6 Water-resisting design

6.1 Groundwater

Waterproofing measures should be designed on the basis of water to the full height of the retained ground at some time during the structure's life where:

• a) no detailed geological or hydrogeological assessment has been undertaken;
• b) the results of the soil investigations are inconclusive with respect to groundwater;
• c) the ground drainage characteristics are unreliable;
• d) the drainage measures (either internal or external) are unreliable or un-maintainable and infiltration cannot be controlled.

Protection against water ingress from the following three sources should be considered:

• 1) the inflow of surface water, ranging from percolation of rain to inundation of water from burst water mains (see 6.3);
• 2) the water pressures acting on the external retaining wall system;
• 3) the water pressures below the base slab.

The water-resisting design should enable the system to withstand a pre-determined head of water or control the water before it reaches the structure.

One or both of the following methods may be used, in conjunction with the waterproofing protection (see 6.2), to reduce water penetration, depending on the conditions of the site and the required internal environment:

• i) exclusion of surface water (see 6.3);
• ii) sub-surface drainage (see 6.4).

6.2 Waterproofing protection

6.2.1 General

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 need for combined protection (see 6.2.2);
• 2) the water table classification and required performance level (see 6.2.3);
• 3) the need for continuity in the protection (see 6.2.4).

NOTE Examples of the three types of waterproofing protection are given in Figure 2.

There is a range of waterproofing systems that can be incorporated in each type of waterproofing protection and these should be assessed in accordance with Clause 8, Clause 9 and Clause 10, as appropriate, and relevant manufacturers' data sheets to confirm that the system selected is suitable for the structure to which it is to be applied.

It is noted that the manufacturer's recommendations for installation, including provision of protection, should always be followed. Similarly, recommendations for fixings where proprietary products are used should be followed.

In cases where the below ground structure is fully buried or the substructure extends beyond the superstructure, protection should be provided against water ingress through the roof slab, for example by: i) encouraging water to drain away from the structure; ii) providing drainage above the roof slab; iii) using an external barrier.

For existing structures, the following types of waterproofing systems should be considered, subject to their suitability for application and their ability to be repaired:

• an internal Type A waterproofing barrier on a structure of suitable strength and stiffness, built of concrete or masonry (subject to the condition of the surface) [see Figure 2a)];
• drained cavity to the walls and floor, using a Type C cavity drain system [see Figure 2c)].

In situations where the use of a waterproofing system covered by this standard is either not achievable or not cost effective, other methods may be used if they can be shown to lead to similar results. However, the risks and implications of such methods should be investigated and recorded.

6.2.2 Combined protection

Consideration should be given to the use of combined protection (i.e. Type A and Type B, Type A and Type C or Type B and Type C) where in a single system:

• a) the assessed risks are deemed to be high (see Clause 5);
• b) the consequences of failure to achieve the required internal environment are too high; or
• c) additional vapour checks are necessary for a system where unacceptable water vapour transmission can occur.

Although structures with Type B protection are designed to be water resistant, additional waterproofing systems may be applied internally or externally to control water vapour movement, where appropriate.

An in-situ "liner" wall designed to provide Type B protection can be cast inside an embedded retaining wall to provide combined protection. In some cases, a fully bonded barrier might also be provided between the two elements.

Although structures with Type C protection are designed to control and manage seepage into a structure, where this is unacceptably high the water resistance of the structure should be improved prior to the installation of the Type C protection, by the application of either Type A or Type B protection.

When combining types of protection, the compatibility of the different protection types should be assessed in order to minimize the risks and negate the need for remedial measures.

a) Type A (barrier) protection

Key

• 1 External waterproofing
• 2 Masonry or concrete wall, as appropriate (see Table 1)
• 3 Concrete floor slab
• 4 Sandwiched waterproofing
• 5 Loading coat
• 6 Internal waterproofing

b) Type B (structurally integral) protection

Key

• 1 Water-resistant reinforced concrete wall and slab
• 2 External or internal (within wall) waterstop, as required
• 3 Waterstop required at junction between wall and slab and at all construction joints
• 4 Concrete/steel piled wall
• 5 Water-resistant reinforced concrete floor slab or slab with added barrier
• 6 Waterstop at junction to follow profile of wall
• 7 Piled wall might need to be faced to achieve desired water resistance (see Table 1)

NOTE Seek the manufacturer's advice with respect to waterstops to suit the specific construction.

