filter,
and not strongly dependent on the shapes of the particle size distribution curve for the fil-
ter as a whole.
The test set-up used by Kenney et al. (1985) is similar to the USSCS apparatus shown
in
Figure 9.6
, but the seepage gradient was between 3 and 50. The cell was tapped lightly
during the test to induce vibration. As might be expected, this was found to have a signif-
icant effect on the results, with vibration dislodging particles and facilitating their pene-
tration into the filter.
Kenney et al. (1985) suggest that for filtering cohesionless base soils the following rela-
tionships should be applied:
D4 D
5F
(9.2a)
50B
and
(9.2b)
D5 D
15F
50B
This is based on the requirement that the cohesionless bases and filters have a uniformity
coefficient Cu
6, and the coarser of the filters given by the relationships should be
adopted. If the base soil has a potential for “grading instability”, i.e. it will not self filter,
Kenney et al. suggest that consideration of allowable loss of fines may be necessary.
(b)
Filters for cohesive soils.
The work of Kenney et al. (1985) was largely on cohe-
sionless base soils. They describe personal experience of two dam cores composed of
widely graded glacial tills with particles from gravel to clay size, where silt and clay size
particles (finer than 0.060 mm) were removed through the filters by seepage. They attrib-
ute this to low seepage velocities being unable to transport the coarser sand particles in
the soil to form a “self filter”. They suggest a rather arbitary selection of particle size in
the base soil (0.020 mm in the example given) which must not pass through the filter (i.e.
