Structure: Approved Document A

O N L I N E V E R S I O N A1/2 FOUNDATIONS OF PLAIN CONCRETE Table 10 M inimum width of strip footings Total load of load-bearing walling not more than (kN/linear metre) Type of ground 20 30 40 50 60 70 (including Condition Field test engineered fill) of ground applicable Minimum width of strip foundations (mm) I Not inferior to Requires at least a pneumatic In each case equal to the width of wall Rock sandstone, limestone or other mechanically operated or firm chalk pick for excavation II Requires pick for excavation. 250 300 400 500 600 650 Gravel or sand Medium dense Wooden peg 50mm square in cross section hard to drive beyond 150mm III Can be indented slightly by thumb 250 300 400 500 600 650 Clay Stiff Sandy clay Stiff IV Thumb makes impression easily 300 350 450 600 750 850 Clay Firm Sandy clay Firm V Can be excavated with a spade. 400 600 Sand Loose Wooden peg 50mm square in Silty sand Loose cross section can be easily driven Note: Clayey sand Loose Foundations on soil types V and VI do not fall within the VI Finger pushed in up to 10mm 450 650 provisions of this section if the Silt Soft total load exceeds 30kN/m. Clay Soft Sandy clay Soft Clay or silt Soft VII Finger easily pushed in up to 25mm Refer to specialist advice Silt Very soft Clay Very soft Sandy clay Very soft Clay or silt Very soft 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 impair the 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. Approved Document A Structure O N L I N E V E R S I O N 37