c) Type C (drained) protection

Key

• 1 Cavity drain membrane
• 2 Inner skin (render, dry lining or walling, depending on system)
• 3 Maintainable drainage channel with pipe connection to suitable discharge point
• 4 Sump formed in situ or pre-formed
• 5 Pump
• 6 Wall cavity
• 7 Reinforced concrete/steel pile or diaphragm wall
• 8 Drainage channel
• 9 Waterstop at junction to follow wall profile
• 10 Internal block wall
• 11 Access point(s) to drainage
• 12 Floor slab with integral protection and/or added membrane (internal or external)

6.2.3 Water table classification and grades of waterproofing protection

When selecting a type of waterproofing protection, Table 1 and Table 2 should be taken into account, in conjunction with the following points.

• a) During the life of the structure, some degree of groundwater pressure is likely to build up against the chosen waterproofing system.
• b) Cracking or defective construction joints can provide a potential path for water ingress.
• c) Water ingress can occur where there is groundwater pressure. If this is not consistent with the required performance level (see Table 2):
• 1) consideration should be given to the form and feasibility of remedial work;
• 2) if remedial work is not possible, the design should be altered.
• d) There are a number of risks associated with not carrying out planned maintenance for structures with Type C protection, e.g. pump failure (see 10.3).

The designer should discuss these points with the client prior to deciding on which type(s) of waterproofing protection to use. The following should also be taken into account:

• 1) initial capital costs compared with costs for future maintenance and any necessary upgrades;
• 2) the scope for testing during installation;
• 3) the risks associated with aggressive groundwater and other ground contaminants, which might require the use of a specific protection barrier;
• 4) the need or ability to provide heating and/or ventilation and the consequences arising in terms of water vapour.

NOTE This might call for the adoption of an improved grade of waterproofing protection (see Table 1 and 6.2.2) or active environmental control in order to manage water vapour (see Table 2).

Risk associated with water table	Water table classification (see Note)	Waterproofing protection
		Type A	Type B		Type C
			Piled wall	Reinforced concrete wall to BS EN 1992
Low High	Low	Acceptable	Acceptable	Acceptable	Acceptable
	Variable	Acceptable if the "variable" classification is due to surface water. The manufacturer's advice should be sought.	Acceptable where: a) the piled wall is directly accessible for repair and maintenance from inside thestructure; or b) the piled wall is combined with a fully bonded waterproofing barrier; or c) the piled wall is faced internally with a concrete wall to BS EN 1992.	Acceptable	Acceptable
	High	Acceptable where: a) an appropriate cementitious multi-coat render or cementitious coatings areused; b) the wall is of concrete to BS EN 1992.		Acceptable	Acceptable
Measures to reduce risk		Use combined protection (see 6.2.2). Incorporate appropriately designed sub-surface drainage and ensure that this is maintained (see 6.4). Use a fully bonded waterproofing barrier (see Figure 6). Lower the permeability of the main structural wall. Use concrete with a waterproofing admixture, e.g. to BS EN 934 (see 9.2.1.5). Ensure that discharge systems, e.g. pumps, are maintained so that the system remains effective (see 10.3.1).
NOTE The water table classifications are defined as follows (see also 5.1.3). 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. High - where the water table or perched water table is assessed to be permanently above the underside of the base slab. Ground permeability might affect risk under a low or variable water table (see 5.1).

6.2.4 Continuity of waterproofing protection

The need for continuity in the waterproofing protection should also be considered when selecting a type of protection. In most circumstances.

the protection should be continuous. In certain situations, e.g. where a drained cavity is combined with an underslab membrane, discontinuity with respect to waterproofing can be acceptable subject to careful detailing and an appropriate assessment of risk (see 5.1.2 and Note 2). Any build up of water should be permanently controlled by a water management system.

The proposed type of foundation and its suitability for providing continuity of waterproofing (where so required) should be assessed.

NOTE 1 Continuity can be provided in situations where the surface or structure of the wall and foundation provides uninterrupted positioning of the waterproofing measures.

In existing structures, assessment of any direct or potential discontinuity should be undertaken in order to determine the need for special waterproofing details, e.g. to overcome the effects of future movement.

NOTE 2 Discontinuity of waterproofing protection might not be acceptable if there is a need to manage radon, methane and other ground gases and contaminants (see 6.5).

6.3 Exclusion of surface water

Where practicable, provision should be made to prevent or reduce percolation of rainwater into the ground.

