For the viaduct's central waterfront replacement, there is an ongoing environmental process that is reviewing three alternatives - a bored tunnel, cut-and-cover tunnel and an elevated structure. If the bored tunnel alternative is chosen, we would move SR 99 into a bored tunnel beneath downtown, reconnect the street grid at the ends of the tunnel and remove the viaduct along the waterfront.
As preliminary design and the environmental process moves forward, we will benefit from lessons learned by other tunnel projects that have been completed or are underway. Several international tunnels have been completed in similar environments and with sizes comparable to the proposed SR 99 bored tunnel. From projects in the United States, including Seattle’s Sound Transit Beacon Hill Tunnel and the downtown transit tunnel, to projects in Germany and China, advances in tunnel technology and experience are being made around the world.
Tunnels with similar sizes
There are a number of successful tunnel projects with sizes similar to the proposed SR 99 bored tunnel. The bored tunnel itself would be designed with an approximate interior diameter of 52 feet and approximate length of 9,100 lineal feet. The tunnel’s depth would range between 60 and 200 feet. Tunnel projects of similar sizes include:
- Shanghai Yangtze River (China): Includes two bores, each about 5 miles long with a 50.6-foot diameter.
- Fourth Elbe River Tunnel (Germany): Includes a single bore with a length of about 2 miles and a 46.6-foot diameter.
- Madrid M30 (Spain): Includes bores with a length of about 5 miles and a 49.9-foot diameter.
Tunnels constructed in similar environments
Numerous tunnel projects, including several in Seattle, have successfully excavated ground conditions similar to those anticipated for the proposed bored tunnel. The ground conditions along the proposed tunnel route include soft soils at the tunnel’s southern entrance, then hard and dense glacier-deposited soils for the remainder of the alignment and at the north entrance. During tunnel construction, we would expect to find materials normally present in glacial soils, such as small rocks and boulders.
More than 150 tunnels have been completed in Seattle since 1890, mostly in glacial soils. International and local tunnels constructed in similar soil as the proposed bored tunnel include:
- Sound Transit Beacon Hill tunnel, downtown transit tunnel and Mercer Street wastewater tunnel (Seattle): Ground conditions of glacial sand, silt, clay and other soils similar to the hard and dense soils along most of the proposed SR 99 bored tunnel alignment.
- Grauholz Tunnel (Switzerland): Ground conditions of glacial sand, clay and silts.
- Shanghai Yangtze River Tunnel (China): Ground conditions of clay, sand and rubble that are similar to soils at the south entrance of the proposed SR 99 bored tunnel.
- Fourth Elbe River Tunnel (Germany): Ground conditions of sand and clay with boulders and obstructions.
Advances in technology
As more projects are completed, tunnel boring technology will continue to experience advances and improvements. Tunnel boring machines have been developing at a rapid rate with a major increase in diameter, better ground control, and improved reliability. These machines can now safely excavate almost any type of soil, rock or groundwater conditions.
Technological advances can be expected to continue into the future. For example, a 62.3-foot diameter boring machine to construct a Russian tunnel is in design.
Advances in technology, as well as lessons learned from other tunneling projects, would help us build the proposed bored tunnel safely, efficiently and successfully.
Return to central waterfront