What Is a Drilled Shaft Foundation, and When Do Commercial Projects Need One?

When shallow soils can not reliably support the loads a structure will impose, engineers look deeper. Drilled shaft foundations transfer structural loads from weak near-surface soils to competent bearing material at depth, without the vibration, noise, or displacement associated with driven pile installation.

For commercial construction on variable soils, soft coastal deposits, and deep-fill conditions common across the Southeast, drilled shafts are among the most frequently specified deep foundation types.

What Is a Drilled Shaft Foundation?

A drilled shaft, also called a drilled pier, bored pile, or caisson, is a deep, cylindrical, cast-in-place concrete foundation element constructed by drilling a hole into the ground and filling it with reinforced concrete. The resulting element transfers structural loads from the column, wall, or grade beam above it down through weak or compressible soils to bearing material at greater depth.

Drilled shafts are designed to carry load through two mechanisms:

• End bearing: The base of the shaft bears against competent soil or rock at its toe, transferring load through direct compression. Shafts bearing on rock or very dense soil rely primarily on end bearing
• Skin friction: The shaft's surface area in contact with the surrounding soil develops friction resistance that contributes to load-carrying capacity along the full embedded length. In soft to medium soils, skin friction may carry the majority of the applied load

In practice, most drilled shafts develop capacity through a combination of both mechanisms, with the relative contribution of each depending on the soil profile, shaft geometry, and design load. The geotechnical engineer determines the design load capacity through analysis of soil boring data, laboratory test results, and applicable design methods referenced in AASHTO, FHWA, and IBC standards.

Drilled Shafts vs. Shallow Footings vs. Driven Piles

Foundation type selection is one of the most consequential decisions in commercial building design, and it flows directly from the subsurface conditions established by the geotechnical investigation. The table below compares the three most common commercial foundation types across the criteria that drive selection decisions.

	Shallow Footing	Drilled Shaft	Driven Pile
Load capacity	Moderate; limited by bearing area	High; skin friction and end bearing over full depth	Moderate to high; primarily end bearing at toe
Lateral load resistance	Low; relies on passive soil pressure at shallow depth	High; full-depth embedment provides strong moment resistance	Moderate; similar to drilled shaft but less common for tall structures
Diameter range	Wide footings, 2 to 10+ feet across	18 inches to 10+ feet diameter	Typically 10 to 24 inches diameter
Installation method	Excavation and concrete pour	Rotary drilling, rebar cage, concrete placement	Driven by hammer; vibration and noise
When preferred	Competent shallow bearing material; low to moderate loads	Deep to competent material; high loads; tight access; soft or variable soils	High-production projects; uniform soil conditions; suitable where vibration is acceptable
Vibration during installation	None	None	Significant; may affect adjacent structures

The decision between shallow footings, drilled shafts, and driven piles is not a matter of preference; it follows from the soil profile. When shallow bearing material is adequate, shallow footings are the most economical choice. When weak or compressible soils extend to significant depth, or when column loads exceed what shallow footings can carry within acceptable settlement tolerances, deep foundations are required. Between drilled shafts and driven piles, the choice depends on soil conditions, site access, proximity to existing structures, schedule, and local contractor availability.

How Drilled Shaft Construction Works

Drilled shaft construction is a four-step process that must be executed with precision because the structural integrity of the foundation cannot be verified after concrete placement. Construction methods vary based on the soil conditions encountered.

Step 1: Drilling the Excavation

A large-diameter rotary drill rig advances a cylindrical excavation to the design depth using a drill bucket, auger, or core barrel, depending on the soil and rock being penetrated. Three construction methods are used depending on how the hole stays open during drilling:

• Dry: Used in stable, cohesive soils that will not cave or flow into the excavation. The hole is drilled dry, inspected, and concrete is placed without casing or stabilizing fluid. Applicable in stiff clays common in Georgia's Piedmont region
• Casing: A steel casing is advanced ahead of the drill to support the borehole walls in unstable soils, loose sands, or zones above the groundwater table where caving is likely. The casing is typically extracted as concrete is placed
• Wet (slurry): A bentonite or polymer slurry fills the borehole, providing hydrostatic pressure that stabilizes the excavation walls. Used in saturated sands, soft cohesive soils, and conditions where casing alone is insufficient. Common in Florida's high-groundwater coastal environments

The choice of construction method directly affects shaft performance. If slurry is used, it must be properly mixed, conditioned, and removed before concrete placement, or it can contaminate the concrete and reduce capacity. Construction method requirements are part of the geotechnical engineer's foundation recommendations.

