drilled pier specs

What is a Drilled Concrete Pier? Home Foundation Repair Spec Sheet

You want to know all about drilled concrete piers used in foundation repair. What are these piers made from *specifically* and what *exactly* are the measurements, ranges, and PSI ratings? You don’t want the simplified version, you want the engineering-level specs.

At Anchor Foundation Repair, we install about 1,000 drilled concrete piers each year and have been repairing Brazos Valley foundations for 35+ years. With well over 30,000 piers under our belts, we know *a thing or two* about drilled piers and can share as few or as many details as a homeowner wants, from the basics to industry-speak standards.

Anchor Foundation Repair

This article is for the numbers-minded individual that wants all the information. We will use professional terminology but we’re thinkin’ you’re gonna like that.

If you’re not like that, that’s okay too. But let me quickly point you in the direction of another article that’s a little less heavy. Check out, “What are the Pros and Cons of Drilled Pier Foundation Repair?

Full disclosure notice: This is the preferred and only method of foundation repair that we employ for a reason. So we can’t say that we’re not just a *tiny bit* biased here since this is the method we choose. But we’ll tone down our pride here and give you just the facts.

Disclosure No. 2: I am not an engineer (and you don’t need to be one either), but we draw from experience and common sense to guide our understanding when talking about and installing drilled piers.

We will explain what a drilled pier is and what it’s made of. We will offer dimensions and other details that help illustrate the qualities that make drilled bell-bottom piers an effective method to raise and stabilize a home’s foundation.

Historical Reference on Drilled Piers

drilled piers to handle difficult Texas soil
This dirt is, well . . . difficult . . .

This technique using drilled bell-bottom piers was designed originally in the early 1900s to deal with the issues of building on difficult Texas soils. It was later refined by the FHA and used in repairs for residential foundations after much of the Central Texas area switched to the shallow slab-on-grade type foundations in the 50s and 60s.

I’m a foundation repairman, not a historian so that’s all the history I got for ya right now . . . 

Drilled Bell-Bottom Pier Construction

Drilled bell-bottom piers are hand-made on site using rebar and poured concrete, and then the shape is created by digging and drilling out the ground at each pier location. 

A 2-foot by 2-foot hole is dug down to 1 foot below the grade beams of the foundation. Next, an 8-inch straight shaft is further drilled out into the ground to approximately 11 feet deep. At the bottom of the shaft, a bell shape measuring 22 to 24 inches in diameter is created by the “beller” — a spinning tool that cuts into the earth, making a wider base for the pier.

A 3-piece cage of ½ inch steel rebar is inserted into the length of the shaft for strength. Next, 3000 PSI concrete is hand poured on location into each pre-drilled hole and poured all the way up to a steel-reinforced concrete pier cap. 

The solid pier structure, which includes the bell shape at the bottom, the straight shaft, and the pier cap rests between 10 to 12 feet below grade, reaching the FHA-approved stable soil depth.

drilled pier specifications

Specs for Drilled Concrete Piers

We put the drilled concrete pier specs in the last section too, but here they are again, organized in bulleted list form.

Dimensional Specifications

  • Pier Shims: Two 8” x 8 “ x 12” concrete blocks, concrete pads, and steel shims
  • Pier Cap: 2’ x 2’ x 1’ deep, placed 1’ below the grade beam
  • Pier Shaft: 8” diameter drilled approximately 11’ deep
  • Pier Base: flared bell-bottom is approximately 22 to 24” in diameter

Material Specifications

  • 3-piece ½” steel rebar, held by ¼” steel ties inserted from pier cap, down through shaft
  • 3000 PSI concrete poured from the bottom all the way through and including the cap
  • 3000 PSI concrete blocks/pads used for preliminary shimming, then steel shims

Functionality of Drilled Piers for Home Raising

The drilled pier creates a stable platform surface on which the foundation raising and leveling can begin. The piers anchor into the stable soil found at the prescribed depth and provide resistance to uplift and can carry any additional load with the large weight-bearing area.

bell-bottom pier foundation repair

Once the solid pier is poured, the concrete is left to cure for 7 to 10 days (or more for heavier homes). The curing time is very important to allow the pier to harden and dry to its strongest possible state before raising.

