What Is a Bored Pile (Drilled Shaft) & When Is It Used in the USA?

A bored pile — called a drilled shaft or caisson in standard US practice — is a deep foundation element formed by mechanically drilling a cylindrical excavation into soil or rock, inserting a steel reinforcement cage, and filling the hole with cast-in-place concrete. Unlike driven piles, bored piles are formed in-place without driving impact forces, making them the preferred deep foundation solution when vibration and noise restrictions apply, when large-diameter high-capacity piles are needed, or when soil and rock conditions prevent driven pile installation. The primary US design references are ACI 318-19, the FHWA Drilled Shafts Construction Procedures and Design Methods Manual (NHI-10-016), and AASHTO LRFD Bridge Design Specifications 9th Edition.

Bored piles transfer structural loads to competent bearing strata through a combination of skin friction (shaft resistance along the pile surface) and end bearing (resistance at the pile toe or bell). In cohesive soils like Chicago Blue Clay, skin friction often dominates — while in gravels, sands, and rock, end bearing is the primary load transfer mechanism. Pile diameter in US practice ranges from 18 inches for light residential caissons up to 10–12 feet for major bridge foundations, with shaft lengths commonly ranging from 15 to 150 feet depending on bearing stratum depth.

How to Calculate Bored Pile Concrete Volume — USA Formula

Calculating the concrete volume for bored piles requires computing the cylindrical shaft volume, adding the bell volume for underreamed piles, applying the overpour factor, and converting to cubic yards for ordering. The rebar cage steel weight is then estimated from the longitudinal rebar ratio and pile dimensions.

Bored Pile Concrete Volume Reference Table — USA Standard Sizes

The table below shows pre-calculated concrete volumes for common US drilled shaft sizes using a 20% overpour factor, for both straight-shaft and belled piles per ACI 318-19 and FHWA design practice.

Bored Pile Types Used in USA Construction

The USA uses several bored pile configurations depending on soil conditions, load requirements, and project type. Here are the most common types encountered in US building, bridge, and infrastructure projects.

Bored Pile Concrete Best Practices — USA Projects

Proper concrete placement and quality control in bored piles is critical — defects are difficult to detect and extremely expensive to remediate. Here are the most important best practices from FHWA Drilled Shafts Manual and ACI 318 commentary for US contractors and engineers.

• Use Self-Consolidating Concrete (SCC) when available — SCC mixes (7–9 inch slump flow) fill around congested rebar cages more effectively than conventional concrete in large-diameter or heavily reinforced drilled shafts. ACI 237R provides guidance on SCC mix design for US drilled shaft applications.
• Clean the bottom before placing concrete — sediment accumulation at the bottom of the borehole reduces end bearing capacity dramatically. FHWA requires bottom cleanliness inspection with a cleanout bucket or airlift prior to rebar cage insertion — maximum 1/2 inch of sediment in the bottom of an end-bearing pile per standard US specifications.
• Specify non-shrink concrete for belled piles — the bell zone is particularly susceptible to cracking from concrete shrinkage and thermal gradients. Many US specifications require expansive cement (Type K per ASTM C845) or non-shrink admixtures for the bell pour in heavily loaded belled caissons.
• Plan Crosshole Sonic Logging (CSL) for critical piles — FHWA and AASHTO require CSL testing for access tubes installed in large-diameter bridge drilled shafts in the USA. CSL detects concrete voids, inclusions, and construction defects that cannot be detected from the surface. Specify CSL access tubes (2-inch diameter steel) during rebar cage fabrication — they cannot be added after the cage is placed.
• Verify bearing stratum before each pile — soil conditions vary from boring to boring and can change significantly across a small site. Confirm the target bearing stratum by visual inspection of drill cuttings, rock core, or field SPT/CPT data at each pile location before finalizing pile length. Premature termination of piles is one of the most common and costly defects on US drilled shaft projects.
• Document concrete mix design approval — ACI 318-19 and IBC 2021 require the concrete mix design to be submitted and approved by the Engineer of Record (EOR) before placement begins. For piles in aggressive soil environments (pH < 5.5, sulfates > 150 ppm, or chlorides), use Type V or Type II/V cement and ensure w/cm ≤ 0.40 per ACI 318-19 Table 19.3.3.