Environmental Engineering Reference
In-Depth Information
Table 9.7.
Segregation criteria
If D
10F
(mm)
Then maximum
Base soil category
is:
D
90F
(mm) is:
All categories
0.5
20
0.5-1.0
25
1.0-2.0
30
2.0-5.0
40
5.0-10
50
10
60
on the assumption that the slope of the line connecting D
15F
and D
10F
should be on a coef-
ficient of uniformity of about 6.) Determine the maximum D
90F
using Table 9.7. For Zone
ZB filters, use the coarse limit D
10F
in Table 9.7.
Sand filters that have a D
90F
less than about 20 mm generally do not require special
adjustments for the broadness of the filter band. For coarser filters and gravel zones that
serve both as filters and drains, the ratio of D
90F
/D
10F
should decrease rapidly with
increasing D
10F
sizes.
Step 9
: Connect the control points to form a preliminary design for the fine and coarse
sides of the filter band. Complete the design by extrapolating the coarse and fine curves
to the 100 percent finer value. For purposes of writing specifications, select appropriate
sieves and corresponding percent finer values that best reconstruct the design band and
tabulate the values.
For situations where filters close to the allowable limits are being contemplated, or
being checked in an existing dam, it is recommended that no-erosion tests as described by
Sherard et al. (1984b) be carried out. This is particularly important for fine-grained soils,
where the separation of the no-erosion and continuing erosion boundaries is small (see
Section 9.3
)
and for dispersive soils, because of the limited amount of testing upon which
the design criteria are based.
9.2.5
Recommended method for design of less critical and non-critical filters
9.2.5.1
Filters upstream of the dam core
The filter Zone 2C upstream of a central core earth and rockfill dam is not subject to con-
tinuous seepage exit gradients or to the risk of high exit gradients if the core cracks.
Zone 2C filters can therefore be designed less conservatively than Zone 2A and 2B filters.
It is common only to require that “Zone 2C filters shall be constructed of well graded rock-
fill with a maximum size of 150 mm”. This will result in most cases in a sand/gravel/cobble
size mixture of rock fragments, which provides a transition between the earthfill core and
the upstream rockfill. Since this zone will be won from the quarry (probably after passing
through a “grizzly” to remove oversize rock) it is relatively inexpensive and it would be nor-
mal to place at least 4 m width, possibly 6 m on a larger dam. The added width is important
so as to counter the effects of likely segregation.
However, care should be taken in adopting this approach. A more robust and defensible
approach, which the authors would recommend, would be to accept a relaxed specification
for these filters, but to ensure they are finer than that required to satisfy the excessive (or,
at worst, continuing) erosion criteria. Be aware that using filter/transition materials with
rocks up to 150 mm is a recipe for segregation problems and it would be better to limit the
size to a maximum of 75 mm.
Even if a relaxed specification is adopted, the fines content (passing 0.075 mm) should
not exceed 5% and the fines should be non plastic.
