The Case of the Leaky Lagoon

Synopsis:
A broken underground pipe gave the appearance that ground water pressure ruptured a large concrete tank, but investigation showed that ground water was not involved, thus resulting in coverage for the loss.

Background:
A 71’ diameter wastewater treatment plant clarifier tank had been drained for preventive maintenance and was in the process of being refilled when several loud, but muffled, noises were heard and tremors were felt by people standing on the rim of the clarifier as well as on the ground outside the clarifier. The tank was again drained and the 10” thick concrete floor was found broken in several places. Repair cost was about $500k.

Analysis:
TI was requested to determine the extent of damage and specific failure mode for coverage consideration. Sewage is brought to the tank in a 20” diameter pipe that passes under the tank to the center, and is then turned upward to empty into the lagoon at the top. In video inspection of the interior of the 20” pipe it was found broken in two places under the tank bottom.

Investigation found that when the tank was originally drained it was noted that the center column had risen 3.5”. At this time, the floor was not supported on the soil under the tank but was suspended from the central column out to the tank walls. When the tank was about 1/3 full of water on refilling, the floor could no longer carry the weight and broke up, creating the earthquake-like scene. When the tank was drained subsequent to partial refilling the central column was found to have risen 12”. The floor and central column were lifted by hydraulic force resulting from soil saturation from the leaking 20” sewage pipe, not an influx of ground water.