Why not a Duwamish tunnel? 20 questions answered

Editor’s Note: When Westside Seattle published Bob Ortblad’s Op-Ed regarding the construction of an Immersed Tube Tunnel (ITT) to replace the now damaged and closed West Seattle Bridge, it got the attention of people around the entire region including Mayor Durkan, the Seattle City Council and others. Questions about how it could work, where and how it could connect, what it would cost and how long it would take emerged. We asked Ortblad to explore his concept further and answer the top 20 questions he’s gotten. 

Our original coverage can be found here

Why not repair the bridge?

Cracks in the two box girders have formed a 4,600-ton “inverted keystone” in the center of the main span. SDOT consultants WSP have reported that the girder cracking will likely continue. Any repair will have to carry this massive dead weight, plus the structural repair’s weight, plus the traffic live load. It would be like building a new bridge to hold up an old bridge plus the traffic live load.

Why not replace the bridge?

A 2007 United State Geological Survey report locates the Seattle Fault (first recognized in 1992) directly below the bridge and that the soil is likely to liquefy in a moderate earthquake. Any solution now must meet a 2500 Year event standard. The new bridge would also have over a half a mile of 6% grade. Let’s not repeat a past mistake.

The Seattle Fault runs through West Seattle and across the southern tip of Harbor Island. Graphic by the University of Washington Why not a Deep Bored tunnel like Bertha?

As Seattle learned with Bertha, a bored tunnel requires a massive expensive boring machine that can get stuck deep underground.  A boring machine would have to go very deep, some 200 feet to find solid ground, making a bored tunnel very long and expensive.

What is an Immersed Tube Tunnel and how is it built?

An immersed tube tunnel is a very simple design. There are like an oversize concrete box culvert sunk into a shallow trench on the bottom of a river.  The concrete tube segments are constructed in a dry-dock, towed to the crossing site, lowered into a pre-dredged trench then connected to cut-and-cover built down-ramps.  The cut-and-cover excavation will need a method to support the trench wall in soft soil. Sheet piling, Secant piles, or a Slurry wall are three options.

Is an Immersed Tube Tunnel a new concept?

The first immersed tube tunnel was a railroad tunnel built in 1906 under the Detroit River. It was round and made of steel. The first concrete box style tube tunnel was built in Holland in 1942.  It is the model for more than 150 similar worldwide tunnels.

Where on the Duwamish can an Immersed Tube Tunnel be built?

The best site for a Duwamish tunnel is just north of the Swing Bridge. City Light has a transmission line crossing at this site. The power lines could be placed in a utilidor in the tunnel. Only two 250-foot tunnel segments are needed to cross the river.

Where can we build the Immersed Tube Tunnel segments?

The tube segments can be built in the Aberdeen Casting Basin used for the SR520 pontoons or at Concrete Tech’s dry-dock in Tacoma. Each of the two segments will weight about 27,500 tons, half the weight of several recent European tunnels.

How long would it take to build an Immersed Tube Tunnel?

The British Columbia 2,000-foot immersed tunnel under the Fraser River was built in 1959 in two years and two months. A Duwamish tunnel would be one fourth as long, at approximately 500 feet. A tunnel can be started immediately and could save a year or more in construction time. The tubes can be built off-site, and the down ramps are outside of the bridge’s defined fall-zone meaning it could start much sooner.  A new bridge must wait for the demolition of the current bridge.

Can an Immersed Tube Tunnel also include Sound Transit light rail?

Yes, an immersed tube tunnel is ideal for light rail. A tube can be configured in any combination of vehicle and transit lanes.  A tube tunnel can handle the weight of the train much better than a long-span bridge. Sound Transit has one underwater bored tunnel under the Montlake Cut and tunnels in Beacon Hill and out to the University of Washington.

How much would an Immersed Tube Tunnel cost?

Cost is a relative question.  A tunnel will cost less than a new traffic bridge, plus the cost of a new bridge for light rail.  A tunnel may be less or about the same as a new combined bridge for traffic and light rail.  Any bridge would require 200-foot pilings, 150-foot piers, and a 600-foot center span that can carry over 800 tons of vehicles and light rail.  A tunnel can easily carry this load, with a 5-foot concrete floor spread over 1.6 acres, and is a much safer and economic design. A tunnel is also a good long-term value. A bridge is designed for a 75-year life. A tunnel is designed for a 150-year life.

