Where is the real water table and a perception of what can cause it to be called potentially variable?
It’s difficult not to talk about a new or existing basement in the same breath without also talking about the local water table. Obviously the word local is key as invariably basements end up in locations which can be entirely unique for many reasons. Soil type, natural drainage, fissures in the ground. These can all combine to fight with the best laid of plans, designs and systems.
But what about the techniques that are often used to backfill a site? Can they also create or have an affect on assumptions about the ubiquitous local water table? The short answer is yes, they can.
A quick look at BS8102 confirms that water will or can often stand well above the true water table which may well be further underground than that of what can be a ‘perched’ temporary or permanent area of standing, ponded water or water that can or will slowly travel down to the actual ground where the true water table is. And that ground – when water gets there – may be also be impervious, particularly if it is clay soil.
As that upper water table travels down or exerts pressure on the actual lower water table or the ground above the real water table, hydrostatic pressure can then increase on the new structure. The potential result? A leak of varying potential severity. Interestingly, current construction methods can be responsible for exasperating this problem.
Not all basements are obviously constructed in the same way, but where loose stone material is used to backfill around the newly constructed basement this can cause issues. That well considered permeable stone can allow the upper level water to sit potentially on or near possibly impervious ground below that backfill.
If a basement suffers water ingress after periods of prolonged or heavy rainfall or run off water presence; the culprit can often be that backfill helping the upper water to sit on the edge of the true water table. As mentioned, this can create pressure on the new structure by virtue of having non permeable ground below the backfill for the upper water to dissipate through.
Where backfilling with the original ground type is used instead of a new stone material, issues can still occur.
Understanding all aspects of the local ground and soil conditions will mean that there is a much better chance of foreseeing issues before they turn into unwanted remedial costs. With the correct drainage approach and choice of other waterproofing systems – using membranes and seals that can take and are designed for high bar pressure; even heavy impermeable clay soil can be dealt with – with water being shut out.
Working with a structural waterproofing specialist will help you get an overall picture of potential hydrostatic risk and what additional measures should be employed to avoid later problems. There’s always a way to create a watertight basement. Working with someone who has seen it all but treats every project as unique will ensure the required result. The types and grades of basement are one thing, but constructing them to meet the required standard and that work with the local conditions is another.
Why make assumptions when you can get an expert to make considered decisions based on the local evidence?