How do drilled bell-bottom piers work on expansive clay?
In short: A drilled bell-bottom pier is a steel-reinforced concrete column cast in an augered hole, with the bottom widened into a bell. It is designed to bear below the clay that swells and shrinks with the seasons, and the bell anchors it to minimize uplift from swelling clay. Its performance depends on reaching the right depth, which comes from site-specific soil data.
How does the system work?
Trade words used on this page:
- Auger: a large drill that bores a round shaft into the ground.
- Under-reaming (belling): widening the bottom of the shaft with a special tool to form the bell.
- Rebar cage: a bundle of steel reinforcing bars lowered into the shaft before concrete is placed.
- Pier cap: the top of the pier, on which a beam or footing rests.
- Active zone: the depth of soil whose moisture, and therefore volume, changes with the seasons.
The sequence is to drill the shaft to design depth, form the bell, set the reinforcement, and fill the hole with concrete. The building or a new beam is then supported on the pier caps.
Why do depth and the bell matter?
Engineering guidance for swelling soils calls for piers to extend well below the zone of seasonal moisture change and to be anchored in non-swelling soil against uplift, using belled bottoms, grooved pier holes or greater depth 1. Researchers report that seasonal moisture change can extend from 3 to 40 ft deep, and that designers should assume the upper part of a pier will lose contact with the soil 2.
Uplift is the upward drag that swelling clay puts on the sides of a pier. Corner piers are often lifted during the wet season and can lose side friction and drop when the soil shrinks in the dry season 2. Six-foot piers that handle normal annual cycles may fail after a multi-year drought followed by a very wet year 2; see OK Data - Active Zone and Drought Depth for Oklahoma drought depths.
Where are drilled piers used?
- New construction on swelling soil: drilled piers with a grade beam are generally recommended for moderately to highly swelling soils, with a 3 to 6 in gap between the soil and the beam 1. See stem wall and grade beam.
- History: drilled piers have been used on expansive soils in California, Colorado and Texas since the late 1950s 2.
- Oklahoma research: an Oklahoma Department of Transportation study built eight drilled shafts at two load-test sites to develop design methods for shafts socketed in weak rock, publishing recommended tip and side resistance relations in 2024 4.
- Code basis: the residential code sends foundations on expansive soil to the building code's expansive-soil design section, and soil survey data from USDA NRCS is the minimum resource for locating expansive soils 5.
What can go wrong with drilled piers?
| Problem | Effect | Source |
|---|---|---|
| Improper pier depth or diameter | Listed as a common design problem | 3 |
| Mushroom-topped pier | Listed as a common construction problem | 3 |
| No or inadequate void under the grade beam | Clay pushes up on the beam directly | 13 |
| Long drought followed by a very wet year | Six-foot piers that withstand normal annual cycles may not be adequate | 2 |
When piers move after construction or repair, the mechanism is described in pier uplift or movement after repair.
What practical considerations apply?
- Design input: depth and bell size come from a site soil investigation, and swelling-soil guidance calls for the foundation to be designed by an engineer who specializes in soil and foundation engineering 1; depth to rock from borings summarizes Oklahoma boring data.
- Engineering: drilled piers are an engineered system; how homeowners work with an engineer is covered in working with engineers.
How drilled piers compare with pressed concrete, steel push and helical piers is laid out in the method selection matrix.
Related
- Stem Wall and Grade Beam
- Method Selection Matrix
- Pier Uplift or Movement After Repair
- Shrink-Swell and Active Zone
- Working with Engineers
Common questions
What is a drilled bell-bottom pier?
It is a reinforced concrete pier cast in an augered hole, with the bottom of the hole widened into a bell shape. The pier carries load down below the clay that swells and shrinks, and the bell anchors it against being pulled up.
Why does a pier need a bell in expansive clay?
Swelling clay grips the upper part of a pier and can lift it. Engineering guidance calls for piers on swelling soil to be anchored in non-swelling soil by belled bottoms, grooved holes or greater depth.
How deep do drilled piers need to go?
Well below the zone of seasonal moisture change, which researchers report can extend from 3 to 40 feet deep. The design depth comes from a geotechnical investigation for the site.
Have drilled piers been studied in Oklahoma?
Yes. An Oklahoma Department of Transportation study published in 2024 built and load-tested eight drilled shafts socketed in weak rock at two Oklahoma sites to improve design methods.
Sources
- Home Construction on Shrinking and Swelling Soils (Special Publication 11) Colorado Geological Survey (Holtz and Hart, 1978)
- Damage to Foundations from Expansive Soils Missouri University of Science and Technology (J. David Rogers, Robert Olshansky, Robert B. Rogers)
- Expansive Soil Colorado Geological Survey
- Probabilistic Approach for Design of Drilled Shafts Socketed in Weak Rock in Oklahoma (FHWA-OK-23-02) Oklahoma Department of Transportation and Federal Highway Administration (University of Missouri)
- Foundations on Expansive Soils – Residential (IHB TB 10-01) Texas Department of Licensing and Regulation
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