4.2 Building near trees

Also see:

2.1

4.2.10 Heave precautions

Foundations, substructures and services shall be suitably designed and detailed to prevent excessive movement due to heave. Heave precautions shall be incorporated into foundations and substructures in accordance with the design. Items to be taken into account include:

  1. potential for ground movement
  2. minimum void dimensions
  3. proprietary heave materials
  4. heave precautions for foundations
  5. other foundation types
  6. suspended ground floors
  7. paths and driveways.

Where foundations and substructure may be subject to heave, they should be protected by voids, void formers or compressible materials.

Where proprietary materials are used, the design of foundations and substructure should take into account the upward force transmitted through the compressible material or void former prior to collapse (refer to manufacturer’s data).

This section provides guidance on heave precautions for common building elements when located within the influence of trees which are to remain or be removed, including:

  • trench fill foundations
  • pier and beam foundations
  • pile and beam foundations
  • other foundation types
  • paths and driveways
  • new drainage.

Potential for ground movement#

After the felling or removal of trees and hedgerows on shrinkable soils, heave can occur, as the absorbed moisture causes swelling. Heave can also occur beneath a building where:

  • roots are severed
  • water enters the ground from leaking drains and services
  • there are changes in ground water conditions.

Minimum void dimensions#

Voids should be provided to accommodate movement due to heave forces acting against foundations and suspended ground floors in accordance with Table 7.

Table 7: Void dimensions

Volume change potentialVoid dimension against side of foundation and ground beamVoid dimension under ground beams, and suspended in-situ concrete ground floorVoid dimension under suspended precast concrete and timber floors(1)
High35mm150mm300mm
Medium25mm100mm250mm
Low0mm50mm200mm

Notes

1. Under suspended floors, the void dimension is measured from the underside of beam or joist to ground level and includes 150mm ventilation allowance.

Void formers consist of materials that collapse to form a void into which the clay can swell. The void dimension is the ‘remaining void’ after collapse. The thickness of the void former should be in accordance with the manufacturer’s recommendations.

Proprietary materials to accommodate heave#

Compressible material compacts as clay expands; the void dimension is the amount the material should be able to compress to accommodate heave. The thickness of compressible material required should be established from the manufacturer’s recommendations, but generally will be approximately twice the void dimension shown.

Each material should be assessed in accordance with Technical Requirement R3 and used in accordance with the independent assessment and the manufacturer’s recommendations.

The correct placement of heave materials is essential to ensure the foundations and substructure are adequately protected from heave forces.

Heave precautions for foundations#

Table 8 shows where heave precautions are required for trench fill, pier and beam, and pile and beam foundation types which are in the zone of influence of trees (see Table 3b) which are to remain or be removed.

Table 8: Position of heave precautions

Situation (see figures 4, 5 & 6)Trench fillPier and beamPile and beam
External trench fill and pier foundations. Unless NHBC is satisfied that the soil is not desiccated compressible material should be provided to the:Inside faces of external wall foundations deeper than 1.5m, based on the appropriate tree heightAll faces of pier foundations deeper than 1.5m, based on the appropriate tree heightN/A
External ground beams
Unless NHBC is satisfied that the soil is not desiccated compressible material or void formers should be provided to the:
N/AInside facesInside faces
Internal trench fill foundations and ground beams
Compressible material required:
NoNoNo
External and internal ground beams
Compressible material, void former or void should be provided to the underside of:
N/AAllAll
Heave precautions required for proposed trees where the soil is not desiccated:NoNoNo

On pilecaps, heave precaution measures should be assessed on a project-by-project basis. Lightly loaded pilecaps consisting of between one and three piles are more susceptible to heave movement than heavily loaded pilecaps. Omission of heave precaution measures should be justified by the designers, particularly on lightly loaded pilecaps.

Raft foundations constructed in accordance with Clause 4.2.8 and Clause 4.2.9 should provide adequate protection from heave.

Other foundation types#

All foundations not covered in this chapter, but specifically designed to counteract heave, should be:

  • designed by an engineer taking account of this guidance
  • submitted to NHBC prior to commencing work on site.

Suspended ground floors#

Suspended ground floors with voids in accordance with Table 7 should be used in situations where heave can occur within the area bounded by the foundations, including where:

  • foundation depth, determined in accordance with this chapter, is more than 1.5m, unless NHBC is satisfied the soil is not desiccated, or
  • ground floor construction is undertaken when the surface soils are seasonally desiccated (ie, during summer and autumn), unless NHBC is satisfied the soil is not desiccated.

Paths and driveways#

Paths and driveways should be designed and detailed to cater for the likely ground movement.

Last updated: 2nd January 2024

Back to top ⇧