Tunnel Project Boringly Smooth. That's Good.

Story by Mike Larson, Editor | September 28, 2010

Super Excavators' Kobelco CK950 crane prepares to lower the 68,000-pound head of the Akkerman tunneling machine down a 50-foot access shaft at 2nd and Maple streets in Milwaukee

Super Excavators, Inc., Menomonee Falls, WI, is currently ahead of schedule and on budget in a nearly $30-million tunneling project that will install nearly two miles of replacement sewer under the east side of Milwaukee, WI, for the Milwaukee Metropolitan Sewerage District (MMSD).

The project, named the Metropolitan Interceptor Sewer System Barclay/4th/Chase MIS Replacement, is replacing a deteriorated section of sewer with a new run of concrete sewer pipe that will connect two existing sections but follow a different route from one to the other.

The project started in November 2007, with soil testing and designing.

Preparatory site work started in April 2008, and tunneling began in August 2008.

Tunneling is scheduled for completion in October of this year, with the entire project to be finished in December.

By the project's completion, Super Excavator's six-person core team will have installed 6,051 feet of 72-inch ID pipe and 4,200-feet of 48-inch ID pipe by microtunneling.

It will also have installed about 100 feet of 48-inch ID pipe using open-cut trenching.

Since August 15, Super Excavators has installed about 3,600 feet of pipe.

On average, every hour of boring lengthens the tunnel by about eight feet and installs one new 8-foot-long section of precast concrete pipe. The rate is the same for both the 72-inch and 48-inch diameter pipe.

When a run of pipe has been put in place, crews pump grout around the outside to stabilize it in place.

Downshaft Comes First

At the bottom of the access shaft, workers cut a hole through the wood and steel bracing where the tunneling machine will start the next bore. The silver ram, visible at bottom, will push the cutter and pipe sections forward.

But before the crew can bore a section of the horizontal shaft that supports the sewer pipe, it must construct a vertical access shaft in order to lower the tunneling machine to the specified depth in the earth.

On this project, depths range from 22 feet to 80 feet, with an average of about 50 feet.

The downshafts range from 20 feet to 24 feet across. Some are shored with sheet piling, but most on this project are supported by wooden-stave lagging held in place by metal rings.

The shallower sections of each downshaft are dug by a hydraulic excavator. Each time the shaft is deepened by four feet, the crew assembles another section of wooden lagging and metal ring.

When the bottom of the hole becomes too deep for the hydraulic excavator to reach, Super Excavators uses a Kobelco CK550 lattice-boom crawler crane with clamshell bucket to finish the digging.

When the downshaft has reached the specified depth, the crew pours a concrete floor. After the floor has cured, Super Excavators sets up a horizontal ram and prepares it to accept the tunneling machine, which will bore the horizontal shaft from one downshaft to the next.

Tunnel-Boring Machine Is Key Equipment

The key equipment system on this job is Super Excavators' electrically driven Akkerman SL-74-90 tunnel-boring machine (TBM), coupled with its Derrick TBS225 slurry-separation plant.

The business end of the tunneling machine is a 34-ton cylindrical body with extendible stabilizers along its sides and a circular, rotating cutter head mounted in front.

This part of the machine sits horizontally in the ground and cuts its way through the earth to form the tunnel for the concrete sewer pipe.

As the cutter head rotates, it eats away at the earth and high-pressure jets of water turn the excavated soil into slurry that is pumped through a hose to the separator unit sitting above ground.

The separator's vibrating screens and centrifuge separate the slurry into gravel, soil and water.

The gravel and soil drop into dump trucks that carry it to a landfill about an hour away.

The clean water is recirculated to the cutter to make more slurry.

The 900-ton-capacity hydraulic ram at the bottom of the vertical downshaft pushes the cutter forward through the ground.

After the cutter has advanced eight feet, the ram is retracted and Super Excavator's Kobelco 950 lattice-boom crawler crane lowers an 8-foot section of concrete pipe.

