|The new SR 520 floating bridge looking east toward|
Medina, with much of the old floating bridge gone.
A sense of urgency propels this work. The old bridge – completed more than a half century ago under design standards less stringent than today's – is structurally vulnerable. Severe side winds and strong waves could twist and heave the old pontoons to the point where they crack, break free and sink. Or perhaps worse, the old pontoons could wash into the new bridge – just a stone's throw away – or strike its anchor cables and cause serious damage.
So our crews are working against the clock to dismantle and remove the old bridge before the region's storm season arrives later this year. Since July, we've separated and towed 11 of the old bridge's 31 pontoons off Lake Washington, and moved 14 others to the north side of the new bridge while we prepare to tow them off the lake. There are now just six pontoons still in their original location and awaiting removal. In total, 69 percent of the floating bridge has already been decommissioned, including removal of the east and west truss structures in June.
Reuse and recycle
Most of the old bridge is being reused or recycled. All 31 of its concrete pontoons will be towed off Lake Washington by winter, transferred to a third party, and hauled elsewhere for use as docks, piers or other marine structures. The old pontoons represent about three-quarters of the bridge's total bulk. So by repurposing all the pontoons, we're taking approximately 70 percent of the old bridge off the lake without breaking apart the associated concrete.
|What’s left of the old floating bridge sits to the left of the new SR 520 floating bridge looking west toward Seattle.|
The bridge's other structures – mainly concrete girders, roadway barrier and columns from the east and west high-rises – are being dismantled on the lake, barged to an industrial site in Kenmore, off-loaded to trucks, and hauled away for reuse on other construction projects or to landfill sites as appropriate.
Commitment to environmental stewardship
Prior to starting construction of the new floating bridge in 2012, we worked with regulators to conduct the required environmental studies and obtain the necessary regulatory permits we needed both to build the new bridge and to take apart and remove the old one. Some of those permits have since been updated to reflect new conditions and, where needed, additional controls.
We are committed to environmental stewardship on all our projects. We have rigorous protocols that our contractors must follow to protect the environment and, if accidents do happen, to limit their effects.
Our contractor is using several construction techniques to keep the concrete contained while removing it from Lake Washington. Demolition best management practices include:
- Spraying water to control dust while we break up concrete columns, barrier and girders – the bridge "superstructure" – atop high-rise pontoons.
- Draping large screens around the elevated demolition segments to contain the material.
- Placing barges under the old road deck while separating over-water girders for removal.
- Bracketing pontoons with barges to catch debris while breaking up the concrete columns and road deck.
- Placing curbs and fencing around the perimeter of demolition barges to keep material out of the lake.
- Using water-tight structures to contain water used for dust-control and keep it out of the lake.
|Tugboats tow a pontoon from the old SR 520 floating bridge through the Hiram M. Chittenden Locks in July as we began|
removing the old bridge from Lake Washington. All 31 pontoons from the old bridge will be reused elsewhere.
Low levels of hazardous materials
Environmental tests conducted prior to bridge-deconstruction work showed that some of the old concrete contains various heavy metals, such as arsenic, barium, lead and mercury. A January 2016 test was performed on the concrete by an independent laboratory in Seattle. The lab used a U.S. EPA-approved test method to determine whether the concrete met the landfill criteria of Public Health Seattle & King County. The concrete met the criteria and fell well below the approved limit for disposal.
Some community organizations, however, have voiced concerns about the level of arsenic in the old bridge’s concrete. They recently questioned a presentation our bridge contractor submitted to the state Department of Ecology last March. In that presentation, the contractor reported results of arsenic levels both from the January 2016 test and from one conducted in 2015:
- The 2015 test, which determined the molecular composition of the concrete, identified an above-normal level of arsenic. This test was performed to determine if the concrete was suitable for recycling.
- The 2016 test was designed to assess if the level of arsenic and other elements from the old bridge’s hardened concrete could leach into the environment if the concrete were placed in a landfill. This second test, called a Toxic Characteristic Leaching Procedure (TCLP), is the EPA-approved analysis used by Public Health Seattle & King County for determining whether to allow disposal at Reserve Silica, a special landfill for inert materials. The TCLP result for arsenic was 0.2 parts per million – well below the EPA’s regulatory standard of 5 ppm for disposal.
- As a result, some of the old bridge’s concrete is being disposed at the Reserve Silica landfill as approved.
|A look at the timeline for decommissioning the old SR 520 floating bridge.|
Finishing the job
The permits we obtained from federal, state and local agencies to deconstruct the old floating bridge establish rigorous standards our contractor must follow. For example, the permits prohibit unauthorized discharges of any kind into Lake Washington. While mishaps can and occasionally do happen on a large construction project, we’ll continue working hard to protect our natural resources while creating a safer, more reliable transportation system for our state.
Want to know more?
You can learn more about deconstruction of the old floating bridge on our website. Some of the information you’ll find here includes:
- A folio describing the key procedures and timeline for removing the old bridge.
- A list of environmental studies, regulatory permits, and agency correspondence related to the bridge-removal effort.
- A presentation showing key activities involved in bridge deconstruction.
- A report describing the environmental protocols and best management practices the contractor follows during bridge-removal work.