Drilled shafts and caissons are large concrete columns built deep underground to support heavy structures such as bridges, towers, and stadiums. They are used when shallow soil near the surface is too weak or too compressible to carry the load safely. By extending downward to stronger soil or rock, they transfer forces to deeper, more reliable bearing layers.
This makes them essential for construction on soft ground, near water, or in dense urban sites where high loads are concentrated.
Key Facts
- Axial stress in a shaft is σ = P/A, where P is load and A is cross-sectional area.
- For a circular drilled shaft, A = πd^2/4, where d is the shaft diameter.
- Total load capacity can come from end bearing plus side friction: Q_total = Q_tip + Q_side.
- End bearing resistance acts at the bottom of the shaft against firm soil or rock.
- Side friction, also called skin friction, develops along the contact surface between concrete and surrounding soil.
- Concrete volume for a straight shaft is V = πd^2L/4, where L is shaft length.
Vocabulary
- Drilled shaft
- A deep foundation made by drilling a large vertical hole and filling it with reinforced concrete.
- Caisson
- A deep foundation element or watertight construction chamber used to place support below weak surface ground or water.
- Reinforcement cage
- A framework of steel bars placed inside the drilled hole to strengthen the concrete column.
- End bearing
- The support force created when the bottom of a foundation presses on firm soil or rock.
- Side friction
- The resistance created along the sides of a deep foundation as soil grips the concrete surface.
Common Mistakes to Avoid
- Using surface soil strength for the whole foundation design is wrong because drilled shafts depend on changing soil layers with depth.
- Ignoring the shaft diameter is wrong because cross-sectional area grows with d^2, so small diameter changes can greatly affect stress and concrete volume.
- Assuming all capacity comes from the bottom tip is wrong because many drilled shafts carry a major part of the load through side friction.
- Forgetting reinforcement is wrong because plain concrete is strong in compression but weak in tension, bending, and cracking during construction and service.
Practice Questions
- 1 A drilled shaft has a diameter of 1.2 m and carries an axial load of 3600 kN. Calculate its cross-sectional area and average axial stress.
- 2 A straight drilled shaft is 1.5 m in diameter and 18 m deep. Estimate the volume of concrete needed using V = πd^2L/4.
- 3 A building site has 6 m of soft clay over dense sand and weathered rock. Explain why engineers might choose drilled shafts instead of shallow spread footings.