Preparatory Works at Allegheny River Tunnel in Pittsburgh Awaiting TBM Arrival in AugustThe work to prepare for Pittsburgh's first underwater tunnels is marching forward on schedule. Excavators, front loaders and other heavy equipment are busy at work on the North Shore of Pittsburgh. They are getting their area ready for the arrival of the tunnel boring machine, especially designed and built by Herrenknecht, that will create Pittburgh’s first ever underwater tunnels. Visit www.herrenknecht.comThe staging areas for the USD435 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. Today, the focal point of the work on the North Shore is next to General Robinson Street within side PNC Park. That is where excavators are digging a giant launch pit, 16.8 m deep and 24.4 m long, where the slurry shield machine will be assembled and start worming its way beneath the shore and the Allegheny river.To keep the pit from collapsing and to provide a firmer surface for the TBM to bore into, cement grout is being injected into the soil to stiffen it. A jet grout rig is injecting the grout for a 6.7 m-wide block in front on the face of the launch pit to provide ground support so that when the TBM is launched, it gives ground firm support as it excavates the soil when it penetrates the face of the wall. At this point, the launch pit is about halfway finished and on schedule to be completed by August. The 6.7 m-diameter TBM is supposed to arrive to the site in August on about 20 truckloads. It is being tested now at the Herrenknecht headquarters in Schwanau, Germany. After it is assembled, sometime in August or September, it will start digging a 730 m tunnel through the rocky ground underneath the Allegheny. At no point will the river water actually touch the tunnels, which will be at least six metres beneath the riverbed, and even deeper in the middle. When it gets Downtown, the TBM will turn around and dig a second tunnel back to the North Side. The tunnel segments themselves will be laid in place behind the machine. Guided by lasers, the machine will churn through about six to 12 metres 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. The USD156 million tunnelling portion of the project is being constructed by a joint venture called North Shore Constructors, a team of Trumbull and Obayashi. Click us/66.During the tunnel boring itself, bentonite slurry will be injected in front of the cutting head 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 TBM. In that stretch, the walls of the tunnel 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. There will also be a new glass-roofed Gateway Center station next to Stanwix Street and two new stations on the North Side - an underground site next to the Sports and Exhibition Authority garage between PNC Park and Heinz Field, and an elevated station at Allegheny Avenue and Reedsdale Street.Both tunnels will be finished by 2010. Opening for operation is scheduled for June 2011. Once the twin tunnels are finished and new stations are built, people will be able to travel on light rail trains between Downtown and the North Shore hot spot that features PNC Park, Heinz Field, the Carnegie Science Center and the new casino. Eventually, the North Shore extension could be stretched all the way to Pittsburgh International Airport. Visit www.portauthority.org 25/07.