Quick answer
Drilled piers are cast-in-place concrete foundation elements made by drilling a deep round hole, placing reinforcing steel and concrete in it, and carrying the building load down to a soil or rock layer strong enough to support it.
Some have an enlarged bell at the base for more bearing area. In clay regions they are drilled below the layer where soil moisture changes. A geotechnical engineer and a structural engineer set the depth, size and steel for each site.
Quick facts
The key facts about Drilled Piers, with sources:
- Made by
- Drilling a hole, placing steel, placing concrete 1
- Main use in houses
- New-construction support on expansive clay, below the moisture active zone 2
- Construction methods
- Dry, casing and wet (slurry) methods 1
- Bell limits
- Only in soil or rock that stands open, dry and cuttable 1
- Price
- Not quoted here; see the cost drivers section
Key takeaways
- A drilled pier is a hole filled with reinforced concrete that carries load to a deeper layer. It is made on site, not manufactured.
- Piers can be straight or have a bell at the base that adds bearing area, but a bell can only be cut in ground that stays open 1.
- In expansive clay, piers are drilled below the moisture active zone so swelling soil does not lift them 2.
- Construction depends on ground and water: dry, cased or slurry methods each exist for a reason 1.
- Design values such as depth, diameter and steel come from a geotechnical report and a structural engineer, never from a rule of thumb.
What a drilled pier is, and what it is not
General information, not a diagnosis of your house. The Foundation Performance Association describes drilled and underreamed piers as cast-in-place concrete foundation components with an enlarged bearing area, extending down to a soil layer able to support the loads 2.
The Federal Highway Administration’s manual calls the same family drilled shafts, and says an underreamed shaft, also known as a belled shaft, has an enlarged base to gain more end bearing than a straight shaft at the same depth 1.
The names overlap: drilled pier, drilled shaft, caisson, belled pier and bored pile all appear in the sources 2. A drilled pier is different from a pier made of a steel pipe or a screw. It is poured in place.
In homes, drilled piers are mostly a new-construction foundation. Repair uses are covered in foundation underpinning, and the steel options are covered in push piers and helical piers.
Cross-section of a footing, stem wall, sill plate and floor framing sitting on soil, with the load from the wall passing down through the footing into the soil.
Illustrative drawing, not to scale and not a design. It shows the load path from framing to footing to soil. A drilled pier replaces the shallow footing with a deeper concrete shaft that reaches a stronger layer; size and depth come from an engineer.
Drilled pier or pressed concrete pier?
A drilled pier is a hole filled with concrete on site. A pressed concrete pier is a stack of precast concrete cylinders pushed down under the footing. They share the word concrete and little else; the page on concrete pier foundation repair covers the pressed kind.
How a drilled pier is built
The FHWA manual describes drilled shaft construction in general. House-scale work follows the same logic with smaller rigs.
- Locate and drill. A rig drills a round hole to the planned depth, using the dry, casing or wet method for the ground 1.
- Cut the bell, if there is one. A belling tool is attached and the soil at the base is cut out to form an enlarged base 1.
- Clean and inspect. The base of the hole must be clean because the design relies on end bearing, and dirty bases are a quality problem 1.
- Place reinforcing steel. A cage of vertical and tie steel is lowered into the hole. In expansive soil, vertical steel resists tension from swelling soil gripping the shaft 2.
- Place concrete. Concrete is placed in a way that avoids voids. The Foundation Performance Association recommends concrete workable enough to avoid honeycombing 2.
- Let it cure, then build on it. Load is not applied until the concrete has gained enough strength 2.
No method fits every house
Pier depth, diameter, bell size and steel depend on the geotechnical report, the building loads and the local code. They are not set by rule of thumb. Never dig or drill next to a house yourself.
