From now on, a floating construction crane being used to build the new Highway 520 will be tethered to deep-rooted pilings, so it never roams again.
On the night of Sept. 29, a barge supporting a tall crane drifted on the southern winds. The crane barge, and two smaller barges attached, nearly rammed a Laurelhurst resident’s dock. The cluster finally beached in a shallow part of Lake Washington before causing any damage. Fortunately, the flotilla didn’t change course and ram the old Highway 520 bridge.
A post-incident review found that the barge’s shallow retaining posts, known as spuds, broke free from the silty lake bottom. They were mounted on a steep slope 20 to 25 feet below surface, without much sediment to embrace them. The spuds must be hydraulically lifted and lowered, to frequently reposition the crane. Therefore, spuds weren’t driven into the firm glacial till, but nestle in softer soil that’s only six to eight feet deep.
The contractor, Mowat-American, plans to drive a few deep pilings into the lake, then attach its floating derrick to the pilings whenever winds exceed 30 mph, a DOT inspector’s message says. Also, upcoming work will occur over a flat lakebed and closer to wind-sheltered Foster Island, adding protection, said spokesman Andrew Richardson.
Mowat-American is building the short West Connection Bridge, which will link the 2016 floating highway to the fixed part of the 1963 bridge, until DOT can fund and build a full Seattle connection.