The Great Highway Coverup

by Rita Robison, Assoc. Ed.; Civil Engineering—ASCE, New York, NY 10017,

Serial Information: Civil Engineering—ASCE, 1987, Vol. 57, Issue 4, Pg. 51-53

Document Type: Feature article


Public opinion has forced changes in highway design since the first ones were built 30 years ago. In the Seattle area, this environmental sensitivity led to federal court action and to scuttling original plans for the last 6.9 miles of I-90. The solution was to depress the highway and reunite the neighborhoods thaat had been split by its predecessor. In two areas on Mercer Island one on the west side of Mt. Baker Ridge, the highway is covered with landscaped lids created by cut-and-cover tunneling methods. Piers and abutments are generally double cast in place walls to accommodate ventilation ducts. The lid structures consist of prestressed concrete girders acting compositely with cast in place topping slabs. On Mercer Island, excavation was a problem because the contractor had to remove all spoil to the mainland, then import backfill and landscaping soil. The entire $1.4 billion project is broken up into several separate segments and two major phases. Phase 1 is scheduled for completion in 1989, phase 2 in 1994.

Subject Headings: Highways and roads

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