WSDOT continues decommissioning of former SR99 tunnel

22 July 2020

Infill operations are continuing on Seattle’s Battery Street Tunnel (the old SR99 highway tunnel) which was rendered obsolete – along with the Alaskan Way Viaduct – when the city’s new SR99 tunnel opened 18 months ago.

The 1950s-built tunnel formed part of the old SR99 route through downtown Seattle and is as seismically liable as the old viaduct was. Washington State Department of Transportation (WSDOT) said that to continue using it would require expensive renovation. It therefore decided to close it to improve safety and mobility in the vicinity.

In May 2019, contractor Kiewit began filling the lower half of the tunnel with crushed concrete salvaged from the demolition of the viaduct. This was poured through hoppers placed over grates in the street above. Vibratory rollers spread and compacted the material in layers inside the tunnel. Crews then filled the tunnel with controlled density fill (CDF) concrete poured around new utilities to protect them from heat and impact.

The final 3m-layer will be created by pumping more than 30,000m3 of low density cellular concrete (LDCC) into the tunnel through ventilation grates and holes in the crown; this will fill-in the headroom between the new utilities and the tunnel roof. Crews will mix the LDCC on site, with infilling starting at the tunnel’s north end, after which work will switch to the south portal area. The entire operation is scheduled for completion in late 2020 or early 2021.