Footings are placed at some depth below the ground surface with minimum embedment for the following purposes (Lambe and Whitman):
1. To gain additional bearing capacity derived by the embedment.
2. To place the footing below the zone of seasonal moisture variations for protection against swelling and shrinkage cycle in case of clayey soils and against the frost action in cold climates.
3. To enable the removal of top organic soil layer.
4. To avoid excessive rise in the elevation of finished floor level.
5. To prevent loss of support to the foundation by soil erosion.
The depth of foundation mainly depends on the soil profile at the construction site. Depth of foundation is the first and most important parameter to be decided before choosing suitable foundation for the structure.
Following are the criteria used to decide the depth of foundation:
1. Minimum Embedment Depth:
IS – 1904-1986 recommends a minimum depth of 0.5 m for the foundation. It is, however, conventional to use a minimum depth of 0.9 m for the foundation.
2. Top Soil:
The top soil at any site is usually organic soil or waste, which should be completely removed and the foundation is placed below the top soil. If the top soil extends to a larger depth, it may be replaced with a suitable backfill material and the foundation may be placed at the minimum depth.
3. Filled-Up Soil:
Certain sites are previously used as dumps for municipal waste or old mine waste. The foundation in such sites should be taken below the bottom of the waste dump to avoid excessive settlement of the foundation and structure. Alternately, the waste may be completely removed and replaced with suitable backfill and the foundation may be located at a suitable depth.
4. Depth of Hard/Stiff Soil:
The foundation should be taken to a level where soil with reasonably high bearing capacity and less compressibility is available. This is the usual rule unless the depth to the hard stratum is very large. This will mainly ensure that the size of the foundation is not excessively large and hence economical.
5. Type of Structure:
For heavy, large, and important structures, such as multi-story structures, sky scrapers, and bridges, the foundations should always be taken to the hard soil/rock.
6. Type of Soil:
For structures located in sites with deep soft clays, the foundation should usually be taken below the bottom of the soft clay, unless the ground improvement proves more economical and viable solution with a shallow depth of foundation.
7. Groundwater Table:
Wherever feasible, the foundation should be located above the groundwater table (GWT). This is because of the undesirable effects of the ground water on the foundation as well as the foundation soil. However, the GWT level changes with seasons and it may not always be possible to consider this criterion.
8. Frost Depth:
In cold regions, where water near ground surface is likely to be frozen during winter, the foundation should be located below the frost depth to avoid damage to the foundation from frost heave.
9. Active Zone:
In black cotton soils, the high volume changes within depth of active zone due to seasonal moisture variations have detrimental effects on the foundation, floors and walls of the building. Hence, the foundation is located below the depth of active zone, which varies from 1.5 to 2.5 m or more from place to place.
10. Damage to Adjacent Structures:
The depth of foundation should be so selected as to avoid the overlapping of the pressure bulb of the new foundation with that of the foundations of the adjacent structure. This is to avoid damage to the existing structure due to the construction of the new structure.
11. Scour Depth and Grip Length:
For all bridges, the foundations should be located below the maximum scour depth plus the minimum grip length below the highest flood level.
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