Site Assessment: Geology and Basement Waterproofing

Risk Assessment

Part of the process of waterproofing design laid out within BS8102 Code of practice for the protection of below ground structures against water ingress, relates to assessing the nature of a given site, and then using this information to influence your design.

In essence the ‘nature of the site’, can be interpreted as ‘how wet is it, and how wet is it likely to be’?

This is important because the greatest driver of risk, is essentially how much water there is, or potentially will be in the ground, to pressure upon the basement structure and waterproofing system(s).

Your objective should always be to achieve effective low risk waterproofing, and if you can interpret site conditions, you then have the best opportunity to know what you’re designing for, which then facilitates appropriate design.

Topography &

Dealing with a retaining structure high on a hillside in well drained ground (unlikely to come under much pressure) can be treated differently to a residential basement in the bottom of a river valley, constructed in permanent high water table.

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The requirement for this assessment was highlighted particularly in the (previous) 2009 revision of BS8102, because what had happened in some instances, was that project Designers, provided with generic standard design details, would just add a dashed line to a drawing and a note for X product to be installed as per manufacturer recommendations/section detail.

This could result in designs which worked in some low risk sites, and then those same designs could subsequently have been used in wet high risk sites, on the logic that ‘I’ve used this system successfully before, therefore if I use it again I’ll get the same result’.

Not always the case, and the design should have been better considered in light of the nature of the site.

Iwe’ve dealt with failures in external tanking systems (for example) applied to reinforced masonry structures in very wet ground, installed by dedicated manufacturer approved waterproofing ‘specialists’. It was the wrong system for the site, it didn’t work and in one case a client had to pay £24k for a remedial cavity drainage system.

My point is that getting the design right is important, because if even the dedicated tanking installers doing it day in day out can’t do it successfully 100% of the time, it just proves that you can’t use the same approach 100% of the time (unless you go for substantial protection every time – which actually is a reasonable consideration, covered under the ‘combined protection’ section of BS8102, which I’ll come back to in another post).

So, if you can assess the site then at least you can hopefully understand it and then design appropriately.

So what are we looking for?

In existing basements there may be a history which gives you an idea of conditions over time, with this being influenced by what if any waterproofing exists, i.e. could be a wet site but a dry space because of effective waterproofing, versus some thing like an unfinished Victorian cellar, where you can reasonably know that no waterproofing is present.  Looking at the history is useful to an extent, but just because it has always been dry, it does not mean that it will always stay that way..!

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