Shallow SEM Tunneling with Limited Clearance to Existing Structures: Design, Construction and Observations

摘要

The excavation and initial lining has just been successfully completed for the Downtown Bellevue Tunnel using Sequential Excavation Methods (SEM). The soft ground tunnel, approximately 2,000 feet long and 38 feet wide with very shallow cover, faced several unique challenges and design refinements that were managed and implemented during final design and construction. This paper provides an overview of the tunnel construction, then focuses on three design refinements; tunneling under an existing utility trench with 4 feet of vertical clearance, tunneling within 4 feet of an existing building basement, and eliminating pipe canopy and replacing 12-foot thick overburden soil with controlled low strength material (CLSM) at the tunnel's north portal. Details of the analysis, design, and construction of these design refinements, together with the comparisons between the predicted and observed settlement results are presented.