4 Design philosophy
It is essential for the success of any project involving below ground structures that strategies for dealing with groundwater, soil gases and contaminants are considered from the very earliest stages of the planning and design processes.
For new structures, it is recommended that the structural design, overall weatherproofing design, waterproofing design and construction processes are considered together, as they generally interact.
In addition, it is recommended that, during the design process and at all stages of the construction process, the designers, specialists, manufacturers/suppliers and installing contractors establish and maintain effective channels of communication. Regular and clear communication coupled with good site supervision allows variations and amendments to the design to be planned and executed without compromising the performance of the waterproofed structure (see also 4.2).
4.2 Design team
The advice of a geotechnical specialist should be sought on the geology and hydrogeology, the external drainage options and groundwater conditions (see Clause 5).
A waterproofing specialist should be included as part of the design team so that an integrated waterproofing solution is created. The waterproofing specialist should:
- a) be suitably experienced;
- b) be capable of devising solutions that accommodate the various project constraints and needs;
- c) provide the design team with information and guidance that assists with and influences the design, installation and future maintenance of the waterproofed structure.
NOTE The waterproofing specialist could be the manufacturer or material supplier, provided that the manufacturer/supplier has the relevant expertise.
All design decisions made by others 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 the designer.
4.3 Principal considerations
In order to develop a robust design for protecting a structure against groundwater, the following factors should be assessed:
- a) the likely highest level of the water table, the drainage characteristics of the soil and other site-specific properties (see Clause 5);
- b) the appropriate waterproofing measures (see Clause 6), i.e. Type A, B or C protection and, where necessary, external drainage based on:
- 1) the results of the site evaluation, including the classification of the water table; and
- 2) the intended use of structure, with consideration given to any requirement for future flexibility. This should be undertaken in consultation with the client;
- c) the appropriate type of primary waterproofing system (see Clause 8, Clause 9 and Clause 10).
NOTE 1 The general principle is to assess the risk of water reaching the structure and then to select a waterproofing system capable of achieving the required internal environment.
NOTE 2 Mechanical heating and ventilation often play an important role in creating the internal environment, particularly where higher grades are required. The design of such systems is a specialist activity, outside the scope of this standard.
Condensation may also be controlled by the provision of adequate ventilation (assisted by heating), coupled with the suitable treatment of floor and wall surfaces.
A three-dimensional review of structure and waterproofing should be undertaken so as to identify any complex geometries, which are not readily identified from normal two-dimensional details.
Service entries are particularly vulnerable to water penetration; where they cannot be avoided, they should be carefully detailed, incorporating sealing, to minimize the risk of water ingress (see 8.1.3).
NOTE 3 The three-dimensional review needs to include detailed information on the proposed waterproofing system (e.g. the effect that this has on wall base details, laps in membranes and waterstops). Examples of complex geometries are corner details and where the wall adjoins: the base slab/foundation; the superstructure; differing floor levels; and windows below ground.
4.3.2 Defects and remedial measures
An ideal waterproofing solution would be defect-free. However, it should be noted that two types of defects might occur in any waterproofing, where a structure is subjected to water pressure, and this could mean that the required internal environment is not achieved. These defects are as follows:
- a) defects owing to poor workmanship or the inappropriate use of materials;
- b) defects owing to the specific properties of the materials being used.
NOTE Reference to "defects" does not apply to normal designed flexural cracks or surface crazing in concrete elements but only to through cracking, which might need to be locally sealed.
It is essential that the construction methods and materials used to realise the design are such that the defects in a) are avoided.
The defects in b), which are generally minor, should be recognized and catered for in the design. Contingency planning for dealing with any localized defects or system failure that arise should be included as part of the overall water-resisting design for the structure (see also Clause 11).
In either case, the issue of repairability should be taken into account and the feasibility of remedial measures assessed.
COMMENTARY ON FIGURE 1
Figure 1 outlines the principal factors and stages that need to be addressed in order to produce a robust waterproofing solution for a below ground structure.
It demonstrates that some matters are interrelated and that a degree of iteration might result from a need to address buildability and repairability. The principal issues (boxes) do not necessarily need to be addressed in the order shown but all need to be understood and evaluated.
5 Site evaluation
COMMENTARY ON CLAUSE 5
Attention is drawn to the fact that many of the issues addressed in this clause are also relevant to the design of the structure itself. For further guidance, see the relevant Eurocodes, e.g. BS EN 1992 or BS EN 1997.
5.1.1 Desk study
A desk study should be carried out in accordance with BS 5930 and BS EN 1997:
- a) to assess the geology and hydrogeology, including soil permeabilities, flood risk, radon, methane and other ground gases and contaminants (e.g. chlorides and acids);
- b) to assess the topography of the surrounding ground in relation to the below ground structure;
- c) to establish the likely highest level of the water table and the potential for the occurrence of a perched water table; and
- d) to identify any missing ground and groundwater information, which should then be obtained by undertaking a site investigation in accordance with BS 5930 and BS EN 1997.
