What Is a Caisson Foundation?
A caisson is a large-diameter drilled shaft (typically 3-12+ feet) excavated to bearing strata and filled with concrete — the heavy-duty deep foundation for bridges, towers, and industrial structures. The term covers drilled shafts, and historically, pressurized pneumatic caissons (workers excavating under air pressure — the Brooklyn Bridge method, now rare). Modern caissons are drilled with large rigs, often with slurry or casing support, and concreted by tremie.
Best price · Quotes back fast · PE stamps backed by Apex Grid Engineering
How caissons are built
A large drill rig excavates the shaft (through soil and into rock where end-bearing is needed), temporary or permanent casing supports the excavation, slurry stabilizes below the water table, the rebar cage is lowered, and concrete is placed by tremie (from the bottom up, displacing the slurry — never free-falling concrete through water). Base grouting (pressure-grouting beneath the base) enhances end-bearing capacity. Shaft diameters to 12+ feet and depths to 300+ feet are routine in bridge work.
Design
Capacity from end-bearing (on rock or dense strata) plus side friction — the geotechnical engineer provides both from borings and load tests (Osterberg cells or statnamic testing for major caissons). The structural engineer designs the shaft for the axial loads, lateral loads (caissons resist lateral by their stiffness — the analysis models soil-structure interaction), and the column-to-caisson connection. Group effects where caissons cluster.
Inspection and integrity
Cross-hole sonic logging (CSL — tubes cast in the shaft, ultrasonic testing between them) verifies concrete integrity — the standard QC for major shafts. Concrete placement records (tremie embedment maintained, no segregation), cage placement, and base cleanliness (drill cuttings left on the base destroy end-bearing — cleanout verified by inspection). Caissons concentrate enormous loads on single elements — the QC matches the stakes.
The bottom line
Caissons are large-diameter drilled shafts for heavy structures — built with casing/slurry support, tremie concrete, and CSL integrity testing. Design for end-bearing plus side friction; verify base cleanliness.
Torch Engineering PLLC · (480) 490-0064 · info@torchengineering.com — PE stamping backed by Apex Grid Engineering.