THE STUBBORN (AND UNFINISHED) RESTORATION OF A ROCK TUNNEL INTERCEPTOR

A project was recently undertaken to attempt to relieve chronic surcharging in a 65-year-old interceptor paralleling the scenic Niagara Gorge. The interceptor, a rough-cut, rock tunnel designed and constructed during the depression, conveys combined wastewater to a pumping station, which in turn transports it to the Niagara Falls Water Board's wastewater treatment plant. Chronic surcharging and wet weather geysers prompted an investigation into the causes, which used various methods including remote observations, entry by divers, chemical tracer testing, ground penetrating radar, imaging sonar and sub-bottom profiling sonar. The effort determined that a several thousand-foot section had become occluded with a long-term accumulation of debris.A cleaning project was undertaken where a choice of several methods would be permitted; the successful bidder chose to bypass pump around the section, allowing manned entry and manual cleaning with equipment. Once cleaning in the mainline tunnel, however, it was discovered that the tunnel roof and walls had severely deteriorated over time. This created a larger cavity within which a much greater quantity of debris existed. The approach and project budget could not accomplish a complete tunnel cleaning as anticipated. The cleaning effort was suspended with about half of the total amount of debris removed.The construction bypass pumping system was modified and expanded in a successor project, aimed at supplementing tunnel hydraulic capacity around the most critical section. This temporary measure would permit the owner to reevaluate the feasibility of other candidate corrective measures, now knowing the actual condition of the tunnel. Work to complete the project is expected within the next few years.