Quantitative risk assessment applied to sludge lagoon embankments

Following privatisation in 1989 Severn Trent Water (STW) inherited a number of waste water treatment works from Severn Trent Water Authority and its predecessors. A number of the works contain raised sludge lagoons, some of which date back over 100 years. Ageing, elevated, sewage sludge lagoons can pose a significant risk as the tip failure at Deighton, Yorkshire in 1992 (Claydon et al, 1997) proved. In learning from this incident, STW is adopting a proactive approach to reduce the risk of failure of sludge lagoons. Whilst associated risks to life or property are very low and not subject to reservoir safety legislation, the catastrophic failure of a sludge lagoon would have a major impact on business reputation as well as attracting punitive remedial costs and litigation. This has led STW to implement a proportionate response to the potential risks posed; significant investment in investigating the nature of these assets has been made, leading to the design and implementation of risk reduction measures over a number of years. Regular and effective monitoring of the assets is also an important part of this process, assisting in detecting changes in behaviour so that problems can be spotted early before they become potentially unstable and expensive to fix.

The sludge embankments have been in place for many decades without major slope failures. The non-engineered nature of the embankment materials, un-recorded construction methods and lack of knowledge of the nature of the materials retained leaves them inherently vulnerable to internal erosion due to seepage, rotational slips and internal combustion. The current lack of any secondary engineered defence could lead to rapid failure in the event of a serious internal erosion or slope stability incident. The uncertainties associated with the non-engineered and variable nature of the embankment materials means that they are difficult to characterise even with extensive intrusive geotechnical investigations. This makes it challenging to assess the future stability of the lagoons using conventional engineering analysis. This paper will describe how the level of risk associated with the various sections of a large sludge embankment were assessed utilising quantitative slope risk assessment techniques applied to infrastructure embankments.