Assessing Sites with a View to Assessing Risk.

Part of the process of waterproofing design laid out within BS8102 Code of practice for the protection of below ground structures against water from the ground, 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).

Our objective is always to provide 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 better allows you to design appropriately.

Dealing with a residential part 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.

The requirement for this assessment was highlighted in the 2009 revision of BS8102, because what happened historically, is that designers, provided with standard design details by manufacturer reps (selling wonder-products that block out all water) 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 resulted in cookie cutter design, treating all sites in the same fashion often on the logic that ‘I’ve used this system successfully before, therefore if I use it again I’ll get the same result’. Sites that were low risk would be fine but issues would occur in more aggressive wet sites, where more comprehensive protection should have been provided, and the design should have been better considered.

Quality of installation plays a part, but we’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. Travesty.

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..!

wet basement
This basement was converted without any waterproofing, on the assumption that it was a dry space…

In new construction, site investigation should be undertaken and we would ask for the S.I. report which includes bore hole results indicating the makeup of the ground and any standing water levels encountered at that time, and in some cases boreholes are capped and monitored over time to assess seasonal impact.

As indicated topography is a factor, so obviously low lying sites, sites at the bottom of a slope, top of a hill, next to bodies of water etc. etc.

The advice within BS8102 is that water tables should be classified into low variable or high, with low being permanently below slab level, variable being above/below periodically, or high being water permanently above slab level.

Irrespective, BS8102 also states that designers should consider the risk that any structure comes under pressure at some point in time, even where site examination indicates dry conditions, and so you should never look at a space and decide that it will forever be dry.

I wouldn’t say that I seek to classify the type of water table specifically, I’m just looking at whether they find water, if not then what the ground type/makeup is, then using this to assess risk of future water while always assuming that water will pressure.

It’s difficult to explain what to do in every situation, apart from getting the right advice(!) but it may be useful to look at a couple of interesting examples.

This was a new-build house with reinforced concrete raft and walls formed in reinforced masonry, which were tanked externally using adhesive bitumen sheet membranes. They had obviously assumed the requirement for waterproofing, which is great, but what I found interesting was a note on the drawing which states:


This is a shot of the interior:


This demonstrates that even in permeable ground there is still risk of groundwater pressure and penetration , and the waterproofing design must reflect this.

This is another example, low lying ground (bottom of river valley), site investigation showed the highest standing water level at more than a metre below slab level. Obviously during construction, conditions had changed significantly!

Precast wall panel system underwater (it leaked).
Precast wall panel system underwater (it leaked).

Technically this might be considered variable water table under the BS102 definitions but would you not look and design for ‘high’ water table even though it might not be permanently above basement slab level as per the BS8102 definition? I would.

In this site the water was literally moving through the ground, i.e. It was permeable ground and this means that the ground would be saturated across a wide area. This is interesting from a design perspective in that you couldn’t rely on land drains for numerous reasons:

Firstly the ground is permeable, meaning that water can move through it easily, so as quick you drain it away, potentially more could flow/rush in and still pressure on the structure.

Secondly, the ground is permeable (!) and so the water present will extend across a wider area, the implication being potentially massive volumes of water and obviously they don’t want to be trying to de-water (drain down) the ground across half the county.

Many waterproofing systems rely on effective land drainage.  Land drainage is OK in certain circumstances in my opinion (house on a hillside being a good example), but this was not one of them.  I can see a general move away from reliance on land drainage in time, at least from a waterproofing perspective, although Engineers may rely on them to control hydrostatic pressure, so that they do not have to design for substantial loading, which is a economics/cost argument.

Incidentally they were not included in the example above (no wonder) despite being part of the original design, and the structure, a Type B integrally waterproof concrete structure, formed in precast panels leaked, and we successfully remedied using internal Type C cavity drainage.

So there you go, permeable sites which might be considered well drained, can have their issues too.

This leads us on to one of the other conditions that I look for and that is clay ground.  Clay is impermeable, meaning that water does not easily move through it.  Rain falling onto clay ground will take time to drain down through it, and with enough rainfall the ground can become saturated.

The weight of water in a standing column exerts a hydrostatic pressure, and it is this pressure which tends to force water through structures, into basements.

Therefore, where we see clay we anticipate that water will ‘perch’ and pressure.  This is referred to as perched water table, and it is this which in my opinion, results in the majority of situations where pressure comes to bear.

We once treated a spiral wine cellar in a property right at the top of a hill.  The cellar is a concrete cylinder in a waterproof liner, dropped into an excavation and back-filled around.  The structure physically popped (floated) up out of the ground, as a result of floatation and hydrostatic pressure.  Clearly it was wet ground, but being high on a hill, could only be perched water table.

Perched water and pressure upon the structure can be encouraged where structures are formed using the open excavation approach, because when the excavation is back-filled, it is usually with granular (and therefore permeable) stone.

This can create a zone of permeable ground around the outside of the basement, which readily accepts water/rainfall.  When that water drains down to the base of the back-fill it may then perch upon impermeable strata/ground at that point (if clay…etc.), resulting in pressure.  This is what land drains are supposed to remove, but often do not to the point that pressure is prevented.

basic illustration of perched water table encouraged by granular back-fill around a basement structure formed in an open excavation.
Basic illustration of perched water table encouraged by granular back-fill around a basement structure formed in an open excavation.
Example open excavation.
Example open excavation pre-backfill.
Consolidated clay site and granular backfill... expect perched water.
Consolidated clay site and granular back-fill… expect perched water.
Sometimes even small details can encourage water to perch and pressure.
Sometimes even small details can encourage water to perch and pressure.

A good waterproofing designer can interpret a site investigation and design appropriately.  A designer will carry professional indemnity insurance to cover their advice, and will take responsibility for the advice that they provide on a paid basis.  Many product/manufacturer reps will happily provide advice, but won’t have the P.I. to back it up and won’t want the responsibility.  The good guys (manufacturers) will push you towards designers/installers with P.I. cover that can provide the right advice.


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