REPORT | NORTH AMERICA
Above:
Tunnelling of Jefferson Barracks was suspended in September 2019 when the TBM encountered a karst feature. A replacement TBM is now being prepared
Opposite top:
The cutterhead is lowered on the Clearwater Project, a new seven-mile tunnel for the Los Angeles County Sanitation District
Opposite bottom: On Canada’s Trans Mountain project, the Herrenknecht dual-mode TBM can be changed from EPB shield mode to an open-mode configuration
Multi-service vehicles will carry up to two tunnel
segment rings, personnel and other equipment through the tunnel, while multiple slurry booster pumps will manage the supply and discharge flows. Vakharia states that the majority of the northern
portion would be post-tensioned to handle internal pressures once the tunnel is in service. “As most of the post tensioning will be conducted
concurrent with mining, two integrated work platform gantries are included in the TBM trailing gear. Within these work platforms, the post-tensioning strands will be threaded through the segment ducts, tensioned and grouted. Due to the additional support features required, the total tunnel-machine length is close to 800ft (244m),” says Vakharia. The lining is a single-pass 18ft (5.48m) ID and 20ft
(6.1m) OD, six-piece precast segmental lining. In several sections, including the fault zones, a secondary steel lining will be installed. “Since this is an effluent outfall tunnel, which can
encounter internal pressures during operation and varying native ground support along the alignment, different types of precast tunnel lining reinforcement will be used. This includes combinations of fibre reinforcement and steel rebar reinforcement, and different types of post-tensioned configurations. The concrete strength of the segments also varies in the
24 | November 2021
southern and northern portions of the alignment,” explains Vakharia.
DIGINDY TUNNEL SYSTEM In Indianapolis, the US$2bn, 45km-long DigIndy Tunnel System is nearing completion. Excavation has finished on five of the six, 5.48m-diameter tunnels which are designed to reduce combined sewer overflows (CSOs), and the sixth – the 11.8km-long Pleasant Run tunnel – is around 20% complete.
DEEP ROCK TUNNELS IN ST LOUIS In Missouri, the Metropolitan St Louis Sewer District (MSD) is building eight, mainly deep rock tunnels and storage facilities as part of MSD Project Clear, a US$6bn, 28-year programme to improve water quality and wastewater management. Excavation of the 6.2km-long Deer Creek tunnel
was completed in January 2020 and now the next two tunnels – Jefferson Barracks and Lower Meramec – are under construction. Tunnelling of Jefferson Barracks was suspended in
September 2019 when the 3.35m-diameter Robbins main beam TBM, which had mined around 2km of the 5.39km tunnel, encountered a karst feature. “The tunnel is located in limestone bedrock. Karst was identified as the most important geotechnical issue as
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