Construction of the Seattle Lid and connecting Mount Baker Ridge Tunnel along Interstate 90 reestablished physical continuity and brought together neighborhoods separated years ago. The Lid combines two different types of structural designs. The west end uses a system of prestressed concrete girders supported on continuous box-shaped walls, which serve as ventilation ducting. At the eastern end where the Lid joins with the tunnel, the structure walls resist shear and are tied into aboveground and underground support buildings. The finished ceiling above the roadways is a 4-inch concrete slab suspended by stainless steel hangers.
The Mount Baker Ridge Tunnel is 1,476 feet long. Because of the difficulty and expense of creating a large-diameter tunnel in this kind of soil, a ring of 24 small tunnels or drifts was bored through the ridge and stacked against one another in a circle, which ultimately became the liner of the larger tunnel. As soon as one of the drifts was lined, it was backfilled with concrete and then pressure grouted at 100 psi to fill any remaining voids. Within the tunnel liner a concrete structure was built. The Seattle Lid and Mount Baker Ridge Tunnel project include two new bridges, the tunnel, three lidded sections, and four major interchanges, totaling 1.4 billion dollars in costs and 1.1 million cubic yards of concrete.