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Chapter 9 — Drilled Shafts & Caissons

9.1 Drilled shafts

A drilled shaft (bored pile) is a cast-in-place reinforced-concrete element constructed by drilling a hole and filling it with concrete. Diameters range from about 0.5 m to 3 m or more, and lengths can exceed 60 m. Drilled shafts offer:

  • high axial and lateral capacity from a single element;
  • low noise and vibration — suited to urban sites;
  • the ability to inspect and instrument the excavation before concreting;
  • a single element that can replace a pile group, avoiding a pile cap.

9.2 Construction methods

The method depends on the ground and groundwater:

  • Dry method — in stable, dry ground, the hole stands open.
  • Casing method — a temporary or permanent steel casing supports the hole through caving soils or under water.
  • Slurry method — a bentonite or polymer slurry stabilizes the hole, displaced by the concrete as it is placed by tremie.
  • Continuous Flight Auger (CFA) — a hollow auger is drilled and withdrawn while concrete is pumped through it.

Each method affects capacity: the slurry method can leave a filter cake and softened zone that reduces shaft friction if not cleaned.

9.3 Capacity

Drilled-shaft capacity follows the same shaft + base framework as piles, but with different coefficients:

  • Shaft resistance in clay is lower than for driven piles (no densification); in sand it depends on construction method and cleaning.
  • Base resistance requires a clean base; debris or softened soil drastically reduces end bearing.
  • Base grouting can be used to improve base contact and capacity.

9.4 Caissons

Caissons are deep foundation elements installed by excavation from within:

  • Open caissons — a shaft sunk by excavating from the bottom.
  • Box caissons — a prefabricated box floated to site and sunk onto prepared ground.
  • Pneumatic caissons — a sealed working chamber under compressed air (rarely used today).

Caissons suit deep water, river beds, and situations where a large, monolithic foundation is required.

9.5 Inspection and integrity

Because drilled shafts are formed in the ground, integrity must be verified:

  • Cross-hole sonic logging (CSL) — ultrasonic probes in tubes cast into the shaft detect defects.
  • Low-strain integrity tests (PIT) — a hammer tap and reflection analysis.
  • Core drilling — direct sampling where defects are suspected.

9.6 Key takeaways

  • Drilled shafts give high capacity in a single element, with low vibration and the ability to instrument before concreting.
  • Construction method (dry, casing, slurry, CFA) controls capacity — cleaning is critical.
  • Base cleanliness governs end bearing; base grouting can help.
  • Caissons are excavated-in-place foundations for deep water and large monoliths.
  • Verify integrity with CSL, PIT, or coring.