Drilled Shafts & Caissons
Large-diameter cast-in-place concrete piles for the highest structural loads.
Drilled shafts (also called drilled piers, caissons, or bored piles) are large-diameter, cast-in-place concrete deep foundation elements constructed by excavating a cylindrical hole, inserting a steel reinforcement cage, and filling with concrete. Diameters range from 18 inches to over 12 feet, with depths routinely exceeding 100 feet. Drilled shafts can support enormous loads — individual shaft capacities of 1,000+ tons are common on major infrastructure projects. They transfer load through a combination of side friction (skin friction along the shaft length) and end bearing on competent rock or dense soil. Drilled shafts are the foundation of choice for bridges, high-rise buildings, transmission towers, and other structures requiring very high axial and lateral load capacity. Construction methods include dry hole, wet hole (with slurry), and cased hole techniques depending on groundwater and soil stability conditions.
Technical Specifications
Common Applications
Installation Process
- 1Set up crane-mounted drill rig; position over layout point with casing if required
- 2Excavate shaft using auger, bucket, or core barrel to design depth
- 3If wet method: maintain slurry level above groundwater; clean base with cleanout bucket
- 4Lower prefabricated reinforcement cage into excavation using crane
- 5Place concrete via tremie pipe from bottom up, displacing slurry
- 6Extract temporary casing (if used) while maintaining concrete head
- 7Perform integrity testing (CSL, thermal, or PIT) after concrete cures
- 8Construct pile cap or column connection at shaft head
Frequently Asked Questions
What is the difference between a drilled shaft and a caisson?
The terms are often used interchangeably. Historically, 'caisson' referred to a hand-dug excavation, while 'drilled shaft' implies machine-excavated. In modern practice, both refer to large-diameter, cast-in-place concrete deep foundations.
How are drilled shafts tested for integrity?
Common integrity tests include Cross-hole Sonic Logging (CSL), thermal integrity profiling, Pile Integrity Testing (PIT/low-strain), and full-scale static or bi-directional (Osterberg cell) load tests.
What is the maximum load a drilled shaft can support?
Individual drilled shafts can support over 5,000 tons. The world's largest drilled shafts exceed 12 feet in diameter and 300 feet in depth, supporting major bridge and high-rise structures.
How long does it take to construct a drilled shaft?
A typical 4-foot diameter, 80-foot deep shaft takes 1–3 days to complete including drilling, cage placement, and concrete pour. Larger shafts or difficult ground conditions may require longer.
What soil conditions require casing or slurry?
Casing or slurry is required when the excavation cannot stand open unsupported — typically in loose sands below the water table, soft clays, or any soil where groundwater inflow would cause hole collapse.
Can drilled shafts be installed in rock?
Yes — drilled shafts are commonly socketed into rock to develop high end-bearing and side-shear capacity. Rock sockets are excavated using core barrels or rock augers.
What is an Osterberg cell test?
An Osterberg (O-cell) test is a bi-directional static load test where a hydraulic jack embedded in the shaft separates it into upper and lower segments, testing side shear and end bearing simultaneously. It eliminates the need for massive reaction systems.
How much concrete goes into a drilled shaft?
A 4-foot diameter shaft at 80 feet deep requires approximately 28 cubic yards of concrete. Larger shafts can require 100+ cubic yards per shaft.