Collapse of a fluid storage tank in a biogas plant: an investigation on possible causes

This paper is referred to the collapse of a reinforced concrete (R. C.) wall of a rectangular digester, which was filled by fluid waste (whose density is almost equal to water) and by pressurized biogas above the fluid surface. The overall height of the wall was 6.00 m, its width was 26.00 m and its thickness was 0.35 m. The collapse, which involved a significant portion of the wall around its center, took place three years after commissioning of the plant. At the moment of the collapse, neither earthquakes nor other environmental extreme events were reported in the area. Moreover, the plant seemed to work properly when failure occurred. A simple elastic analysis, in the presence of the standard service loads, revealed that the stress levels (both in concrete and in reinforcing steel) did not exceed the elastic thresholds, as they have been measured on extracted specimens. Consequently, the possible causes of collapse have been investigated by considering several aspects. The analyses led to the conclusion that the structural collapse can be enclosed in the framework of damage accumulation for the reinforcing steel bars.