DigIndy TBM over halfway on 45km route 29 Aug 2019

Mining of the 6.1km (3.8 mile) Fall Creek section of the DigIndy deep-level CSO storage tunnel under Indianapolis is to begin in September 2019, following the completion of excavation on the White River route in April 2019, according to Mike Miller, Construction Manager for owner, Citizens Energy Group. About 27km (17 mile) of the 45km (28 mile) network has now been bored by a refurbished Robbins TBM that was previously used on the Second Avenue Subway in New York City, with the project on target to meet its Federally-mandated completion date of 2025.

Fig 1. Six sections of the DigIndy CSO system

The DigIndy project comprises six alignments. The 12.2km (7.6 mile) Deep Rock Tunnel Connector, DRTC, and 2.7km (1.7 mile) Eagle Creek elements have been operational since 29 December 2017, so far capturing 5.7 million meters³ (1.5 billion gallons) of CSO. Mining is complete on the 9.3km (5.8 mile) White River and 2.9km (1.8 mile) Lower Pogues Run sections. Lining work and adit/deaeration chamber construction is now underway on both.

Fall Creek will be the penultimate part to be excavated, after which the TBM will be refurbished and set to bore the 11.8km (7.3 mile) Pleasant Run alignment by contractor, a joint venture between Shea and Kiewit. The entire route is bored at a diameter of 6.2m (20ft) at a depth of 76.2m (250ft) with a 30cm (12in) cast-in-place concrete lining. When complete, the system will capture CSO from most of the 134 outflows in the city, holding about 1 million meter³ (250 million gallons) at any one time and preventing about 22.7 million meter³/year (6 billion gallons/year) of CSO flowing into the river.

Drive strategy

Despite the length of the DigIndy network, the project is using only one TBM and requires only eight large-diameter shafts, thanks to a drive strategy that has seen the machine mine three bifurcations to the main DTRC-White River-Fall Creek main alignment (Fig 1), before backing up and continuing. In contrast, the 41km main sewer in Abu Dhabi, which had an excavated diameter of 6.3m, used eight TBMs.

Breakthrough on White River section

Earlier in the project, after mining the DRTC, the contractor proposed mining Eagle Creek, which was originally a shallow alignment, as a deep-level spur to the DRTC. This saved time and reduced surface disruption, while adding about 17 million gallons of storage capacity. To do so, the contractor backed the TBM down the DRTC, excavated Eagle Creek, then backed up again and walked the TBM to the end of the DRTC, avoiding the need for a retrieval shaft.

Mining on White River began in 2016. The machine bored north, up the White River alignment, before diverting east to excavate the Lower Pogues Run element of the project. It was then backed up and relaunched north, until diverting again to mine a northwestward spur of White River. The TBM was again backed up and relaunched to complete the main White River alignment. It is currently about 274m (900 ft) into the Fall Creek alignment, awaiting restart of excavation. When it reaches the end of Fall Creek, which Miller expects relatively quickly, in about March 2020, due to the favourable geology along that portion of the route, the TBM will be retrieved and refurbished. It will then mine Pleasant Run, which Miller expects to begin later in 2020.

“This drive strategy means we have only needed eight large diameter shafts,” said Miller. “Because we have been able to back up the TBM, the dead ends on Eagle Creek, Lower Pogues Run and the White River spur only required small-diameter shafts to allow flow down from the closest CSOs.”

Terminal face on Lower Pogues Run

The large diameter shafts are located at the Southport launch site, at the end of the DTRC, at the White River-Fall Creek boundary, and at the end of Fall Creek, as well as at the launch site, halfway up and about 914m (3000ft) from the end of Pleasant Run. “We were not able to put a large-diameter shaft where Pleasant Run terminates, so we are going to have to mine to the end of the alignment and then back the TBM up to the retrieval shaft.”

Groundwater infiltration during tunnel construction

For the most part, the geology has been fairly consistent limestone/dolomite bedrock, but some sections of higher groundwater infiltration have been encountered. As a risk mitigation measure, the owner and contractor worked together to develop a strategy for the implementation of probing ahead of the TBM face. If infiltration above a certain tolerance is encountered, the owner pays for the additional time and materials needed to resolve the issue. “This shared risk approach provides value to the owner based on expected levels of groundwater infiltration, but also protects the contractor should higher levels be encountered,” described Miller.

Pump station, ventilation and drop shafts

The project also included construction of a new underground pump station at the Southport treatment facility. Constructed by an Oscar Renda-Southland Contracting joint venture, it includes a 30m long x 18m wide x 23m high (100ft long x 60ft wide x 75ft high) cavern to house four 30 million gallon/day pumps. The cavern was constructed by drill+blast and is supported by 700 rock bolts. A waterproof membrane and then shotcrete finishes the cavern. The pump station is connected to the original launch shaft, which now functions as the screen and grit shaft, by a 1.8m diameter x 55 m long (6ft diameter x 180 ft) long tunnel.

White River landing/Fall Creek launch shaft

Managing the ventilation was challenged by the various bifurcations along the route, explained Miller. “We do not want air from the mainline to enter the smaller runs because we only have a small diameter shaft at the end of those. So we have built 1.5m (5ft) diameter bulkheads from the crown to redirect any air and make sure it vents at the launch or retrieval shafts.”

There are 33 drop shafts along the route to bring surface discharges to the tunnel alignment with limited air entrainment. With the exception of those at the dead ends, these drop down into deaeration chambers that are connected to the mainline through adits. All of the adits and deaeration chambers are constructed by drill+blast, while the drop shafts themselves are primarily excavated by raise boring through the bedrock and by an innovative oscillation excavation method through the overburden.

Crane-mounted oscillator installs drop shaft cans

“We have a mixed bag of overburden, as well as a high water table,” explained Miller. “This means we need a watertight retention system to be able to build the drop shafts through to the bedrock. To achieve this, we have used a crane-mounted rig that can take a 3cm thick x 3.7m diameter (1.25in thick x 12ft diameter) can and oscillate it through the overburden using a specialised clam bucket. It is a very efficient system: they can install two 30m-33m (100ft-110ft) shafts in less than two weeks.”

The efficiency of this system, combined with its rarity in the US, with only one or two in the country, prompted DigIndy to construct all the drop shaft overburden excavations at the same time, including those adjacent to sections that had not yet been excavated. “We have put all of the support of excavation cans in for the remainder of the program,” said Miller, completing the final one in late August 2019.

“DigIndy is one of the largest civil works projects in Indianapolis history,” Jeffrey Harrison, President and CEO of Citizens Energy Group, told TunnelTalk. “Citizens employees work hard every day to find innovative ways to keep DigIndy on schedule and below budget, and we are fortunate to have highly skilled contracting partners that continue to provide effective solutions to complex issues. When fully implemented, the DigIndy system is going to help restore water quality in local waterways to levels not seen in more than 100 years. The utilisation of our waterways as an asset to our community will improve the quality of life for central Indiana.”