Indianapolis’ digindy tunnels WILL stretch a vast 45km in total, traveling below the area’s water bodies and right-of-ways. But the complexity of the system underlies a simple fact: All of the tunnels—more than a marathon’s worth of excavation—are being bored by one machine. The decision not to use multiple machines—and to use a Robbins Main Beam TBM in service since 1980—has redefined thinking for large scale projects and created a primer for how to get the maximum life out of a given TBM.
A SINGULAR DECISION
The EPA-mandated DigIndy project is part of a consent decree targeting Combined Sewer Overflows (CSOs) in various parts of Indianapolis and surrounding areas that currently flow into the White River and its tributaries. Once online, the deep tunnel project will reduce the amount of raw sewage overflows and clean up tributaries along the White River.
Mike Miller, manager of construction for project owner Citizens Energy Group, describes the environmental benefits the project will provide. “It’s going to be huge—we have about 140 CSOs and this project will abate over 100 of them. The city of Indianapolis and surrounding areas currently have 5 to 6bn gallons of overflows per year going into area waterways. We are aiming for greater than a 95 per cent capture rate, which is over the consent decree requirement. This project will make a greater impact on our surface waters in central Indianapolis than any other project constructed to date.”
For Miller, the benefits are not only environmental. “Indianapolis is often criticized for not having lakes, mountains, or oceans. But, we have this extensive river system that is highly underutilized. With cleaner water, this project could drive economic development. We could see recreation and tourist activities along our waterways. It’s ultimately just the right thing to do for the health of our people and for the river system.”
The contractor on the project, a joint venture of J.F. Shea and Kiewit (S-K JV), completed work targeting three critical CSOs that will go online by the end of 2017. The 12.2km-long, 5.5m-diameter Deep Rock Tunnel Connector (DRTC) will have a capacity of 289M liters for overflow events, and has enough slope that it will be self-cleaning with minimal maintenance once operational. That tunnel, as well as the 2.8km-long Eagle Creek Extension Tunnel, was mined in between 2013 and 2015. Citizens Energy Group then issued a separate, competitive solicitation for the remainder of the work. “Originally when the project was conceived there was a feeling that we would need two TBMs to comply with the consent decree date of 2025,” Miller explains. “But we are a public trust; a pseudo-private entity. We look for best-value proposals, not low bids based solely on price. The credit really goes to S-K JV: they came back with a scenario and said if we do it this way, we can use just one TBM for the rest of the project.”
The new scenario required some sequencing of the various tunnels to be constructed and modifications to the design. “S-K JV had already mined 15.5km of our tunnels over 24 months so we knew we could trust them.
“From an owner’s perspective we are excited to have a good partner like S-K locked in long term to complete the rest of our system. We took a lot of unknowns off the table by bundling the four tunnels together and working with S-K. We saved a significant amount of money this way.”
With the winning plan underway, S-K JV is now excavating the White River Deep Tunnel, an 8.5km stretch north of the completed DRTC and pump station. The Lower Pogues Run Deep Tunnel will split off 3km from the White River Deep Tunnel heading east. The two tunnels are expected to be online by 2021, with the final two tunnels, the 5.9km-long Fall Creek drive and 11.7km Pleasant Run, anticipated to be online by 2025.
A RECORD-SETTING DRIVE
The 6.2m diameter TBM, owned by S-K JV, was refurbished and redesigned in 2013 for the first leg of the DigIndy project—the DRTC. Originally built in 1980, the TBM had previously bored at least five other hard rock tunnels including New York City’s Second Avenue Subway.
Design updates for the DRTC included new 19-inch disc cutters, variable frequency drive (VFD) motors, a backloading cutterhead, and a rescue chamber. “The age of the machine wasn’t a concern for us, it was a positive. We knew it could perform in harder, more abrasive rock. We rebuild machines all the time, it’s just a part of what we do,” says Stuart Lipofsky, project manager for S-K JV.
The 36-year-old TBM was launched from a deep shaft and utilized a unique continuous conveyor system, also manufactured by Robbins. The system spanned 25km (two times the tunnel length) of belt and two unprecedented 90-degree curves, hauled muck up a 76m-deep shaft using a vertical belt and deposited it in piles using a radial stacker. “This was one of the most complex conveyor systems in tunnelling construction in North America,” Lipofsky says. “The belt was going through two 90-degree curves in opposite directions and S-curves in other places. It was very unusual and amazing to see a belt system perform as well as this one did.”
