Understanding water table and basements

Defining water table

There is a distinction between what is referred to as ‘perched' water table, versus what we refer to as 'true' water table.

Perched water table is defined within the 2022 revision of British Standard 8102 protection of below ground structures against water ingress, as a ‘reservoir of water in the ground maintained temporarily or permanently above the standing water level in the ground below it, caused by the presence of an impervious soil or stratum of low permeability’.

Or in other words, water may stand (perch) in the ground at a level, before draining down to true ‘water table’, which may be at a much lower level.

Why are some properties more prone to coming under groundwater pressure than others?

It is this ‘perched’ effect which can encourage many below ground structures to come under hydrostatic pressure.

It is as a result of the way in which most modern basement structures are constructed.  Typically the formation of a basement/cellar is achieved by excavating an oversized ‘hole’ for want of a better word, in which the structure can be formed while granting a working space around the exterior of a structure.

Once the basement structure is formed, it is common practice (but note that this is not true of every design) for the excavated working area around the exterior of the basement to be in-filled using loose/granular stone, which is included to encourage water to drain down to lower level, by virtue of the inherent porosity of the material.

In the event that the ground is impermeable compared to the loose stone, which in many cases is likely, there is a probability that given sufficient water from rainfall/surface runoff etc., it will stand in the excavation (within the porous backfill), upon the underlying and less-permeable ground, then pressuring on the basement structure and waterproofing.

Land drainage is commonly included to address this water, however that is a topic for another article; the important aspect to consider here is that of water standing temporarily in the ground, predominantly in the backfill, this creating the potential for hydrostatic pressure to bear upon a basement structure.

If your basement or cellar suffers issues of liquid water penetration shortly after periods of heavy rainfall, there is a probability that this is as a result of perched water table.

To find out more about water table issues on property you can learn more from our Waterproofing design specialists here.

The influence ground conditions

The risk of such a situation occurring is influenced by the drainage characteristics of the ground in which a given structure is constructed, in that for example, a granular low cohesion soil will allow water to move through it (encouraging drainage away from the structure) to a much greater degree than a impermeable cohesive clay soil.

While in older properties it is less likely that a granular stone backfill is employed around a structure, it is more likely that the material excavated from the site is reintroduced around the basement, and this can also encourage pressure, either as a result of the backfill material being impermeable (clay) and therefore encouraging water to ‘perch’ against the structure at high level, or becoming more permeable (broken up when excavated), and therefore accepting water more readily than the surrounding ground which has not been broken up.

How excavation/reintroduction affects permeability

We have been involved in new-build projects where we have backfilled around the structure using the material that was excavated from the site (the design was well rationalised), and the backfilled ground effectively ‘bounced’ prior to compaction, versus the undisturbed ground which did not, this illustrating how excavation/reintroduction affects permeability.

‍What about true ‘water table’?

Simplistically and for the purposes of this explanation, we consider that ‘water table’ is the standing water level in the ground, i.e. the ground below that position is saturated (all pores full of water), while those above it are not.  It is the level of the water within the ground which is the key consideration applicable to waterproofing and basement design.

True ‘water table’ is less likely to vary with intermittent rainfall, being influenced to a greater degree by increased seasonal rainfall, which is implicit with the UK climate.

If your basement suffers sustained periods (i.e. not just following rainfall) of flooding/standing water during the winter, this is more likely to be as a result of true water table (as opposed to perched water table).

The design guides ‘Basements for Dwelling’ and BS8102 classify water table levels in relation to basement structures as ‘Low, Variable & High’, with low being permanently below basement slab level, high being permanently above, and variable being in-between, for example as a result of seasonal variation.

There is little clarification in respect of the difference between high and variable i.e. if water table is above basement slab for 6 months of the year – is it high, or is it variable (?), however simplistically, the higher it is and the greater the duration, the greater the risk, and the greater the consideration and probable investment required, in the provision of structural waterproofing measures, and/or a structure which will resist potential loads.

Our recommendations

In new build basement waterproofing construction, a site investigation report is commissioned which informs a waterproofing design specialist everything they need to know about the situation.

In respect of the impact on waterproofing design, the main implication that designers & specifiers should consider, especially when appraising site investigations is that the standing water level identified during bore hole testing may not be indicative of the risk of water pressure coming to bear, or the depth of water which would pressure.

Specifically, we would not recommend making assumptions that groundwater is not a factor on a given site, and that consideration should be given to addressing it, even if only via basic measures such as sub-surface drainage, with greater protection being recommended the greater the consequences of water penetration on the internal basement environment.

This may sound obvious; however we do have experience of dealing with failed waterproofing systems, where project designers made such assumptions, only for these to prove incorrect, with this resulting in costly remedial work, with one such project landing the Architect in a legal battle.

An additional recommendation would be to at least revisit bore-holes some time after formation to determine whether a standing water level is evident, as it may take time for water to move through the ground, following the path of least resistance into the bore-hole depending on the porosity of the ground.

While we are not structural Engineers, we certainly understand the implicit load associated with hydrostatic pressure and that where such loads are anticipated they must be designed for or reduced if viable with appropriate maintainable drainage.

‍

‍

‍