Drilled pier types and construction methods
| Choice | What it is | When it is used |
|---|---|---|
| Straight shaft | A hole of constant diameter filled with concrete 2 | Quality control is simpler than for belled piers 2 |
| Underreamed (belled) shaft | A shaft with an enlarged base cut by a belling tool 1 | Ground that stands open and dry gives more end bearing than a straight shaft 1 |
| Dry method | Drilled with no fluid in the hole 1 | Soil and rock above the water table that will not cave 1 |
| Casing method | Steel casing keeps the hole open 1 | Caving or deforming soil, or water-bearing layers over a stable layer 1 |
| Wet method | Hole kept filled with drilling fluid, concrete placed through it 1 | Ground that cannot be dewatered 1 |
| Auger-cast pile | Grout pumped through a hollow auger as it is withdrawn 2 | A related option for sand or below the water table 2 |
Example: how ground and water change the way piers are made
The example below pairs site conditions with the construction approach the sources describe. It shows why a geotechnical report comes first. It does not recommend a method for any site.
| Site condition | Approach described | Source |
|---|---|---|
| Stiff clay above the water table that stays open | Dry method; stiff clay can often be drilled this way | 1 |
| Sand layers that cave into the hole | Retrievable casing can allow drilling through sandy layers | 2 |
| High groundwater | Slurry displacement method can be used; concrete integrity is harder to confirm | 2 |
| Very stiff clay with fissures or slickensides | Bells can slough, so belling may not be reliable | 1 |
| Expansive clay near the surface | Shaft extends below the moisture active zone; vertical steel resists swell-induced tension | 2 |
- InvestigateBorings and lab tests
- DesignDepth, size, steel
- DrillDry, cased or wet
- Reinforce and pourCage then concrete
- Cure and buildLoad after strength
A general order from engineering references, not a schedule for any house. Source: [1] [2] [4]
Each row is a reminder that the ground decides the method. See geotechnical soil investigation for what borings reveal.
Where drilled piers are used
The Foundation Performance Association lists drilled and underreamed piers among deep support systems commonly used in new construction on expansive soils, and calls them the most commonly used deep foundation component in the Houston area 2. Deep support systems hold the building on soil below the layer that swells and shrinks 2.
Bridges and other large structures use the same technology at larger scale 1. In houses, piers are tied to a slab or grade beam that spans between them 2.
Using a drilled pier to repair an existing house raises extra questions, such as drill rig access and how the new pier connects to the old foundation.
The Foundation Performance Association’s guideline on pressed concrete piles even addresses what to do when an existing drilled pier is tied to the foundation 5. An engineer answers those questions.
What drilled piers do well and where they struggle
Strengths
- A long track record in expansive clay regions, in use for about a century 2.
- Straight shafts give better lateral resistance than slimmer deep foundations 2.
- A bell gives large end bearing economically where ground allows it 2.
- No driving vibration or noise, which the paper lists as a problem for displacement piles 2.
- Piers can be drilled below the moisture active zone to anchor against swell 2.
What a drilled pier cannot do
A drilled pier supports weight. It does not control soil moisture, and it does not keep soil from moving around it. In expansive ground, swelling soil can grip the shaft and pull upward.
The FHWA manual explains that swelling generates stress increases at the soil and shaft interface, and the resulting shear acts upward on the shaft 1.
Designers manage this by isolating the shaft with casing, by reducing skin friction with coatings or liners, or by designing the shaft to resist uplift tension 1. These are design choices and not guarantees.
Piers often come with a structural slab separated from the surface soil by a void space, so swelling soil cannot push on the slab 2. That is a design feature that must be built correctly. A pier does not make a foundation immune to movement.
What drives the price of drilled piers
We do not give prices. A fair figure requires dated local quotes for a designed scope. The drivers are the pier count, depth, diameter and whether each has a bell, plus reinforcing steel and concrete volume.
Ground adds cost. Casing, slurry and groundwater control all add steps. Hauling away excavated soil is a line item 2.
Deeper borings may be required, because the Foundation Performance Association notes straight-shaft piers need deeper soil borings than underreamed piers 2. Engineering, inspection, concrete testing and permits add more.
The Foundation Performance Association lists foundation options in order of increasing risk and decreasing construction cost, and puts deep support first, which tells you the cost is higher and the risk lower than for shallower systems 2.
See the cost guide and cost calculator for how quotes are built.
What a homeowner should expect in a drilled pier project
- Geotechnical report. The Foundation Performance Association’s practice recommends borings, field tests and lab tests, including special items when drilled piers are expected 4.
- Structural design. A structural engineer designs depth, size, steel and the connection to the slab or grade beam.
- Inspection during drilling. Someone independent of the driller should check depth, base cleanliness and steel.
- Concrete placement and curing. Concrete must be placed to avoid voids 2.
- Records. Keep the pier log, concrete tickets and test results.
- Permits and inspections. Follow the code your jurisdiction adopted.
