Deep Tunnel Construction Under Way

By Mike Larson, Editor | September 28, 2010

Working 320 feet underground, tunnelers from Shea/Kenny JV are beginning to carve out the 23-foot-diameter horizontal tunnel that will become the newest reservoir in the Milwaukee Metropolitan Sewerage District (MMSD) deep-tunnel system.

This is the third tunnel that has been cut into bedrock beneath the city to temporarily hold the exceptional volume of sewage and stormwater that sometimes comes from heavy rainfall or snow melt.

New Tunnel Adds Capacity

Running for two miles under North 27th Street, from W. Hampton Avenue north to Mill Road in Glendale, the new $65-million tunnel will add 27 million gallons of storage capacity to the 494 million gallons provided by two existing deep tunnels built in the 1980s and 1990s, respectively.

The three tunnels will be connected to provide one 521-million-gallon holding tank.

When finished, the new deep tunnel will measure 21 feet in diameter by 11,000 feet long. At each end, a vertical shaft 24 feet in diameter and 320 feet tall will connect the tunnel's horizontal shaft to the earth's surface.

In building the tunnel, Shea/Kenny will initially bore a 23-foot-diameter hole through bedrock, then line it with a 1-foot layer of concrete, yielding the 21-foot finished diameter.

Work Moving Ahead On Schedule

Crews started constructing the tunnel's first vertical shaft in May 2007 at North 29th Street and Hampton Avenue.

Construction of the vertical shafts is an interesting story in itself.

The first 70 feet below the surface consisted of soft earth. So Shea/Kenny sank a 24-foot circle of refrigerant tubes all the way to bedrock in order to freeze a ring of earth and form a solid wall within which they could excavate a shaft and concrete its walls.

After excavating down each 10 feet, the crew poured a concrete wall around the face of the hole to form a concrete shaft.

When the shaft had reached bedrock at 70 feet, the technique changed to drilling and controlled blasting until the shaft reached its 320-foot final depth.

When the first vertical shaft had been completed in March, Shea/Kenny's crews immediately began carving out the horizontal main tunnel.

First Work Done By Hand

Most of the 2-mile-long tunnel will be cut through the rock by a 2,100-horsepower tunnel-boring machine (TBM) that's 23 feet in diameter and up to 385 feet long.

But right now, tunneling crews are blasting, and using power drills, pry bars and a bulldozer to dig out a 23-foot by 450-foot underground chamber in which to assemble the TBM so it can be ready to work by the end of July.

This initial work consists of drilling several 9-foot-deep holes into the front wall to check for ground water. If water comes out of the probe holes, the crew fills them with grout to stop the leakage.

Then several 8-foot-deep holes are hand-drilled into the rock face at prescribed locations, filled with explosives, and rigged with a detonator for each hole.

The detonators set off their charges in a precisely timed pattern so the middle blows first and the outsides blow milliseconds afterward. The controlled blasting causes the rock to all fly inward, preserving the outside walls.

When the blasting is done, the crew checks the walls and ceiling of the new area. Small loose pieces are chipped out with a handheld pry bar.

Larger loose areas are secured by drilling a hole and inserting an 8-foot-long rod that has a flat plate on its inside end to help secure the loose rock. The rod is glued securely into its hole with high-strength resin.

Pieces of the blasted rock typically measure about 3 inches to 5 inches in diameter. They are loaded into a container, hoisted up the entry shaft by a crane, and hauled away for use in roadbeds, for making concrete, or in other construction applications.

The work goes on around the clock, five days a week, with about 15 people working on each shift.

TBM Coming In June

The manual digging of the chamber will go on until early June, when the TBM will arrive at the site on 45 trailer trucks.

Its components, weighing up to 209,000 pounds each, will be lowered down the entry shaft and assembled in the chamber.

The assembly is expected to take about a month.

TBM Custom-Built

For the past several months, the TBM has been undergoing upgrades and modifications under the direction of Shea/Kenny's equipment superintendent Keith Walter, who has more than 24 years of experience working with TBMs and other heavy construction equipment.

Working in an unused factory in Milwaukee, the refurbishing team integrated components from two TBMs to create a customized machine built specifically to meet the needs of this job.

They then completely assembled the 23-foot by 385-foot TBM, made further modifications and fully tested all of its electrical, hydraulic, water, and lubrication systems before dismantling it for shipment to the tunnel site.

The machine features a rotating cutter head that measures 23 feet in diameter and carries 48 metal cutting discs on its face. As the cutter head rotates, the cutting discs put up to 35 tons of pressure on the rock wall, causing the rock to fracture and fall off the face.

Two 350-horsepower water-cooled electric motors drive the head, while four more identical motors operate the conveyors, hydraulics and other systems.

Scheduled Completion Is End Of 2009

While the TBM is being assembled at one end of the new tunnel's route, Shea/Kenny will begin building the tunnel's second vertical shaft two miles away on Mill Road in Glendale, where the tunnel will end.

The process will be the same as that used to build the first vertical shaft. In fact, the ground-freezing ring is already in place and doing its work.

Pouring of the tunnel's 1-foot-thick concrete liner is scheduled to begin in April 2009, with the tunnel put into operation later that year.

MMSD Public Information Manager Bill Graffin explains, "The Milwaukee Metropolitan Sewerage District serves 28 communities that have a total of 3,300 miles of sewers. Our two regional treatment plants can process all the waste generated by the 411-square-mile area we serve. If sewage were all that the system had to handle, there would never be any overflow."

Graffin says, "In addition to sewage, however, the system has to process millions of gallons of water that leak into the sewer lines from the ground and streets. It even handles that extra water quite well."

"But when even more runoff pours down some combined sewers during heavy downpours of rain, the total amount of water and sewage will sometimes add up to more than the two regional treatment plants can handle all at once."

"The deep tunnel system," he says, "forms a reservoir that can temporarily store the sewage and extra storm water until the treatment plants catch up."