4.2.9 Foundation depths for specific conditions in shrinkable soils
Foundations in shrinkable soils shall be designed to transmit loads to the ground safely and without excessive movement. Issues to be taken into account include:
- strip and trench fill foundations in non-shrinkable soils overlying shrinkable soil
- measurement of foundation depths
- granular infill beneath raft foundations in shrinkable soils
- steps in foundations.
4.2.9.1 Strip and trench fill foundations in non-shrinkable soils overlying shrinkable soil
Non-shrinkable soils such as sand and gravel may overlie shrinkable soil. Foundations may be constructed on overlying non-shrinkable soil if all the following are satisfied:
• conditions of Chapter 4.3 Strip and trench fill foundations are met
• consistent soil conditions exist across each plot, and this is confirmed by the site investigation
• depth of the non-shrinkable soil is greater than ¾ foundation depth X, where X is the foundation depth determined using charts in Clause 4.2.12, tables in Clause 4.2.13 or the Foundation Depth Calculator App, assuming all the soil is shrinkable
• the thickness T of non-shrinkable soil below the foundation is equal to, or more than, the width of the foundation B, Figure 4
• proposals are submitted to, and approved by, NHBC prior to work commencing on site.
Where any of the above are not met, foundation depths should be determined as for shrinkable soil.
4.2.9.2 Measurement of foundation depths
Where ground levels are to remain unaltered, foundation depths should be measured from original ground level.
Measurement of foundation depths where ground levels are reduced or increased, either in the recent past or during construction, should be as shown in Figures 5, 6 and 7.
Figure 5: Levels from which foundation depths are measured where trees or hedgerows are to remain
Figure 6: Levels from which foundation depths are measured where trees or hedgerows are removed
Figure 7: Levels from which foundation depths are measured where trees or hedgerows are proposed
4.2.9.3 Granular infill beneath raft foundations in shrinkable soils
Granular infill together with raft slab foundations can be used as an alternative to deep trench fill foundations. Where adopted, granular infill should be placed beneath raft foundations on shrinkable soils as shown below.
Infill should:
- be at least 50% of the foundation depth determined for a trench fill foundation and not more than 1.25m deep (measured from ground level determined in accordance with Clause 4.2.9.2)
- extend beyond the edge of the foundation by a distance equal to its natural angle of repose, plus 0.5m
- be placed in accordance with specification within Chapter 4.6 Engineered fill.
4.2.9.4 Steps in foundations
On sloping ground, foundation trenches can be gradually stepped so that the required foundation depth is reasonably uniform below ground level.
Where foundations are to be stepped to take account of the influence of trees, hedgerows and shrubs, they should be stepped gradually, with no step exceeding 0.5m.
Last updated: 2nd January 2025