This update provides guidance on the requirements associated with the design and construction of basements and other below ground structures. Principally, this concerns the process by which the risk of ground water penetration is appraised and addressed so that problems associated with penetration do not occur while consideration is also given to economic construction.
This process and rationale is primarily detailed within BS 8102 (2009) Code of Practice for protection of below ground structures against water from the ground (and other associated design guides). A risk assessment is to be provided prior to the submission of drawings to ensure that the site specific constraints are considered in the design proposals submitted.
The early involvement of a waterproofing designer is an important consideration because the waterproofing design typically has an influence on elements of the structural and/or architectural design. Early involvement allows the waterproofing to be duly considered in association with these other aspects and prevents situations where design fees are increased as a result of necessary redesign, or where waterproofing is compromised by working within the constraints of an ill-considered structure relative to achieving the required standard of environment.
For warranty purposes it is required that all below ground structures including partial, semi and full basement are be designed and constructed to a minimum of Grade 2 to BS 8102 (2009), with Grade 3 to BS8102 (2009) being necessary for habitable occupied spaces. An exception to this is a basement used solely for underground car parking, where a Grade 1 is accepted.
In order to meet the requirements of BS8102 the following must be considered:
The risk assessment is to be produced in order to demonstrate the risks and then compare this to the proposed design. Assessment of a site must be based on the results of the site investigation and other site-specific factors. Seasonal variations in the water table must be accounted for unless long-term monitoring is undertaken. However, even where standing water levels are not noted during such pre-start site investigation, the drainage characteristics of the ground must receive particular attention.
Soils with low permeability represent a risk of waterlogging or encouraging a ‘perched water table’, where water stands temporarily or permanently within the ground against a structure, and arguably this affects more properties with basements versus the true water table level.
It could be assumed that, based upon a site investigation, the risk of water pressure ever occurring is low, BS 8102 advises that consideration is given to the effects of climate change and burst water mains and sewers, as well as stating that it should be assumed that there is risk of waterlogging “even where site examination indicated dry conditions”.
In summary, the site investigation guides the design, but it should never be assumed that some degree of water pressure will not occur. We will require a specific design risk assessment of the geology, hydrology, services, water ways which will have an impact on the basement structure by causing the structure to be subjected to a head of water in the event of leaking services, rise in water table, etc.
The purpose of a land drain is to relieve hydrostatic pressure from the structure walls to prevent water from bearing against the structure. The incorporation of maintainable land drains can be used under appropriate ground conditions. Scheduled maintenance of land drains is required.
The potential implications on the moisture content of the soil and the potential impact on adjacent structures must be considered in the design. Provision of a land drainage is not to be classed as one form of water protection e.g. when two forms are required AND that they must be connected to a reliable point of discharge.
A maintainable land drain must not to be positioned closer than a line of 45 degrees from under side of slab/blinding of with invert above upper surface of floor slab.
The following guide is intended to support design teams as to what should be included in a below ground structural waterproofing risk assessment.
All design decisions made by others e.g. change of the structural design of the retaining wall, that might have an impact on the waterproofing design should be brought to the attention of the waterproofing specialist/designer and installing contractors. Final decisions and any recommendations should be approved by named the designer above.
Where relying on the use of waterproofing product manufacturer ‘standard details’, they typically disclaim design responsibility, so it is incumbent on the waterproofing design specialist to ensure that such details are correct and appropriate for the site and structure or offer suitable variation and it is the responsibility of the stated designer to ensure that the structural waterproofing design appropriate based on the site specific constraints encountered.
Note: One, or a combination, of the following types of waterproofing protection should be selected:
a) Type A (barrier) protection;
b) Type B (structurally integral) protection;
c) Type C (drained) protection.
When making this selection, consideration should be given to:
1) The potential need for combined protection
2) The water table classification and required performance level
3) The requirement for continuity in the structural waterproofing protection.
Where a tanking product is proposed by the contractor and is outside of the waterproofing specialist design report, the waterproof specialist should endorse its use within their design remit.
Note: Even when the site investigation indicates dry conditions, the risk of some waterlogging (see Note 2) in the future should be assumed.
Note: Details of maintenance, commissioning and handover pack proposed are critical to the long term success of any “Type C” system specified.
Note: Within BS 8102, waterproofing specialists are advised to consider the probability that systems may not be installed perfectly and that defects may occur as a result of this, or defects may be present in the supplied materials.
Please include details and specific products proposed for the scheme, not limited to but including:
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Every care was taken to ensure the information in this article was correct at the time of publication. Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.