Great Hill Tunnel Rehabilitation: A Comparison of ROV and Manned Inspections of a 1927 Water Tunnel

摘要

The Great Hill Tunnel (GHT) is owned and operated by the South Central Connecticut Regional Water Authority (RWA). It is a 100 MGD, 3,600-foot-long, 6-foot horseshoe-shaped raw water transmission tunnel that cannot be taken out of service and dewatered without installation of a bypass system. After completion of the tunnel in 1927, the first documented inspection was by divers in 1973. Subsequently over the past seven years, the tunnel was inspected by remote operated vehicle (ROV) three times, most recently in the fall of 2017.During the 2017 inspection, routine leakage from the tunnel was noted to have increased dramatically. The owner became concerned that known defects observed during the ROV inspections—including a large defect at the transition from the tunnel to a 48-inch-diameter cast iron pipe—were not only getting larger, but also that the increased leakage was a sign of impending failure of the system. Therefore, an emergency manned inspection and repair program was planned based on the information from the three ROV inspections. This manned inspection and rehabilitation work was completed in the spring of 2018. Regular inspection of water infrastructure, especially manned, is often challenging because of lack of redundancy, limited shutdown windows, or aging infrastructure that could be compromised by dewatering. ROV inspections can alleviate some of these issues but do have limitations. This paper will compare the differences between ROV and manned inspection results and how such results can be applied to tunnel rehabilitation.