The Best Foundations for 6 Common Soil Types

Marwa Hasan
Written by Marwa Hasan
Updated January 3, 2022
large home on green land
Photo: Iriana Shiyan/ Adobe Stock

Know your soils to keep your home stable

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That home you cherish sits on soil, but not all soil is the same. Some soils are strong enough to support a skyscraper, while others can't even support a person’s weight. 

Here are a few of the most common soil types and how they affect a home’s foundation.

1. Clay Soil

Clay is an expansive soil. When exposed to moisture, clay can greatly expand, but it will shrink significantly when it’s dry. A drought impacts a home’s foundation significantly if the house sits on clay.

These extreme changes can cause a foundation to shift and eventually crack, making clay a less-appealing foundation soil.

The Best Foundation for Clay Soil

Drilled pier foundation: Putting anchors deeper will increase stability.

A slab-on-grade foundation: It can resist clay soil’s tendency to shrink or expand.

2. Sand and Gravel Soil

Sand containing gravel has large particles that drain easily. When mixed with gravel and other materials, sand does not retain water. This means there is less risk for the building to shift around. Compacted sand and gravel offer even more stability and are a great option to build a foundation.

The Best Foundation for Sand and Gravel Soil

Compact mix: The mix of sand and gravel must be compacted correctly before pouring the foundation.  

Helical piers: While sand doesn't hold water, it can lose its fraction and shift when water runs through it, leaving gaps beneath the foundation. In these cases, helical piers (aka screw anchors) make a suitable foundation for sandy soil.

3. Rock Soil

There are many rock types, such as bedrock, sandstone, and limestone.

Rock soil makes for an excellent foundation thanks to its stability, its high bearing capacity, and its resistance to water damage. It’s ideal for supporting residential or commercial buildings. 

The Best Foundation for Rock Soil

As long as the rock is level and the foundation is well supported. The most suitable foundations for rock soil are:

  • Strip foundation

  • Pad foundation

  • Raft foundation

4. Loam Soil

Loam contains sand, silt, and some clay. Loam has a dark color, soft, dry, and crumbly to the touch.

Loam generally doesn't shift, expand, or shrink. It drastically handles the presence of water very well. It incorporates the best qualities, making it ideal for supporting a foundation.

The Best Foundation for Loam Soil

Isolated pad footings would work perfectly with loamy soil, depending on the depth of the groundwater table.

5. Peat Soil

Peat is usually dark brown or black, often found in bogs and wetlands. It consists of decaying or organic matter. 

Peat may be the worst soil to build a structure on. It is highly compressible and can absorb a lot of water, but it also dries out easily, causing it to shift and move, risking the stability of the foundation.

The Best Foundation for Peat Soil

Replacing soil: In some cases, when the peat layer is shallow, it can be removed and replaced with engineered fills to build over a stronger soil and avoid toxic gasses during decomposition.

Concrete piles: Pads and beam foundations are extended to a firm soil layer below.

Raft foundation: If there is no firm subsoil, a raft foundation will extend to support the structure beyond the peat soil.

6. Silt Soil

Silt is poor soil for building a foundation due to its poor water draining ability. This causes silt to shift and expand, which does not provide the building any support and puts it under repeated, long-term stress, causing structural damage or failure. 

The Best Foundation for Silt Soil

Concrete piers can go down to solid earth under the silt to support the building without compromising the structural integrity. 

Why Considering Soil Type Before Construction Is Important

construction site new home
Photo: Lev/ adobe stock

The strength and stability of the soil depend on its physical properties. It's important to obtain a soil report, which will help better calculate the soil's bearing capacity. Determining the soil type before starting a project can make the job more efficient. 

Failing to pick the best foundation type for the soil can cause immediate or future foundation issues. Even if your house was built using the wrong foundation, it's never too late for a foundation repair. Contact a local foundation repair professional to assess the situation and suggest a solution that will protect your house and save you future costly repairs.

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