Step 2: Cleaning and Inspecting the Base

Before concrete is placed, the base of the excavation must be cleaned of loose cuttings and disturbed material. In end-bearing shafts, the condition of the base material determines the shaft's capacity; soft or disturbed material at the toe can dramatically reduce end bearing. A construction inspector observes the cleaning operation and verifies base conditions before the reinforcing cage is set.

Step 3: Placing the Reinforcing Cage

A prefabricated steel reinforcing cage is lowered into the cleaned excavation. The cage is designed by the structural engineer to resist bending moments, shear, and tensile forces that the drilled shaft may experience under lateral or seismic loads, or in uplift conditions. The cage must be held at the correct position and cover the borehole wall. Construction materials testing during this phase verifies rebar size, spacing, and cage configuration against the approved drawings before concrete placement begins.

Step 4: Placing Concrete

Concrete is placed using a tremie pipe, a rigid tube that extends to the bottom of the shaft and is gradually withdrawn as concrete rises from below. Tremie placement prevents concrete free-fall, which can cause segregation of aggregate and paste, and ensures that contaminated slurry or water is displaced upward rather than trapped within the concrete column. Concrete mix design for drilled shafts requires high workability (high slump or use of self-consolidating concrete) to flow around the reinforcing cage without vibration.

Load Testing

For major commercial projects, drilled shaft design capacities are verified through full-scale load testing. Static axial load tests apply compressive or tensile loads to a test shaft and measure the load-settlement response. Dynamic testing methods, including high-strain dynamic pile testing (HSDT) adapted for drilled shafts, provide a faster alternative. Load test data, refine design assumptions, confirm capacity, and provide documentation required by some building officials or owners for high-consequence foundation systems.

When Do Commercial Projects Require Drilled Shafts?

The trigger for drilled shaft foundations is almost always the subsurface investigation. The soil-boring program that characterizes the site identifies the conditions that render shallow foundations impractical and necessitate deep foundations. Common triggering conditions in the Southeast include:

Deep Weak Soils

When soft clays, loose sands, organic deposits, or variable fill extend to significant depth below the proposed foundation elevation, shallow footings would either fail in bearing capacity or settle unacceptably under design loads. Drilled shafts bypass the weak zone and bear against or develop skin friction in deeper competent material.

High Column Loads

Multi-story commercial buildings, parking structures, industrial buildings with heavy equipment loads, and bridge structures impose concentrated column loads that exceed what individual spread footings can carry within reasonable plan dimensions. Drilled shafts concentrate load-carrying capacity in a smaller footprint, which is particularly valuable in constrained urban sites.

Significant Lateral Loads

Structures subject to significant lateral loads from wind, seismic events, vehicle impact, or earth pressure require foundation elements that resist both vertical and horizontal forces. Drilled shafts' deep embedment and large cross-sections provide the moment resistance that shallow footings cannot. High-mast lighting, transmission towers, cellular towers, retaining structures, and bridge abutments commonly rely on drilled shafts specifically for lateral load resistance.

Expansive or Collapsible Soils

High-plasticity clay soils in Georgia's Piedmont and other Southeast locations swell when wet and shrink when dry, producing heave forces and differential movement that can damage shallow foundations. Drilled shafts extend below the zone of seasonal moisture change and can be designed with smooth-sided sleeves in the upper portion to isolate the shaft from expansive soil uplift. This strategy is commonly used for commercial construction in Atlanta's expansive clay soils.

