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Geotechnical News • December 2014
51
GEOSYNTHETICS
presented results from longer-term
laboratory flow tests that supported
filter design criteria established from
shorter-term testing, and Giroud et al.
(1977) reported on observations over
a period of 6 years on the performance
of geotextile filters at the Valcros Dam
in France.
The general findings of early labora-
tory work led Hoare (1982) to note
that “for unidirectional laminar flow
conditions, particle retention and
permeability criteria... are well estab-
lished. All the approaches currently
adopted tend to give similar answers
and are dependent on the fabric being
the ‘catalyst’ in the formation of a
‘self-induced’ filter within the soil”.
Lawson (1982) further observed that
research had validated the underlying
assumption that “a geotextile having
an indicative pore size equal to the
average pore size of an ‘equivalent
granular filter’ gave similar perfor-
mance”, and affirmed the concept of
a self-induced filter by means of a
bridging network in the soil-geotextile
composite zone (see Fig. 3).
In contrast to a granular filter, the
opening size distribution of a geotex-
tile is controlled directly through the
process of manufacturing. Accord-
ingly, the properties of a geotextile
filter are specified with reference to
a characteristic value of opening size
(On) in the fabric that is established
by means of an inverse-sieve analysis,
with additional consideration given
to the polymer type and also to the
strength of the fabric. For ease of
comparison, the schematic illustration
of Figure 4 depicts the causal relations
between characteristics of a geotextile
filter, and functional requirements
against which performance is assessed.
Geofilters: Part 1 - Concluding
Remarks
The path-of-discovery through which
the use of granular filters has evolved
in engineering practice shares many
similarities with the origins and devel-
opment of practice for the specifation
of a geotextile filter. Comparison of
Figs. 1 and 4 readily identifies the
common functional requirements of (i)
base soil retention, (ii) permeability,
and (iii) placement/installation and
durability. The same comparison also
draws attention to the fact that, by
virtue of its manufacturing process, a
geotextile does not exhibit the suscep-
tibility to material segregation, during
construction, nor internal instability,
arising from seepage-induced migra-
tion of finer grains, that can occur in
a granular filter. Indeed, it has been
argued that segregation, together with
seepage-induced internal erosion of
a granular filter, constitute one of the
greatest risks to be managed in dam
safety engineering (ICOLD, 2014). In
this regard, the potential for a geotex-
tile to serve as an adjunct to a granular
filter in critical applications, wherein
the two materials provide a compos-
ite filter layer, may yield significant
benefit and is deserving of careful
consideration.
References
Barrett, R.J.(1966). Use of plastic fil-
ters in coastal structures. Proceed-
ings 10th Int. Conf. on Coastal
Engineering, Tokyo, Sept. 1966,
pp.1048-1067.
Bertram, G.E. (1940). “An experi-
mental investigation of protective
filters.” Graduate School of Engi-
neering, Hard Univ., Cambridge,
Mass., Soil Mechanics Series, No.
7, 21p.
Burenkova, V.V. (1993). Assess-
ment of suffusion in no-cohesive
and graded soils. Proc. 1st Int.
Conf. on Geo-filters, Karlsruhe,
Germany 20-22 Oct. 1992 (eds.
J. Brauns, U. Schuler and M.
Heibaum), p.357-360.
Figure 3. Base soil filter interface (from Lawson, 1982). Figure 4. Functional requirements of a geotextile filter.