Substructure and ground-bearing floors

Also see:

5.1.4Ground conditions

Substructure and ground-bearing floors shall not be adversely affected by ground conditions, and take account of:

  1. ground hazards
  2. bearing capacity of the ground
  3. nature of the ground
  4. effect of sloping ground on depth of infill and wall construction
  5. site works and construction.

Ground hazards

Hazards likely to affect substructure and ground-bearing floors include contaminated materials, waterlogged ground and chemicals, particularly sulfates.

Where it is necessary to reduce the entry of radon gas, which should be identified in the site investigation, such precautions should be acceptable to NHBC.

Bearing capacity

Ground-bearing floors may not be suitable where the bearing capacity and nature of the ground varies, even where the depth of infill is less than 600mm. Special measures may be needed to restrict settlement, such as the use of suspended floor construction.

Nature of the ground

Where there is shrinkable soil, expansive materials or other unstable soils, suspended floor construction may be necessary.

Shrinkable soils are classified as those which contain more than 35% fine particles (silt and clay) and which have a Modified Plasticity Index of 10% or more. A soil testing laboratory should be consulted to verify the Plasticity Index of the soil.

The effect of sloping ground

Sloping ground may require steps in the substructure and possibly different floor levels.

Where more than 600mm of infill is required at any point in a self-contained area, the floor over the whole of that area must be of suspended construction.

Construction on steep slopes may involve walls below DPC level acting as retaining walls and should be designed by an engineer where (H) is greater than four times (T).

  • (H) = height difference between floor/ground levels
  • (T) = the total thickness of the retaining wall.

Site works and construction

Special precautions may be needed to prevent damage to the substructure from site operations on adjoining ground such as ground treatment, or surcharging due to infill.