Downtown in Soft Ground

by John W. Critchfield, Sr. Tunnel Engr.; Parsons Brinckerhoff, Inc. Two Waterfront Plaza, Suite 220, 500 Ala Moana Boulevard, Honolulu, HI 96813,
John F. MacDonald, Proj. Mgr.; Guy F. Atkinson Co., 448 South Hill, Suite 206, Los Angeles, CA 90013,


Serial Information: Civil Engineering—ASCE, 1990, Vol. 60, Issue 2, Pg. 36-39


Document Type: Feature article

Abstract: Critchfield and MacDonald describe the shield excavation of twin bus tunnels 21.25 ft in diameter and 5,000 ft long as part of the Downtown Seattle Transit Project. This $450 million project, scheduled to begin Service in September 1990, includes construction of a 1.3 mi tunnel to carry dual-powered buses and light rail trains. A variety of innovative engineering practices were included in the tunnel construction. Initial support for the tunnels consisted of expandable precast concrete segments, followed by a final lining of cast-in place concrete. Waterproofing was provided by a PVC membrane installed between the initial support and the final lining. During the course of the excavation a variety of methods were used to control the ground and limit settlement. These included deep wells, vacuum well points, eductor wells, chemical grouting, compaction grouting, jet-grout underpinning and extensive instrumentation.

Subject Headings: Buses | Construction | Excavation | Innovation | Soil settlement | Tunneling | Tunnels | Washington

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