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Geotechnical News • December 2014
www.geotechnicalnews.com
GEOSYNTHETICS
Introduction by Jonathan Fannin, Editor
Professor of Civil Engineering, University of British Columbia
Geofilters
In marking the return of this col-
umn in the GN:June 2014 issue, I
reproduced the quotation attributed
to Albert Einstein that “the value of
a good education is not the learning
of many facts, but the training of the
mind to think of something that cannot
be learned from textbooks”. In the
subsequent GN:September 2014 issue,
I noted that we have an extensive body
of technical and case-study informa-
tion on geosynthetics, in the form of
conference papers and peer-reviewed
journal articles, much of which has
been published through the auspices of
the International Geosynthetics Soci-
ety (IGS) and its joint USA-Canadian
chapter, The North American Geosyn-
thetics Society (NAGS). The body of
technical and case-study information
on geosynthetics represents a valu-
able source of knowledge. As Karl
Terzaghi noted of knowledge develop-
ment in soil mechanics and foundation
engineering, such information allows
us to “to discriminate between what
we really know and what we merely
believed”. The underlying premise of
Terzaghi’s observation informs this
column on the subject of Geofilters.
In Geofilters: Part 1, I compare the ori-
gins of current practice for the specifi-
cation of a geotextile filter with those
for the specification of a granular filter.
I make the comparison now because
the South African National Commit-
tee on Large Dams (SANCOLD) has
just finished preparing, in September
2014, a very substantial revision
of the 1985 ICOLD Bulletin 55 on
“Geotextile Filters in Dams”. The new
draft bulletin, which is currently in a
review-consultation process, reaches
the conclusion that “Geotextiles can
thus be used in non-critical aplications
as primary filters and can be used as
adjuncts to granular filters in criti-
cal applications to form a composite
filter material”. This latter statement
represents a significant advance in
geosynthetics engineering practice.
Accordingly, it is timely to contrast the
path-of-discovery through which our
practice in granular filters has evolved,
with the origins and development of
our practice in specifying a geotextile
filter. The comparison of these two
materials provides an opportunity to
discrimate between what we really
know, and what we merely believed,
about the development and relative
merits of using a geotextile filter.
The companion Geofilters: Part 2 that
will appear in the GN: March 2015
issue will review select guidance that
is currently used for geotextile filters,
and also that for granular filters, plac-
ing specific emphasis on laboratory
tests recommended for evaluation of
soil-filter compatibility, as well as
considerations for material placement
and durability. Thereafter GeoFilter:
Part 2 will describe the SANCOLD
activities and outline the contents of
the revised bulletin. The objective is
to place in context the proposed use, in
dam engineering, of a geotextile filter
as an adjunct to a granular filter in
critical applications.
Granular filter: origins of
current practice
The principle of using a filter material
to control groundwater seepage and
protect against subsurface erosion was
first studied in a systematic manner
by Karl Terzaghi, for whom it had
become a special interest through
some of his early consulting experi-
ence on seepage control for small
weirs, most notably the weir at Hallein
in Austria (Fannin, 2008). He first
completed a series of fundamental
laboratory permeameter tests (Terza-
ghi, 1922) to examine the concept of
a graded granular filter. Thereafter,
his laboratory testing was conducted
in partnership with industry in order
to establish empirical design rules for
larger weirs and zoned earth fill dams,
most notably the Bou Hanifia dam in
Algeria (Terzaghi, 1939). Over time,
Jonathan Fannin