clock menu more-arrow no yes

Filed under:

Inside the Highway 99 tunnel’s safety engineering

Now that the tunnel is bored, how is it being built?

A view of the tunnel behind Bertha the tunnel-boring machine from February 2017.
Via WSDOT

Now that Bertha is out from underground and being taken apart, the next step is to finish building the tunnel the boring machine left behind—and it takes a lot more than building a road to make the underground highway ready for cars.

Underground, safety concerns are different: With all the car exhaust that collects in a closed space full of cars, the tunnel needs to be ventilated, especially during peak hours. Fires are a particular challenge in tunnels both because of ventilation and location. Earthquakes are always a concern around the region.

Subcontractor HNTB is responsible for many of the tunnel’s components that allow for safe, smooth operation. Curbed Seattle spoke with HNTB project manager Brian Russell about the details in the tunnel’s design that are being built to keep the tunnel safe.

Curbed Seattle: As an overview, what components of the tunnel is HNTB working on?

Brian Russell: HNTB is responsible for the design of the tunnel structure—including cut-and-cover sections, open cut sections and bored tunnel structure—mechanical and electrical components of the tunnel structures, ventilation buildings, fire [and] life safety, civil work, roadway design, and building settlement mitigation measures for structures along the tunnel alignment.

The Seattle Times reported in 2011 that both fires, floods, and earthquakes were taken into account in the design. Can you talk about these safety components?

The tunnel is designed to withstand earthquakes, sustain large amounts of water and suppress fires. Systems were designed and implemented to allow WSDOT to manage all sorts of day-to-day scenarios, ranging from normal operations to emergencies.

Fire detection systems and pollutant concentration monitoring systems were designed to mitigate fire hazards and maintain safe pollutant concentration levels within the tunnel facilities.

The tunnel is equipped with lighting systems for both daytime and nighttime driving, strategically designed to provide a smooth transition upon entering and exiting the tunnel as well as the drive through the duration of the tunnel.

Linear heat detectors situated above the roadway will trigger a fire detection signal when the air temperature rises faster than 28 degrees per minute or reaches 190 degrees. The extraction and jet fans will pick up speed and a sprinkler system will be activated once a fire is detected.

Variable message signs throughout the tunnel will provide motorists with real-time instructions to address the many scenarios that may occur in the tunnel, such as lane closures, accidents, disabled vehicles, etc.

How much earthquake? How much water?

The precast liner ring’s concrete reinforcement, bolted connections, and rubber gaskets were designed to safely resist the ground motions predicted for an MCE (maximum considered earthquake, which are expected to occur once in approximately 2,500 years) with no water leaks.

The tunnel has a pumping capacity of 1,090 gallons per minute, sufficient to keep up with an MCE storm, and emergency storage below the roadways that can hold up to 480,000 gallons.

Can you speak to the fire risk in tunnels?

The tunnel itself does not create an increased risk of fire, rather tunnel fires can be more dangerous for motorists. That is why the tunnel is designed with systems in place to detect and suppress fires.

There are also operating procedures implemented by WSDOT to reduce the risk of fire, such as restricting petroleum tankers from using the tunnel.

Design features include video detection systems, which monitor the tunnel, and heat detection systems, which can activate the sprinkler systems should a fire gain sufficient intensity.

Let's say there's an accident or even a pile-up in the tunnel—how do emergency responders access the site?

The tunnel roadways are two lanes wide in either direction, complete with an eight-foot-wide shoulder on one side. In the case of an emergency, responders could use this extra width to bypass traffic. Signage in the tunnel will inform motorists of any special instructions.

What volume of car exhaust could be collected in the tunnel? What technology are you using to clear it out?

The tunnel ventilation system was designed to withstand and respond to emergency incidents in addition to controlling and limiting pollutant concentrations during normal traffic operating conditions. The large size of the extraction fans allows them to meet emissions ventilation requirements during day-to-day operations.

Pollutant concentrations vary depending on vehicle speed, and HNTB calculated airflow to maintain pollutant criteria limits at incremental vehicle speeds from zero to 70 miles per hour. Calculations determined grade effects, airflow created by vehicle piston effect, and the number and location of extraction dampers and jet fans. Extraction fans were implemented to control emissions at various speeds and to maintain surrounding pollutant levels.

An extraction duct extends the entire length of the tunnel, which collects exhaust and smoke through ventilation dampers. The extraction duct and exhaust dampers allow greater safety during fire incident operation, where smoke can be removed at the fire start location rather than being pushed forward to the exit portal.

Do fire prevention and pollution dispersal work in tandem in this design?

The tunnel’s ventilation system is designed to remove polluted air from the tunnel while pulling in fresh air from the portals. Ventilation is activated when a low level of carbon monoxide is detected, and continues until the concentration decreases.

The ventilation system is also designed to remove smoke in a specific fire zone. Dampers are built into the tunnel walls at regular intervals so air can be pulled out quickly from the roadway zones.

Cooling and ventilating the tunnel seems like it would take a lot of energy. Were there steps taken to keep the tunnel greener?

The large ventilation fans will not operate at all times. For example, the fans will not operate at normal traffic speeds of 50 miles per hour, rather, only during high traffic congestion or in the case of a fire.

How often do you expect the fans to be on?

About two to three hours per day, during congested traffic periods.

How hot is the tunnel expected to be under normal conditions?

The tunnel interior will be a few degrees warmer than the outside air temperature under normal conditions.


When completed, the State Route 99 tunnel will replace the existing Alaskan Way Viaduct. It’s expected to open to traffic in early 2019.