24
Geotechnical News • September 2016
GEOSYNTHETICS
Geotextile filter case study:
Alouette Dam spillway rehabilitation
British Columbia, Canada
Jonathan Fannin, Editor
Professor of Civil Engineering, University of British Columbia
Readers of this column may recall
that, in the GN: June 2015 issue, I
reported on filter applications with
a return to geotextile “basics” that
examined the measurement and report-
ing of pore size opening, cross-plane
hydraulic conductivity (permittivity),
tensile strength and soil-geotextile
compatibility. In projects where the
cost of remedial works is anticipated
to be significant, I acknowledged that
the state-of-practice is first to identify
a candidate geotextile on the basis of
index tests for pore size, permittiv-
ity and strength, and then to evaluate
its suitability for the proposed con-
struction application from laboratory
compatibility testing of a sample of
the base soil in combination with the
candidate geotextile filter. In this cur-
rent article, I describe a case study that
illustrates the approach with reference
to the specification of a geotextile
used in rehabilitation of the drainage
system beneath the spillway of the
Alouette Dam in Canada.
Alouette Dam, British Columbia
The Alouette Dam forms a part of the
Alouette-Stave Falls-Ruskin Hydro-
electric Complex, a sequence of three
dams that is located about 65 km east
of Vancouver, Canada. It is operated
by BC Hydro, a Crown corporation
that supplies most of the electrical
energy for the Province of British
Columbia. BC Hydro operates more
than 70 dams, with maximum heights
up to 240 m, and the continued safety
and operation of those dams is an inte-
gral part of the Provincial economy.
The Alouette Dam controls the level
of Alouette Lake. Impounded water
is diverted through a tunnel near the
head of the lake to a powerhouse and
hence into Stave Lake, from where it
passes sequentially through the Stave
Falls powerhouse into Ruskin Lake
and, lastly, through the Ruskin power-
house. In total, the three powerhouses
have the capacity to produce 205 MW
of electricity. The Stave Falls dam was
the first of the sequence to be con-
structed. It is a concrete-gravity and
rock-fill dam, approximately 120m
long, that was completed in 1912
and subsequently raised to a height
of approximately 24 m in 1925. The
Alouette Dam is an earthfill dam,
approximately 290 m long and 20 m
high, completed in 1928. The Ruskin
Dam is an overflow concrete gravity
structure, approximately 110 m long
and 60 m high, completed in 1930.
The Alouette Dam is located on a gla-
cial outwash formation. The earthfill
structure incorporates a cut-off trench,
a low level outlet and a spillway. The
majority of material used in construc-
tion of the dam was obtained from
excavation of soil for the adjacent
spillway. The construction method
involved excavation by steam-shovel
and placement by dumping from rail-
way wagons at or near the outer slopes
of the dam, followed by washing
toward a central pool in the embank-
ment in a manner of “semi-hydraulic
filling” (Carpenter, 1927) for which
“all the material in the core area was
thus deposited under water and the
voids in the coarser materials form-
ing the slopes at the edges of the core
were well filled with fines”. In 1983,
the need for a seismic upgrade of the
structure to accommodate revised
design seismic loading resulted in the
Jonathan Fannin