NOTE 1 BS EN 752 gives guidance on collecting and disposing of surface and sub-surface water.

NOTE 2 Burst water mains and leaky sewers can provide additional sources of surface water. These can affect perched water tables. The drainage behind the wall needs to be able to cope with the highest inflow rates, e.g. the burst water main, which might not be practicable in coarse-grained soils.

6.4 Sub-surface drainage

Where sub-surface drainage is deemed necessary to lower the potential for hydrostatic pressure on the waterproofing system and lessen the risk of water ingress through defects, it should be provided by one of the following methods:

• a) permeable granular fill;
• b) no-fines or hollow blockwork;
• c) geosynthetic drainage composite;
• d) underslab drainage.

NOTE 1 Figure 3 gives examples of the positioning of land drains, drainage channels and sub-surface drainage.

Such provisions should be made maintainable where they are used to control the level of water in a structure that does not in itself provide adequate water resistance.

Where practicable, water should be kept from prolonged contact with perimeter structure walls or base slabs by porous or open jointed land drains, combined with a geosynthetic drainage composite installed to the full height of the earth-retaining wall and laid to proper falls around the perimeter of the structure, adjacent to the wall footing and, where appropriate, beneath the slab itself.

The sub-surface system should be graded to an open outlet below the level of the lowest slab, such as to a stormwater drain protected by a pumped surcharge device or to a pumped sump, and can also provide a suitable outfall for any sub-floor drainage.

Perimeter drainage at floor level is likely to lower the groundwater table to a degree that varies with the permeability of the subsoil and the possible consequences of this (such as permanent lowering of the water table in the surrounding area) should be taken into account. Any existing system of land drains should be tested, checked and only retained if both appropriate and maintainable. Any local diversions necessary should retain the existing geometry so far as practicable with new and easily maintainable pipework.

Care should be taken so that no damage is caused in nearby structures. Where deep structures are contemplated in built-up areas, groundwater lowering should not be undertaken without careful investigation in conjunction with a groundwater specialist (see 4.2). Perimeter walls providing a cut-off into an impervious layer or stabilization of granular subsoils by grout injection or similar alternative treatments may be considered instead.

NOTE 2 For structures in coarse-grained soil, with variable and high water tables (see 5.1.3), the flow rates are likely to make it impracticable to pump for the design life of the structure. In these cases, a hydraulic cut-off wall into fine-grained soils is required to isolate the ground below the base slab. This enables an underslab drainage system to relieve the water pressure below the base slab.

NOTE 3 For structures in fine-grained soil, with variable and high water tables (see 5.1.3), the flow rates are more likely to make it practical to pump for the design life of the structure. A drainage system may be provided outside the retaining wall to control the water pressures. An underslab drainage system may also be provided below the base slab to control the water pressures.

Cut-off walls are formed from diaphragm walls, secant piles or steel piles. Over the excavated wall depth remedial works can be carried out on leaky walls. This is not feasible for the length of wall below formation. Therefore, specified tolerances should be such that there is adequate intersection between piles or provision of continuous water bars at diaphragm wall panel joints and particular attention should be given to workmanship.

In interbedded soils, relief wells may be used below formation level. The cut-off wall may also be used to reduce the flow rate and control the drawdown outside the site.

a) Construction without a toe

b) Construction with a toe

Key

• 1 Maintainable land drain (see 6.4) not to be positioned closer than a line of 45° from the underside of the slab/blinding or with an invert above the upper surface of the floor slab
• 2 Measures to control water vapour might be necessary where the invert of the land drain is above the underside of the floor slab
• 3 Incorrect position of land drain, which can cause hydrostatic pressure on barrier leading to water ingress if defects are present
• 4 Subsoil drainage layer, where appropriate (see 6.4)
• 5 Structural wall and foundation slab

6.5 Ground gases

The insertion of a ground barrier for the prevention of radon, methane and other ground gases and contaminants from entering a structure should be considered in the design, choice of the materials and installation of any waterproofing system.

NOTE 1 Attention is drawn to the Building Regulations [3]. Further guidance on the characterization and remediation of ground gases is given in BS 8485.

NOTE 2 The maps of areas where basic or full protection against radon needs to be provided are contained in the Building Research Establishment (BRE) reports BR211 [4], BR376 [5], BR413 [6] and the Health Protection Agency (HPA) document Radon in Dwellings in Scotland: 2008 Review and Atlas [7].¹⁾