Keeping the Alaskan Way Viaduct safely open to traffic during SR 99 tunnel construction is a top priority. It’s one of the main reasons we’re boring a tunnel – to minimize disruption at the surface as we replace the viaduct.
Recent media reports have stated that ground settlement due to tunnel construction could cause us to close the viaduct. These reports are inaccurate.
The viaduct is still vulnerable to earthquakes, but it remains safe for everyday use. The viaduct settled as a result of the Nisqually earthquake and we strengthened the viaduct in those areas so it remains safe for drivers.
We have no plans to close the viaduct until after the tunnel opens to traffic. Crews inspect the structure four times per year – the next inspection is scheduled for this weekend – and have taken a number of steps to protect it during construction.
We anticipated some ground settlement would occur during construction of the tunnel, which is taking place near the viaduct. That is why the viaduct has been strengthened in this area and more than 100 monitors have been installed on the viaduct to measure how much the ground beneath the structure moves.
Those monitors told us that the viaduct settled up to four-tenths of an inch at one location along the viaduct near where tunnel construction is underway. This settlement is well within the limits established by WSDOT’s structural engineers.
We will continue to carefully monitor the viaduct during construction and will take additional steps to reinforce the viaduct if needed.
We’ll be posting additional details soon about the work we’ve done to protect the viaduct. In the meantime, please feel free to contact us if you have questions or concerns.
Learn more about the Alaskan Way Viaduct Replacement Program.