IMPACT OF THE NEWLY DISCOVERED INDIANAPOLIS REEF ON CSO TUNNEL CONSTRUCTION IN INDIANAPOLIS, IN

Citizens Energy Group (Citizens) implemented a long-term control plan to address combined sewage overflow (CSO) in the city of Indianapolis, Indiana. The DigIndy Tunnel System mitigates potential health risks associated with CSOs through the nearly 28-mile-long tunnel system. The 20-foot diameter tunnels store at least 250 million gallons, removing an average of 6 billion gallons of CSO annually from local streams and rivers. All tunnels were excavated using a main beam tunnel boring machine (TBM).

During excavation of the Lower Pogues Run Tunnel (LPgRT), Citizens and the contractor, Shea-Kiewit Joint Venture, encountered a zone of highly porous dolomitic limestone within the Wabash Formation that produced diffuse groundwater inflows of 1 to 2 gpm/LF along 4500 LF of the LPgRT, resulting in difficult excavation, excessive pumping, mucking issues, schedule delays and increased costs. Brierley Associates reviewed data related to the LPgRT to better understand the adverse subsurface conditions that lead to extensive diffuse tunnel inflows as well as the powdering/crushing of the rock during TBM excavation.

Two distinct carbonate rocks, both part of the Wabash Formation, were identified within the problematic reach of LPgRT. One is a relatively strong and low permeability argillaceous dolomite, while the other is a highly porous, lower strength dolomite that was interpreted to represent a previously unknown, ancient, fossilized reef beneath Indianapolis, named herein the Indianapolis Reef. Known and well-documented occurrences of reef (e.g. the Huntington Reef Lithofacies) occur in outcrops, quarries, and subsurface boreholes throughout Indiana. The reef facies can be differentiated from the argillaceous facies of the Wabash Formation based on color, texture, mechanical behavior during coring, hydraulic conductivity, and strength parameters. Proper identification of reef facies is crucial for the success of future excavations in carbonate geology encountered by deep underground projects in the Midwest.