A deep foundation element comprising a large‑diameter bored shaft (drilled shaft) constructed by boring into the ground, placing reinforcement, and filling with concrete to transfer structural loads to competent strata by end bearing and/or shaft (skin) friction; used where shallow foundations cannot meet load or settlement requirements.

Principle

A caisson carries axial loads by a combination of base bearing (if bearing on competent strata) and shaft friction developed along the bored shaft circumference; its capacity and settlement performance are functions of shaft length/diameter, soil shear strength profile, and construction quality.

Demonstration

Illustrative scenario: Situation — A heavy column load must be supported on a site with thick compressible deposits underlain by a dense sand layer at depth. Recognition — Spread footing would cause excessive settlement. Action — Drill a caisson to the dense layer, place reinforcement cage and tremie‑placed concrete to form a continuous shaft. Consequence — Column load is transferred principally to the competent sand by end bearing and skin friction; settlements are reduced to acceptable levels compared with a shallow foundation.

Misapplication

Assuming a caisson is identical to a driven pile in behavior or ignoring negative skin friction (downdrag) from settling surrounding soils. The semantic error is to treat installation method as indicating identical load transfer mechanisms; drilled shafts typically rely more on in situ shaft bond and may behave differently for lateral and settlement response than driven piles.

Consequence

Properly designed caissons provide high vertical and lateral capacity with controlled settlement, making them suitable for heavy loads and unfavorable near‑surface soils. They require specialized drilling, casing or slurry control, and stringent quality assurance; poor construction or improper design assumptions can reduce capacity, cause construction collapse or excessive deformation.

Reversal

If competent bearing strata are shallow, spread footings are more economical. In very cobbly or bouldery ground, or where drilling equipment is impractical, alternative foundations (driven piles, socketed rock anchors, or ground improvement) may be preferred. In seismic conditions, shaft‑to‑rock connections and liquefaction effects must be carefully considered and may alter the choice.

Boundary

Clearly within — A bored, cast‑in‑place shaft of large diameter filled with concrete and reinforcement designed to transmit loads by end bearing and shaft friction. Boundary case — Continuous flight auger (CFA) piles or bored displacement piles, which are constructed differently but can produce similar load capacities; differences lie in installation method and quality control. Clearly outside — Driven piles (rammed or vibrated) and shallow spread footings that rely primarily on near‑surface bearing.

Semantic Tension

Capacity vs. constructability/safety: achieving required axial and lateral capacity often demands deep borings, casing/slurry control and strict QA, increasing cost and risk; drilled solution vs. alternative methods: trade between predicted performance and practical site constraints (boulders, groundwater, vibration limits).

Synthesis

A caisson is a bored deep foundation that combines end bearing and shaft friction to meet capacity and settlement criteria when shallow foundations fail; its success depends equally on correct geotechnical evaluation and disciplined execution rather than on nominal diameter alone.