Crews were able to keep up the good advance rates despite significant water inflows that hovered around the 2500 GPM rate for months and an extensive grouting program. Ultimately the TBM broke through one year ahead of schedule, and the Eagle Creek Deep Tunnel was added onto the contract as a change order. That 2.8km stretch was completed in less than a year—before the original contract date to complete the DRTC.
ROBUST REFURBISHMENT
With the initial phase of work completed, S-K JV set about refurbishing the TBM for the nearly 28km of tunnels the TBM would bore. “We put in a new bearing, motors, gearboxes, electronics, chillers, etc. We went through everything. We did that on the jobsite. We got the machine out in May, and the refurbishment lasted through August, about four to five months,” Lipofsky says.
In addition to the refurbishment, S-K JV, with the assistance from Robbins, added an automatic grout system and three extra decks to the back-up system. Based on what was experienced at the DRTC, S-K JV knew they would encounter water and needed to replace the manual grout system used on that job. “It’s a cement pre-excavation grout used with an automated grout plant on the trailing gear. Going forward we knew we would need to do grouting,” Lipofsky explains.
The new high-capacity grout system was very similar to one that S-K had previously used on California’s Arrowhead Tunnels. The system was fitted with new components manufactured by Hany, including a grout mixer, agitator, and pump, as well as new PLC controls to automatically adjust the auger feed rate, water, additives, mixer discharge rate, and the rate of material flowing to the high-pressure grout pump. The system also measures grout volume and pressure as it is injected into the probe holes. Robbins designed a system to move grout bins into place on the back-up in order to feed the mixer. With the new design elements in place, the TBM was ready to be launched. After completing its newest phase of tunnelling work in 2025, the machine will have bored perhaps more mileage than any other Robbins TBM currently in service: 51 km of tunnel over multiple projects since 1980. The long life the JV has achieved with the TBM, says Lipofsky, “requires a deep understanding of the machine and knowing how to take care of it.”
THE NEXT PHASE
With the TBM design finalized, the machine was launched to bore the White River Tunnel from the 67m deep DRTC retrieval shaft. Since September, the machine has excavated more than 1,200m in similar limestone and dolomite rock. The TBM is nearly at a point approximately 1.6km into the White River Tunnel, where the drive will bifurcate eastwards to bore the 3km Lower Pogues Run Tunnel in front of Lucas Oil Stadium in downtown Indianapolis. The machine will then be backed up to the bifurcation point before continuing north for completion of the White River Tunnel.
As the machine bores, continuous conveyors remove muck in an extensive system that was highly successful at the DRTC. Much of the conveyor structure remains the same for the new tunnels, with new horizontal and conveyor belting provided. The conveyors will wind through curves as sharp as 300m in radius, as the tunnels follow the path of the White River overhead. The path is slightly altered from its original design and includes another branch to the west–a move that Miller explains: “We tweaked the alignment of the White River Tunnel. What happened was where that branch ends there is a big CSO that we need to collect. Based on what we learned with the conveyor on the DRTC, we decided not to do a deep curve of 110 degrees, which was the original design.
“Robbins recommended against it for optimal conveyor performance, and so we went back to the drawing board at the 11th hour in order to ensure the most efficient use of the conveyor. We are getting ready to mine Lower Pogues, and then we will back up, mine the tunnel branch, back up again, and mine the rest of the tunnel alignment.”
Thus far, Lipofsky says the conveyor is “performing really well. Excellent.” However ground conditions have been challenging and include steady water inflows. “We’re in mixed face rock, with limestone in the bottom third and dolomite in the top two thirds where all the water is coming in. It is difficult ground to grout but easy ground to mine.”
The JV is installing rock bolts with occasional wire mesh, but says very little support is needed as the rock is of good quality.
Tunneling is currently on schedule, and spirits are high among all parties involved.
Breakthrough on the White River Tunnel into a retrieval shaft will take place on CEG land—it’s a location that happens to be in the back parking lot of Miller’s office. “I’m really looking forward to seeing that,” he says
