what is the main function of a bell on the bottom of a pier?

What Is the Main Function of a Bell at the Bottom of Bell-Bottom Piers?

You’re researching home foundation repair methods and have this very specific question: What is the main function or purpose of putting a bell shape at the bottom of a drilled bell-bottom pier?

First off, bravo. This is an amazing question that you should be congratulated for asking. Not many homeowners will dive this deep into understanding how drilled piers work. Great job, you’re one in a million.

Anchor Foundation Repair Bryan College Station

At Anchor Foundation Repair, we understand the stress and anxiety that come with overwhelming repair projects as well as the need to comprehend the techniques used to repair your home. After 35+ years in business, we want you to know as much about foundation repair as we do and we’ve got no secrets to hide about our methods.

In this article, we review the key features of a drilled bell-bottom pier used in home foundation repairs. Then we detail the purpose and essential function of the bell-shaped base and explain how it works to benefit the stability of your slab foundation.

What Is a Drilled Bell-Bottom Pier?

I realize you might already know this part, but we’ll cover it quickly to help anyone who hasn’t gotten these details elsewhere. 

For foundation repair on a home, the repair essentially involves digging underneath the house and lifting it up from below all while avoiding damage to the rest of the structure. This is no small feat. You need a durable and resilient base from which to raise the home.

pressed piles vs bell bottom piers

A drilled bell-bottom pier is a steel-reinforced concrete structure created underground to raise, support, and stabilize a home foundation. Bell-bottom piers are monolithic, meaning that it’s one continuous piece of concrete rather than a bunch of sections pieced together like some other foundation repair methods.

Typically, these piers are added well after a home is built to help *repair* a slab foundation that has settled or sunk. The pier allows the foundation repair contractor to return your home to its original elevation by lifting from this new stronger and more stable base. 

Drilled bell-bottom piers are similar to the footings of larger commercial structures. Only with commercial buildings, footings are part of the original construction instead of added as a remedy for foundation settlement later on.

Bell-Bottom Pier Specs and Construction

Drilled piers are created onsite by digging and drilling out the needed shape of the pier and then pouring concrete into the empty space in the ground. It’s kinda like making a candle mold and then pouring the wax into the empty container to create the candle.

The key dimensions of our bell-bottom piers include:

  • An 8-inch diameter shaft,
  • Drilled to a depth of 10 to 12 feet from the top of the slab,
  • The bell shape is 22 to 24 inches in diameter at the base of the shaft, and
  • Also includes a 2-foot by 2-foot pier cap on top.

drilled pier specs

Other companies might do things a little differently, but there’s a diagram around here somewhere to show you how we do it. We will first dig a box-shaped hole to reach a couple of feet underneath the perimeter beam of your home, which could be 2 or 3 feet below the surface. So the starting point for the pier cap is already more than a few feet below ground. 

The rest of the bell-bottom pier is created by drilling the straight shaft down to the desired depth. Then, a “beller” is used to cut out the bell shape at the bottom of the shaft with a spinning attachment.

To be clear, the shaft itself is not 10 to 12 feet in length. The goal is for the base of the pier (aka the bell) to rest 10 to 12 feet below the slab. The actual measured length of the pier shaft is not what’s important, it’s reaching the depth of 10 to 12 feet from the top of the slab that’s important. 

This *magic depth* is where the expansive clay soil is less reactive and the moisture content is more consistent throughout the year. The base of the pier resting in less reactive soil makes for a more stable foundation environment.

What is the Main Function of the Bell-Shaped Base of the Pier?

give me the real truth about bell bottom piers

The bell-shaped base of a bell-bottom pier serves several important functions. All of these functions ultimately work together for the overarching purpose of creating a strong, stable base from which to raise your home back to its original position.

In general, the wider diameter at the bottom of the bell compared to the shaft creates a 3 to 1 dimensional ratio for maximum strength and stability, and:

  • Maximizes the pier’s footprint for increased stability, and
  • Minimizes the potential for settlement and sinking in dry conditions.

The graduated cone shape of the bell also plays an important role by:

  • Locking in the pier’s position using the soil’s weight and compression,
  • Resisting uplift in wet or saturated conditions due to the locked position, and
  • Creating structural strength and durability to withstand expansive clay forces.

