North American Tunneling Industry Firsts in the Midwest: The Blacksnake Creek Stormwater Separation Improvement Tunnel

摘要

The Blacksnake Creek Stormwater Separation Improvement Tunnel intercepts Blacksnake Creek stream and stormwater flows up to 18 cubic meters per second (650 cfs) from the City's combined sewer system and redirects the flows to the Missouri River through a dedicated stormwater conveyance tunnel approximately 2.0 kilometers (1.2 miles) in length and 2.7 meters (9.0 feet) in finished diameter. Mixed faced conditions of soil and shale bedrock along portions of the tunnel alignment required excavation and initial support methods capable of transitioning between the varying ground conditions. An Earth Pressure Balance Machine (EPBM) was selected to provide better control over the excavation face and reduce risks associated with settlement and groundwater inflows. The particular EPBM used was designed by Lovsuns and manufactured in China by Lovsuns' parent company, their first EPBM delivered to the North American tunneling industry. The tunnel was lined with precast concrete segments solely reinforced with macro synthetic fibers (BarChip, Inc. BarChip 54 fibers) in lieu of steel fibers or traditional bar reinforcement, another first for the North American tunneling industry, to comply with the American Iron and Steel provisions of the Contract. The project also includes a baffle drop shaft, a separate shorter tunnel underneath active main line BNSF railroad tracks, and an energy dissipation structure. This paper will focus on the design decisions and lessons learned throughout construction including addressing mixed face ground conditions and groundwater during the single pass tunnel excavation and installation of the precast concrete segmental tunnel lining.