Problems with Pre-Loaded Tanks
In 1977 severe corrosion was discovered in the stressing wires of a banded tank in Kent that had been commissioned in 1967. As a result of the discovery, a large number of the Preload tanks were inspected both superficially and intensively. It was concluded that all the banded tanks were subject to structural failure due to premature corrosion of the stressing wires. The straight walled tanks or un-banded tanks were performing satisfactorily; however, the wires supporting the dome were vulnerable and periodic checks were necessary.
Several years were spent searching, in vain, for a satisfactory solution to the problem. The current solution is to remove the Preload stressing wires in the bands and replace them with polypropylene covered tendons. A 2 million gallon Preload reservoir repaired in this way in 1987 and has not shown any distress or deterioration since.
In the un banded tanks, there have been cases where due to loss of protection to the stressing wires, some corrosion develops. Experience has shown that the corrosion tends to be localised, and successful repairs have been carried out to these tanks, and many of these tanks are still in operation. This is mainly because the remaining wires are still in tact and are held securely by the bond between the gunite and the wires.
Unfortunately at Lanner Hill reservoir, constructed for The Camborne-Redruth UDC the corrosion in the wires supporting the dome had progressed so far that the wires were unable to hold the dome ring together. This led to a major collapse of the domed roof.