D50 (Filter )
< 40
(1)
(6.24)
D50 (Base)
D15 (Filter )
(2) 5 <
< 40
(6.25)
D15 (Base)
D15 (Filter )
<5
(3)
(6.26)
D85 (Base)
If the base material is a fine-grained cohesive soil, such as fat or lean clay, these requirements
are not applicable, and the stability criterion is that the D15 size of the filter cannot exceed 0.4
mm.
When the base material is very fine, the required filter material may also be quite fine, and
more than one layer of filter (a graded filter) may be needed. In such a case, each layer must
satisfy the stability and permeability requirements relative to the underlying layer.
If the filter is designed for protection against the upward flow of water, the graded filter is
constructed so that each layer is coarser than the one beneath (a "reverse" or "inverted" filter).
Geosynthetic Filters. Geosynthetic materials are also used as filters, replacing a component of
a graded filter. Detailed information on the use of geosynthetic filters can be found in Holtz et
al. 1995 (FHWA HI-95-038) who define permeable geosynthetics as "geotextiles." Numerous
geotextiles are on the market, with a wide variation in size and number of openings and in
strength and durability. Geotextiles which provide opening areas of 25 to 30 percent are
desirable to minimize the possibility of clogging and to reduce head loss.
When geotextiles are used, care must be taken not to puncture the material during
construction. If the filter fabric is placed on top of the base material, gravel can sometimes be
placed directly on the fabric, eliminating the need for filter sand. If the paving materials is
dumped or cast stone, however, it is desirable to place a protective blanket of sand or gravel
on the filter, or to take care in placing the rock, so that the filter fabric is not punctured. Stones
weighing as much as 1,360 kg (3,000 lbs) have been placed on synthetic filters with no
apparent damage. If a protective covering is not used, the size and drop of the rock should be
limited. The sides and toe of the filter fabric must be sealed or trenched so that base material
does not leach out around the filter fabric. Care is also required in joining adjacent sections of
filter fabric together; sewn, overlapped, and welted seams are used. See Holtz et al. 1995 for
AASHTO specifications and construction guidelines.
6.8 OVERTOPPING FLOW ON EMBANKMENTS
6.8.1 Introduction
Floodwaters which exceed the crest elevation of roadways, approach embankments, levees,
and similar earth embankment structures result in a hydraulic condition referred to as
overtopping flow. Overtopping flow can be broadly characterized into two categories
according to the level of tailwater on the downstream side of the structure: (1) "submerged
flow," where tailwater is higher than the embankment crest and presents a backwater
condition sufficient to affect the discharge over the crest; and (2) "free flow," where tailwater
may be present against the downstream embankment slope but at an elevation low enough
such that interference with flow over the crest does not occur.
6.44
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