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shallow foundation, types and factor
The document outlines shallow foundation engineering, defining shallow foundations as those with width greater than depth, and describes various types including strip, spread, combined, strap, and mat foundations. It also discusses factors affecting foundation depth such as topsoil depth, frost depth, water table level, and underground defects. Identifying these factors is crucial for ensuring the stability and effectiveness of foundations in construction.
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shallow foundation, types and factor
- 1. SUBJECT FOUNDATION ENGINEERING TOPIC DEFINITION OFSHALLOW FOUNDATION. TYPES OF SHALLOW FOUNDATIONS. DEPTH AND FACTOR AFFECTING IT. PREPARED BY ER. SANJEEV SINGH SNJV432@GMAIL.COM
- 2. CONTENT Definition ofshallow foundation. Types of shallow foundations. Depth and factor affecting it.
- 3. SHALLOW FOUNDATION Ashallow foundation, according to Terzaghi, is one whose width is greater than its depth ( i.e, 𝐷𝑓/B ≤ 1). Shallow foundation are located just below the lower part of the wall or a column which they support. Footing are structural member, made of brick work, masonry or concrete, that are used to transmit the load of the wall or column such the load distributed over a large area.
- 4. TYPES OF SHALLOW FOUNDATIONS Strip Foundation Spread or Isolated Footing Combined Foundation Strap or cantilever foundation Mat or Raft Foundation
- 5. STRIP FOUNDATION Thefoundation whose length is considerably greater than its width is called strip or continuous foundation.
- 6. SPREAD OR ISOLATED FOOTING A spread footing is provided to support an individual column. A spread footing is circular, square or rectangular slab of uniform thickness. Sometime it is stepped or haunched to spread the load over a large area.
- 7. COMBINED FOUNDATION Acombined footing supports two columns. It is used when the two columns are so close to each other that their individual would overlap. Combined footing may be rectangular or trapezoidal in plan.
- 8. STRAP OR CANTILEVER FOUNDATION A strap footing consists of two isolated footings connected with a structural strap. The strap connects the two footing such that they behave as one unit. The strap act as a connecting beam and does not take any soil reaction. The strap is designed as a rigid beam. A strap footing is more economical than a combined footing when the allowable soil pressure is relatively high and the distance between the columns is large.
- 9. MAT OR RAFT FOUNDATION A mat or raft foundation is a large slab supporting a number of columns and wall under the entire structure or a large part of the structural. A mat is required when the allowable soil pressure is low or where the columns and walls are so close that individual footing would overlap or nearly touch each other. Mat foundation are useful in reducing the differential settlement on non-homogeneous soil. Where there is a large variation in the loads on individual columns.
- 10. DEPTH AND FACTOR AFFECTINGIT Depth of top soil. Frost depth. Zone of soil volume change. Water level. Scour depth. Underground defects. Root holes. Minimum depth.
- 11. DEPTH OF TOPSOIL The footing should be located below the top soil consisting of organic matters which is eventually decompose. The top soil should be removed over an area slightly larger than the footing.
- 12. FROST DEPTH Thefooting should be carried below the depth of frost penetration. If the footing is located at insufficient depth, it would be subjected to the frost damage due to formation of ice lenses and consequent frost heave. During summer, thawing occurs from the top downwards and the melted water is entrapped. As the soil water freezes and melts, the footing is lifted during cold weather and it settles during warm weather. The shear strength of the soil is also decreased during warm weather due to an increase in water content. To prevent frost damage, the footings should be placed below the frost depth, which may be 1 m or more in cold climate.
- 13. ZONE OF SOILVOLUME CHANGE Some soils, especially clays having high plasticity, such as black cotton soil, undergo excessive volume change. Such as soil shrinks upon drying and swell upon wetting. The volume changes are generally greatest near the ground surface and decrease with increasing depth. Large volume changes beneath a footing may cause alternate lifting and droping.
- 14. WATER LEVEL Thefooting should be placed above ground water table as far as possible. The presence of ground water with in the soil immediately around a footing in undesirable as it reduce the bearing capacity of the soil and there are difficulties during construction.
- 15. SCOUR DEPTH Thefootings located in stream, on water fronts or other locations where there is a possibility of scouring, should be placed below the potential scour depth.
- 16. UNDERGROUND DEFECTS The depthof footings is also affected by the presence of underground defects, such as faults, caves and mines. If there are man made discontinuities, such as sewer lines, water mains, underground cables, these should be shifted or the footing relocated.
- 17. ROOT HOLES Ifthere are root holes or cavities caused by burrowing animals or worms, the footing should be placed below such a zone of weakened soil.
- 18. MINIMUM DEPTH Is:1904-1978 specifies that all foundation should extend to a depth of at least 50 cm below the natural ground surface. However, in case of rocks, only top soil should be removed and the surface should be cleaned. Where q is the intensity of loading (𝐷 𝑓)min= 𝑞 γ ( 1−𝑠𝑖𝑛 ’ 2 1+𝑠𝑖𝑛 ’ )