The final ingredient in the layer cake that is filling up Seattle’s Battery Street Tunnel is being mixed starting this month. Crews have begun pouring a special type of concrete into the tunnel through ventilation grates and holes in the tunnel’s roof. This flowable material will fill in the remaining space within the old SR 99 highway tunnel.

Crews working for the contractor, Kiewit, are starting the pouring at Denny Way, the tunnel’s north end. The low-density cellular concrete (LDCC) is mixed on-site with mobile equipment staged adjacent to Borealis Avenue. The mixing plant will stay there for several weeks, then move to the Battery Street Tunnel’s south portal, which is adjacent to First Avenue. Over the next several months, crews will pump LDCC from the south portal area using a series of hoses placed along Battery Street.

Above: LDCC mixing equipment staged along Borealis Avenue, just south of Denny Way

What is low-density cellular concrete (LDCC)? LDCC is produced by mixing water and slurry (a liquid form of concrete) and then injecting a foaming agent. This process produces a kind of concrete meringue that is lightweight and does not get as hard as typical concrete. The material’s lightweight property helps protect the utilities beneath it from excess weight, while its lower strength will allow future crews to dig through it when required to reach those utilities. A 5-gallon bucket of LDCC weighs about 20 pounds, versus 100 pounds for standard concrete. This final stage of filling will use approximately 40,000 cubic yards of LDCC to fill the roughly nine vertical feet left in the tunnel. This is a lot of material – by comparison, CenturyLink field reported using about 10,000 cubic yards of concrete in its construction. The LDCC is the third type of fill material crews have used in the Battery Street Tunnel. First, crews poured crushed rubble produced from viaduct rubble into the tunnel with trucks from the surface. This spring and summer, crews have been filling the tunnel with Controlled Density Fill concrete (CDF) around the new utilities to protect them from heat and impact. The LDCC is the final layer in the cake, filling in the headroom between those utilities and the tunnel’s roof.

What should I expect during construction?

People traveling in the area should expect single-lane closures on Battery Street and cross streets between First and Sixth avenues, and along Borealis Avenue between Sixth Avenue and Denny Way. The batch machinery and idling trucks will also produce an increase in noise and possible vibration.

Above: The mobile LDCC mixing plant staged along Borealis Avenue.

More work to come

Fully filling the Battery Street Tunnel is not the end of the job. Once the LDCC is poured, crews will be able to turn to improving the surface of Battery Street. This work has already begun on some blocks, and includes patching over the tunnel’s ventilation grates, building new sidewalk and ADA-compliant ramps, and installing new street lighting. The tunnel’s south portal has been the construction staging yard for the job and will be turned into a slope and then handed over to the City of Seattle. All work on the project is expected to conclude in 2021.