Sometimes when the capacity of a secondary containment bund is inadequate, a quick fix solution is to raise the height of the bund walls.
Before making such a decision, we need to be sure that this solution does not impair wall stability or impact adversely on bund operations. We may need to consider alternatives such as increasing the bund area.
If changing an existing bund, we need to ensure that the modified structure conforms to the latest guidance (i.e. Ciria C736: Containment Systems for the Prevention of Pollution). This tells us that bund wall height should not exceed 1.5m for reasons of observation, fire-fighting, access and egress and prevention of confined spaces. Therefore, if we are to go above this height we need to assess the risk and be certain that there are no practical alternatives.
We need to satisfy ourselves not only that the bund wall will stay up when the bund is full of water but even when subjected to a surge wave from the collapse of a tank. (This could happen during tank testing). This surge wave is known as the ‘hydrodynamic effect’.
To assess this effect, Ciria 736 allows us to assume that our bund is 250mm higher and must resist the hydrostatic loading arising from this height. However, if the risk of bund wall failure is so severe as to justify special measures, we need to consider a more rigorous analysis.
Our structural analysis will need to assess the following parameters:
• The stability of the bund wall against overturning.
• The stability of the bund wall against sliding.
• The pressure exerted by the bund wall on the underlying formation.
• The structural strength of the bund wall components – particularly the wall stem.
To do this effectively, we need to know the dimensions, materials and structure of our existing wall. Often reliable drawings will not be available, and we will need to employ a specialist firm to investigate the wall. They will dig exploratory pits, core the concrete and probe the size and depth of the reinforcement.
We also need to know the strength of the existing soil that the wall sits on. If a full geotechnical investigation is not practical, we may have to use conservative assumptions based on our experience.
If we find that we need to strengthen the wall, this may be done in a number of ways – as shown in the diagram below. Widening the base and providing a downstand resists overturning, reduces bearing pressure
and also provides a key to resist sliding. If space is available, we may consider the alternative of construction of a compacted earth berm to the external face of the wall. To strengthen the stem, we can increase its width and add a layer of reinforcement.
The connection between our wall extensions and the existing wall needs to be able to resist the pressure from the retained fluid. Most times we can do this with dowels chemically anchored into the existing concrete. We also need to consider the watertightness of the new joints and ensure that they are equipped with adequate waterstops. Where necessary, the joints will need to be fire resistant.
Our bund wall extension may also require the raising of existing pipework, cable routes or access structures.
This work should be planned to coincide with the main construction works.