Click image to larger view of tunnel's path
	Listen In: Hear the PG's Mark Roth and Port Authority engineers discussing the tunnels planned for under the Allegheny River: Talking about the plans and impact of a historic project Section of Tony Dorsett Drive to close A one-block stretch of Tony Dorsett Drive on the North Side will close for two months starting tomorrow morning as part of the new light-rail tunnel project. The closure will allow contractors to start building a new underground rail station near the Sports & Exhibition Authority parking garage, as well as a portion of the tunnel heading west toward Heinz Field. The street will reopen in August with metal plate coverings as construction continues beneath the roadway.
	Darrell Sapp, Post-Gazette
	Looking west toward Heinz Field from the Sports & Exhibition Authority parking garage roof, the mound of sand in the foreground is the point at which Port Authority light rail trains will emerge from their underground tunnels on the North Shore. Click photo for larger image.

Amid dust, vibration and almost constant noise, the work to prepare for Pittsburgh's first underwater tunnels is marching forward on schedule.

In four more years, if all goes according to plan, Port Authority Transit will have two 2,400-foot-long tunnels snaking beneath the Allegheny River, carrying light rail passengers to the North Shore and back again.

The staging areas for the $435-million project are concentrated on Stanwix Street, Downtown, and along General Robinson Street on the North Side, between PNC Park and Heinz Field.

The Stanwix Street work is quiet right now, consisting mainly of utility line relocation carried out in the evening.

But the General Robinson portion of the project is booming.

The focal point is a giant hole, 55 feet deep and 80 feet long, that's being carved into the earth beneath the street.

This is the "launch pit," the point from which a $10-million tunnel-boring machine from Germany will start burrowing beneath the shore and then deep into riverbed beneath the Allegheny, probably in September.

The boring machine, built especially for the Pittsburgh project by Herrenknecht AG in southern Germany, is essentially a 22-foot-diameter worm with teeth.

The complex mechanical cylinder is being tested now at the Herrenknecht headquarters in Schwanau, and will be shipped here in August.

Guided by lasers, the machine will churn through about 20 to 40 feet of earth a day, which means the southbound tunnel should reach Downtown some time around February, and the northbound will hit the North Shore in July 2008.

At no point will the river water actually touch the tunnels, which will be at least 20 feet beneath the riverbed, and even deeper in the middle.

At this point, the launch pit is about halfway finished and on schedule to be completed by August, said Kirk L. Thompson, civil engineering manager at the Port Authority.

Stiffening the soil
One day last month, two huge excavators were chomping and disgorging earth from the launch pit, but as Mr. Thompson explained, it wasn't just soil they were moving.

To keep the pit from collapsing and to provide a firmer surface for the tunnel machine to bore into, cement grout was being injected into the soil to stiffen it.

Other parts of the job will use a milkshake-thick slurry of bentonite, a kind of absorbent clay.

During the tunnel boring itself, bentonite slurry will be injected in front of the cutting heads to help hold back the rock and soil and then to mix with it so it can be transported through pipes back to the surface.

One other part of the job will use a different type of bentonite slurry. The portion of the tunnels from General Robinson Street to the Heinz Field parking lot will actually be excavated from ground level rather than using the boring machine.

In that stretch, the walls of the pit will be made by injecting bentonite slurry into the excavations to hold the soil in place, and then pumping in concrete grout to slowly displace the lighter slurry.

To cut down on the dust on the North Shore, each truck entrance has been lined with stone, and the trucks' wheels are washed off before they head out onto the street.

Several of the supports beneath I-279 and its ramps also have solar-powered instruments attached to detect any shifting that might occur as the construction proceeds.

Underwater tunnels common
While a project of this type is a first for Pittsburgh, a city of bridges and through-the-hillside tunnels, underwater tunnels are common elsewhere.

The Lincoln and Holland automotive tunnels in New York City are underwater. So is the light rail Trans-Bay Tube from San Francisco to Oakland, and the Ted Williams Tunnel from Boston to Logan International Airport.

The Trans-Bay and Ted Williams tunnels were made by creating tunnel sections and then lowering them from the surface of the water into trenches that were then covered over.

The Port Authority considered that approach, said Henry Nutbrown, the authority's manager of engineering and construction, but decided against it.

Not only did the bored tunnel turn out to be cheaper, he said, but the immersion method would have endangered the habitats of several vulnerable species of fish, including the river redhorse and the mooneye.

The Port Authority did consider a bridge rather than tunnels, he said, but rejected that idea because the bridge would have had to rise so steeply Downtown to get above Fort Duquesne Boulevard that much of Stanwix Street would have been clogged by bridge supports, and a similarly large chunk of land would have been consumed by the bridge where it touched down on the North Shore.

Is it worth the cost?
The cost of the project, which will include a new Gateway Station Downtown and two new stations on the North Shore, works out to $355 for every man, woman and child in Allegheny County.

That has generated sharp criticism from some at a time when the Port Authority is paring routes, laying off employees and talking about raising fares to keep the system afloat.

Mr. Nutbrown has two answers to that complaint.

First, the money for the extension, including $348 million from the federal government and $72.5 million from the state, can be used only for this project, and would have to be given back if the tunnels were abandoned.

Second, and more importantly, he said, the extension will be vital in helping boost the development that is taking place around the two stadiums and the new casino on the North Shore, ferrying people in both directions for work and recreation.

Eventually, he said, the North Shore extension could be stretched all the way to Pittsburgh International Airport.

"We know there are still people out there who find this kind of a project unappealing," Mr. Nutbrown acknowledged, "but I don't think they're looking at how we've paid these dollars into the federal government and now this is our opportunity to get it back and turn it into something worthwhile."

The $156-million tunneling portion of the project is being supervised by a joint venture called North Shore Constructors, a pairing of Trumbull Corp., of West Mifflin, and Obayashi Corp., of San Francisco.

The southbound tunnel will run under one corner of the new Equitable Resources headquarters before worming beneath the river. The northbound tunnel will parallel its twin, slightly upriver.

New stations planned
Workers will build a new, glass-covered Gateway Station along Stanwix Street, as well as two new stations on the North Side -- an underground site next to the Sports & Exhibition Authority garage between PNC Park and Heinz Field, and an elevated station at Allegheny Avenue and Reedsdale Street.

All trains running into the new Gateway Station will head to the North Side and back again, eliminating the horseshoe turnaround at the current Gateway site.

Just as the current light rail line emerges into daylight near the Allegheny County Jail, the extension will do the same thing next to Heinz Field and then run on elevated tracks more than 20 feet above ground until reaching its final destination.

But the focus right now is on the launch pit, all in anticipation of the arrival of the Herrenknecht machine, which will be delivered to Pittsburgh on about 20 trucks.

The project's general contractors are leasing the machine, which the Port Authority insisted should be brand new.

Why?

"This is the first time it's ever been done in Pittsburgh," Mr. Thompson said, "and we wanted to get rid of as much risk as possible, so instead of having a rebuilt machine, we asked for a new one."