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If the soil investigation shows the water table to be below basement level, should the basement still be protected against water from the ground? This note discusses this question.

What is waterproofing

In the context of a basement, waterproofing is the application of waterproof/ water-resisting material to protect it from water ingress.

Water table and types of groundwater

Water table is the level at which the soil is saturated with water. However, this is not the only type of water found in the ground.

Perched water is another type of groundwater that should be considered. Perched water is a water reservoir in the ground maintained permanently or temporarily above the water table, caused by an impermeable or poorly permeable stratum.

Soil investigation can determine the water table. However, it is not always possible to assess whether perched water exists. Also, the basement itself could cause a change in the existing water path resulting in perched water.

Therefore, although the water table may be lower than basement level, there is still risk of perched water penetrating the basement. The probability of this should be assessed and considered.

BS8102

The best practice for dealing with and preventing entry of water from ground into basements is detailed in BS8102 Code of practice for protection of below ground structures against water from the ground.

The current version of this code (2009) stipulates that a risk assessment should be undertaken to determine the extent and type of protection, taking into consideration[i]:

• The long-term water pressures, the effects of surface water infiltration, and the use of external drainage and cut-off walls.

• The effects of climate change, burst water mains and sewers, adjacent trees, sulfates, radon, methane and other ground gases and contaminants.

• Where external drainage is proposed (cavity wall system), the effects of drawdown on adjacent structures, the potential silting of drainage and biofouling issues.

• Even when the site investigation indicates dry conditions, the risk of some waterlogging in the future should be assumed as groundwater requires time to drain away, and this can cause limited pressure periodically coming to bear against the structure.

The above should be assessed in the context of the basement’s usage, including considering potential future uses where applicable, the acceptable internal environment for this usage and the impact on finishes[ii]. The internal environment in this context includes considerations of the temperature, humidity level, presence of ground gasses and contaminants, and tolerance towards water penetration and damp.

Also, even if the investigated water table is low, BS8102 only allows risk associated with groundwater to be considered as low if the structure is on free-draining strata[iii]. 

Risks and liabilities

When assessments are done in accordance with BS8102, it is rare for basements to be deemed not to require waterproofing.

In the instance where the water table is lower than basement level, choosing not to waterproof would not cause immediate structural collapse. However:

• Should there be any change to the water level from the scenarios detailed, water ingress could occur.

• This, in return, could cause the concrete and reinforcement to deteriorate from being in contact with water, leading to structural distress and even eventual failure.

• Water ingress matters are difficult and expensive to remediate after a building is constructed.

The building owner/ developer must consult the project’s Professional Engineer with Practicing Certificate (PEPC) in deciding not to provide waterproofing and consider the acceptable level of risk. If a building owner/ developer chooses not to comply with BS8102, the PEPC is within his rights to discharge his liabilities relating to the consequent potential detrimental impact on the structure.

Reference: Definitions and content for this note are based on BS8102.

[i] BS8102:2009 Clause 5.1.2

[ii]Definition of finishes: Surface treatment to floors, walls and ceiling. For example floor tiles and wall claddings.

[iii]BS8102:2009 Clause 5.1.3

Rev 00 l July 2021 l © ohr 2021 

Disclaimer

ohr notes are designed and intended for built environment stakeholders who are not structural engineers, as general information in summary form on engineering topics, current at the time of publication, for general information purposes only. The material may not be applicable in all situations. The contents do not constitute professional advice, are not intended to be a substitute for professional advice and should not be relied upon as such. You should seek professional advice in relation to any particular matters you or your project may have.