Great care should be taken to ensure foundations are constructed correctly as they are the most important structural element of any building. Outlined is guidance which should be followed to ensure correct construction.
The locations of underground pipes and services should be established prior to digging. If the foundation trench is waterlogged, any water should be collected in a sump and pumped away from the foundation area.
This should be taken down to a firm bottom prior to filling. This requires the removal of all organic matter including roots and timber and the removal of all vegetable soil from the entire area.
You should remove all loose material and soft spots prior to pouring foundation. If required, backfill with lean mix concrete in order to make up levels prior to pouring.
"Photo - Excavating"
Area of foundations should be set out accurately. Ensure trench is clean, dry and level. Site engineer should provide pegs for level of top of foundation.
Please ensure that pegs are provided showing level of top of foundation and that the trench is clean, dry, level and compact. This is especially important in wet weather.
Diagram B2 - Setting out
If designed reinforced strip foundations are required due to ground conditions, steel should be placed in accordance with design details and specifications. If foundations are stepped, reinforcement should be designed and placed to take such steps into account.
Ensure there is no debris in the trench before pouring. Groundwater should be allowed to discharge to sump.
Provide a sump and clean trench bottom in situations where an excavation becomes waterlogged. Foundations should be stepped correctly.
Diagram B3 - Sump provision
Diagram B4 - Typical step foundation detail
Soil Bearing Pressure
As the ability of a soil to support foundations varies greatly depending on the type of soil, a method of measuring this ability is required. This is known as the soil’s bearing pressure and is measured in kN/m^2^. The larger the bearing pressure, the larger the loads the soil can carry. The maximum bearing pressures for a given foundation are determined by a site investigation.
The subsequent table provides information on 5 broad soil classifications. This can be used to provide a preliminary indication of the bearing capacities of soils and details of the field tests that may be carried out for a given type of soil. The loading imposed by the party wail of a two-storey house of traditional construction is in the order of 75 kN/m^2^.
Based on the subsequent table it can be seen that problems can be expected with:
Loose silty/clayey loose sand and sand
Soft and very soft sandy/silty/clays
Fill or made ground
Table B1 - Guideline bearing pressures based on soil composition
The table above is for guidance purposes only and not a substitute for field and laboratory tests – which should still be carried out by professional site investigation personnel.