How Caissons Can be an Alternative to Concrete Foundations

Updated 10/9/2026

When it comes to designing structures, the foundation is a critical component that contributes to the stability and longevity of the structure. While concrete foundations have been the conventional choice, caissons can be a better alternative depending on soil conditions. With their quicker, more cost-friendly installation, caissons may be the better choice for your next project.

How Caissons Can be an Alternative to Concrete Foundations

When designing steel transmission structures, the foundation is a critical component that contributes to the stability and longevity of the structure. While concrete foundations have traditionally been a common choice, caissons can offer an alternative depending on soil conditions and project requirements.

Caissons can provide faster, more cost-effective installation in suitable conditions. Two types of caissons used for steel structures are vibratory caissons and socket caissons. Each has different installation methods, connection requirements, and design considerations.

What is a Caisson Foundation?

A caisson foundation uses a steel pipe or bent plate tube installed into the ground to support a structure. Depending on the type of caisson, the structure may be connected through a base plate connection or positioned inside the caisson and secured with grout or concrete.

Caissons can be an alternative to conventional concrete foundations, particularly when soil conditions and installation requirements make them a practical option.

What is a Vibratory Caisson?

Vibratory caissons are one of the most common types of caissons used for steel structures. They are often selected for their relatively fast installation and cost-effectiveness.

Vibratory caissons consist of pipes or bent plate tubes that are vibrated into the ground using driving ears. Once installed, the caisson is connected to the structure through a base plate connection. This is accomplished by bolting the flange of the structure to the inside or outside of the caisson.

What Soil Conditions Are Suitable for Vibratory Caissons?

Vibratory caissons have been successfully used in various soil types but are most often used in soils with poor cohesion.

These soil conditions can create challenges for drilled shaft foundations because of difficulties maintaining the stability of the drilled hole walls while placing concrete.

However, rocky soil conditions can cause a vibratory caisson to hit refusal, preventing it from being driven farther into the ground.

A thorough assessment of soil conditions is essential when determining whether a vibratory caisson is an appropriate foundation method.

What Are the Key Design Considerations for Vibratory Caissons?

Several components must be considered when designing and installing vibratory caissons.

1. Anchor Bolt Connections

While different bolt sizes can theoretically be designed for the structure-to-caisson connection, foundation loads typically dictate the use of larger bolts.

Since A615-GR75 14J and 18J bolts are specifically mentioned in ASCE 48 for anchor bolts, they are often used for caisson connections. These bolts can be milled down and fully threaded to provide the connection while maintaining the leveling capabilities used in conventional concrete foundations.

Anchor bolt circles can be positioned inside or outside the caisson wall depending on project specifications. Neither configuration is inherently superior.

For additional information on connection design, see our guide to calculating anchor bolt embedment for transmission poles. 

2. Foundation Sizing

The diameter and embedment depth of a vibratory caisson depend on the soil conditions and foundation reaction loads.

Maintaining a relatively similar size between the caisson and the bottom of the structure is advisable. The goal is to keep the anchor bolt circle close to both the structure wall and the caisson wall, allowing for a more efficient flange plate design.

3. Driving Ears

Driving ears are critical components of vibratory caissons and are mounted on top of the flange plate used for the structure connection.

The welds and construction must withstand the forces generated by the vibratory hammer during installation. A simple fillet weld is typically not sufficient to withstand these forces.

A minimum wall thickness of 3/8 inch is recommended.

Once the caisson is installed, the driving ears are either cut or knocked off. Touch-up galvanizing is then applied to the exposed bare metal created by their removal.

What is a Socket Caisson?

While vibratory caissons are commonly used, socket caissons provide another foundation alternative.

A socket caisson can be vibrated into place or installed in a drilled hole. When vibrated into place, several feet at the top must be excavated because the structure sits inside the socket caisson on a floor after installation.

Bolts are sometimes added to the sides of the caisson to help plumb the structure. Once the structure is properly aligned, the space between the structure and caisson, typically between 3 and 9 inches, is filled with grout or concrete that adheres to both surfaces.

The primary benefit of a socket caisson is that it eliminates the need for a flange plate connection between the caisson and the structure. This can reduce costs associated with the thickness and width of the flange plate.

What Are the Design Considerations for Socket Caissons?

As with vibratory caissons, a foundation engineer must determine the appropriate diameter and embedment depth based on the soil conditions at the job site and the foundation reaction loads.

The depth at which the structure is socketed into the concrete allows loads to transfer from the structure into the caisson and then into the ground.

With socket caissons, the driving ears must be placed on the caisson walls rather than the top because the top must remain open for the structure to enter.

Vibratory vs. Socket Caissons. What's the Difference?

Both vibratory and socket caissons can be used as alternatives to conventional concrete foundations, but they differ in how they are installed and connected to the structure.

When Should Caissons be Considered Instead of Concrete Foundations?

Caissons can provide an alternative to conventional concrete foundations when soil conditions and project requirements support their use. Foundation reaction loads are one consideration in this decision.  

Their relatively rapid installation and potential cost savings can make them appealing for projects with tight schedules and budgets. However, soil conditions, foundation reaction loads, installation methods, and structural connection requirements must all be considered.

For example, vibratory caissons may be suitable in soils with poor cohesion, while rocky conditions can prevent the caisson from reaching its required depth.

The appropriate foundation method should be determined by a qualified engineer based on the specific requirements of the project.

Frequently Asked Questions About Caisson Foundations

What is a caisson foundation used for?

A caisson foundation supports a structure by transferring loads into the ground. For steel transmission structures, caissons can provide an alternative to conventional concrete foundations when site conditions are suitable.

What is the difference between a vibratory caisson and a socket caisson?

A vibratory caisson is driven into the ground and typically connected to the structure through a base plate and anchor bolts. A socket caisson allows the structure to sit inside the caisson, with grout or concrete filling the space between them.

Are caissons less expensive than concrete foundations?

Caissons can be more cost-effective than conventional concrete foundations because of their relatively quick installation. However, the actual cost depends on project requirements, soil conditions, and foundation design.

What soil conditions are suitable for vibratory caissons?

Vibratory caissons are often used in soils with poor cohesion, where maintaining the walls of a drilled shaft can be difficult. Rocky soil conditions can cause installation challenges if the caisson hits refusal.

Do socket caissons require anchor bolts?

The socket caisson connection described here eliminates the need for the flange plate connection used with vibratory caissons. The structure is instead placed inside the caisson and secured with grout or concrete.

Choosing the Right Foundation for Steel Transmission Structures

Both vibratory and socket caissons offer alternatives to conventional concrete foundations, with different advantages depending on soil conditions, structural loads, and installation requirements.

Vibratory caissons offer relatively straightforward installation through vibration and a base plate connection, while socket caissons eliminate the need for a flange plate connection by allowing the structure to sit inside the caisson.

Ultimately, foundation selection depends on the specific conditions and requirements of the project. Consulting with a qualified foundation engineer can help determine whether a caisson is an appropriate solution for a steel transmission structure.

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Bill Elliot, P.E.

Senior Engineer

Bill graduated from Texas A&M University in 1994 and has been a PE for 27 years. He brings 25 years of experience in the utility industry, including five years with a substation design consultant, where he designed standard-shape substation structures and foundations. For the past 20 years, Bill has worked at DIS-TRAN Steel, designing standard-shape and tapered tubular structures for the high-voltage industry.