Collapse of a Reinforced Concrete Dome in a Wastewater Treatment Plant Digester Tank

Exponent Failure Analysis Associates was retained by a municipal government to perform an independent study of a fatal collapse in a local sewer treatment plant. The reinforced concrete dome shell covering a cylindrical prestressed concrete digester tank collapsed due to sludge overfilling. The collapsed dome, similar to domes on two adjacent identical tanks, showed radial and circumferential cracking prior to the collapse. Analysis showed the dome cracks were a result of temperature and shrinkage stresses, and of internal overpressure happening at some time prior to the overpressure resulting in the collapse. Inspections of the postfailure debris and analysis of the dome cracking preceding the collapse show practically no relationship between the dome condition prior to collapse and the failure. Failure of the sludge level monitoring system and the lack of a sludge overflow system are cited as the primary reasons for the failure.