Some underground projects go smoothly, and others are a challenge. Dry sand may flow into your trench and require you to shore it quickly. Often, you have to fight water. You may have to drill and jack pipe more than 300 feet into the soil and then dig out the last few feet by hand because the drill rig can’t reach that far.
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.