Conditions relating to the ground
2E1. There should not be:
- A: Non-engineered fill (as described in BRE Digest 427) or wide variation in ground conditions within the loaded area; nor
- B: Weaker or more compressible ground at such a depth below the foundation as could impair the stability of the structure.
2E2. The following design provisions relate to foundations:
- A: The foundations should be situated centrally under the wall;
- B: For foundations in chemically aggressive soil conditions guidance in BS 8500-1 and BRE Special Digest 1 should be followed. In non-aggressive soils, concrete should be composed of Portland cement to BS EN 197-1 and -2 and fine and coarse aggregate conforming to BS EN 12620 and the mix should comply with one of the following recommendations:
- i: In proportion of 50kg of Portland cement to not more than 200kg (0.1 cubic metres) of fine aggregate and 400kg (0.2 cubic metres) of coarse aggregate; or
- ii: Grade ST2 or grade GEN I concrete to BS 8500-2;
- C: Minimum thickness T of concrete foundation should be 150mm or P, whichever is the greater where P is derived using Table 10 and Diagram 23. Trench fill foundations may be used as an acceptable alternative to strip foundations;
- D: Foundations stepped on elevation should overlap by twice the height of the step, by the thickness of the foundation, or 300mm, whichever is greater (see Diagram 21).For trench fill foundations the overlap should be twice the height of the step or 1m, whichever is greater;
- E: Steps in foundations should not be of greater height than the thickness of the foundation (see Diagram 21);
- F: Foundations for piers, buttresses and chimneys should project as indicated in Diagram 22 and the projection X should never be less than the value of P where there is no local thickening of the wall.
Minimum width of strip foundations
2E3. The recommended minimum widths of foundations given in Table 10 may be used.
*****Image Diagram 21 Elevation of stepped foundation
*****Image Diagram 22 Piers and Chimneys
*****Image Diagram 23 Foundation dimensions
*****Image Table 10 Minimum width of strip footings
The table is applicable only within the strict terms of the criteria described within it.
Minimum depth of strip foundations
2E4. Except where strip foundations are founded on rock, the strip foundations should have a minimum depth of 0.45m to their underside to avoid the action of frost. This depth, however, will commonly need to be increased in areas subject to long periods of frost or in order to transfer the loading onto satisfactory ground.
In clay soils subject to volume change on drying (‘shrinkable clays’, with Modified Plasticity Index greater than or equal to 10%), strip foundations should be taken to a depth where anticipated ground movements will not impairthe stability of any part of the building taking due consideration of the influence of vegetation and trees on the ground. The depth to the underside of foundations on clay soils should not be less than 0.75m on low shrinkage clay soils, 0.9m on medium shrinkage clay soils and 1.0m on high shrinkage clay soils, although these depths may need to be increased in order to transfer the loading onto satisfactory ground, or where there are trees nearby