SEATTLE (AP) Seattle Tunnel Partners (STP), the state’s contractor on the Alaskan Way Viaduct Replacement, is preparing to build what will be the world’s largest deep bore tunnel, with an outside diameter of 56 ft. (17 m).
But before construction can start, the team needs to get a record of decision from the Federal Highway Administration on the final environmental impact statement.
STP is led by Dragados USA and Tutor Perini Corp. The contract is worth $1.4 billion, including incentives.
Construction is scheduled to start in October, though actual tunneling won’t begin until mid-2013.
This year, the team will relocate utilities and install geotechnical instruments. Early next year, it will begin more visible work such as open-cut excavations and closing streets.
Chris Dixon, deputy project executive of STP, said he is excited about the project and confident it will succeed. He said the team must balance many needs but it is prepared for that.
Dixon said the ends of the tunnel are the riskiest parts because they are more shallow and the soils are fragile.
There are 158 buildings above or near the route of the tunnel. No matter what STP does, there will be some amount of ground loss by tunneling, but different buildings can handle different amounts.
The Washington State Department of Transportation initially identified 20 buildings that were at greater risk of damage by tunneling and might need extra protection. STP re-calculated and determined that only seven structures need extra work to prevent excess settling.
It said the other 151 structures can withstand more settling and remain sound.
Half an inch of settlement is acceptable for the seven structures determined to be more at risk. One inch of settlement is acceptable for the rest of the buildings. Dixon said people in buildings along the tunnel route should not feel any vibrations or movement from tunneling.
Of the seven buildings, ones that are directly above the tunnel will receive compensation grouting for protection. These are the Polson Building, Commuter Center Garage and the Commuter Building, all on Western Avenue.
The Western Building will likely get compensation grouting as well, depending on discussions with its owner. Originally, the Western Building was to be torn down, but now the plan is for it to be rehabilitated.
Compensation grouting involves sinking shafts beneath a building and inserting grout tubes. Crews will pump in grout to strengthen the soil and fill any voids before the tunneling machine passes through. They will observe how soils react as the tunneling machine approaches to determine how much grouting is necessary.
“This essentially provides a barrier between what’s happening around the tunnel and the structure above,’’ Dixon said.
At-risk buildings that are near but not directly above the tunnel will get micro pile walls. These buildings include 1 Yesler Way, which houses Al Bocalino Restaurant in Pioneer Square, and three buildings at the north end of the tunnel.
To build micro pile walls, the team will drill a series of 6 in. (15 cm) diameter pipes into the ground and fill them with concrete to form a barrier. Any settlement that occurs will stop at the wall.
STP also will install micro pile walls around the existing viaduct, and do other work to protect its foundations.
All 158 buildings will get devices to monitor vibrations and ground loss.
STP now is seeking permission from each property owner to do a pre-construction condition survey. Workers will do interior and exterior surveys of the buildings, document existing conditions and prepare a report on each structure.
They will conduct interviews with owners and tenants, and install gauges on any existing cracks more than a quarter inch (.63 cm) wide.
Dixon said the survey will establish a baseline for judging any future damage claims. STP is trying to get the surveys done as soon as possible.
Dixon said STP wants to start putting monitors in buildings in September. Hundreds of instruments also will be installed in the street along the tunnel’s pathway. Dixon said they will provide a baseline before tunneling begins.
Instruments include extensometers that detect ground movement, piezometers that measure groundwater elevation and pressure, and deep benchmarks, which are pipes installed lower than the tunnel to measure ground elevation change.
As the tunnel machine moves closer to a point, monitoring in that area will increase.
During tunneling, a construction monitoring task force will meet daily. The task force will prepare daily reports and interpret data to ensure structures are safe. Dixon said all performance data from the machine will be linked to geotechnical information, allowing the team to interpret the causes of movement.
In order to bore beneath buildings, WSDOT must acquire 55 subsurface property rights. Ron Paananen, project administrator, said the agency has done some appraisal work but cannot make offers until it receives a decision on the EIS. After that, WSDOT will begin making offers to owners.
The U.S. General Services Administration, managers of the historic Federal Building between Western and First avenues, had said it would not let WSDOT drill under its building unless several issues were resolved.
Paananen said there is “a path forward’’ to acquiring permission from the federal government, but no agreement is in place. He said he expects to have the issue worked out next year.
The tunnel won’t pass under the Federal Building until late 2013.