Strip and trench fill foundations

Also see:

4.3.3Ground conditions

Strip and trench fill foundations shall be adequate, of a suitable depth and taken to a suitable bearing stratum. Issues to be taken into account include:

  1. the home design and layout
  2. ground conditions
  3. frost susceptible soils and cold weather construction
  4. shrinkable and volume change soils.

The home design and layout

Foundation design should take account of site conditions, shape, size and construction of the homes. Foundations for terraced homes may require special precautions to prevent damage from differential settlement, while stepped foundations or suspended floors may be required for sloping sites.

The depth of foundations should provide a clean, firm and adequate bearing for the design loads.

Ground conditions

All relevant information about the history of the site, plus the nature and load-bearing capacity of the ground, should be available before the foundations are designed. Information may be available from:

  • NHBC
  • local authorities
  • gas, water and electricity companies
  • aerial photographs, Ordnance Survey maps and geological maps and surveys.

Site assessment surveys may require supplementary investigations involving trial pits and boreholes.

Frost susceptible soils and cold weather construction

In frost susceptible soils, e.g. chalk, the depth to the underside of the foundation should be at least 450mm below finished ground level, to avoid damage from frost action.

Additionally, when construction is undertaken during cold weather the foundation should either be at least 450mm below finished ground level, or alternatively, precautions should be taken to prevent freezing of the ground.

Where the finished ground level is to be above the existing ground level and cold conditions are expected, the foundation depth should be taken from the existing, not finished, ground level.

Shrinkable and volume change soil

The design should specify the minimum foundation depth.

Table 1: Minimum foundation depths in shrinkable soil

Modified Plasticity IndexVolume change potentialMinimum depth (m)
40% and greaterHigh1.0
20% to less than 40%Medium0.9
10% to less than 20%Low0.75

Shrinkable soils are classified as containing more than 35% fine particles (clay and silt) and have a Modified Plasticity Index of 10% or greater.

These minimum depths may only be used where any existing or proposed trees or shrubs are outside the zone of tree influence. Heave is possible in shrinkable soil where trees have been, or are being, removed.