The depth of foundation below ground surface is on the most important criteria on which the type of foundation depends. If the depth of foundation is less than the width of foundation then it is known as a foundation otherwise it is the deep foundation.
Shallow foundation
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when the depth of foundation ‘D ’ is less or equal to the width ‘B’ it is called as a shallow foundation or open foundation i.e. D < B. shallow foundations are used if the supporting soil or rocks layer near to the ground surface, has sufficient load-bearing capacity.
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It is placed immediately beneath the lower parts of the upper structure. The main object of this type of foundation is to spare the load of the upper structure over a larger area to bring the pressure intensity within safe limits.
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These are generally used for all ordinary building which carries light or moderate loads and where good bearing capacity is available at shallow depth i.e. D< 5 m
The various types of shallow foundations are as follows:
- Wall Footing
- Column Footing
- Inverted arch Footing
- Raft or mat Footing
- Grillage Foundation
Deep foundations
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when a stratum of good bearing capacity is not available at a reasonable depth and where other types of foundations such a grillage or raft foundations are not suitable then depth foundations must be adapted to attain a bearing stratum which will be suitable all respect.
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They are generally provided when the depth of foundations is more than 5m. In addition to the above, there may be many other conditions which may require deep foundations for souring, stability, and durability of the structure.
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for example, in bridge constructions, the pier must be founded well below the scouring depth, even though good bearing stratum may be available at shallow depth.
Types of the deep foundation are
- Pile foundation
- Piper Foundation
- caisson