Site Access Constraints

In urban infill sites, interior renovation projects, and constrained construction environments, the overhead clearance and footprint required for pile driving equipment are unavailable. Drilled shaft rigs can be configured for restricted overhead clearance and smaller footprints. For interior underpinning projects in existing buildings, specialized compact drilling equipment makes drilled shafts feasible where driven piles are not.

Vibration-Sensitive Adjacent Structures

Driven pile installation generates impact vibration and ground displacement that can damage adjacent structures, disturb sensitive equipment, and affect residents or tenants in neighboring buildings. Drilled shaft construction produces no significant vibration or lateral ground displacement during installation, making it the preferred deep foundation method in urban environments and adjacent to existing structures.

Drilled Shaft Design: What the Geotechnical Engineer Provides

Drilled shaft design begins with the subsurface data established by the geotechnical investigation. NOVA's geotechnical engineering team produces foundation recommendations that include all of the following parameters that the structural engineer and foundation contractor need to execute the design:

• Recommended shaft diameters: Based on design load, soil profile, and constructability with available equipment
• Design tip elevations: The minimum depth to which shafts must be drilled to reach competent bearing material or develop adequate skin friction, established from boring logs and lab test data
• Allowable axial capacities: The design compressive and tensile loads each shaft can carry, derived from soil strength parameters and published design methods
• Lateral load capacities: Soil resistance parameters for lateral loading, provided for structural analysis using programs such as LPILE or COM624P
• Construction method requirements: Whether dry, casing, or slurry methods are appropriate based on groundwater conditions and soil stability
• Concrete mix requirements: Minimum slump or flow, f'c, and any special mix requirements for slurry displacement or aggressive groundwater chemistry
• Special inspection requirements: IBC Chapter 17 triggers for drilled shaft installation, including observation of drilling, base cleaning, rebar cage placement, and concrete placement

Drilled Shafts in the Southeast: Regional Considerations

The Southeast's diverse geology creates a range of drilled shaft design and construction challenges that geotechnical engineers must address explicitly.

Georgia Piedmont: Rock Variability and Residual Soils

The weathered granite and gneiss profiles of metro Atlanta and surrounding counties produce highly variable subsurface conditions: partially weathered rock zones (PWR) that look competent in the field but compress under load, and transitions from soft residual soil to hard rock over short vertical distances. Drilled shaft design in Piedmont soils requires conservative criteria for shaft tip elevation verification during construction, because rock surface elevation can vary significantly from one shaft location to the next within the same foundation grid.

Coastal Plain: Soft Soils and High Groundwater

The coastal plain soils of Georgia, South Carolina, and Florida include near-surface soft clays, silts, and organics with low bearing capacity and high compressibility. Drilled shafts in these conditions rely primarily on skin friction developed in deeper competent sands or stiff clays, with slurry construction methods required to stabilize the borehole against caving in saturated, cohesionless zones.

Florida: Limestone and Karst

Florida's limestone geology creates specific design challenges for drilled shafts. Rock quality can vary dramatically over short distances due to dissolution cavities, and drilled shaft design must account for the possibility of encountering voids at the design tip elevation. Florida DOT and FDOT specifications for drilled shafts in limestone include requirements for core drilling within the shaft excavation to verify rock quality before accepting the tip elevation.

Coastal Markets: Corrosive Environments

Chloride-bearing groundwater in coastal markets aggressively attacks concrete and reinforcing steel. Drilled shafts in marine or coastal exposure environments require increased concrete cover over reinforcing steel, the use of epoxy-coated or stainless-steel reinforcement in critical zones, and concrete mix designs with reduced permeability to limit chloride penetration. These requirements are established in the geotechnical report and carried into the structural drawings and specifications.

Need Geotechnical Engineering for a Deep Foundation Project? NOVA's geotechnical engineers design drilled-shaft and deep-foundation systems for commercial projects across Georgia, Florida, North Carolina, and South Carolina. We provide the subsurface investigation, laboratory testing, and engineering recommendations that foundation contractors need to build on. Since 1996. Learn more: usanova.com/what-we-do/geotechnical-engineering