Environmental Engineering Reference
In-Depth Information
8.4.5
Filters
Purpose
Filters are used to reduce flow velocities and prevent the migration of fines, clogging of
drains, and piping of adjacent soils where flow passes across zones with significant dif-
ferences in permeability. When water flows across two strata of widely differing grada-
tion, such as from silt to gravel, the silt will wash into the gravel and a pipe or cavity will
be created that could lead to structural collapse. In addition, the silt may clog the gravel,
stopping flow and causing an increase in water pressures.
Filters are used commonly with blanket and trapezoidal or triangular drains, and often
with trench drains. Circular drains including wellpoints or slotted pipe are installed with
filter materials, except in the cases of subhorizontal drains and galleries.
Design Criteria
Objectives
A filter is intended to permit water to pass freely across the interface of adjacent layers,
without substantial head loss, and still prevent the migration of fines. See Bennett (1952),
Cedergren (1967) and (1975), Lambe and Whitman (1969), Terzaghi and Peck (1967), and
USBR (1974).
Design Basis
Empirical relationships have been developed to satisfy the above criteria based on grada-
tion characteristics as follows:
Piping criterion
: The 15% size of filter material must not be more than 4 to 5 times
the 85% size of the protected soil
●
Permeability criterion
: The 15% size of the filter material must be at least 4 to 5
times the 15% size of the protected soil, but not greater than 20 to 40 times,
expressed as
●
D
15
(filter)/
D
15
(protected soil) must be > 4 or 5 but < 20 to 40
(8.15)
D
15
(filter)/
D
85
(protected soil) must be > 4 or 5
The filter soil designed may be too fine-grained to convey enough water, provide a good
working surface, or pass water freely without loss of fines to a subdrain pipe. Under
these circumstances, a second filter layer is placed on the first filter layer and the first
layer is then considered as the soil to be protected, and the second layer as the soil to be
designed.
Discharge through Drain Pipes
Water flowing through filters is often carried away through a subdrainage pipe, of which
there are many forms including:
Plastic pipe (corrugated or smooth) with holes or slots
●
Asbestos cement pipe with holes or slots
●
Corrugated metal pipe (bituminous-coated) with holes or slots
●
