There are different types of foundations for building construction and the selection of a particular type depends on soil conditions and loads from the structure. It is advisable to know the suitability of the chosen type of foundation before making any decision on its selection. The selection of a particular type of foundation is often based on a number of factors, such as:
The foundation depth depends on the following factors:
- Getting an adequate allowable bearing capacity of subsoil.
- For clayey soils, depth should be below the zone where shrinkage and swelling due to seasonal weather changes are likely to cause appreciable soil movements.
- For fine sands and silts, founding depth should be below the frost zone.
- For bridge piers, the maximum depth of scour should be considered with the foundation located sufficiently below this depth.
- Depth of foundation shall be at least below the top soil, miscellaneous fill, vegetation i.e approximately 300 mm below NGL.
All foundations should be taken down to a minimum depth of 0.5 m below natural ground level as a matter of sound engineering practice. In filled-up ground it may be necessary to go beyond the depth of fill or take special precautions to avoid settlement issues for the structure. In such cases, it may be necessary to have a shallow foundation i.e. at a higher level, and replace the soil in between the base of foundation and the level of excavation either by:
(a) Lean concrete,
(b) Well compacted structural fill to required specifications. The width of fill should not be less than the width of foundation. This is necessary for dispersion of load on either side of the foundation.
In sloping grounds, the horizontal distance from the footing edge (shallow side) to ground surface shall not be less than 60 cm for rock sub-base, and 90 cm for soil. In addition, a line drawn at 30 degrees to the base from the outer edge should not intersect the sloping surface of the ground.
For foundations near existing structures, the minimum horizontal distance between existing and new footings shall not be less than the width of the wider footing. An analysis of bearing capacity and settlement may be required to study pressure bulb overlaps and the likely impact on the foundations.
BEARING CAPACITY FAILURE
Bearing capacity of the soil is its capacity to support the structure loads. It is the maximum pressure the soil can without failure.
TYPES OF BEARING CAPACITY FAILURES
Bearing capacity failures of foundations can be grouped into three categories:
(a) GENERAL SHEAR
This involves total rupture of the underlying soil. There is a continuous shear failure of the soil from below the footing.
General shear failure ruptures and pushes up the soil on one or both sides of the footing, often resulting in a subsequent tilt in the structure.