Opportunity
In 2000, a single-track, hard-rock rail tunnel connecting the port of Whittier, Alaska, to the Seward Highway was converted to dual rail/highway use. Two and a half miles (four kilometers) long, the converted tunnel is still the longest road tunnel in North America.
Solution
As lead designer for this design-build project, Mott MacDonald achieved several firsts. The Whittier Tunnel was the first to combine rail and highway use in North America, the first to combine jet and portal fans in its ventilation system, and the first to use a unique traffic control system that ensures no conflict between rail and highway service along the same alignment.
After 15 years of uninterrupted service, Mott MacDonald was retained a second time to help meet maintenance and rehabilitation needs. We conducted assessments, made recommendations, provided cost estimates, and completed design and bid-ready construction documents for the upgrade of the groundwater collection and conveyance systems.
Outcome
The converted tunnel is the longest road tunnel in North America. Its dual function is accomplished with a precast concrete roadway surface with integral rails and a sophisticated traffic control system that allows both control of the single-lane highway traffic flow direction as well as the safe operation of train traffic.
The tunnel was designed for temperatures as low as –40°F and winds up to 150 mph, and portal buildings were built to withstand avalanches.
This landmark project boasts several “firsts,” including these:
- Longest highway tunnel in North America.
- First combined-use rail-highway use tunnel in North America.
- First tunnel to use a ventilation system that combines jet and portal fans in US.
- First tunnel with a unique traffic-control system that regulates rail and vehicular traffic.
- First tunnel designed to operate in temperatures down to –40°F and winds up to 150 mph, and with portal buildings able to withstand avalanches.