Big blasts at harbor rock out near Big Gig

By DON BEHM
dbehm@journalsentinel.com

Posted: July 6, 2007

Harbor Tunnels

Photos/Jack Orton Miners and Milwaukee Metropolitan Sewerage District workers are lowered into the joining of two 17-foot-diameter tunnels for sewer pipes this week near the Milwaukee harbor.

"Fire in the hole," yells MMSD senior project manager Larry Ellis (left) as Paul Nolan sets off dynamite to blast through a tunnel.

About 250 feet underground, crews inspect progress of one of the tunnels this week in the harbor siphons project intended to reduce sewer overflows into Lake Michigan.

Graphic/David Arbanas Click to enlarge

Breaking Ground

One tunnel will extend 2,050 feet north from Jones Island to an access shaft at Erie St. Another tunnel will extend 2,400 feet west to a shaft near Scott and Barclay streets.

As musicians pounded rhythms and rocked the stages of Summerfest during the past nine days, construction crews 255 feet below the Milwaukee harbor were shaking the earth in their own way by blasting through bedrock to create two 17-foot-diameter tunnels for sewer pipes.

There is no precise tempo to the six daily underground blasts, three in each tunnel.

Before each one, a machine drills 80 holes precisely 10 feet, 6 inches into the rock face. Several footlong sticks of dynamite are packed into the 2 1/2 -inch-wide holes with long rods, then workers are evacuated by stepping into a steel cage. A crane lifts the cage out of an access shaft.

An air horn at the surface sounds one long sustained note when all are clear. Blasting caps start the multiple explosions, and workers waiting on the rim of the shaft feel Jones Island shake beneath their feet as a quick succession of booms reverberates the length of a tunnel and out of the hole.

"See if the Big Gig can compete with that," Chuck Kennedy said this weekas he walked away from the shaft to return to a nearby trailer office. Kennedy is a resident engineer with consulting company Earth Tech Inc. and supervises daily activities.

The Milwaukee Metropolitan Sewerage District hired a joint venture of JF Shea Construction Inc. and the Kenney Construction Co. to create the two tunnels as part of a $138 million harbor siphons project intended to reduce sewer overflows into Lake Michigan.

One tunnel will extend 2,050 feet north from Jones Island, nearly the length of a football field beneath the Milwaukee River, to an access shaft at the south end of Erie St., west of the Summerfest grounds, said Larry Ellis, a senior project manager with the sewerage district. The other tunnel will reach 2,400 feet west, the full 255 feet beneath the Kinnickinnic River, to a shaft near Scott and Barclay streets.

By Friday morning, blasting had pushed the north tunnel 870 feet and the west tunnel had progressed 920 feet, Ellis said. Before tunneling could begin, workers first excavated a vertical shaft from the surface of Jones Island to a depth of 255 feet. The two tunnels extend horizontally from the bottom of the shaft.

A cluster of pipes, or siphons, placed in each tunnel will carry sewage and storm water collected by several large regional sewers directly to the Jones Island sewage treatment plant during heavy rains.

Offsetting the bottleneck

The siphons will reduce the risk of sewer overflows in two ways: pushing more wastewater to the plant and saving space in the deep tunnel storage system for wastewater from other areas, said Mike Martin, director of technical services for the sewerage district.

In 2002, a state Legislative Audit Bureau evaluation of sewer overflows criticized a bottleneck that occurs during storms: existing siphons could deliver only up to 260 million gallons of wastewater a day to the plant, though its treatment capacity is 330 million gallons a day. That resulted in as much as 70 million gallons a day of wastewater unnecessarily going into the deep tunnel during heavy rains. When the underground storage caverns reach their capacity, sewers overflow.

In a year's time, the lack of siphon capacity results in an estimated 1 billion to 2 billion gallons of wastewater being diverted to the deep tunnel that could have been treated immediately, according to the audit. Wastewater reaches the Jones Island plant only from the existing siphon or from the deep tunnel.

The additional siphons, rehabilitation of regional collection sewers that connect to the siphons and the construction of a 2 1/2 -mile-long wet weather relief sewer beneath Canal St. in the Menomonee Valley that will feed the siphons will offset the bottleneck. The three projects will save 1 billion to 2 billion gallons of capacity in the deep tunnel during a year, Martin said.

On Monday, the sewerage district's governing commission will be asked to award a $34 million contract to Michels Corp. for construction of the Canal St. sewer. It is scheduled to be completed in 2010 as part of a court-ordered overflow reduction plan.

The new harbor siphons could be operating by summer 2009.

In creating the 17-foot-diameter tunnels for the siphons, crews blast 10 feet of rock at a time. Monitors on the surface measure ground vibrations throughout the construction area.

Each explosion yields about 160 cubic yards of shards, said Darrell Vliegenthart of JF Shea, the project's general superintendent.

Crews must wait about 30 minutes after a blast for the smoke and dust to clear before returning to the tunnels. Rock is scooped up and taken to the access shaft, where it is dumped into a steel bowl capable of carrying up to 13 cubic yards to the surface.

A crane operator tips the bowl and spills the rock fragments onto the ground. A bulldozer carries the shards to the site of a former car ferry dock on the west shore of Jones Island.

The rock gradually will fill 2 1/2 acres of open water where the dock had been. This landfill will be turned over to the Port of Milwaukee.