What a foundation does
A foundation is the part of a structure that transfers its load into the ground. It must do two things: the ground must not fail in shear under it, and the settlement must stay small enough, and even enough, that the structure is not damaged.
On firm soil near the surface, a spread footing or raft enlarges the contact area so that the pressure on the soil is within its allowable bearing pressure. When good ground lies deep, piles or well foundations carry the load down, by friction along their sides and by bearing at their tips.
Choosing a foundation starts with a site investigation: boreholes, standard penetration tests, sampling and lab tests to find the soil layers and the water table. Settlement often governs rather than strength, particularly on soft clays, where consolidation settlement may continue for years.
Differential settlement, one part sinking more than another, is what cracks walls and tilts structures. Foundations are proportioned so that pressures, and therefore settlements, are similar across the building.
Required footing area A = service load / allowable bearing pressureContact pressure q = P / A (concentric load)
Problem. A column carries 900 kN service load (including self-weight of footing). Allowable bearing pressure is 150 kPa. Size a square footing.
- A = 900 / 150 = 6.0 m².
- Side = √6.0 = 2.45 m, so adopt 2.5 m × 2.5 m.
Answer. 2.5 m × 2.5 m
Quick quiz
1Two criteria a foundation must meet?
Adequate safety against shear failure and acceptable settlement.
2When are piles used?
When competent ground is deep or loads are large.
3What cracks walls more, uniform or differential settlement?
Differential settlement.
- IS 1904 (India): Design and construction of foundations in soils: general requirements.
- IS 6403 (India): Determination of bearing capacity of shallow foundations.
- IS 8009 (Part 1) (India): Calculation of settlement of shallow foundations.
Cited by number and clause only. We do not reproduce clause text, tables or figures (BIS Act 2016, s.10(5) and s.11). Check the current edition and amendments before use.
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References and further reading
Standard texts, listed in no order of merit. We do not copy text, figures, tables or solved problems from any book; our explanations and examples are original.
- B. M. Das, Principles of Foundation Engineering, Cengage, latest edition.
- J. E. Bowles, Foundation Analysis and Design, McGraw Hill, 5th edition.
- M. J. Tomlinson, J. Woodward, Pile Design and Construction Practice, CRC Press, latest edition.
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