Installation/Raising Process for Drilled Piers

From the flat “platform” surface created by the pier cap, two precast 3000 PSI concrete blocks are placed on top of the cap with a 20-ton bottle jack in between. The bottle jacks are raised intermittently at all pier locations to the appropriate elevation to restore functionality to the home and alleviate the problematic symptoms of foundation settlement.

Once the bottle jacks reach the optimal raising point, shims are added to fill the space. First using various thicknesses of concrete pads, then steel shims are hammered in between the precast pads and the bottom of the slab foundation. The shims hold and secure the home in place, and then the bottle jack is removed from the top of the pier cap.

raising and leveling home foundations

After the jacks are removed, hydrostatic pressure testing is performed on the home’s plumbing system to check for leaks. Any identified under-slab leaks would need to be repaired by a plumber, which can be accessed by under-slab tunneling

Then the entire repair is backfilled with topsoil and then secured by mud-pumping slurry underneath the home to fill voids and fully support the underside of the foundation.

Why Are The Piers and Process Designed This Way?

The pier structure and all its parts and processes are by design. It’s on purpose that the pier goes to a depth of 10 to 12 feet because that is the depth where the clay soil becomes more stable and less susceptible to ground-level moisture conditions. It is by design that there is a very strong, one-piece, reinforced pier with smaller replaceable parts resting on top too.

The pier has been thoughtfully and carefully designed using 1:3 ratios between the shaft and the bell measurements, and the ratio of materials between the steel and the shaft.

mud-pumping used in drilled pier foundation repair

Some people might ask why topsoil and slurry are used rather than filling it all in with additional concrete. Or you might wonder why pre-cast concrete blocks are used on top of the cap. The reason for both of these things is because if pier adjustments are needed later, this enables both easy access and easy replacement to make the repairs quickly and with significantly less intrusion.

If everything were encased in concrete, you couldn’t easily uncover and reaccess the existing piers. You would just have to bust it all up and start again from scratch. Using topsoil and mud-pumping with slurry makes it easy to dig out when needed, but still provides support and stability while in place and in use. 

Using the two concrete blocks spaced apart allows room for a jack to be reinserted and a home to be re-lifted with very little additional legwork. More shims can easily be hammered in when needed as well.

Drilled Pier vs. Pressed Pile Methodology

We have talked a little about how slab-on-grade shallow foundations became prevalently used in the 1950s and 60s. So 10 or so years into things, these slab foundations started having settlement problems and the drilled pier method of foundation repair was developed sometime in the 60s. Then in the 80s, a newer method of foundation repair came on the scene using pressed pilings.

pressed piles vs bell bottom piers

Pressed pilings were faster and cheaper to install and gained favor with many foundation repair companies for speed and higher profit margins.

In 1991, the founder of Anchor Foundation Repair asked two highly qualified Aggie engineers to evaluate the characteristics and functionality of the drilled pier vs. pressed pile methods of foundation repair. 

The conclusions of this report, prepared by M. Lewis Coody, P.E. and Ronald J. Kruhl, P. E., are why Anchor Foundation Repair continues to use only the drilled bell-bottom pier method of foundation repair today. To sum up this report really quickly: it basically says drilled piers are better for a number of reasons, from the way they are made and installed, to the way they function in the ground.

But don’t just take our word for it, here’s the original Coody Kruhl Report document for you to see and read for yourself if you like. Warning: it’s heavy with engineering language so not for everyone’s pleasure.

Anchor Offers More Than Just Drilled Piers

Anchor uses drilled piers because we know that they are the highest quality foundation repair product available to handle the expansive clay soils in our Central Texas area. Despite the fact that drilled piers are not the easiest, fastest, or cheapest to install, we continue to use them because we can stand behind this process.

who can you trust for foundation repairs?

At Anchor Foundation Repair, we have been installing drilled piers in Bryan, College Station, and surrounding Brazos Valley communities since 1985. We aren’t going anywhere and have confidence in our chosen method by offering a lifetime transferable warranty and service agreement.

Yes, we proudly use drilled piers, but there are a few other things that make us stand out as well. Check out the 4 Service Features That Set Anchor Foundation Repair Apart from other companies.