Dig Indy Tunnel System: Pleasant Run Deep Tunnel Update
Nick Maynard – Citizens Energy Group
Leo Gentile, P.G. – Black & Veatch Corporation
Maceo Lewis, P.E. - Black & Veatch Corporation
Abstract
Pleasant Run Deep Tunnel is one of the larger segments of the 27-mile long Dig Indy System network of deep rock tunnels to reduce combined sewer overflows into Indianapolis’ waterways. This paper describes how Citizens Energy Group has been working internally and collaboratively with its engineering partners to optimize often conflicting drivers within urban constraints while maximizing storage, improving constructability, reducing cost, managing risk and meeting federal consent decree requirements.
The proposed alignment will be approximately 38,800 feet long and a finished inside diameter of 18 feet. Tunnel size was determined by constructability considerations to provide adequate room for ventilation, conveyor systems, and the utilities and equipment necessary for tunnel construction. The remaining Dig Indy Tunnel System is also sized at 18 feet, so uniformity in size was recommended by Citizens. Additionally, tunnels in this size range are fairly common and a number of contractors are capable of constructing this size tunnel.
Adits within the bedrock will be required at some drop shaft locations to convey captured CSO flow from the drop shafts to the tunnel. The number and length of required adits were minimized by optimization of the tunnel alignment and selection of drop shaft locations in close proximity to the proposed tunnel alignment.
Ten drop shaft sites were originally planned to capture a total of 28 CSOs along the PRDT alignment. Citizens modeling team developed options to maximize the capacity of existing infrastructure along the PRDT route while eliminating CSOs by measures, such as raising or adding weirs on existing interceptor and CSO sewers instead of installing new diversion structures and consolidation sewers. The flows from these shafts were consolidated and will be transferred to nearby drop shafts. In addition, the original 19,500 feet of consolidation sewer and appurtenances has been reduced to 14,600 feet of sewer.
Multiple alternative sites for the TBM working shaft and retrieval shaft were evaluated. The TBM working shaft will be the launch point for the TBM and the location where the majority of tunneling operations are based and generally requires a minimum of three to four acres to accommodate the tunnel construction operation. The proposed Bluff Road and West Pleasant Run Parkway North Drive site has approximately 4.5 acres of open area.
Two alternative locations were proposed for the retrieval shaft based on the preferred alignment. The first alternative location for the TBM retrieval shaft is at Ellenberger Park, which has a proposed size of 4.5 acres. The second alternative location proposed for the TBM retrieval shaft is located near Pleasant Run Golf Course. Both alternative TBM retrieval shafts can be used for drop shafts upon completion of tunnel construction.
Citizens is assessing possible reuse of tunnel spoils to help with construction near-surface infrastructure associated with tunnel construction. Up to 500,000 cubic yards of rock will be produced during mining of Pleasant Run Deep Tunnel. With minimal processing, the tunnel spoils can be used as controlled low-strength material for consolidation sewer backfill. Tunnel spoils could also be used for cover and grading of legacy Citizens facilities as they are being redeveloped for other uses.