Drilled Piers for Foundation Repair

Drilled piers — also called caissons or bored piles — represent a deep foundation intervention used to transfer structural loads through unstable or compressible soils to competent bearing strata below. This page describes how drilled piers function as a foundation repair method, the soil and structural conditions that make them appropriate, the engineering and permitting requirements that govern their use, and the boundaries that separate them from competing deep foundation systems. The scope covers both residential and light commercial applications across the United States.

Definition and scope

A drilled pier is a cast-in-place concrete element installed by boring a cylindrical shaft into the ground using rotary drilling equipment, then filling that shaft with reinforced or plain concrete to form a structural column. In foundation repair contexts, drilled piers are installed adjacent to or beneath an existing structure to arrest settlement, re-level displaced foundation elements, or supplement a failing shallow foundation system.

The primary classification distinction in drilled pier design is between straight-shaft piers and belled piers (also called underreamed piers). Straight-shaft piers transfer load primarily through skin friction along the shaft and end bearing on the bearing stratum. Belled piers include an enlarged base — typically 2 to 3 times the shaft diameter — that increases end-bearing area, making them well-suited for stiff clay or rock where base resistance is predictable. A third variant, the helical pier, uses steel shaft sections with welded helix plates advanced by torque rather than drilling, though it occupies a distinct product category addressed separately in helical and driven pier systems.

Applicable design standards include:

ACI 336.3R — Design and Construction of Drilled Piers (American Concrete Institute), which governs materials, reinforcement, and construction tolerances
AASHTO standards for drilled shafts in transportation-adjacent or highway-adjacent applications
International Building Code (IBC), Chapter 18 (International Code Council) — which sets minimum requirements for deep foundation elements including pier dimensions, bearing capacity verification, and inspection protocols
ASTM D1143 — Standard Test Methods for Deep Foundation Elements Under Static Axial Compressive Load, applicable when load testing is required by the engineer of record

Pier diameters in residential repair applications typically range from 10 to 18 inches, with depths determined by geotechnical investigation. Commercial applications commonly use diameters of 18 to 48 inches or greater.

How it works

The installation sequence for drilled piers in a foundation repair project follows discrete phases:

Geotechnical investigation — Soil borings or cone penetration tests establish bearing strata depth, soil classification, and groundwater conditions. The engineer of record uses this data to specify pier diameter, reinforcement, and tip elevation.
Permitting — A structural permit is required in virtually all jurisdictions. Permit documents typically include a geotechnical report, a structural engineer's pier schedule, and site plan. Permit issuance triggers mandatory inspections at defined stages.
Excavation and access preparation — Working space is established adjacent to the structure. For interior slab applications, slab sections are saw-cut and removed to allow drilling access.
Shaft drilling — A truck-mounted or compact drilling rig bores the shaft to the specified tip elevation. Casing may be used in unstable soils or high-groundwater conditions to prevent shaft collapse prior to concrete placement.
Reinforcement placement — A rebar cage — sized per the structural engineer's design — is lowered into the shaft. Minimum concrete cover is governed by ACI 318 and IBC §1810.
Concrete placement — Concrete is placed by tremie method in wet conditions or direct pour in dry shafts. Minimum compressive strength is typically 3,000 psi (20.7 MPa), though engineers frequently specify 4,000 psi or greater.
Load transfer and leveling — After piers achieve design strength (typically a 28-day cure cycle or verified by cylinder testing), a bracket or cap is installed. Hydraulic jacks transfer the structure's load to the piers, and the foundation is lifted incrementally to target elevation.
Final inspection — The jurisdiction's building inspector verifies pier installation records, concrete test results, and connection details before the work is accepted and the permit closed.

The critical variable throughout is achieving the specified bearing stratum — confirmed by the drilling crew's observation of soil type, drilling resistance, and, in some specifications, a hammer-sounding or probe test at the shaft tip.

Common scenarios

Drilled piers are applied across a defined range of failure conditions and site types visible across the foundation repair listings:

Expansive clay settlement — Common in Texas, Oklahoma, and the Gulf Coast region, where shrink-swell soils cause cyclical heave and settlement of shallow foundations. Piers extend through the active zone — typically 10 to 25 feet — into stable clay or rock.
Fill settlement — Structures built on uncontrolled fill or debris-laden sites experience long-term compression. Piers bypass the fill to native bearing material.
Undermined foundations — Erosion, utility trench failure, or adjacent excavation removes lateral and vertical support from existing footings. Pier installation re-establishes bearing without demolishing the footing.
Slope instability — Structures on hillside lots with creeping soils benefit from piers drilled through the unstable zone to bedrock or dense material, which also provides lateral resistance.
Historic structure stabilization — Masonry and unreinforced concrete foundations in older buildings require pier systems that can be installed with compact equipment in confined access conditions.

Decision boundaries

Drilled piers are not universally applicable. Several conditions affect feasibility and shift the decision toward alternative systems. The foundation repair directory purpose and scope addresses how contractors with deep foundation specialization are categorized.

Drilled piers are favored when:
- Bearing strata are at depths greater than 15 feet
- Soil profiles include expansive or compressible clay with a definable stable layer below
- Large-diameter shafts are needed to carry high column or wall loads
- Belled ends can be formed in stiff, cohesive soils without shaft collapse

Alternative systems are indicated when:
- Groundwater or running sand makes open drilling impractical without prohibitive casing costs (driven piles or helical piers may be preferred)
- Access is too restricted for drilling equipment (helical piers with compact hydraulic drives are an alternative)
- Bearing strata are at shallow depths (10 feet or less), where underpinning or spread footing extension may be more cost-effective
- Rock is present at the surface or within a few feet — rock-socketed drilled shafts require specialized tooling and different cost thresholds

The boundary between repair and replacement is governed by structural assessment and, for substantially damaged structures, by the International Existing Building Code (IEBC) — specifically the 50-percent damage rule that can trigger full code compliance requirements (International Code Council, IEBC 2021). Structural engineers of record make the determination in jurisdictions that adopt the IEBC.

Permitting thresholds vary by jurisdiction. Some municipalities exempt minor underpinning from full structural permits; most require a permit for any drilled pier installation. The how to use this foundation repair resource page explains how contractors with documented deep foundation credentials are differentiated in directory listings.

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