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Those are just a few
of the problems solved by the contractors in the following stories.
Flowing sand presents the challenge on the first project, Point
of the Mountain Aqueduct in the Salt Lake City area. Contractor
W.W. Clyde & Co. is installing 12 miles of 60-inch welded steel
water pipe through both rural and residential areas.
“We have to maintain
traffic and utilities through the corridor,” says Randy Lingwall,
Clyde’s project manager. “We’re trenching and
shoring the sections in the paved areas, but we can open-cut and
slope the trench out in the sagebrush.”
At the peak of construction
last summer, Clyde was running six different headings, each with
two or three excavators. For a typical crew, a Caterpillar 385 excavator
opened up the trench, a smaller excavator installs shoring, and
a third ’hoe handles backfilling. Pipe depth ran between 10
and 29 feet, and the shored sections were 12 to 14 feet wide.
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PHOTO: CLARK CONSTRUCTION |
| Complex shoring in underground site |
Shoring has proven to
be labor-intensive, because the fine sand flows so readily. “It’s
like trying to dig a trench in water,” says Lingwall.
So Clyde went to United
Rentals and bought slide rail shoring made by Efficiency Production
Inc. The shoring consists of driven posts and panels that fit down
between the posts, or rails. The excavator drives the rails into
place at specified intervals. Laborers connect the posts and panels
as construction proceeds. “It’s working as it was designed
to work,” says Lingwall. “This shoring is the only thing
we could find that would stop the sand from caving in, keep current
utilities in place, and let us maintain a narrow excavation.”
Finished Early
Adding a second construction crew is helping The Industrial
Company (TIC) complete a cut-and-cover box culvert job five months
ahead of schedule in Savannah, GA. When it’s complete, the
1-mile-plus project will provide double and triple box culverts,
and a stretch of concrete pipe, to drain stormwater that has often
flooded this otherwise charming coastal city.
“The biggest factor
behind finishing early was to start up with a second crew,”
says Carl Kleeman III, vice president of TIC’s Savannah office.
“We outfitted a whole second crew with John Deere equipment—two
large excavators, a mini-excavator, a 544J loader, and a 450J bulldozer.
Using just one crew would have taken another eight months.”
TIC used sheet piling
in two parallel lines spaced 32 feet apart, to support the trench.
Both the first and second crews excavate the trench with a John
Deere 330CLC excavator. Working in shallower ground, the second
crew has also used its excavator to drive sheet pile.
“The 330CLC works
great with the pile hammer,” says Mark Waltz, TIC’s
equipment manager. “It’s a lot faster to mount the pile
hammer on an excavator than a crane, because we’ve got the
hammer fitted with a quick coupler. He can drive pile, and then
in about 20 seconds he can put the hammer in its stand, grab the
bucket, and start excavating.”
After crews place bedding
stone and build a 48-foot length of floor slab for the box culvert,
a traveling concrete form moves into place. The form is shaped like
an upside-down U for each box of the culvert. The form fits between
the sheet pile and serves to build both the roof and the walls of
the culvert in one pour.
Building a culvert down
one street presented a challenge because of the narrow widths—essentially
residential front lawns—located outside the sheet piling.
Houses precluded driving concrete trucks alongside the sheet piling
and dumping concrete into the forms. Instead, concrete trucks had
to park at the forward edge of progress and then rely on a pump
to move material back to the forms.
Deep Benching
For a sanitary sewer project near Rockford, IL, contractors
are using both open-cut trenching and pipe-jacking methods. The
project includes a total of 4,800 feet of heavy-walled PVC pipe
of diameters varying among 18 inches, 15 inches, and 8 inches. “We
had to go through an 1,800-foot distance where there’s a 35-
or 36-foot cut,” says Steve Schlichting, one owner of the
general contractor, Schlichting & Sons Excavating Inc.
“If we had a 35-foot
cut, we’d first bench down 17 feet, but if we had a 26-foot
cut, we’d only bench down 8 feet so that we had a constant
18-foot trench,” he says. “Then we’d come back
and bench down 3 or 4 more feet. We had a 14-foot-high stacked trench
box, with an 8-footer and a 6-footer. The excavator would hit pockets
of sand and I’d have to back up and bench myself down, because
the trench box would pull me back into the hole.
“Safety is the
most important thing,” Schlichting says. “Because this
trench is so deep, it’s all the more life-threatening.”
The project’s
biggest challenge was the groundwater. “If you don’t
get ahead of the water, it gets ahead of you,” says Schlichting.
“If you let it get ahead, it’s fight, fight, fight the
water.”
