Drilled Shaft Construction: The Cast-in-Place Deep Foundation for High-Capacity Loads

Drilled shafts (also called caissons or bored piles) are large-diameter cast-in-place concrete deep foundations. Drilling rigs excavate holes ranging from 18 inches to 12+ feet diameter and depths from 20 to 200+ feet. Reinforcement cages installed; concrete placed by tremie or pump. Capacities can exceed 5,000+ tons per shaft — substantially exceeding driven piles. Bridge foundations, large buildings, and specialty applications use drilled shafts. ACI 336 provides standards.

Understanding drilled shaft construction helps contractors coordinate this specialty deep foundation. This post covers drilled shaft construction.

Drilled Shaft Construction Methods

Multiple construction methods:

Construction methods vary by conditions. Dry method when soils stable and no groundwater — simplest approach. Casing method temporarily or permanently inserts steel casing to maintain hole. Wet method (slurry) uses bentonite or polymer slurry to maintain hydraulic head and stabilize soils. Combinations common. Method selection per soil and groundwater conditions.

Drilling Equipment

Specialty drilling rigs:

Specialty drilling rigs. Crane-mounted (kelly bar drilling) traditional. Hydraulic drilling rigs increasingly common. Auger tools spiral in and remove soil. Bucket tools fill with spoils for removal. Core barrels for rock. Casing oscillators install permanent casings. Slurry circulation equipment for wet method. Major specialty equipment investment.

Reinforcement

Reinforcement cages substantial:

Reinforcement cages assembled at grade then lifted into holes by crane. Vertical bars and ties form cage. Splices required for long shafts (assembly limited by transport length). Spacers center cage in hole maintaining cover. Specific cover requirements per code. Substantial scope for large shafts.

Concrete Placement

Concrete placement specialized:

Concrete placement specialized. Tremie placement in wet method — tremie pipe extends to bottom, concrete pushes slurry up displacing it. Free fall in dry shafts limited to specific heights to prevent segregation. Continuous placement without interruption — stoppage causes weak zones. SCC sometimes used for self-leveling. Specific mix design for tremie placement (high flow, segregation-resistant).

Slurry Construction

Slurry method specialty:

Slurry construction stabilizes holes when soils unstable. Bentonite or polymer slurry. Maintains hydraulic head preventing soil collapse. Slurry properties (viscosity, density, sand content) tested per specifications. Slurry replaced during concrete placement — concrete pushes from bottom. Slurry disposal after with environmental considerations.

Quality Control

QC critical underground:

Quality control critical because shaft is underground and not visible. Drilling logs document depth and conditions. Bottom inspection where possible (dry shafts, downhole cameras). Slurry quality testing throughout. Concrete cylinders. Crosshole Sonic Logging (CSL) verifies concrete integrity through embedded tubes. Thermal Integrity Profiling (TIP) detects defects. Required testing per project specifications.

Bridge Foundation Application

Bridges use drilled shafts widely:

Bridge foundations widely use drilled shafts. Pier foundations transferring loads to bearing rock or hard soil. Abutment foundations. Long-span bridges. Marine environments over water using barges. DOT specifications detailed. Specific testing protocols. Inspection programs throughout construction.

Building Foundations

Buildings use for high loads:

Building foundations use drilled shafts for high loads. High-rise buildings. Heavy industrial. Specialty structures (water tanks, transmission towers). Underpinning of existing structures. Limited overhead applications use specialty low-headroom rigs. Design per project specific structural requirements.

Bottom Line

Drilled shaft construction provides large-capacity cast-in-place deep foundations. Construction methods include dry, casing, and wet (slurry). Specialty drilling rigs and tools. Reinforcement cages assembled and lifted. Concrete placement by tremie or other methods. Slurry construction for unstable conditions. Quality control critical — CSL or TIP testing common. Bridge foundations widely use drilled shafts. Building foundations for high loads. Specialty contractors handle. For heavy civil and large building projects, drilled shaft expertise serves substantial market. Quality construction protects long-term performance of structures depending on shafts.