The Indianapolis Department of Public Works is due to advertise its 7.5-mile (12.2km) Deep Rock Tunnel Connector (DRTC) project at the beginning of May. Designed by AECOM, the DRTC will be capable of conveying combined sewer overflows (CSOs) of up to 550 million gallons/day, to capture sewage from three of the city’s largest overflow locations. The 18ft (5.48m) i.d. tunnel will be bored at depths of 270ft (82m) through limestone and dolomite using a main beam TBM and lined with un-reinforced concrete.
The estimated $200-$300 million contract will include a TBM launch shaft, located at the city’s Southport Advanced Wastewater Treatment Plant, a retrieval shaft, 3 x utility shafts and 3 x vortex drop shafts. In addition, 5,600ft of levee improvements are required to protect the site from a 500-year flood event.
John Morgan, Tunnel Program Manager for Indianapolis Department of Public Works, told Tunnelling Journal that ground investigations show excellent material for tunneling, with projected high boreability and low abrasivity. “I don’t know that we could be in a better medium,” says Morgan.
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Average advance rates of 100ft (30m) per day are expected on the basis of 24hr working with a 4-6hr maintenance shift. “If you match mucking rates to that, your looking at 20 to 25 trucks an hour,” says Morgan. However, 7pm-7am noise restrictions would require night time stockpiling, which would mean 50-60 trucks per hour during the day. Another possible option would be sale of the material to American Aggregates, who operate a surface mine adjacent to the Southport site. “It would be very simple to run a conveyor over to that property,” says Morgan. “But at the end of the day it’s the contractor’s decision what he does.”
The DRTC is the first phase of Indianapolis’ federally mandated Consent Decree and $1.7 billion Long Term Control Plan to curb combined sewer overflows. Four additional deep storage tunnels will also be built: The White River and Fall Creek tunnels (which will commence in 2015); and the Pleasant Run and Pogues Run tunnels (which will begin in 2020). The entire Deep Rock Tunnel System is due to be completed in 2025.
DRTC bids are scheduled for opening in July, with Notice To Proceed (NTP) expected approximately 60 days later, in September. Final completion of the project is scheduled for August 2017. At that point in time DRTC’s retrieval shaft will become the launch shaft for the White River Tunnel, which is currently at the 30% design stage by Black & Veatch.