Foundations are the parts of a structure that transfer loads from columns, walls, and slabs into the ground. They matter because even a well designed building can crack, tilt, or fail if the soil cannot safely support its weight. Engineers choose foundation types by studying loads, soil strength, groundwater, settlement limits, and nearby structures.
The main categories are shallow foundations and deep foundations.
Key Facts
- Bearing pressure under a footing can be estimated by q = P/A, where P is load and A is contact area.
- Shallow foundations are usually used when strong soil is close to the surface and settlement can be controlled.
- Deep foundations are used when weak surface soils require loads to be transferred to deeper strong layers or through skin friction.
- Total load on a foundation includes dead load, live load, environmental loads, and sometimes seismic load.
- Settlement is vertical ground movement, and differential settlement is uneven movement that can damage a structure.
- Factor of safety for bearing is often written as FS = q_ult/q_allow, comparing ultimate capacity with allowable bearing pressure.
Vocabulary
- Shallow foundation
- A foundation that transfers building loads to soil near the ground surface, often using spread footings or mat foundations.
- Deep foundation
- A foundation that transfers loads to deeper soil or rock using piles, drilled shafts, or caissons.
- Spread footing
- A widened concrete base under a column or wall that spreads load over a larger soil area.
- Pile
- A long slender structural element driven or drilled into the ground to carry load by end bearing, skin friction, or both.
- Bearing capacity
- The maximum pressure soil can support before shear failure or unacceptable deformation occurs.
Common Mistakes to Avoid
- Choosing a shallow foundation only because it is cheaper, which is wrong because weak or compressible surface soil can cause excessive settlement and failure.
- Ignoring differential settlement, which is wrong because small uneven movements can crack walls, jam doors, and overstress structural frames.
- Assuming piles carry load only at their tips, which is wrong because many piles also transfer load through friction along their sides.
- Using building weight alone in foundation design, which is wrong because live loads, wind, earthquakes, water pressure, and construction loads can control the design.
Practice Questions
- 1 A column carries a service load of 900 kN. If the allowable soil bearing pressure is 150 kPa, what minimum square footing area is required, and what is the side length?
- 2 A building load of 2400 kN is supported by 8 identical piles. If the load is shared equally, what load does each pile carry? If each pile has an allowable capacity of 350 kN, is the pile group adequate?
- 3 A site has 2 m of loose fill over soft clay, with dense sand starting at 12 m depth. Explain whether a shallow mat foundation or deep piles would likely be more appropriate, and justify your choice using load transfer and settlement.