Twelve Tons C Impedance Bridge De

A new raw-water intake line for the City of Wyandotte, Michigan, installed last year, consisted of 1700 feet of 42-inch concrete pipe laid in a trench 12 feet deep, below the bottom of the Detroit River. Before the trench was filled to cover the pipe, tests showed a leakage of only one gallon per minute, which was negligible compared to the leakage allowed by the specifications.

During the process of backfilling, a constant head of approximately 20 feet was maintained in the pipe. After about 800 feet of the work had been completed, a leak developed, so serious that two pumps, delivering 60,000 gallons per hour, were unable to fill the 12-inch riser pipe used as a test head.

To repair the leak, it was first necessary to locate it. Pumping tests indicated that the leak was in the covered portion of the pipe.

The method chosen to locate the leak consisted of pumping into the pipe water containing known concentrations of

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