Completion of a Fourteen – Year Rock Tunnel Interceptor Rehabilitation Project

The Niagara Falls Water Board (NFWB) owns and operates water, wastewater, and stormwater infrastructure in the Niagara Falls, New York area. One component of this infrastructure is the North Gorge Interceptor (NGI), an unlined rock tunnel constructed in 1937. Problems with the NGI became evident following prolonged wet weather overflow and surface observations of a significant surcharge within access shafts. An investigation involving several innovative measures indicated a nearly-complete tunnel obstruction with sediment and debris, upstream from a principal pumping station.A cleaning contract was designed and bid. The contractor elected to bypass pump interceptor flow around the work area, permitting conventional mining techniques for debris removal. The rock tunnel was discovered to have enlarged over time, creating a larger cavity within which a much greater quantity of debris had accumulated. The contractor was able to remove 4,300 cubic yards (yd3) of debris and rock under this contract; however the remaining debris continued to create the flow restriction and surcharge. The NFWB utilized a portion of the bypass pumping system post-construction to supplement tunnel flow to the pumping station until a final resolution to the problem could be evaluated and implemented.The award of American Recovery and Reinvestment Act (ARRA) funding enabled the project's concluding phase to proceed. The mining-in-the-dry approach was used again. A tunnel bulkhead isolated the work zone from upstream flows, enabling debris removal from three locations. Restricted access to the work zone, crumbling rock, odor and ventilation issues, and occasional wet weather surcharges further complicated project progress. One change order to increase debris removal quantities and extend the schedule was necessary to completely clean the subject stretch. After cleaning was completed, various rock stabilization measures were emplaced to slow the rate of continuing rock deterioration. Inspections are planned on a periodic basis to verify the condition of the tunnel and plan smaller–scale cleaning efforts.