CSO project in Portland benefits from environmental engineering
Paul G. Cummins,
Baker Tanks
A diver prepares to go into the Nicolai Shaft in
early May 2003. Photo: Sue Bednarz
It is hard to imagine the type of
municipal infrastructure required
to manage the Pacific Northwest’s
annual heavy rainfall. One of the
situations plaguing a seasonally waterdrenched
city is a combined sewer
overflow (CSO) which occurs because
combined sewer pipes are not large
enough to carry both stormwater and
sanitary flow to the treatment plant.
During a CSO, stormwater fills the
combined sewers. The overflows
carry bacteria from the
untreated sewage, as well as
other stormwater pollutants
directly into the river.
In 1991, the city of Portland,
Oregon’s Environmental Services
division recognized the
need to reduce its CSOs from
the nearby Columbia and
Willamette Rivers. To date,
Portland has reached the halfway
mark in its 20-year effort
to stem CSOs - completely
stopping the CSOs from the
Columbia Slough and now
concentrating on the Willamette
River projects.
Geology proves challenging
One of the main projects is
the building of the West Side
pipeline, a $293 (US) million
sewer improvement dubbed by
locals as the “West Side Big
Pipe.” One of the main contributors
to the West Side CSO’s
construction success is Baker
Tanks, who provide temporary
liquid and solid containment
rentals.
The West Side CSO is a project
designed to reduce the volume of
untreated combined sewage that overflows
into the Willamette River during
rainstorms. Attesting to the environmental
effectiveness of this operation,
the prior Columbia Slough project
reduced CSOs to the Slough by 99 percent.
Additionally, over the past 11
years, Portland’s Environmental
Services has removed 1.8 billion gallons
of stormwater from the combined
sewer system and has reduced total
CSO volume by 53 percent annually.
The undertaking - the largest public
works project in the city’s history -
includes building an 18,500-foot
pipeline on the west side of the
Willamette River, and a large pump
station on the east side of the river on
Swan Island in North Portland.
The Nicolai Street shaft is the first
of four access shafts to be built that
connect to a 120-foot deep, four-mile
long, 14-foot diameter tunnel at various
locations along the alignment. The
shafts will be used for a variety of
functions such as providing personnel
and equipment access, surge storage
capacity and air venting. The Swan
Island pump station will eventually be
the connecting point between the east
and west side sections of the CSO
project. The east side section is still in
the design phase.
The general contractor for the project
is a joint venture between two companies,
Impregilo and S.A. Healy.
Impregilo is an Italian firm with extensive
mining experience, while S.A.
Healy, Impregilo’s Chicago-based sister
company, has extensive underground
construction experience. There
are only a handful of firms in the world
that can do this kind of specialized
work.
“The geology on this job is
extremely challenging,” notes Jim
McDonald, project manager for
Impreglio/S.A. Healy. “All of this
work is being performed in silt
and sand that is below the water
table. The soil is extremely wet
and very permeable which
makes it difficult to excavate.
To support the excavation, we
are using a variety of deep foundation
techniques that are cutting
edge, such as slurry wall
construction and jet grouting.”
McDonald adds that they
have generally been able to control
the water from the deeper
shaft and tunnel construction
without having to discharge it.
However, surface excavations
have resulted in large amounts
of water that must be filtered
and disposed of in an environmentally
compliant manner.
“We use Baker Weir tanks
for filtering the sediment from
the groundwater,” he says. “A
big advantage is that we’re
working in an urban area and
these steel tanks take up a
smaller footprint than others.
We’re able to run our discharge
water through the Baker Weir
tanks, then release into the city sewer.
The six sites on this job each have a
number of their tanks because even if
we don’t get water when we excavate,
we have to be prepared in case we do.”
Providing an efficient solution
To stabilize the porous ground and
construct the shafts, Impreglio/S.A.
Healy called upon a joint venture of
subcontractors Bencor, Petrifond, and
Pacchiosi to perform the $50 (US) million
slurry wall construction and jet
grout ground improvement.
“We have been using as many as
seven Baker EZ Access tanks in each
location,” says Vince Luongo, Bencor,
Petrifond, and Pacchiosi project manager. “The Baker
tanks contain the bentonite that stabilizes the trenches that
we’re excavating to form the Nicolai shaft construction for
the tunnel-boring machine (TBM) entry.” In August 2003,
the contractors used the Nicolai shaft as a launching point
for two TBMs - one tunneling north and the other tunneling
south - to build the CSO tunnel. The tunnel-boring
machines were specially manufactured in Germany for
this project.
To assemble the sophisticated equipment below
ground, the Impreglio/Healy crew lowered the machine
parts - the largest weighing in at 85 tons - a piece at a time
to the bottom of the 60-foot-wide shaft. The 10-man team
of mechanics and labourers then hooked up motors,
attached hydraulic hoses, programmed the guidance system,
and connected the water and slurry pipes.
Custom manifolds connect pumps and filtrations systems
on the east and west sections of the project.
Once assembled, the TBMs began chewing a tunnel 16
feet in diameter through the earth. “The machines bore at
one end and spit out the 3- to 5-foot-long concrete pre-cast
pipe rings at the other,” says McDonald. “After we install
the 14-foot diameter concrete lining at the tail of the
machine, hydraulic jacks move the cutter forward and
make room to install the next ring.”
Using this method, it is estimated that it will take about
11 months to bore the north tunnel to Swan Island; the
south tunnel is expected to take 23 months.
Baker's local branch in Portland, Oregon provided the
21,000-gallon EZ Access tanks to assist with the solids
and liquids containment on the construction site. The
tanks’ four proprietary hinged deck lid panels lift up for a
visual inspection of all the internal contents. Additionally,
Baker installed 6-inch valves to customize each tank for
the job.
Creating innovative solutions
Of special note is Bencor, Petrifond, and Pacchiosi’s jet
grouting process. The jet grout process involves pumping
high-pressured air, water and cement into the 330 feet
deep holes. This has only been done a few times at this
depth. The air and water erode the soil in a column form.
Then the soil is flushed out of the ground to the surface
and the jets backfill behind the flushed soil with the grout
to form a stabilized, cemented ground. The result is a
water-impermeable ground well suited for excavation and
construction.
