Giovanni Valle is a licensed architect and LEED-accredited professional and is certified by the National Council of Architectural Registration Boards (NCARB). He is the author and managing editor of various digital publications, including BuilderSpace, Your Own Architect, and Interiors Place.
The early stages of construction begin with the foundation – determining whether the soil is suitable for the structure. When looking at construction sites, clay soil is one of the most common soil types you’ll encounter.
Clay soil isn’t particularly good for construction because it’s susceptible to moisture fluctuation. It expands when it’s moist and contracts when it’s dry, resulting in uneven floors and fissures in the foundation. Clay creates poor soil support, and you’d need a proper structural foundation to keep it stable.
Although it’s not ideal for construction, sometimes there is no alternative since changing your land’s soil composition is often not feasible. So, let’s discuss what options you have to make your construction more stable, even with clay as soil support.
Characteristics of Clay Soil
Clay soil is capable of holding weight – it’s certainly better for soil support than, say, sand. It’s also highly pliable when moist, making it easier for anyone to move and manipulate according to the construction needs. However, when it’s dry, it can be challenging to move and often need heavy equipment to manage it.
Besides this, clay soil is made up of tiny particles that can absorb and store water, making it prone to moist fluctuations. It expands when it gets enough moisture and contracts when it gets dry. When you’re working on a construction, this characteristic is something that you’d want to avoid because it makes your building susceptible to frequent changes in your soil’s condition.
Why Clay Soil is Not Ideal for Construction
When you’re in the planning stage of construction, soil support is the first thing you need to consider. It determines the foundation that you’ll need and the size of the structure that you can build. Some soil types are strong enough to support a skyscraper, while others can’t even support a person’s weight.
In the case of clay soil, it can support a structure’s weight, but it doesn’t have the stability to maintain the foundation’s strength. Since it’s prone to moist fluctuations, it can put a lot of pressure on the foundation, making it move up and down. It won’t be a problem when the building is new, but it’ll happen several times every year, creating fissures and cracks that weaken its structural integrity.
Clay soil isn’t ideal for construction because its characteristics can weaken the building’s foundation. It makes the project more complicated and will take a lot of time to complete. That’s why it would be best to conduct soil tests before you even purchase land, to ensure that your structure will have the ideal soil support.
Can You Still Use Clay Soil for Construction?
In short, yes. It’s impossible to have the best soil type for every construction project. That’s why different buildings use different foundations to ensure that it can support the weight and adapt to different soil characteristics. It’s possible to still use clay for construction, but your design stage will take a different approach to the building’s foundation.
There are two ways to build the foundation for structures on clay soil, and both of them have varying degrees of strength and complexity. So a successful project starts with understanding the options that are available for similar endeavors.
Drilled pier or drilled shafts are typical for structures with large lateral and axial loads. It uses a combination of end bearing resistance and side friction to create enough support for the building. If you’re going to use this foundation on clay soil, the more moisture the soil has, the deeper and bigger the drilled pier should be.
The diameter for drilled piers can be anywhere from 24 ft (7.3 m) to 120 ft (36.6 m). Using it for the foundation of a structure built on clay soil is perfect because you can install it to a depth of more than 300 ft (91.4 m). Of course, you’d have to factor in the structure’s weight, but it’s an excellent solution for most residential buildings.
Drilled piers create better stability for the structure because it’s anchored deep into the clay. Remember, the key to getting better stability with this option is to anchor it deeper as the building gets higher and the soil gets more clay content.
If a drilled pier gives your structure better stability, a slab-on-grade or floating slab foundation counters the clay soil’s tendency to expand and compress. This technique uses slabs that are laid directly on the earth, hence the term “floating slab.” It acts as the foundation for the structure, and you’re not required to have more footings for it.
The minimum thickness for a slab-on-grade foundation is 4 inches (10.16 cm). However, the thickness required for your construction would depend on the building’s size and the soil’s clay content. If there are concerns or tendencies for the structure to be unstable, we use a layer of gravel and bitumen (asphalt) before laying the slab. This process prevents moisture from reaching the slab, making it more effective against structures on clay soil.
When you’re building a structure, you have two options for slab-on-grade. The choice between the two will depend on the soil’s condition and the building you want to have. Below are the two types of slab-on-grade that you can use for your construction.
A supported slab-on-grade is an option that we use if the foundation already has an existing footing. The gravel and moisture barrier protects the slab from clay soil’s instability, while the footing supports the foundation.
The slab also has joints laid out in a grid to prevent random cracks from appearing. It also reduces the stress that the foundation gets when the clay soil experiences extreme temperature fluctuations.
A monolithic slab-on-grade is what we use if the structure doesn’t have an existing footing. If you use this as the foundation for your building, the slab will serve as the footing. The slab’s perimeter is thicker than the rest of the surface because it creates more stability for the structure. This design also helps in equal distribution of weight across the surrounding soil.
The rest of the parts for a monolithic slab-on-grade is similar to a supported slab; one layer of gravel and moisture barrier. These layers help relieve the tension whenever the clay soil shrinks or compresses while preventing moisture from reaching the slab. The surface also has a planned grid to prevent random cracks from appearing, which is a common concern when building on clay soil.
You can use both techniques without reinforcement, and they’ll do well for clay soil. However, if you want to create more stability for your structure, you may opt to use post-tension cables or reinforcing steel.
These materials will provide your structure with additional strength that your foundation may need. It’s a crucial factor that you need to consider if there’s high clay content in the soil. Using reinforcing steel, together with slab-on-grade, ensures that your foundation can withstand the constant shifting of soil.
Clay soil isn’t the ideal soil type for construction, but it doesn’t mean that you can’t build on it. Although the process will be more complicated than building on bedrock or loam, it’s still possible to use clay soil for construction. The only thing that you need for it is to have the right foundation for better stability.
Drilled piers can help your building be more stable, while slab-on-grade helps counter clay soil’s instability. Both are excellent options for your foundation and ensure that your building has the soil stability that it needs to last longer.
- Soil’s Matter: What type of soil is good for a foundation for buildings or houses?
- Foundation Support Specialists: Choosing the Right Foundation for Clay Soil
- Uretek-Gulf Coast: Types of Soil for Your Building Foundation | URETEK Gulf Coast
- Ram Jack: Different Soils & How They Affect Foundations
- Nicholson Construction: Drilled Shafts for Deep Foundations
- Civil Read: Slab on Grade or Grade Slab Foundations
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