Milwaukee's Deep Tunnels—No Clone

Serial Information: Civil Engineering—ASCE, 1987, Vol. 57, Issue 12, Pg. 70-73

Document Type: Feature article


Tunnel boring machines are excavating nearly 19 miles of tunnels 17-32 ft in diameter as part of Milwaukee's $1.9 billion upgrade of its sanitary sewage system. Laid out to run up to 300 ft deep under three rivers, the tunnels will relieve overflowing sewers, and store their overflow until the sewage-treatment plants can process it. Four innovative approaches in design and construction are described. To the extent possible, the tunnel will be unlined, saving an estimated $50 million. Flows up to 3,300 cfs will enter the dropshafts, which will feed the deep tunnels up to 300 ft below. Rather than using massive aeration and deaeration, the flows entering the dropshaft will create a vortex as they spiral down the shaft. Cutting the falling sewage's energy 60%. Many miles of smaller (72-120 in.) sewers will also be built. One option permitted contractors is the single-pass precast concrete liner. Because it avoids the necessity of a second (cast in place concrete) liner, this design may cut sewer costs up to 30%. Ground freezing is being used to stabilize the ground and control groundwater when excavating access shafts up to 180 ft deep, and to create frozen-ground cofferdams underground to build the dropshaft approach channels.

Subject Headings: Sewers | Soil stabilization | Excavation | Overflow | Shafts | Precast concrete | Linings | Boring | Wisconsin | United States

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