Section 3: Subsoil drainage

3.1 The provisions which follow assume that the site of the building is not subject to general flooding (see paragraph 0.8) or, if it is, that appropriate steps are being taken.

3.2 Where the water table can rise to within 0.25m of the lowest floor of the building, or where surface water could enter or adversely affect the building, either the ground to be covered by the building should be drained by gravity, or other effective means of safeguarding the building should be taken.

3.3 If an active subsoil drain is cut during excavation and if it passes under the building it should be:

a. re-laid in pipes with sealed joints and have access points outside the building; or

b. re-routed around the building; or

c. re-run to another outfall (see Diagram 3).

Diagram 3 Subsoil drain cut during excavation

3.4 Where there is a risk that groundwater beneath or around the building could adversely affect the stability and properties of the ground, consideration should be given to site drainage or other protection (see Section 4: Floors).

3.5 For protecting low lying buildings or basements from localised flooding where foul water drainage also receives rainwater, refer to Approved Document H (Drainage and waste disposal). In heavy rainfall these systems surcharge and where preventative measures are not taken this could lead to increased risks of flooding within the property.

3.6 Flooding can create blockages in drains and sewers that can lead to backflow of sewage into properties through low level drain gullies, toilets, etc. Guidance on anti-flooding devices is given in a CIRIA publication79.

3.7 General excavation work for foundations and services can alter groundwater flows through the site. Where contaminants are present in the ground, consideration should be given to subsoil drainage to prevent the transportation of water- borne contaminants to the foundations or into the building or its services.