The new section of pipe is inserted between the ram and the cutter or a previously installed section of pipe. The ram then pushes the train of pipe sections, with the cutter in the lead, forward eight more feet.

The cutter and ram are operated from an enclosed control center above ground.

Maintenance Leads To Reliability

After the tunneling machine completes a run of pipe, ranging from 300 to 1,100 feet, it is lifted to the surface for complete inspection, maintenance and testing.

Says Tunneling Superintendent Brian Strane, "When the TBM is in the ground, you want it to work perfectly. Having a malfunction while it is between access shafts would cause a host of delays and problems. This is a case where you need to take every preventive step you can."

Thanks in large part to that stringent maintenance, Super Excavators has so far had 100-percent availability from the tunneling machine on this job.

Soil Variation Presents Challenges

Strane, who has been in the tunneling business for 14 years and with Super Excavators for the past four, says this project is pretty typical for the company.

"This is about the 103rd tunnel I've done in my career," says Strane. "In many ways, the process is pretty typical. But it does have one or two special challenges."

Says Strane, "The largest challenge here is the quick and extreme change in soils. We are tunneling in an uneven transitional layer seam that rises and falls rapidly. We've encountered 0-blow-count organic material that provides no resistance and also tough 30-blow-count clay, as well as cobbles, boulders and a sand-silt mix.

"Later in the project, we will even be going right from solid rock into soft ground," says Strane.

"The varying ground resistance and obstacles make steering the cutter a little more challenging," says foreman Nate Wiedmeyer, who has operated this machine for about three years on a series of jobs. "The laser guidance system works incredibly well, but you still have to pay continuous attention to what's going on."

A video camera and microphone mounted to the cutter feed a steady stream of pictures and sounds to the operator's station in the control cabin above ground. The camera can tilt, pan and zoom to let the operator see any area. These eyes and ears let the operator see and hear what's going on at the cutter head so he or she can make adjustments to keep the cutter on target.

Wisconsin Winter Presents Challenges, Too

Wisconsin's winter weather can present the crew with some challenges, too. Says Strane, "We work through most winter weather. Snow doesn't really affect us too much, but extreme cold can make things difficult."

Though the tunnel cutter is in the ground, which stays about 55 degrees all year round, the system's slurry sorter, pumps and electrical equipment sit above ground.

Because so much of the operation depends on water, freeze-ups are a real possibility.

To counteract them, the crew keeps two or more of every hose on hand. If one freezes up, they replace it and thaw the frozen hose inside a heated 8-foot by 8-foot by 20-foot container. Rotating the hoses lets the crew keep working through all but the coldest of winter days.

Cold affects the cranes, loaders and other equipment too. For those machines, Super Excavators provides heaters, more-frequent lubrication, and long warm-up times.

Safety Focus Eliminates Lost-Time Accidents

On this project, MMSD manages safety through an owner-controlled insurance program (OCIP), in which it sets the safety standards for all contractors working on the project.

But high safety standards are routine for Super Excavators. Strane says that he cannot remember a single lost-time accident on any of his projects during the four years he has been with the company.

He credits the company's strong focus on safety awareness and training. "If some of the days next week get as cold as predicted and we can't work," he says, "it will be a good opportunity to have the crew get more training from our company safety officer."

Safety measures on the job site include having gas detectors in the shafts, pumping fresh air from above ground into the work area, and having video cameras and microphones both at the cutter head and in the downshaft.

"A good safety record," says Strane, "is not only great for your crew and productivity, it also helps your bonding rates, insurance premiums and bidding opportunities."

Minority Contractors Involved

Two minority contracting firms are currently working as subcontractors to Super Excavators on the project.

Benavides Construction, Waterford, WI, is building the permanent concrete manholes for the finished access shafts.

Liberty Contractors, Inc., Franklin, WI, hauls away the sand, gravel and other solids excavated by the tunneling.