Paula Wallis, TunnelTalk

Governor Chris Gregoire and Mayor Greg Nickels at the signing

The signing of an agreement between the State of Washington and the City of Seattle brings to a close eight years of debate over how to replace the Alaskan Way Viaduct along the City’s waterfront. Governor Chris Gregoire and Mayor Greg Nichols signed the agreement to replace the viaduct with a deep bored tunnel on Saturday October 24, 2009.

At the signing ceremony Mayor Nickels said future generations would judge the decision to build the tunnel as the right one. The tunnel played a significant role in Seattle’s mayoral race, and Nickels, a two-term incumbent, lost his bid for re-election in the August primary. This was due largely to his unwavering support for the tunnel. "There are issues that are worth losing elections over," Nickels said at the signing ceremony. "This is one of them. We've made history here today."

During the event Governor Gregoire also announced that the State will install a system to safeguard drivers on the crumbling viaduct in the event of an earthquake. "A system with sensors, signs and gates on the entrance ramps, will close down the structure in an emergency until our engineers know whether it is safe to travel on it again," Gregoire said.

On Monday the state also released a 2007 video simulation of what could happen to the viaduct during an earthquake. Opponents of the tunnel question the timing of the video’s release just days before the November 3 general election. Critics say keeping the viaduct open is unsafe and argue that replacing the viaduct with a surface highway would be less costly and be in service sooner, removing the viaduct by 2012 instead of 2015 under the tunnel option.

The bored tunnel will be about two miles (3.2km) long and is planned as a stacked-deck four-lane highway with two lanes in each direction. Drivers will bypass the downtown by entering the tunnel near the sports stadiums in the south and connect to highway SR-99 north of the existing Battery Street Tunnel (Fig 1).

Fig 1. Tunnel alignment

According to the signed agreement, State funding for the estimated $4.24 billion replacement project is capped at $2.4 billion. Civil construction of the tunnel is estimated at $1.9 billion (subject to reconsideration after an expert panel review) and under the agreement, no more than $400 million of any additional costs will be financed with toll revenue. Any costs in excess of $2.8 billion will be borne by property owners in the Seattle area who benefit from the replacement of the viaduct.

Candidate for Mayor Mike McGinn, who largely lead a single issue campaign opposing the tunnel, said Monday that the City was now committed to the plan and if elected he would "uphold and execute the agreement". However, he said he would fight to renegotiate the State mandate that makes Seattle taxpayers responsible for any cost overruns.

The Port of Seattle plans to allocate $300 million to the project. A $930 million commitment by the City will pay for a new Elliott Bay sea wall, a new promenade and other surface improvements along the waterfront. The City and County are still identifying local funding sources for their funding commitments.

A panel of experts for WSDOT (the Washington State Department of Transportation) is currently conducting a Cost Estimate Validation Process for the bored tunnel, including value engineering and risk management, with the results due by January 1, 2010. The three panel members are: Richard Sage, Sound Transit Construction Manager, who is familiar with local conditions and costs and has managed the tunnels and complex underground construction for Seattle's Link light rail project; Donald Hilton, independent consultant, who is experienced in heavy civil and tunnel construction with responsibility for cost estimating on many projects and Robert Goodfellow, Black & Veatch Director of Tunneling, who is experienced in management and risk management for major tunnel projects and underground structures.

In deciding on the bored tunnel option WSDOT studied successful highway tunnel projects around the world with similar sizes. These included China’s 5 mile x 50.6ft (8km x 15.4m) diameter Shanghai Yangtze River twin bored tunnel; Germany’s Fourth Elbe River 2 mile x 46.6ft (3.2km x 14.2m) diameter single bored tunnel; and Spain’s 5 mile x 49.9ft (8km x 15.2m) diameter Madrid M30 highway tunnel. It also looked at more than 150 tunnels completed in Seattle's history since the 1890s many of them in ground conditions similar to those on the proposed new tunnel alignment. These include soft soils at the tunnel’s south entrance, and hard and dense glacier deposits for the remainder of the drive.

The 9,100ft x 54ft (2.7km x 16.4m) diameter tunnel will run 60ft to 200ft (18-60m) below the surface. The south portal, to be located in the vicinity of First Avenue South between Charles and Dearborn Streets, will serve as the starting location for the TBM. The proposed tunnel alignment extends under First Avenue South, passing under a rail tunnel, to the intersection of Pike Street where it will make a sweeping turn to the east going beneath a sewer tunnel and numerous buildings and ends at the north portal on highway SR 99, in the vicinity of John Street.

Cross section of tunnel

The tunnel will be constructed using a closed, pressurized-face TBM and supported with a bolted, gasketed, precast concrete lining. Contracts for construction of the south and north cut-and-cover connection works, retaining walls, and roadway sections will be awarded separately.

WSDOT started the procurement process for the design-build project last month with Requests for Qualifications announced on September 15, 2009. Statement of Qualifications (SOQ) are due November 16, with the three or four short listed qualified contractors announced before the end of the year.

With the signing of the agreement the State wasted not time tendering the first major contract. On Monday October 26, WSDOT issued a call for bids for the South Holgate Street to South King Street Viaduct Replacement Stage 2 contract "Olympia Headquarters Projects." The $2 million to $2.5 million contract includes construction of bridges, relocation of utilities, retaining walls and demolition of the existing SR99 elevated roadway.

If all goes to plan the tunnel construction contract will be bid in September 2010 with the best value proposal announced in December 2010.

References

Seattle's tunnel for viaduct replacement approved -TunnelTalk April 2009
Alaskan Way frustration -TunnelTalk Sept 2007

Washington State Department of Transportation