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.
Understanding Construction Machines: Drilled Shafts and Caissons
A drilled shaft begins with careful investigation of the ground. Engineers collect soil and rock samples from borings near the planned foundation. These samples show layer thickness, water level, strength, and possible obstacles such as boulders or old buried structures.
The design team uses this information to choose the diameter and depth. A larger diameter gives more concrete area to carry compression. Greater length can provide more contact with the surrounding ground.
The required capacity is checked with force divided by area for stress in the concrete. The result must remain within safe limits for the concrete and the ground.
The main machine is usually a rotary drilling rig. Its large auger, drilling bucket, or core barrel removes soil as it turns. Different tools suit different ground.
Buckets work well in many soils. Rock tools break hard material. A temporary steel casing may support the upper part of the hole where loose soil could cave in.
In wet or unstable ground, crews can fill the excavation with slurry. This dense fluid presses outward against the hole walls and helps hold them in place. The slurry must be checked often because dirty or weak slurry cannot support the ground properly.
After drilling reaches the planned level, the bottom needs special attention. Loose cuttings left at the base can act like a soft pad and reduce support. Crews clean the hole with buckets, airlifting equipment, or pumps.
They then lower a steel reinforcement cage into the excavation. The cage gives the finished concrete column strength against bending and tension, which can occur from wind, earthquakes, uneven ground movement, or sideways loads from bridge structures. Concrete is commonly placed through a long pipe called a tremie.
The pipe outlet stays buried in fresh concrete during the pour. This prevents the concrete from mixing with water or slurry and keeps the shaft continuous.
A shaft resists load in two main ways. The base can press on a strong layer, while the sides can grip the surrounding soil through friction. The balance depends on local geology.
Rock at the base may provide much of the support. In clay or dense sand, side resistance may be very important. Engineers consider settlement as well as ultimate strength.
A foundation that does not fail can still cause trouble if it settles too far. Quality checks include measuring depth, checking vertical alignment, recording concrete volume, and testing concrete samples. Some shafts are examined with sound wave tests that can reveal voids or narrow sections.
Students should notice that success depends on the full sequence. Good calculations cannot compensate for a poorly cleaned hole, damaged reinforcement cage, or interrupted concrete placement.
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.