Ground conditions favor Indianapolis progress
06 Aug 2013
Peter Kenyon, TunnelTalk
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As the refurbished Robbins TBM excavating the first of five deep rock CSO tunnels under Indianapolis hits the landmark half way point, the story so far is one of record-setting progress. Peter Kenyon of TunnelTalk speaks to Citizens Energy Group (CEG) Director of Special Projects John Trypus, and discovers that both designer and Owner had pre-launch reservations about using the contractor's own refurbished TBM for the project.
- By any stretch of the imagination progress on the Indianapolis Deep Rock Tunnel Connector (DRTC), which forms the 7.6-mile (12.2km) long heart of a planned wider network of five CSO tunnels totaling 25 miles (38km) in length, has exceeded all expectation.
- Sources close to the project have told TunnelTalk there was a sense of unease about selecting for the job a contractor JV (Shea/Kiewit) that proposed to complete excavation of the 20ft 2in o.d tunnel using a 37-year-old refurbished Robbins Main Beam Gripper machine.
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Shea/Kiewit JV has completed 4 miles (6.4km) of DRTC
- Such concerns, it appears, were not isolated. "It is fair to say that both the designer (Aecom) and the owner (Citizens Energy Group) had reservations about using a refurbished TBM," conceded John Trypus, CEG's Director of Special Projects. "But we performed four separate visits to the TBM site during refurbishment and were happy with the effort the contractor put into the rebuild. The machine has performed well on our project."
- Indeed it has. Six months into the 250ft (76m) deep drive - which launched from the shaft at the Southport Advanced Wastewater Treatment Plant in February – the tunnel contractor is already three months ahead of schedule. Advance rates to date have averaged more than 100ft/day (30m/day), with a world record daily progress in the 6m-7m diameter range of 124.7m achieved on June 12.
- "A big part of the reason behind such good progress is that the rock we are mining through is characterized very well. We have exceeded a lot of the rates anticipated before the project got under way, and the contractor has performed well," said Trypus.
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Water ingress has averaged 120,000 gallons/day
- "If anything the rock has been tighter than predicted, and water ingress, which we have been pumping out at an average rate of 120,000 gallons per day, has been low," Trypus reported to TunnelTalk from Indianapolis. Water penetration has, however, increased the longer the heading has progressed and as sections of more fractured rock have been encountered. The drive is mostly through geology comprising competent limestone and dolomite rock. A two-pass system is being used for installation of the tunnel lining, which is mostly unreinforced. Grouting and rock bolting are being carried out as necessary.
- "We encountered some layering of the rock at the very beginning of mining that required some additional support in the form of rock bolting and wire mesh to protect for overbreak," explained Trypus. "Within 500ft of the start of mining, we began to run out of these layers. We have been in a very standard support pattern since then. All the mining conditions we have encountered to this point have closely resembled the geotechnical information we obtained during design. Our quality of rock is projected to improve as we move further up the alignment. We have no reason to doubt this information based on our experience to this point."
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7.6-mile (12.2km) DRTC alignment
- The alignment was initially designed at concept stage as an inter wastewater plant tunnel in soft ground under high groundwater conditions at depths of 35ft-75ft (10.7m-22.9m) below surface level, and with a diameter of 12ft (3.66m). By moving the alignment deeper into a more stable geology, designer AECOM has been able to make use of more favorable excavation conditions, avoid contaminated areas that were discovered along the initial project route, removed the need for a second pumping station, and make cost savings by avoiding wells, gas lines, existing sewers and other utilities.
- The Shea/Kiewit JV that won the DRTC construction contract with a low bid of US$180.2 million, selected a refurbished, rebuilt and redesigned Robbins Main Beam Gripper TBM within its proposal, setting apparently ambitious production rates that enabled it to come in at nearly $100 million less than the Chief Engineer's estimate of $280 million for the excavation work.
- "Contractors were free to choose between using a new or refurbished TBM within their proposal," said Trypus, who agreed that the Chief Engineer's estimate of a daily production rate of 75ft (23m) had been "conservative."
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Inside the TBM operator's cab
- "The contractor assumed a much greater production rate and they are achieving it. It also might be the case that the very competitive bid reflects the fact that this is the first 7.6-mile tunnel of what will eventually be approximately 25 miles [in five tunnels] and that the nine contractors and JVs who bid really wanted to be involved in the first project with a view to future bids."
- The extensive rebuild of the 37-year-old contractor-owned machine was completed by the S-K JV and Robbins and included retrofitting a new backloading cutterhead with 19in disc cutters and installation of a variable frequency drive motor system. So far, everything has gone smoothly. "Cutterhead wear has been less than anticipated (approximately four dozen cutters have been replaced) - half as much as predicted by the contractor," said Trypus.
- "It's kind of scary. We are excited at the rate of progress but at the same time we are wary of saying that because as soon as you do things can go wrong."
- Work is progressing on a 24-hour three shift system on a five days a week basis, and downtime has so far been minimal: "No scheduled maintenance shutdowns were factored into the contract. So far the contractor has shut down mining production for a few short durations to contend with belt maintenance and navigating the first 1,000ft radius that we drove," said Trypus.
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Single 250ft (76m) deep working shaft
- "Our greatest challenge to date has been only having the one working shaft to pull out all of the excavated material and water and there have been a couple of cases of modifications being made to the conveyor system."
- If progress continues at current rates final breakthrough could be achieved by the end of 2013 or early in 2014, but attention is now turning to other elements of the $1.6 billion project.
- "Eleven firms have been invited to make proposals for the deep tunnel pump station, with bids expected to come in the $70-$90 million range," said Trypus. The deadline for final bid submissions is September (2013), with contract award expected later this year.
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US$1.6 billion scheme comprises 25 miles (38km) of tunnels
- In the meantime work continues by Black & Veatch on the final design of the White River (5.3 miles) and Fall Creek (3.7 miles) tunnels (60% complete), with contractor prequalification expected to take place in 2015 for a 2016 construction start. Construction of these tunnels is estimated at $389 million (2011 price). Parsons Brinckerhoff, as lead designer of the Lower Pogues Run Tunnel (1.9 miles), is just moving into the advanced design stage, while the final piece of the 25-mile jigsaw is the Pleasant Run Tunnel (6.6 miles), final design for which has not yet been awarded.
- "The geological profile for the other four tunnels is anticipated to be similar to that of the DRC," said Trypus. "Currently we are still evaluating how we plan to perform future tunneling sections, but the notion of having two TBMs working at the same time is still an option that is being considered."
- Once completed, the DRTC will remove some of the city's worst offending CSO outfalls, enabling the City to meet stringent Federal clean water targets agreed as part of its Long Term Control Plan. The tunnels will act as a storage system for the collection of billions of gallons of raw sewage that would otherwise flow into state waterways during periods of heavy rainfall when the existing wastewater treatment plant cannot meet capacity.
- The Owner for the DRTC (part of the bigger DIG INDY scheme), Citizens Energy Group, took over responsibility for Indianapolis' water and wastewater utlilities from the City's public utlilities department in 2011.
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Mobilizing for Deep Rock Tunnel Connector start - TunnelTalk, December 2012
Slim spead in Indianapolis' CSO tunnel bids - TunnelTalk, August 2011
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