Smart safety for Seattle underground highway 24 May 2018

TunnelTalk Reporting

With the last of the lower deck road panels installed to create the two-lane northbound carriageway of the double-deck SR99 TBM-bored highway in Seattle, attention by design-build contractor STP (Seattle Tunnel Partnership) has turned to installing and testing the operational and safety systems.

Jet fans installed to provide longitudinal ventilation
Jet fans installed to provide longitudinal ventilation

Systems will include more than 300 CCTV cameras to monitor traffic and security at all times as part of an incident-detection system as well as automatic ventilation systems to keep air quality and visibility high, and automated sprinkler systems designed to extinguish fires at source.

The tunnel is designed for all types of traffic, including heavy goods vehicles, and its ventilation system is based on longitudinal jet fans for normal operation and supplemental exhaust systems to operate in emergencies. As vehicles push fresh air through the tunnel using the piston effect, electronic sensors on the tunnel walls will sample air quality constantly. When air quality falls due to an increase in exhaust emissions, the sensors will automatically activate the ceiling mounted jet fans to pull fresh air from the portals and direct along the two road decks of the tunnel.

If this is not sufficient, or if there is a vehicle fire within the tunnel, large exhaust fans at the base of the ventilation stacks at the tunnel portals will be activated to suck exhaust and smoke through louvers on the tunnel wall, into the ventilation corridor on one side of the tunnel and out through the stacks at the ventilation buildings.

Exhaust ventilation will activate in event of in-tunnel fire
Exhaust ventilation will activate in event of in-tunnel fire

A similar smart fire fighting system will operate in the case of an in-tunnel fire. The ceilings of the 2km long double-deck road decks are lined with more than 8 miles (13km) of fibre optic heat sensors. These will activate fans and alert tunnel staff in the Traffic Management Centre when they detect heat.

If this is detected by a fire, tunnel operators will activate the system of tunnel sprinklers, which are installed in 100 separate safety zones allowing only sprinklers in the zone of the fire incident to activate.

Adding to safety in the tunnel, operators can communicate with drivers through the use of strategically placed electronic signs and a public address system with an AM/FM radio override.

If there were to be an emergency, flashing lights lead evacuating vehicle drivers and passengers to the exit doors into the safety corridor where they can wait in a safe area for assistance or use the exit corridor to leave the tunnel. The safety corridor is built into the opposite side for the large diameter tunnel and equipped with its own positive-pressure ventilation system which keeps it supplied with fresh air at all times.

Miles of cabling installed for tunnel operation

When completed the SR99 highway tunnel will have approximately:

  • 95 miles (150km) of electrical wiring
  • 21 miles (34km) of sprinkler pipes
  • 15 miles (25km) of lights
  • 13 miles (21km) of fibre optic cables
  • 8 miles (13km) of linear heat detectors

Once all installed, the thousands of components involved will be tested and tested again. First to make sure they work as they should, then to make sure they work as a system, and then to ensure that all systems are integrated and function as a single integrated operational network.

With all the M&E systems installed, tested and certified, design-build tunnel construction contractor, the Seattle Tunnel Partnership, a JV of Dragados USA and Tutor Perini, will hand the completed tunnel over to the Washington State Department of Transportation for a different contractor to build the final ramp connections at either end and realign the SR99 route through the tunnel and away from the elevated viaduct which is to be demolished. The new tunnel route of the SR99 is estimated by WSDOT to be ready for opening as early as the Fall of this year (2018).


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