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.
(b) LOCAL SHEAR
Local shear failures involve soil rupture immediately below the footing. There is soil bulging on both sides of the footing, but the bulging is small when compared to that in general shear. This mode of failure is characteristic of medium dense or firm soils.
(c) PUNCHING SHEAR
In punching shear, the soil outside the loaded area remains relatively uninvolved and there is hardly any movement of soil on the sides of the footing. This is associated with compression of the soil directly below the footing and vertical shearing of soil around the footing perimeter. A punching shear failure is characteristic of loose or soft soils.
Any foundation must be safe against all of the above 3 types of bearing capacity failures.
All foundations settle to varying extents as the soil beneath them adjusts to the building loads. Foundations on rock bed settle by an insignificant amount, if any. Foundations on other soils, such as clay, may settle more. Where all the foundation elements settle at the same rate, it is termed as ‘uniform’ settlement. Settlement that occurs at differing rates between the various elements of building foundation system, it is termed ‘differential’ settlement.
When all parts of a building rest on one type of soil, and the soil pressures are uniform, differential settlement is usually not a concern. However, where soil pressures between the various areas of a building support system are different, differential settlements are likely. This may result in a tilt or distortion of the building frame and may cause ‘structural’ or ‘non-structural’ damage to it.
The foundation must be of an adequate quality so that it is not subjected to deterioration, for example, by chloride or sulfate attack.
The foundation must be designed with sufficient strength that it does not fracture or break apart under the applied structure loading. The foundation must also be properly constructed in accordance with the design code and project specifications.
ADVERSE SOIL CHANGES
The foundation must be able to resist long term adverse soil changes. Expansive soils, that expand or shrink with moisture content, thereby causing movement of the foundation, may severely damage the structure. In such cases, the foundation level may be considered below such soils.
A foundation must be able to resist excessive settlement or lateral movement in case of an earthquake.
Based on an analysis of all of the factors listed above, a specific type of foundation, for example, isolated footing, combined footing, mat foundation, piled foundation etc may be recommended by the geotechnical engineer.