NOTE Guidance on best practice in ground investigation, laboratory and field-testing for embedded retaining walls is given in CIRIA publication C580 .
The drainage characteristics from analysis of the soil should be determined in accordance with BS 8004.
5.1.2 Risk assessment
NOTE 1 The principal risks with respect to water ingress into structures are the external environmental conditions.
A risk assessment should be carried out which considers the long-term water pressures, the effects of surface water infiltration and the use of external drainage and cut-off walls.
Risk assessment should also consider:
- a) the effects of climate change, burst water mains and sewers, adjacent trees, sulfates, radon, methane and other ground gases and contaminants; and
- b) where external drainage is proposed, 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 (see Note 2) in the future should be assumed.
NOTE 2 Even in a permeable subsoil, groundwater requires time to drain away and this can result in limited pressure periodically coming to bear against the structure.
5.1.3 Water table classification
Where assessment of the water table is undertaken, this should be classed into the following three categories, which can then be used to determine the suitability of different types of waterproofing protection (see 6.2).
- High - where the water table or perched water table is assessed to be permanently above the underside of the base slab.
- Low - where the water table or perched water table is assessed to be permanently below the underside of the base slab. This only applies to free-draining strata.
- Variable - where the water table fluctuates.
NOTE In certain ground conditions, external drainage systems can be used to convert the "high" and "variable" water tables to the "low" condition (see also 6.3).
5.2 Inspection and survey for existing structures
Following an assessment of the external risk (see 5.1.2), a comprehensive survey should be undertaken of any existing waterproofing arrangements.
The structure should also be examined in order to determine any potential movement that might occur between the walls and floor.
NOTE 1 The base slab of many older structures is likely to abut the external walls. This can give rise to movement between wall and floor. Special flexible joint details might be required so that strains in the waterproofing materials are controlled within acceptable limits where bonded or surface-applied barrier materials are used in such situations. Similar details might also be required at other locations where structural movement can occur.
NOTE 2 Less movement would be expected in cases where the floor is set into the wall although horizontal movement can occur unless the floor is reinforced such as to achieve the necessary fixity.
As the space available in below ground structures is often converted into habitable rooms during refurbishments, the survey should also consider the previous use, e.g. this might have been such that dampness was of less concern than for the proposed use.
NOTE 3 Existing waterproofing might have to be removed completely and replaced with an entirely new system, although in some instances it might be possible to apply a Type A barrier or a Type C drained cavity system directly. These systems can be considered where there is no existing waterproofing but the suitability of Type A barrier materials depends on the characteristics of the surface of the wall or floor.
5.2.2 External walls
An old external cavity form of waterproofing protection might be encountered that is not immediately obvious. Where an external cavity is found, it should be inspected to confirm that:
- a) the cavity is not bridged by debris;
- b) any drainage is still functioning and has not been silted up; and
- c) air bricks are not obstructed by soil or vegetation.
NOTE 1 An example of an external cavity is where an inner wall, load-bearing or otherwise, has been built with a cavity between it and an earlier retaining wall. This cavity might be closed/capped with a plinth, and would typically be drained at the base and ventilated by air bricks.
It should be established whether walls are earth retaining or free standing as there might be instances where the waterproofing measures have not been continued along internal walls abutting the external retaining wall.
NOTE 2 Where the external wall is of solid construction, there might be no waterproofing or there might have been previous attempts to mask dampness. Many such applications can be incomplete or ineffective. It is possible that the attempted waterproofing was inappropriate for the prevailing external conditions or the wall surface to which it had been applied.
The survey should include any constructions which abut the main structure, such as garden walls and arches under steps, as these are a potential source of moisture transfer.
Where drainage tiles have been used to cover the floor, they should be assessed to confirm that they have sufficient strength to withstand loadings from walls, plant, equipment, vehicles, etc., appropriate to the intended use.
Where no-fines concrete has been used as a drainage layer, this should be similarly assessed to confirm that it has sufficient strength to withstand loadings and is still draining effectively.
NOTE 1 Drainage tiles are typically made from clay or concrete. Both tiles and no-fines concrete have widely been replaced by cavity drain membranes.
The floor should be thoroughly surveyed for signs of moisture penetration.
NOTE 2 Moisture from the ground can move across abutting constructions at any level and in some conditions running water might be encountered.
Consideration should also be given to:
- a) the ability of any surface to accept the proposed waterproofing;
- b) the effect that any proposed waterproofing system is likely to have on stresses imposed on the existing structure by groundwater once the waterproofing system has been installed.