Caring for a pier foundation
Drilled piers need little upkeep, but the soil beside the house still changes with water.
- Keep water away from the foundation. See drainage and grading.
- Watch large trees and irrigation near the house in expansive clay; the tree distance check helps.
- Look for cracks and sticking doors and photograph them with dates.
- Keep the geotechnical report, pier log and as-built drawings with the house records.
- Call an engineer if floors or walls move noticeably.
Risks in drilled pier work
- Sloughing soil at the shaft or the bell during drilling 2.
- Voids or poor concrete when pouring under water, slurry or in thin steel cages 2.
- Uplift from swelling soil that grips the shaft 1.
- Wrong depth, because contractors believe the base should sit in a color or stiffness of clay instead of the depth shown on the engineer’s drawings 2.
- Drilling or digging near buried lines and pipes.
When to call a licensed structural engineer
Call one when floors or walls move noticeably, cracks widen, doors and windows stick together with new cracks, or a pier-supported house shows signs of lifting or sinking. Never dig or drill beside a foundation yourself.
Codes, design guides and who signs the design
Deep foundations fall under the soils and foundations chapter of the building code your jurisdiction adopted, and the code edition and any amendments control. The model International Residential Code lets the building official require a soil test where expansive, compressible or shifting soils are likely 6. Check your local rules.
Engineers design drilled piers in expansive soil using published procedures. The Foundation Performance Association has published a design procedure for drilled concrete piers in expansive soil, based on experience in southeast Texas 3. It is a guide for engineers, not a home recipe.
Who may stamp the design depends on your state. See foundation design and building codes and verify credentials with the issuing board.
Drilled piers next to other deep support
| Method | Made by | Typical setting | Main limit |
|---|---|---|---|
| Drilled pier | Drill, steel, concrete on site 1 | New houses on expansive clay 2 | Caving soil, water, haul-off, cure time 2 |
| Helical pier | Manufactured shaft rotated in 7 | Repairs and tight sites 8 | Rock and shallow bedrock 7 |
| Push pier | Steel pipe pressed in against the structure 9 | Repairs 8 | Needs firm layer within reach 8 |
| Pressed concrete pier | Precast cylinders pressed in 5 | Repairs in clay 5 | Limited uplift and bending resistance 5 |
Drilled pier myths
- Myth A deeper pier is always safer.
- Reality Depth must suit the soil and the design; a pier in the wrong layer may not help 2.
- Myth A bell is always better than a straight shaft.
- Reality Bells can only be cut in ground that stands open, and quality control is harder 12.
- Myth Drilled piers stop all foundation movement.
- Reality Soil near the piers can still change; piers resist movement of the supported parts only.
- Myth The bearing layer is a color of clay.
- Reality Contractors who judge by color or stiffness can misplace the base; depth comes from the engineer’s drawings 2.
- Myth Every foundation repair uses drilled piers.
- Reality Drilled piers are mainly a new-construction method 2. Repair methods include pressed concrete, push and helical piers, and an engineer chooses among them.
Regional soil: why drilled piers are common in some states
The Federal Highway Administration notes that expansive soils are common in Texas and along the Gulf Coast, in the Appalachian states, the Southwest and the Great Plains, with about twenty states where expansive clays are a problem to some degree 1.
The Foundation Performance Association’s design procedure for drilled piers is based on experience in southeast Texas, so its numbers should not be carried to other regions 3. High water tables change how piers are built; frost depth matters in cold regions.
When drilled piers belong on the list
Drilled piers belong on the list for new construction on expansive clay, or for heavier loads with a stable layer at depth. For repairs, they come up less often because access and connection are harder.
Use the guide below to prepare questions. See repair methods for the repair options and foundation repair for the service overview.
- If Expansive clay is mapped near the site
- Then Ask the geotechnical engineer for the moisture active zone depth
- If Groundwater is high
- Then Ask how the shaft will be made and how concrete will be checked
- If A bell is proposed
- Then Ask why the ground will hold the bell open
- If An existing house needs repair
- Then Ask the engineer which repair methods fit, and why a drilled pier would or would not
Geotechnical first
Ask for the geotechnical report before accepting a pier design. A design without borings is a guess. Our engineer or contractor tool shows whom to ask.
Questions about Drilled Piers
What is a drilled pier?
