Unearthing Mt. Baker Tunnel

by Edgar B. Johnson, (F.ASCE), Partner in Charge; HNTB, Seattle, WA,
Lee J. Holloway, (M.ASCE), Chf. Structural Engrg.; HNTB, Seattle, WA,
Georg Kjerbol, (M.ASCE), Senior Project Engineer; HNTB, Seattle, WA,


Serial Information: Civil Engineering—ASCE, 1985, Vol. 55, Issue 12, Pg. 36-39


Document Type: Feature article

Abstract:

A new tunnel under Mt. Baker in Seattle is part of major improvements to I-90 between Bellevue and Seattle's business district. Feasibility studies began in 1963, but it was not until 1977 that the concept of a single bored tunnel was accepted by WSDOT. Conventional full face tunneling was not suitable for the site, which had silty clays beneath a residential area. Instead, the stacked drift liner system was used, with 24 bores driven in a circular pattern and filled with concrete before excavation began on the 63 ft diameter tunnel. The liner was designed as a semi-flexible support system capable of adjusting to non-uniform external loading by deforming until equilibrium is reached. Structurally, it acts as a ring in compression. Because they are only 9 ft in diameter, the drifts were excavated with conventional equipment. Access pits 90 ft in diameter and 90 ft deep were converted to vertical retaining walls for the cut-and-cover end portions of the 1,500 ft tunnel.

Subject Headings: Tunnels | Linings | Feasibility studies | Excavation | Clays | Tunneling | Silt

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