5.4 Waterproofing of basements and other below ground structures

Also see:

4.1

5.4.4 Ground conditions

The waterproofing system shall take account of ground conditions.

The ground conditions should be fully considered by the engineer and waterproofing design specialist in the design of the waterproofing system.

NHBC may request investigation and a report of the ground conditions where the below ground waterproofed structure:

  • retains more than 600mm of ground, measured from the top of the retained ground to the lowest finished floor level
  • comprises more than 15% of the perimeter of an individual building (eg terraced homes, apartment blocks and detached garages), measured on plan.

The ground conditions report should take into account appropriate investigations, as described in Table 1.

Table 1: Investigation of ground conditions

Further investigationGuidance and information
Desk study, including review of:
• ground water, lost rivers and flooding issues
• flood potential of the site
• available ground water data
• SuDS impact assessment
• flood risk assessment
• topography of the site
• effects of adjacent surface finishes
https://www.gov.uk/browse/environment-countryside/flooding-extreme-weather
www.bgs.ac.uk/geology-projects/groundwater-research/groundwater-data/ www.metoffice.gov.uk/weather/climate/uk-climate climate-change.data.gov.uk
Historical Publications Ltd ‘The Lost Rivers of London’ by Nicholas Barton
Contaminated or aggressive ground and/or ground water conditionsTesting required where there is the potential for chemically aggressive ground and/or ground water
Water level change, including potential for flash flooding and waterloggingIdentifying likely fluctuations and short-term flooding events
Impact assessment of ground water flow where the construction is likely to have a ‘damming’ effectInterpretative report by a qualified engineer, hydrologist or hydrogeologist to include:
• assessment of the direction of ground water flow
• damming effects on the ground water regime
• damming effect of adjacent structures

Where it is necessary to establish the water table, a detailed hydrogeological assessment should be undertaken by a suitably qualified engineer, and include:

  • long-term water level monitoring over at least one year to capture seasonal fluctuations
  • short-term flooding events that typically occur during autumn and spring
  • information based on a suitable number of boreholes monitored at intervals of three months or less.

Last updated: 2nd January 2024

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