what is a drilled pier?

What Is a Drilled Bell-Bottom Pier in Foundation Repair Terms?

You’re researching foundation repair and come across the term “drilled pier” or “drilled bell-bottom pier” and want to know more about it. You might be a little confused because you thought all foundation repairs were basically the same, so the idea of different types or different methods of foundation repair is new. 

You’re about to get schooled in the drilled pier construction and concept . . . in a good way.

Anchor Foundation Repair Bryan College Station

At Anchor Foundation Repair, we have used drilled piers for all our slab foundation repairs for 35+ years. Who better to tell you about this repair type and pier type than us? Methinks nobody . . . Although we are certainly fans of drilled piers since we install them exclusively, we will present this info impartially with a fair listing of pros and cons.

This article will define and describe the drilled bell-bottom pier and technique in simple terms. We will also get into a little history, construction, and pros and cons of this method of foundation repair. Let’s get to the *bottom* of this now . . . 

What Is a Drilled Bell-Bottom Pier? (Definition and History)

When a slab foundation settles (or sinks) from its original elevation, a form of underpinning is used to raise the foundation back up. Underpinning, in general, is extra foundation support added post-construction aka foundation repair. Drilled bell-bottom piers are one type of underpinning used in foundation repair.

what is drilled pier foundation repair

Get ready for some hyphen overload here because drilled piers are one-piece, hand-poured, steel-reinforced supports with a flared-out bell-shaped base. 

The bell shape anchors the pier to resist expansive clay soil uplift, settling, and lateral movement to help maintain its position in the ground in all directions. Reaching a depth of 10 to 12 feet below the ground surface lowers the footing of the foundation so that it can rest on less reactive and more supportive soil.

Piers are spaced apart and placed strategically around the home’s perimeter and under the middle when necessary. The piers create a strong and stable platform to raise the home back to its original elevation and are secured to stay there once the home is returned to its proper position.

Drilled piers are an older form of underpinning developed in the early 1900s in Texas to handle the problem of building on the expansive clay soils that dominate our state. One of our drilled pier contemporaries, Dawson Foundation Repair out of Houston, has put together this interesting history of drilled piers and we don’t need to compete so feel free to check it out.

How Are Drilled Bell-Bottom Piers Made? (Construction)

Drilled piers are constructed by digging below the perimeter beam of the foundation. Then a straight shaft is drilled out that reaches a depth of 10 to 12 feet below ground level. Next, a “beller” is inserted into the shaft that spins and essentially scrapes out the bell shape at the base of the shaft.

beller used for drilled piers

Then a cage of steel rebar is inserted into the length of the shaft to the bottom. Poured concrete is brought in to fill the entire cavity that has been created, including the bell-shaped base, the shaft, and a square pier cap on top. 

This solid concrete pier is then allowed to cure for 7 to 10 days to maximize its strength before the piers begin to bear the weight of the home. Once the concrete is cured sufficiently, jacks are placed on each pier to raise the home back into place and the position is secured by steel shims

The assembly on top of the pier cap can easily be uncovered and the home can be raised again should adjustments be needed in the future.

Here’s a more detailed article about the typical process to install drilled piers: Drilled Pier Foundation Repair: 4 Steps to Expect in a Typical Project.

What’s So Great About Drilled Piers? (Pros and Cons)

We think drilled piers are the best foundation repair method, so in our book, there are lots of pros. With fairness and transparency, we recognize that there are some cons to this underpinning method as well and can share them with honesty.

Pros of Drilled Bell-Bottom Piers

  • The straightness of a drilled pier is verifiable
  • One-piece concrete design is sturdy and long-lasting
  • Steel-reinforced with rebar for strength and durability
  • Bell-shaped base offers superior uplift resistance
  • Bell-bottom piers are also used in the construction of large commercial buildings
  • Effectiveness does not depend on the weight of home or moisture content of soil
  • Set up for future elevation adjustments if needed
drilled piers

Cons of Drilled Bell-Bottom Piers

  • Takes longer to install due to hand pour and concrete curing time
  • More expensive than other methods
  • Harder to find a contractor using this method

Even though drilled piers take longer and are more expensive, the fact that they are effective through changing soil moisture conditions is a big deal. Here in Texas, we have *nothing but changing soil moisture conditions* so it’s better if your foundation repair method works independently and regardless of whether it’s dry or rainy. 

At least that’s what we think about it. Here’s an Aggie engineer report that comes to the same conclusion. Though I will warn you it’s a tough, technical read.

What Is the Main Competitor of Drilled Piers? (Pressed Piles)

concrete pressed pilings

The main competitor of drilled piers is concrete pressed pilings, which is another form of post-construction underpinning used for residential foundation repair

Pressed piles are a newer and more widely used method due to their quick installation. Less time equals higher profitability for the contractor but may or may not benefit the homeowner with goals for long-term home stability.

Concrete pressed pilings are faster to install and less expensive than drilled piers. However, the long-term effectiveness of pressed piles should be questioned due to their dependency on moisture content in the soil and home weight creating friction to hold the home in place. Check out the quick pros and cons of pressed piles here.

Comparing Foundation Repair Methods

drilled piers

Even though pros and cons are a great way to evaluate methods, the pros and cons make even more sense for each foundation repair method when they are fully compared and contrasted with the competition. 

At Anchor Foundation Repair, we have been inspecting and repairing foundations in the Brazos Valley since 1985. We make it a point to educate and inform homeowners about the methods so that they can select the one that will best meet their needs.

For a full comparison workup of these two competing methods, check out Bell-Bottom vs Pressed Pile Foundation Repair Methods: What’s the Difference?