You might be thinking, “Well, that all sounds super, but I don’t get it. What does any of that really mean for my foundation repairs? Like, HOW. DOES. IT. WORK?” Or maybe a better question is, “Why? Why does the bell shape make things more stable, strong, durable, resistant, etc?”

Why or How Does the Bell-Shaped Pier Work?

let's talk about foundation problems

The dad in me wants to say that the bell-shaped pier works because it just does. It’s physics. But instead of leaving it at, “Because I said so or it’s because of science.” We’re going to break each one of these functions down into comparative examples that the everyday homeowner can understand.

Maximizing the Footprint for Increased Stability

Since we’re talking about a footprint, let’s go with an example involving your feet. Think about yourself standing on the ground, straight and tall with your feet together vs. with your feet spread wider than shoulder-width apart. Now imagine someone trying to push you over with your feet together and then with your feet spread apart. 

Q: Which way is it going to be harder for you to get shoved around?

A: When you have a wider base with your feet spread apart, you’re more stable and harder to push over. 

wide bell bottom shape of pier similar to science flask

Another good example is comparing a glass beaker (technically the kind I am thinking of is called an Erlenmeyer Flask) to a glass cylinder. Think back to your science class days . . . The flask has a wide bell-shaped base and the cylinder is the same diameter from top to bottom. Q: Which one is going to fall over easier with just one tap? A: The cylinder. It’s like a pyramid vs. the *Leaning* Tower of Piza.

The same concept applies to bell-bottom piers. Forces from the soil coming at it sideways will be lessened with the wider more stable base. The wider base helps the pier to resist lateral movement, keeping it straight and in place.

Minimizing Settlement In Dry Conditions

So how does the bell shape minimize settlement? Well again, this is about the wider base compared to something less wide. Let’s imagine that the dirt is really dry. Q: Will it be harder to stab a narrow metal rod with a half-inch diameter into the ground or a solid pole with a 4-inch diameter?

A: The pole with the wider diameter will be harder to force into the ground.

Something small without much surface area will go down into the dirt more easily, and something larger with a wider footprint will be more difficult. The more surface area there is on the base, the more resistant it is to sinking. 

concrete piling vs. bell-bottom pier
Concrete pressed pilings are 6″ in diameter, a bell-bottom pier is 22-24″ at its base . . .

The bell bottom pier has a diameter of 22 to 24 inches, while other foundation repair methods use concrete pilings that have a 6-inch diameter. If the weight of your home is resting on something that is either 24 inches wide or 6 inches wide at its base, which one do you think would sink/settle less easily into the ground?

Locking In the Pier’s Position Due to Soil Compression

The bell shape has several key features working for it. The weight of the soil above pushes down on the bell around the angled part of the base. The soil is sort of a passive locking mechanism that helps to keep the pier in place. The pier cannot easily move up or down due to the angle of the bell getting squeezed by the downward force. 

The dirt is kind of like a nut on a screw that smushes everything together and keeps it there.

why bell bottom piers have a bell

To illustrate this concept another way, it’s comparable to dovetailing techniques used in furniture making and woodworking. Furniture makers use dovetail joints to keep the wood from easily pulling apart. Imagine trying to pull apart a wood joint made with straight tails vs. dovetails. The angled dovetail is naturally resistant to separation simply due to its shape.

Dovetailing is a more complicated method than using simple straight joinery. The process for straight joints is faster, cheaper, and easier to do. Whereas, dovetail joints take more time and patience to employ. This is why dovetail joints are commonly used on higher-quality furniture built to last a lifetime and straight joints are more often found on cheaper furniture.

Comparing pressed pilings to bell-bottom piers is very similar. Pressed pilings are the same diameter all the way through, they have no flared base and are much easier and faster to install. Pressed pilings have nothing at the bottom to help lock them into the soil. 

By contrast, bell-bottom piers take more time and skill and therefore cost more. The time taken to use the drilled bell-bottom pier method ultimately makes for a pier that offers more stability due to this locked-in *dovetailed* concept in comparison to other foundation repair methods.

