Well hello, and welcome to another episode of Waterproofing Wisdom.
This time, I’m looking at – ‘Do we really need to think about waterproofing a structure to the full height of retained ground?’
So, this is a project that we’ve just been looking at, and it’s in London – and it has typical London geology, with some London clay that doesn’t really let water through – and that’s leading to some perched water. And here we have a historic vault that is going to get in-filled. And there’s a new basement, new b2, and a new basement fit out over here. And we’ve looked at it, and we’ve said to the rest of the design team that we’d like to have a reinforced concrete wall here, and the very reasonable, legitimate query has come back from the rest of the design team saying – ‘But why? The waterproofing, the water is down here, there’s perched water, we just want to use a block work wall up here and put a cavity drain membrane in’. So, I wanted to explain, well actually, our best practice guidance requires us to consider that water might come to bear to the full height of the structure, and if that was to happen, then that retained water in this area would push over their proposed block work. Hence we’ve requested a reinforced concrete there.
So let me try and explain a little bit more. This is another project I worked on, and it was, I was called in as an expert witness, and it’s in London so would presumably have a similar geology. The vault in that scenario was outside of the scope of the refurbishment contract, and here is a block work wall. This potential scenario sort of did occur – so the reason why we would always anticipate water to the full height of the structure at some point in the building’s life is because you could have a burst water main. In this area it would be Thames water and these are images taken from the news, if you search for ‘Thames water, burst water main’. As you can see, where you put the water table on a section detail is not necessarily the same as where water always is.
In this scenario, it led to water ingress at really quite some flow rate. Although you could have a cavity drain membrane that probably could accommodate that, you should never design for that. Really, that is too much water, and that should be prevented before installing a cavity drain membrane. So, in the section detail I showed a moment ago, the vault is to the left, its filled up with water, and now water is coming through the wall – that white wall there – coming under the wall, and spreading across the internal floor. Which in that case is an art gallery – fortunately all of those pictures were raised slightly above the floor level, which was very fortuitous.
So, that is why we always anticipate water coming to the bare, to the full height of the structure at some point in the building’s life.
My name is Ben Hickman, I’m the Technical Director of CLW. If you ever want to talk about waterproofing, do get in touch! Thank you.
