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Geotechnical News • December 2014
49
GEOSYNTHETICS
a sequence of laboratory studies by
other investigators led to the develop-
ment and verification of the empirical
rules that now govern the specification
of a granular filter for different types
of base soil (including, amongst other
notable contributions, the laboratory
findings of Bertram, 1940; Karpoff,
1955; Lafleur, 1984; and Sherard et
al., 1984a and 1984b).
A granular filter material comprises
one or more select gradations of cohe-
sion less soil, for which characteristic
grain sizes (Dn) are established from
sieve and hydrometer testing. In
effect, for a granular filter, by specify-
ing directly the grain size distribu-
tion, the corresponding opening size
distribution in the porous medium is
determined indirectly. Accordingly,
the properties of a granular filter are
specified with reference to the range
and shape of the particle size distribu-
tion curve, with additional consid-
eration given to the mineralogy of
the grains and also to the thickness
at which the filter layer is placed. A
schematic illustration of the causal
relations between characteristics of a
granular filter and functional require-
ments against which performance is
assessed, is given in Fig. 1 (after Fan-
nin and Moffat, 2002). Those func-
tional requirements are:
• Base soil retention
• Permeability
• Internal instability
The characteristic grain size of the
finer fraction (for example, Dn =
D15) is believed to influence the pore
size distribution, or more strictly the
pore constriction size distribution, of
the filter and, hence, the capacity for
retention of the base soil (see Fig. 2).
The quantity and size of the smallest
particles also exert a major influ-
ence on the hydraulic conductivity,
or permeability, of the granular filter.
Lastly, the shape of the grain size
distribution governs the potential for
any seepage-induced migration of the
finer fraction through the interstices of
the coarser fraction, a form of erosion
attributed to internal instability of the
filter gradation curve (as suggested by
de Mello, 1975, Kezdi, 1979; Kenney
and Lau, 1985 and 1986; Burenkova,
1993; Li and Fannin, 2008; Wan and
Fell, 2008; and Moraci et al. 2012,
amongst others).
In addition, it can be argued there are
functional requirements against which
the ease of construction and service-
ability are assessed, namely:
•
Segregation potential
•
Placement and durability
The quantity and size of the larg-
est particles are believed to exert an
influence on the potential for segrega-
tion of grains during placement of the
filter, as does the shape of the grada-
tion curve (Ripley, 1986; Kenney and
Westland, 1992). Mineralogy of the
granular material, and thickness to
which it is placed, act to control the
durability and construction method
respectively (Wittman, 1979).
The causal relations illustrated in Fig.
1 are commonly described by a series
of design criteria that must be satisfied
by the granular filter (see for example,
USDA, 1994). The specification
criteria used in design are empirical,
insomuch as the criteria were estab-
lished from interpretation of a limited
number of experimental observations,
Figure 1. Functional requirements of a granular filter Figure 2. Base soil filter interface
(adapted from Fannin and Moffat, 2002). (from Cedergren, 1989).