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4.2.8Design and construction of foundations in shrinkable soils

Foundations in shrinkable soils shall be appropriately designed and constructed.

Reference should be made to Clause 4.2.10 to establish the precautions necessary to cater for potential heave.

The following will only be acceptable if they are designed by an engineer and account for all potential movement of the soil on the foundations and substructure:

  • trench fill foundations deeper than 2.5m
  • pier and beam foundations
  • pile and beam foundations
  • rafts.

Trench fill foundations

If trench fill foundations are deeper than 2.5m:

  • the instability of the trench sides can lead to serious construction difficulties
  • the design should take account of soil desiccation and the associated arboricultural advice
  • additional heave precautions may be necessary to cater for lateral and shear forces acting on large vertical areas of foundation
  • concrete overspill or overbreak in excavations should be avoided in order to reduce the possibility of additional vertical forces being transmitted to the foundation
  • compressible material should be correctly placed to avoid excessive heave forces being applied to the foundations
  • construction joints need to be detailed to account for increased lateral forces.

Pier and beam foundations
Pier depths not exceeding 2.5m depth may be derived from Clause 4.2.7. Pier depths greater than 2.5m require site
specific assessment.

Pile and beam foundations

When selecting and designing pile and ground beam foundations, piles should be:

  • designed with an adequate factor of safety to resist uplift forces on the shaft due to heave.
  • reinforced for the length of the member governed by the heave design.

Sufficient anchorage should be provided below the depth of desiccated soil. Slip liners may be used to reduce uplift but the amount of reduction is small, as friction between materials cannot be eliminated.

Bored, cast-in-place piles are well suited to counteracting heave. Most types have a straight-sided shaft, while some are produced with a contoured shaft to increase load capacity. The design should allow for the enhanced tensile forces in these piles.

Driven piles are less well suited to counteracting heave and are difficult to install in stiff desiccated clay without excessive noise and vibration. The joint design of these piles should be capable of transmitting tensile heave forces.

Ground beams should be designed to account for the upward forces acting on their underside and transmitted from the compressible material or void former prior to collapse, and in accordance with the manufacturer’s recommendations.

Raft foundations

Raft foundations in shrinkable soils will only be acceptable where all of the following apply:

  • design is by an engineer in accordance with Technical Requirement R5
  • NHBC is satisfied that the raft is sufficiently stiff to resist differential movements
  • NHBC is satisfied that the raft is founded on granular infill placed and fully compacted in layers and in accordance with the engineer’s specification. Where required by NHBC, site inspections are to be undertaken by the engineer to verify suitable compaction of the fill
  • the raft is generally rectangular in plan with a side ratio of not more than 2:1
  • foundation depth is derived in accordance with Clause 4.2.7, and is less than 2.5m