Geotechnical News • March 2015
51
GEOSYNTHETICS
“In a limited number of cases, geotex-
tiles have been used as deeply buried
filters in France, Germany, South
Africa and a few other nations. Most
notable, is a geotextile installed as
a filter for Valcross Dam which has
been successfully performing for over
35 years. These applications remain
controversial and are not considered to
be consistent with accepted engineer-
ing practice within the United States.
Because geotextiles are prone to
installation damage and have potential
for clogging, their reliability remains
uncertain. …. It is the policy of the
National Dam Safety Review Board
that geotextiles should not be used in
locations that are both critical to safety
and inaccessible for replacement.”
The specific concern of FEMA (2011)
arises from the successful outcome of
research studies, on sand and gravel
filters, and their usage over many
years in dams (Fig. 2). In contrast,
the usage of geotextiles in embank-
ment dams is deemed to be very
limited. Further, a concern is raised
that materials on the downstream
side of a geotextile will not provide
adequate support to the discharge face,
causing the geotextile to bulge and
thereby encourage detachment of base
soil particles that contribute to (i) the
formation of “filtered layer cake with
a very low permeability” and result
in clogging of the geotextile, else (ii)
piping if the opening size distribution
of the geotextile is overly large. Expe-
rience is cited from geotextiles used
under riprap, and in highway drainage
work, in support of the concern. It
leads to the recommendation that “due
to issues with clogging, geotextiles
should only be used in non-critical
areas of embankment dams”.
The scope of the USBR (2011) chapter
of the design standard applies to
“naturally occurring earth materials
or to filters manufactured from such
natural earth materials by grading,
screening, washing, and crushing.
… Filters of woven or nonwoven
fabrics are generally not recommended
for use as protective filters and are
excluded from this chapter.” Refer-
ence is made to Appendix B of the
Chapter 5, in which the content of the
FEMA (2011) publication is largely
reproduced verbatim in which discus-
sion of the shortcomings related to the
use of geotextiles.
SANCOLD (2014) revision of
ICOLD Bulletin 55 (1986)
The history of dam engineering from
ancient through to modern times
reveals the use of an engineered filter
layer is a relatively recent develop-
ment that was formalised by Karl Ter-
zaghi less than 100 years ago. In the
time since then, and more particularly
in the last 25 years, it is reasonable
to claim that considerable advances
have been made in our understanding
of both granular and geotextile filters.
Likewise, there is a more informed
appreciation for the challenging issues
that exist in the sourcing, manufactur-
ing, placement, performance and dura-
bility of both granular and geotextile
filters. Indeed, there are many similari-
ties to be found in case histories that
report on deficient performance.
In reporting on geotextile filters in
dams, the SANCOLD draft revision of
ICOLD Bulletin 55 seeks to address,
amongst other factors:
Figure 2. Current regulatory guidance includes commentary on both granular
and geotextile filters.
