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Classify shallow and deep foundations
Short answer
Shallow foundations are used where firm soil is near the surface: isolated footings for light loads, raft when footings cover more than half the plan area. Deep foundations (piles, wells) are used when weak upper soil overlies a deep strong layer or for large scour. Try the cheapest shallow type first.
Classify the shallow and deep foundations according to their soil conditions (how would you select the suitable type of foundation according to soil conditions?).
Foundations are chosen by comparing the bearing capacity and compressibility of the soil layers with the building load. They fall in two groups.
Shallow foundations ()
Used where a firm layer is near the surface.
| Soil condition | Suitable foundation |
|---|---|
| Good soil at shallow depth, light loads | Isolated spread footing / wall (strip) footing |
| Columns close together, or weak soil needing more area | Combined or strap footing |
| Weak soil, heavy load, columns cover more than ~50% of the plan area, differential settlement problem | Raft (mat) foundation |
Deep foundations ()
Used where the upper soil is weak, compressible, or scour-prone and a good layer lies deep.
| Soil condition | Suitable foundation |
|---|---|
| Soft soil over hard stratum or rock at moderate depth | End-bearing pile / pier |
| Deep soft soil with no hard layer | Friction pile |
| Large river bed, scour, bridge piers | Well (caisson) foundation |
| Very heavy loads, uplift or lateral loads | Bored/driven pile group, drilled shaft |
Selection rule: try the cheapest shallow type first; if the footings would cover more than about half the plan area or settlement is excessive, go to a raft; if still unsafe, use piles or wells.
✓ Answer checked by Claude Sonnet 5.5
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