In one of the largest and most interesting projects in the region, the $3.3 billion Alaskan Way Viaduct Replacement Program will create a safer route through Seattle and allow for redevelopment of the city’s waterfront.

“The current roadway is reaching the end of its life, is not up to current seismic codes and needed to be replaced,” says Brian Nielsen, Administrator, Alaskan Way Viaduct Replacement Program for the Washington State Department of Transportation (WSDOT). “A magnitude 6.8 earthquake, in 2001, damaged the viaduct. While it was repaired and strengthened, that event proved beyond a doubt that the viaduct needed to be replaced.”

The double-deck viaduct was built in the 1950s and is one of only two highways running north-south through the city. After the 2001 Nisqually earthquake, columns settled as much as 5 inches. The department repaired and strengthened the road, imposed weight restrictions and began twice-yearly inspections. 

The Washington state legislature authorized funding in 2009 to replace the viaduct with a bored tunnel. Tunnel preparations began in 2010, and WSDOT replaced the southern mile of the viaduct with a surface road in 2011.

The legislation requires the transportation department to collect tolls on the tunnel to help repay the construction bonds and for future operations and maintenance. The Washington State Transportation Commission will set toll rates, expected time-of-day tolls will range from $1 to $2.50. 

The current replacement program entails a 2-mile-long SR 99 tunnel under downtown Seattle, completed by design-build team Seattle Tunnel Partners, a joint venture of Tutor Perini Corp. of Los Angeles and Dragados USA of New York. Other projects in the program include a 1-mile stretch of new highway south of the tunnel and Seattle’s stadiums, a new overpass at the south end of downtown that allows traffic to bypass train blockages near Seattle’s busiest port terminal and, in 2019, the removal of the seismically vulnerable Alaskan Way Viaduct, Nielsen explains.

“This was a very ambitious project with big challenges,” Nielsen says. “The Washington State Department of Transportation’s goal was to build a double-deck highway underneath Seattle and keep traffic moving above. We accomplished that – keeping SR 99 open to traffic through construction.” 

Boring with Bertha

The department awarded Seattle Tunnel Partners (STP) the $1.46 billion SR 99 tunnel contract and began boring in July 2013. Hitachi Zosen Corp. of Japan built the 57.5-foot diameter tunneling machine, nicknamed Bertha, and HNTB completed the tunnel design. The contract included building the double-deck highway in the tunnel, tunnel portals and two tunnel operations buildings, at the north and south ends of the entrances. 

“At the time we started, the tunneling machine was the largest diameter in the world and pushed the limits of engineering,” Nielsen recalls. “While there was trouble along the way, the machine was ultimately repaired and dug a five-story wide tunnel underneath Seattle’s downtown without any soil movement on the surface.” 

The 367-foot-long, nearly eight-ton boring machine overheated and stopped tunneling in December 2013 with the tunnel about 10 percent complete. STP realized the seals protecting the main bearing had failed, and STP with Hitachi Zosen decided to lift the front end up from underground to make repairs. 

STP dug a 120-foot-deep access pit and used a special crane to lift the 4-mllion pounds of machinery needed for the repair. Hitachi Zosen made a number of repairs and redesigned the seal system. The tunnel-boring machine was then lowered back into the pit, reassembled and tested.

An ongoing lawsuit between WSDOT and STP continues about who is responsible for the costs of the repairs. STP claims an abandoned steel well in the machine’s path caused the damage, while WSDOT “alleges the damage was caused by design and operational issues, in addition to other factors for which STP is contractually responsible,” according to WSDOT Spokesperson Laura Newborn. 

Tunneling resumed in December 2015. The average mining speed was 1 inch per minute.             The tunnel-boring machine built the tunnel one tunnel ring at a time, placing 10 6.5-foot, 172-ton-wide curved segments to form a ring. The machine encountered eight types of soil and excavated about 550 cubic yards of soil for each tunnel ring, more than 850,00 cubic yards all together. Three barges hauled the soil away. 

In addition to boring the tunnel, STP built 1,000 feet of cut-and-cover tunneling at the south entrance, and 450 feet at the north end to allow the roadways to gradually join the bored tunnel where they are stacked.  

The operations building at the north end, along with WSDOT’s traffic management center in Shoreline, Washington, control the ventilation, lighting, traffic management and safety systems. Both buildings have 40-foot-tall, 10-foot-diameter, yellow ventilation stacks, capable of moving 160,000 cubic feet of air per minute. The new tunnel is built to withstand a Cascadia Subduction Zone earthquake as strong as a magnitude 9.0.

“The ultimate accomplishment of the SR 99 Tunnel Project proves that large tunnels can be built underneath cities with minimal traffic impacts above,” Nielsen says.  

The team completed tunneling for the 2-mile-long tunnel in April 2017. STP then spent four months dismantling the tunnel boring machine and later completed the roads inside tunnel. Teams lifted 8,300 tons of machinery from the receiving pit and hauled it away, recycling the 944-ton cutterhead.

The upper roadway inside the tunnel was cast in place behind the tunneling machine as it moved north. The lower roadway was pre-cast and installed after the tunneling machine was dismantled. The lower road is made up of 1,152 panels, which were built by offsite and transported to Seattle.

STP’s final tests of the safety and operational systems inside the tunnel are scheduled to be complete this fall. In early 2019, WSDOT will permanently close the Alaskan Way Viaduct, begin moving SR 99 off the viaduct and into the tunnel and then open the tunnel. The viaduct will be removed after the tunnel opens. Tolling will begin sometime later in the year.

The SR 99 tunnel is the longest in the continental United States, and one of only two single-bored, double-deck tunnels in the world, the other being in Turkey, according to WSDOT.

Constructing the Other Components

After the Alaskan Way Viaduct is removed, the city of Seattle will build a new Alaskan Way surface street along Seattle’s waterfront. 

WSDOT will later begin work to reconnect surface streets near the north end of the tunnel and complete additional street improvements near the south entrance to the tunnel.

“Managing a megaproject of this size requires a large team of talented and committed engineers, contractors and workers,” Nielsen concludes. “I’m most proud of how all the teams worked together, even in the most challenging times, to make this project a success.”