Strip and trench fill foundations

Also see:

4.3.7Safe transmission of loads

Strip and trench fill foundations shall transmit loads to the ground safely and without excessive settlement, and take into account:

  1. dead and imposed loads
  2. foundation width and thickness
  3. stability of any adjoining building.

Dead and imposed loads

Dead and imposed loads should be calculated in accordance with:

BS EN 1991-1-1UK National Annex to Eurocode 1. ‘Actions on structures. General actions. Densities, self-weight, imposed loads for buildings’.
BS EN 1991-1-3UK National Annex to Eurocode 1. ‘Actions on structures. General actions. Snow loads’.
BS EN 1991-1-4UK National Annex to Eurocode 1. ‘Actions on structures. General actions. Wind actions’.
BS 648‘Schedule of weights of building materials’.

All foundations should be:

  • continuous throughout the building, including integral garages, porches, conservatories, bay windows, etc.
  • symmetrical beneath load-bearing elements (i.e. walls should be located centrally on foundations).

Foundation width and thickness

The width of the foundation should:

  • be of sufficient width throughout to avoid overstressing the ground, especially where the foundation is required to support piers or columns
  • depend on the load-bearing capacity of the subsoil and the loads from the building
  • not be less than the wall thickness, plus at least 50mm each side.

The width of strip foundations should account for ground conditions and be in accordance with the following table:

Table 2: Acceptable foundation widths

The thickness (T) of the foundation should be:

  • equal to projection (P) or 150mm (whichever is greater)
  • 150mm to 500mm for strip foundation
  • 500mm minimum for trench fill foundations.

Stability of any adjoining building

Where foundations are taken deeper than an adjoining building, excavation and construction will usually need to be carefully supervised by the design engineer, to check the standard of workmanship. Where necessary, allowance should be made in the design for differential movement.