To dewater the trench,
Schlichting put a 12-inch PVC pipe down into the ground at one manhole
and pumped from that around the clock. At one time, the contractor
also had two 2-inch pumps and one 3-inch pump inside the trench
box to pump water out.
At a junction between
the open-cut trench and one of the pipe-jacking lengths, Schlichting
installed a four-sided Pro-Tec trench box that is 40 feet long,
10 feet wide, and 22 feet high. It works like slide rail shoring
so that a side panel can raise up to allow pipe to be jacked into
the bottom of the hole. “This way, we’re protected on
four sides,” says Schlichting.
The pipe-jacking contractor,
Central Boring, encountered difficulty with drilling one stretch
at about 300 feet into the bore, Schlichting says. The drill kept
hitting rocks and became ineffective at that distance, so the contractor
had to dig out the last 40 feet by hand—and jackhammer the
rocks out. To some extent the rocks were a surprise, Schlichting
says. “Prior to that it was all bank-run sand and gravel.
Then we hit hardpan and in some areas hit pockets of sand that had
water in them.”
Trenchless in
Indiana
You don’t always have to dig a trench. For a $9.2
million sewer rehabilitation project in Madison, IN, contractor
Miller Pipeline Corp. employed a variety of trenchless methods:
- Pipe-bursting, 50,000
feet. Pipe-bursting is accomplished by pulling a bursting device
through an existing pipe. The new product pipe is usually attached
to the bursting device and is pulled into place as the device
advances.
- Cured-in-place pipe
(CIPP), 3,000 feet. With CIPP, water is used to invert a pipe
liner, much like pushing a sock inside-out with water. Once the
liner is in place against the host pipe, the hot water creates
a curing process that hardens the liner. Miller’s CIPP process
is called ToughTube.
- PVC form-and-fold
liner, 34,000 feet. Miller designs and manufactures a form-and-fold
liner known as EX, which resembles the CIPP process. The liner
is pulled into place by a cable and winch and is expanded and
cured with steam, says Miller’s Mark Hallett, vice president
of the utility division.
- Horizontal directional
drilling (HDD), 5,000 feet. HDD involves drilling a small pilot
hole using technology that enables the drill to be steered and
tracked from the surface. The pilot bore is launched from the
surface at an angle, usually 8 to 20 degrees with horizontal,
and moves to horizontal as depth is reached. The pilot hole is
enlarged by pulling back reamers, and in the final ream, the product
pipe is attached.
The City of Madison
allowed Miller an unusual degree of influence over the method selected
for most locations in the sewer system. “We were able to recommend
the best fix for the pipe rehab depending on what the CCTV [closed-circuit
television] survey showed,” says Hallett. “That was
the real success of that project. We work all over the United States,
and it is rare that we have that kind of influence over what is
best for the customer.”
Making Up Lost
Time
To provide a vital trolley connection between California’s
Mission Valley and East County San Diego, Clark Construction Group–California
LP last summer completed work on the $105 million San Diego State
University Tunnel and Station. Clark’s contract consisted
of four main sections: the 900-foot west section of open cut-and-cover
box culvert construction; a 1,070-foot section of rock tunnel; a
670-foot excavation to provide for construction of the main station;
and a 1,000-foot east section of open cut-and-cover box culvert
construction.
“We needed to
close a road to get access to the tunnel construction site, and
the road closing was delayed by about six months,” says Jim
Day, Clark’s project executive. “If we could have had
access to the tunnel site, we would have started the tunnel, the
station excavation, and the east box culvert—all three sections
together—and moved dirt from west to east. Then we would have
taken dirt out of the west section and built that box culvert.”
But the delay in access
to the tunnel caused Clark to reorder the work sequence to make
up the lost time. “Work on the station began before tunnel
construction,” says Day. “Then we modified the sequence
of excavation so that the station and tunnel finished construction
together.” Voilà! The new sequence erased the delay.
To excavate the tunnel,
Clark used the New Austrian Tunneling Method (NATM). The tunnel
was approximately circular in cross-section, 68 feet across. Using
a specialized hydraulic excavator, Clark tunneled out the top half
of the circle first, working alternately on the upper left quadrant
and the upper right quadrant. The contractor could excavate about
3 linear meters of tunnel per 24-hour day in the rock, called stadium
conglomerate. As tunneling progressed, the contractor installed
shotcrete and steel ribs to support the tunnel walls. Following
the upper-half excavation, the lower half of the tunnel was excavated
and shotcreted.
Whether your underground
projects call for trenchless sewer linings or NATM tunnels, certainly
the challenges are out there. So are the solutions, if you look
for them.
Daniel C. Brown
is the owner of TechniComm, a communications business based in Des
Plaines, IL.
GEC
- March/April 2006 |
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