Foundations
Foundations tests shallow and deep foundation types, bearing capacity, setting out, excavation, concrete blinding, reinforcement cover and damp-proofing at plinth level. Expect strip footings, isolated spread footings, rafts and piles, with attention to soil condition and water.
Core rules
- A foundation transfers load to soil without excessive settlement; its area depends on load and safe bearing capacity.
- Shallow foundations include strip, pad and raft; deep foundations include piles and well foundations.
- Founding depth must reach firm strata and be below frost or seasonal moisture movement where relevant; in India, termite and damp control are also important.
- Provide a plain concrete blinding layer, correct reinforcement cover, and compact concrete in layers.
- Do not found on loose fill, roots or organic soil without treatment.
Worked example
A column load is 400 kN and the allowable bearing capacity is 100 kN/m2. Required footing area = 400 ÷ 100 = 4 m2. A square footing needs sides of √4 = 2 m, so use a 2 m × 2 m pad. If the footing is 0.5 m thick, concrete volume = 2 × 2 × 0.5 = 2 m3, before excavation allowance.
Traps that cost marks
Sizing a footing from load alone without bearing capacity, ignoring eccentricity, or placing reinforcement with insufficient cover are common errors. A larger footing is not automatically safe if it bears on soft fill.
Key points
- Footing area = load ÷ safe bearing capacity.
- A 400 kN load at 100 kN/m2 needs 4 m2, e.g. 2 m × 2 m.