Deep foundations: piles vs caissons

One-line orientation

Deep foundations carry load past weak surface soil to a deeper stable layer or rock. Piles are often slender and grouped; they may be driven or drilled. Caissons, or drilled shafts, are generally large cast-in-place shafts formed in drilled holes.

Key points

• When deep foundations are used: shallow footings are not feasible because of poor soil conditions or heavy loads — load must reach a deeper, more stable layer.
• Piles — commonly driven, but sometimes drilled:
  • Slender members, typically used in groups, that transfer load to deeper stable soil or rock. Installation may be driven or drilled, depending on the pile type.
  • Useful where the driven approach can pass through weak or non-cohesive soils to reach a competent bearing layer.
  • Materials: timber (light/temporary, can’t be spliced, often friction), steel (H-pile or pipe, drivable to great depth, weldable to extend), concrete (precast or cast-in-place), composite.
  • Variants: mini-pile (small diameter, pre-drill + grout then insert; tight access), helical pile (screwed in, no pre-drilling; low noise/vibration).
• Caissons / drilled piers — bored then poured:
  • Large, cast-in-place shafts: drill a hole, then fill with concrete. The drilled hole needs stable sidewalls or construction support such as casing/slurry before concrete placement. Reach deep stable soil or rock; common in bridges and where robust support is needed.
  • May have a belled bottom to spread load over a larger area.
• Pile cap: a thick, reinforced-concrete slab on top of a pile group that binds the piles and distributes the superstructure/column load evenly across all the piles, so the group acts as one element.

Confusions / comparison

	Pile	Caisson (drilled pier)
Common installation	Often driven; some piles are drilled	Drilled, then cast with concrete
Diameter / form	Slender, usually in groups	Large diameter, often a single shaft
Construction soil cue	Driven through weak/non-cohesive soils	Drilled hole needs stable sidewalls or casing/slurry
Material	Timber, steel, concrete, composite	Cast-in-place concrete (may be belled)
Load spread at top	Pile cap ties the group	Carries column directly
Typical use	Building load to deep strata	Bridges, heavy/robust support

Related

→ Pile load transfer: end-bearing vs friction (this topic): how the load actually reaches the soil · Settlement (this topic): why weak soil drives the choice of deep foundations · Soil improvement methods (this topic): the alternative to going deep.