A drilled pier is a hole drilled deep into the ground, filled with reinforcing steel and concrete, and used to carry a building’s load to stronger soil or rock. It is also called a drilled shaft, belled pier or caisson. The pier is made on site and carries load by end bearing and side resistance.
What is a belled pier?
A belled pier, or underreamed pier, has an enlarged base cut by a belling tool. The bell adds bearing area for more end bearing than a straight shaft at the same depth. A bell can only be cut in soil or rock that stays open and dry, which limits where it can be used.
Why are drilled piers used in clay soil?
In expansive clay, soil swells when wet and shrinks when dry. Drilled piers are made deep enough to sit below the layer where moisture changes, so the building rests on soil that does not move much. Vertical steel in the pier resists tension from swelling soil gripping the shaft.
How deep are drilled piers?
It depends on the site. Depth comes from a geotechnical report and a structural design, based on the moisture active zone, bearing layer and loads. We do not give a typical depth because any number could mislead. Depth on the engineer’s drawings is the one that counts.
Can drilled piers be used to repair an existing foundation?
Sometimes, but they are mostly a new-construction method. Under an existing house, drill rig access and the connection to the old foundation raise extra questions that an engineer must answer. Other methods, such as pressed concrete, push and helical piers, exist for repairs, and an engineer chooses among them.
What is the difference between drilled piers and push piers?
A drilled pier is a concrete shaft made in a hole on site. A push pier is a steel pipe pressed down under a footing by a hydraulic ram. Drilled piers are mostly built before the house; push piers are used to support an existing house. They carry load in different ways.
Do drilled piers stop foundation movement?
They limit movement of the parts they support, but no source promises that a foundation will never move. Soil beside the piers can still change, and swelling soil can try to lift the shaft. Designers use casing, coatings or steel to manage that.
Does a drilled pier foundation need a geotechnical report?
Yes in practice. A design for drilled piers needs soil data on depth, strength, water and swell. The model residential code allows a building official to require soil tests where expansive or compressible soils are likely. A written geotechnical report is how that data is delivered.
How much do drilled piers cost?
We do not publish prices. The drivers are the pier count, depth, diameter, whether there is a bell, steel, concrete, ground conditions, haul-off, engineering and inspection. Our cost guide and calculator show how quotes are built so you can compare scopes line by line.
Sources
- Federal Highway Administration, Drilled Shafts: Construction Procedures and LRFD Design Methods, GEC 10, FHWA-NHI-10-016 (May 2010), retrieved .
- Foundation Performance Association, FPA-SC-01-0: Foundation Design Options for Residential and Other Low-Rise Buildings on Expansive Soils (2004), sections 4.1 and 5.1, retrieved .
- Foundation Performance Association, FPA-SC-16-0: Design Procedure for Drilled Concrete Piers in Expansive Soil (2017), retrieved .
- Foundation Performance Association, FPA-SC-04-0: Recommended Practice for Geotechnical Explorations and Reports (2001; written for southeast Texas), retrieved .
- Foundation Performance Association, FPA-SC-08-1: Design, Manufacture, and Installation Guidelines of Precast Concrete Segmented Piles for Foundation Underpinning (2014), retrieved .
- International Code Council, 2018 International Residential Code, Chapter 4 Foundations, section R401.4 Soil Tests, as reproduced by UpCodes (model text; adopted editions and amendments vary), retrieved .
- Iowa State University Bridge Engineering Center and InTrans, Helical Pile Foundation Guide for Bridge Structures (October 2023), retrieved .
- Hubbell Power Systems (CHANCE), Technical Design Manual, Edition 3 (2014). Manufacturer document, retrieved .
- Hubbell Power Systems, Foundation Solutions blog: Push Piers, Frictional or End Bearing? Manufacturer document, retrieved .
Expert review
Written by the HyreFoundation Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Definitions, construction methods, belling limits and expansive soil behavior come from the Federal Highway Administration’s drilled shaft manual (written for transportation structures; used for general mechanics).
Residential advantages, disadvantages and uses come from a Foundation Performance Association peer-reviewed paper written for expansive soils, mainly southeast Texas.
Code text comes from the 2018 model IRC as reproduced by UpCodes because the ICC site would not load. Comparison cells cite the sources for each other method. No depths, diameters, loads or prices are given. Rules checked 2026-10-14. General information, not engineering advice.
Suggest a correction. We fix errors and say what changed.