Sixty-three
years of debris had accumulated in the main sanitary sewer
in St. Paul, Minn. Dug under a limestone stratum in 1937,
the 6-foot tall by 2-foot wide semi-oval tunnel runs five
miles through the city's west end.
During the winter of 2000, the St. Paul Sewer Division (SPSD)
began cleaning the entire sewer by removing the two feet
of accumulated sand and gravel with shovels and wheelbarrows.
Crews labored 100 feet below ground in three feet of water
flowing at 700 to 900 gallons per minute. "Our guys worked
hard, and were getting badly beat up," says Jim Sirian,
the shop's maintenance foreman. "It also didn't make
sense to have a $250,000 vacuum truck sucking debris out
of a wheelbarrow."
The
next winter, after paying $100,000 to a contractor to clean
400 feet of line using two cranes and a bucket, the department
again looked in-house. Sirian knew that new machinery would
allow city personnel to do the jog, and he suggested calling
Jeff Schultz of Sewer Equipment Company of America, based
in Glenview, Ill. The two had met at the Minnesota Onsite
Sewage Treatment Contractor's Association's annual conference
in January.
The
amount of water in the tunnel made jetting impractical,
so Schultz recommended an old-fashioned solution--clamshell
bucket machines--to conquer a 21st century problem and save
the department millions of dollars.
Running
Deep
St.
Paul sits on a bluff overlooking the Mississippi River,
and its sewers and storm tunnels run 60 to 100 feet deep,
depending upon whether the manhole is set on a hill or in
a dale.
"When
the pull-in machine and truck loader arrived in the spring
of 2002, they paid us back in the first two weeks,"
says Sirian. Throughout the summer, a two-man crew extracted
two to four cubic yards of debris a day using an 18-inch
bucket.
"That
doesn't sound like much," continues Sirian, "until
you realize that each pull averages 1,000 to 2,000 feet
between manholes, then comes up 100 feet. This is a lot
of work."
In one
1,700-foot section on the outskirts of St. Paul, at least
two feet of sand and gravel with the consistency of fresh
asphalt lay on the bottom. The tunnel is less than 100 feet
deep here because the manholes are in a hollow.
Work's
A Drag
After
parking the pull-in machine over one manhole and truck loader
over the next consecutive manhole, each operator (one per
machine) sets the start and stop meter on his drag line.
The distance between manholes determines the drag line's
length, which is 1,700 feet for this section.
The
clam bucket is in the middle of these two drag lines. Tension
on the hoist cables when raising the bucket tells the men
if they dragged far enough to fill it. By the time the bucket
arrives at the surface, all the water in the debris has
drained out. The bucket then travels up a ramp on the truck
loader and dumps into a 3-cubic-yard garbage container.
When this is full, the crew radios for one of the department's
four Vactor trucks to transport the debris and dump it into
a 12-foot-diameter interceptor pipe near downtown St. Paul.
A 100
mgd flow rate carries the sand and gravel two miles to the
city's wastewater treatment plant, where it passes through
a grit chamber and settling tanks before being mechanically
raked and pumped off the bottom. It is dewatered, then hauled
to a landfill.
OSHA
Compliance
"It's
a big process," says Sirian. "We're moving tons
of sand and gravel. Right now, we're halfway through cleaning
the entire sewer. It's impossible to estimate when we'll
be done, because work is interrupted whenever our two-man
crew is needed elsewhere for priority projects or emergencies."
Pulling
a total of 15 to 20 miles a day for weeks and months is
causing parts on the equipment to wear out. If the bucket
jumps the rollers or a chain snaps, it usually happens in
the tunnel. "We follow OSHA's confined space regulations,"
says Sirian. "Our guys radio for two more, then wait
for them to serve as attendant and entry supervisor. Before
going down, they meter the air in the manhole. They each
have two air monitors with them and carry 15-minute air
packs." The SPSD has had no work-related injuries on
this job.
Since
the hydraulic bucket machines went into operation, Sirian
estimates they have saved the department $2 million. A second
set has been ordered.
Reprinted
with permission from Cleaner, (August) 2003 / COLE Publishing
Inc. / 800-257-7222 / www.cleaner.com
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