Building SR 520, an Eco-Friendly Bridge Across Lake Washington
Working to construct a bridge in an environmentally sensitive area can be very difficult. The new 2-mile section of State Route 520 is being constructed across Lake Washington, adjacent to the existing highway that acts as the west connection for the largest floating bridge in the world. This bridge section is 90% over water, and its foundation consists of 99 drilled shafts varying from 8-12 feet in diameter, with a depth of up to 175 feet constructed from a temporary trestle structure.
A containment area was built at each shaft location using multiple layers of fiber-reinforced plastic, along with a spoils bin for direct spin-off and spoils control. Constant monitoring, inspection, and maintenance were needed to ensure the operations remained contained. Spoils are hauled away in sealed trucks, with concrete moving in shortly thereafter. Since polymer drilling fluid was utilized, in addition to permanent steel casings, piping for all supply lines used high-density polyethylene (HDPE) piping and a secondary containment in high-traffic areas. A group of containment tanks followed the operations down the trestle, allowing for storage, batching, and remixing and ensuring no spillage of the drilling slurry into the lake.
