Seattle Bus Tunnel Construction

This paper describes the shield excavation of twin transit tunnels, through soft ground, in downtown Seattle. The tunnels are 6.5 m (21.25 ft) diameter and 1525 m (5000 ft) long. Initial support 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.