Geotechnical News • September 2016
27
GEOSYNTHETICS
general conformance with an AAS-
HTO Class 2 material. Details of the
three geotextiles that were tested and
found acceptable to B.C. Hydro were
reported in an appendix to the specifi-
cation document.
Prior to ordering the geotextile, the
Contractor was required to submit
for acceptance, a 1m x 1m sample
of the proposed material, including
the manufacturer’s name and product
name. The transportation, storage and
installation of the geosynthetics was
specified in a manner that limited the
maximum total duration of exposure
to ultraviolet light to a period not
exceeding 14 days. Prior to installa-
tion of the geotextile, the Contractor
was required to submit for review and
acceptance, information on the manu-
facturer, product name, lot number
and roll number of each geotextile roll
delivered to the site. No installation
was to occur prior to the acceptance of
all submissions.
Construction and performance
monitoring
The spillway rehabilitation project
commenced in 1992, and was com-
pleted over a period of several months.
The filter was specified to be placed
in a slackened condition such that it
would conform to the subgrade sur-
face area. No construction equipment
was allowed to operate directly on the
geotextile. On the steep side-slopes of
the spillway channel, placement of the
geosynthetic filter and cuspated sheet-
drain was found relatively straightfor-
ward. In contrast, and as anticipated
in the early stages of design, it proved
challenging and time-consuming to
achieve compaction requirements for
the granular filter and drain on the
steeper sections of the spillway profile.
With regard to strength of the geo-
textile, and construction survivabil-
ity, engineering inspection revealed
no physical damage to it during the
period of installation.
The laboratory testing of soil-geo-
textile compatibility had identified
three candidate geotextiles, and the
Contractor elected to select one of
them for use in construction. At the
time of writing the specification
documents, the laboratory evaluation
of soil-geotextile compatibility had
been made for a select combination of
confining stress and hydraulic loading,
and over a relatively short duration of
time. Since completion of construc-
tion, nearly 25 years ago, the perfor-
mance of the drainage system has
been subject to ongoing performance
monitoring. The performance monitor-
ing includes pore water pressure mea-
surements within the drainage system
under the stilling basin. Observations
from the monitoring program indicate
the geosynthetic filter and sheet-drain
are part of a composite drainage
system whose overall performance,
like that of the rehabilitated spillway,
is fully in accordance with design
expectations (B.C. Hydro, personal
communication).
Closing remarks
It is widely-accepted practice to
specify a geotextile for a filtration
application with reference to (i) provi-
sion of adequate material strength and
durability, (ii) an empirical rule gov-
erning base soil retention, and (iii) an
empirical rule governing base soil per-
meability. The development of current
practice is shown to be founded on a
long-standing body of field and labo-
ratory experience, acquired in many
countries, over a period of more than
50 years – something that we have
addressed in earlier GN:Geosynthetics
articles.
Soil-geotextile compatibility is
predicated on the geotextile hav-
ing adequate strength to ensure no
adverse damage during the process
of installation (termed ‘construc-
tion survivability). Thereafter it must
endure the working environment of
the installation over the service life
of the structure (termed ‘durability’).
Recommendations for construction
survivability of geotextiles were
first addressed in a systematic study
conducted in 1972. They have since
been refined over time, with current
practice giving recognition to several
classes of material strength, each of
which is established with reference
to standardized index tests. Durabil-
ity studies have been ongoing for
a similar period of time, with early
contributions associated with specific
case history records dating back to a
1969 revetment application in Florida
and a 1970 dam application in France.
Insights to the governing influence of
thermal-photo-oxidation degradation
mechanisms on material durability are
consistent with field observations over
many years. Indeed, the basis for pro-
vision of adequate material strength
over the service life of the structure is
now well-understood.
“On the steep
side-slopes of the
spillway channel,
placement of the
geosynthetic filter
and cuspated
sheet-drain was
found relatively
straightforward.’’
The requirement for soil-geotextile fil-
tration compatibility is contingent on
there being no unacceptable erosion
as a consequence of soil loss through
the geotextile while, at the same time,
providing for unimpeded flow of water
from the soil through the geotextile.
Empirical design criteria for soil
retention and permeability were first
established for woven and nonwoven
geotextiles in the period 1972-1975.
They have since been developed and
refined, mostly from the findings of
laboratory studies, and with occasional
reference to companion theoretical
analysis. The criteria relate a charac-
teristic opening size of the geotextile
to a characteristic grain size of the
