Possible cause for the partial dam wall collapse at the Benoni CBD

Possible cause for the partial dam wall collapse at the Benoni CBD

Being asked to diagnose concrete failure from photographs is an inexact science but is a growing tendency. Over lunch today Thami challenged me to write an article on the dam failure at Benoni when all I had to go on are the photographs he posted on the website.

First of all, it is not a concrete failure, but could lead to voids forming behind the concrete surface to the spillway. A loss of material supporting the spillway surface could result in an instability of the entire wall, as earth walls of this type rely on their mass to withstand the hydraulic pressure behind the wall imposed by the depth of the water. Bearing in mind that a one metre wide depth of water behind the wall will impose the same pressure as the entire dam.

So, what caused the failure; with the limited information available to me, both from the website and the TV I would say it was cavitation. This is common when concrete linings are placed to conduct water moving at high velocity, such as dam spillways and water channels. It appears that the failure occurred at a joint in the concrete where it is possible that with time a difference in level on the two sides of the joint had developed.

This difference in level may only have been a few millimetres which is sufficient to make the water “jump” over the uneven sides of the joint. Underneath the water a gap will develop which won’t achieve laminar flow, as the water is moving too fast. Air can’t get into the gap and a vacuum forms. This vacuum is able to pluck surface concrete off the concrete lining and get underneath the lining to remove the supporting layers of soil, and hence the collapse of the concrete panel in the spillway lining.

A similar but much worse failure occurred recently in an American dam where water plucked the concrete panels off and destroyed the underlying soil layers.

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