Section 4.2.4

Foundation Depths

Shrinkable soils are widely distributed throughout the UK. The moisture content and the volume change of the soil can change seasonally and through the effects of local tree, hedgerows and shrubs. It is vital that shrinkable soils are identified and foundations adapted based on the level of risk.

  • The foundation depth is dependent on the soil type.

  • The soil/ground investigation report will provide you with the soil type and foundation recommendations.

  • Foundation design based upon soils found across the site.

  • The executive summary or foundation design will provide you with information required.

  • In shrinkable soils such as clays and silts, the minimum foundation depth is determined by the plasticity index.

  • These depths are used when the foundation is outside of the influence of trees.

  • Foundations dug to the required depth.

  • Foundations must have clean level bottoms and

  • vertical sides.

  • Any loose materials inside the trench.

  • Trees will have an influence over the foundation depth in shrinkable soils.

  • Use the NHBC foundation depth calculator to determine the foundation depth OR plot specific foundation design.

  • Please refer to plot specific foundation design.

  • Foundation depths in non-shrinkable soils such as sands and gravels.

  • Follow plot specific design in these scenarios.