Resisting Uplift In Saturated Conditions

rain and concrete

In an expansive clay soil environment, rainfall and moisture levels are important to consider. When the soil is dry, everything shrinks and sinks causing settlement. When the soil is wet for prolonged periods, you get heaving and the soil pushes everything up.

Even though drilled piers are placed at a depth to reach non-reactive soil, something called “wicking” can still occur when it rains. Water penetrates farther into the soil anywhere that a pier or piling penetrates the ground. The object sort of creates a pathway for water to trickle down into the earth. 

Wicking happens with all foundation repair methods and there’s no good way to prevent it. The only thing you can do is create a support system that might:

  • Move the water farther away from the base, and 
  • Better resist any uplift issues that might result from wicking.

That wide base coupled with the angle of the bell continues to prevent the pier from being pushed up from below from the heaving of expansive clay soil that could still occur. 

Creating Structural Strength and Durability

creating a bell-bottom pier

The cone-shaped bell at the base of the pier has yet another key feature that ups its superiority. A bell is hard to break, its shape makes it inherently stronger.

We’ve been talking a lot about a wide base. But if it were just about a wide base, then why don’t they just make a base that’s more like an upside-down T shape? Like why not use a vertical pier with a horizontally laid piece at the bottom? A flat square, circular, or rectangular base that is much wider than the shaft with lots of surface area should give you the wide base you need, right?

Not exactly. While a wide flat base would give you lots of surface area for resistance, it doesn’t give you lots of strength or durability.

Here’s a simple example for you. Let’s compare a *kiss-shaped chocolate* to a flat piece of chocolate. Let’s imagine that the base of your pier is either a kiss shape (very similar to a bell-bottom pier) or a flat bar. Now let’s imagine that some sort of force pushes up or tries to push down on the edges of the chocolate pieces. 

Q: Which chocolate piece is harder to break?

A: The edges of the kiss shape are nearly impossible to break off with your fingers, whereas the flat chocolate simply snaps apart without much difficulty.

bell bottom base of a pier is strong

Even though the flat piece of chocolate has more surface area and makes a “wider base”, the bell-shaped chocolate is stronger. The bell won’t easily buckle under pressure exerted on it from above or below. The case is similar with concrete. Even though all concrete is strong, the shape of the concrete can make it stronger with a bell shape or more susceptible to breakage with a flat-shaped base.

The bell-shaped base offers more strength and durability in its compact structure than something flat with a wider base.

One More Reason for a Bell-Shaped Pier

Not only is the bell-shaped pier ideal for strength, resistance, and durability, but it’s also optimal from an efficiency standpoint. You’ve got to make this stuff strong, but it also has to be doable completely underground by workers on the surface.

buying foundation repair

A repair company has to find a balance between time, cost, and effort to make a repair method affordable for a homeowner and profitable for the contractor too. Sure, we could do something much more complex, costly, and time-consuming to repair your foundation.  But by then, you might as well build a new house instead because the repair expense will be too high.

The bell-bottom pier is easier to make with the tools we have available and offers the key benefits of strength and stability. It works without excess costs, high-tech equipment, or wildly expensive materials. Plus, we can complete the work under your home, while your home remains intact above ground.

While drilled bell-bottom piers are not the fastest technique on the planet, it’s still fairly efficient and can be completed in a reasonable amount of time with relatively moderate costs.

Why We Choose Drilled Bell-Bottom Piers

Now that you know *all about that base* of the bell-bottom pier, perhaps you understand better why it’s effective in our expansive clay soil conditions. Even though it seems like it’s just a simple shape, there’s a lot of science and thought behind this design. The downside: this method does take more time and expense compared to other foundation repair methods.

used to make the bell on the bottom of a pier

We don’t stick with this tried and true method because we like how long it takes or how much it costs. We stick with what some call an “old-school” method because we believe it’s the longest lasting and provides maximum stability for your home. 

You only have one chance to do foundation repair right the first time. At Anchor Foundation Repair, we’ve raised and repaired thousands of Brazos Valley foundations since 1985. We feel so good about the bell-bottom piers we install that we offer a fully-transferrable lifetime warranty and service agreement on all our slab repairs. 

Want even more history, details, and science on drilled piers? Check out: What is a Drilled Concrete Pier? Home Foundation Repair Spec Sheet next!