How deep would the tunnel be?

An NOAA survey has measured the crossing site at -25 to -30 feet MLLW. The navigation requirement is -30 MLLW feet.  The lowest point of the roadway would be about -55 feet MLLW (-62 feet Mean Sea Level).

How much dredging would be required?

A trench will require about dredging of 136,000 cubic yards. This is a little over the 100,000 cubic yards done annually to maintain the Duwamish Waterway. The Port of Seattle plans to dredge 1,155,000 cubic yards to improve the East & West waterways.

Would the dredge and ramp excavation be toxic?

A Port of Seattle map shows Superfund Sites to North, East, and South but none near the Spokane Street bridges. Harbor Island itself is a Superfund site so any construction done there, even two new bridges would require some remediation work. Sampling will need to be done. Average Superfund sites cost around $26 million for soil remediation.

How do you connect to current elevated east and west highway sections?

A tunnel will not connect to, or use the elevated high sections. Elevated highways connect to the current bridge at an elevation of about 130 feet about 400 feet from the river’s edge. It would take impossible 35% down-ramps to connect to a tunnel (maximum is 6%). A tunnel will allow the removal of over a half-mile of elevated highway, freeing about 8 acres of land for development or park land.

How do you connect coming from the East?

To connect vehicle traffic, about 1,500 feet of new elevated highway is required. The connection would start just before the East Waterway, go over the waterway and lower Spokane St. and connect at ground level to the entrance to a 900-foot down ramp to the tunnel.

How do you connect at the West Delridge side?

The current Delridge off-ramp is 1,400 feet long and drops 110 feet.  The current Delridge on-ramp is 1,100 feet long and climbs 60 feet.  The tunnel off and on-ramps would be much shorter and less steep and simplify the current concrete jungle. The tunnel entrance/exits ramps would be at grade level at the end of Pigeon Point directly below the current elevated highway. The main highway would continue to climb to the west at a 6% grade for 400 feet, to an elevation of 34 feet. This will allow the off-ramp for traffic and light rail to continue at grade and turn left under the main highway. About 1,000 feet of new elevated highway would need to connect to existing elevated highway. A tunnel would go under both W. Marginal Way SW & SW Spokane St. before it surfaces near Pigeon Point.

What happens to an Immersed Tube Tunnel in an earthquake?

WSDOT’s web site has a 5-minute YouTube explanation of the way a tunnel is the best place to be during an earthquake. An immersed tube tunnel is almost buoyant so if the ground liquefies it floats on the soft soil. Japan has 25 immersed tube tunnels and San Francisco’s BART has the longest US immersed tube tunnel (3.8 miles long built in 3 years) which has weathered several earthquakes without damage. It is being seismically retrofitted.

Do you need ventilation in the tunnel?

The tunnel will be short, about 1,500 feet long (West 500’ enclosed ramp, 500’ immersed tube, plus East enclosed ramps 500’). Some vent fans and shafts can be located at each end of the tube section.

Will the Native Tribes object to a tunnel?

While they have not yet weighed in on the idea it’s worth noting a tunnel will remove more than a half-a-mile of elevated concrete highway and bridge from the delta and over the river. This should improve the view, reduce traffic noise, and eliminate shadows on the delta and river. Any land lost could be exchanged for the new space. The in-river construction of a tunnel can be scheduled away from seasonal fish runs and would take very little time. British Columbia’s First Nation People demanded a new immersed tunnel over a new high bridge for a Fraser River crossing.

Is a tunnel safer than a bridge?

During an earthquake, vehicles and Sound Transit trains (which each weigh close to 300 tons) could be in the middle of a 600-foot bridge span, 150 feet over the river. A bridge such as this will be supported by 150-foot east and west bridge piers supported by pile caps resting on 200-foot pilings driven in soil subject to liquefaction. An Immersed Tube Tunnel is almost immune to earthquakes. An Immersed Tube Tunnel would also be safer than a bridge because it will have better grades (slopes into and out of it). First, the grade will be half as long. A bridge would need to go up 150 feet. A tunnel only needs to go down 62 feet. Second, the tunnel has an initial 6% down grade that allows acceleration or constant speed, with momentum to carry it into the up grade. A bridge has a long initial 6% up grade that will slow heavy trucks and buses, then a long 6% downgrade that may require braking. A tunnel is also protected from, fog, wind, rain, snow, and black ice.

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