Soil Stability: Understanding How Soils Cause Foundation Problems
The ground beneath a home consists of several layers of soil, each with its own properties. These layers can wash out, soften as they get wet and shrink as they dry out, causing the foundation to settle and crack.
A home’s foundation is built on layers of soil. Soil also comes into contact with a home’s foundation walls. But when the soil underneath and around your home becomes too wet or too dry, it can create a variety of issues — including putting stress on your home’s foundation and causing foundation settling and shifting, cracks and leakage and even bowing walls.
Unfortunately, we rarely think about the soil that’s under and around our home, except when there’s a problem. So, whether you’re planning a site for a new home or maintaining an existing foundation, it’s a good idea to have a basic understanding of the types of soils out there, their characteristics and how they behave.
Types of Soils: Characteristics and How They Affect Foundations
A high percentage of clay can reduce the soil’s load-bearing strength, and the soil can put pressure on a home’s foundation when wet.
According to the Smithsonian Environmental Research Center, there are more than 20,000 soil varieties in the U.S., but the main types are sand, silt and clay. The major difference between these soils is the size of their particles. Most soils consist of a combination of these three main types.
Soil also has air and water mixed into it, and compacting soil densifies and strengthens it. The soil’s composition influences how much water it can hold, determines its load-bearing strength and how much pressure it exerts on a home’s foundation. Generally, you want to pay attention to the soil’s texture and the amount of clay it contains.
Building on poor soil, or building without footings on unstable clay or sand, contributes to foundation settlement and cracking. Below are explanations of the major types of soils and how they affect foundations.
Characteristics: Sand particles are the largest of the three and tend to hold little water. Water passes through sandy soils rather than being absorbed. Instead of expanding as they absorb moisture and contracting as they dry out, sandy soils maintain a fairly consistent volume and density.
How it affects foundations: Because of their stability and good load-bearing qualities, sandy soils are less likely to shift and settle, so they rarely cause foundation problems.
Characteristics: Clay particles are very small and tend to pack down easily, which means water doesn’t drain well. Clay soil absorbs water easily, expanding as it becomes more saturated.
How it affects foundations: Soils rich in clay and silt have the greatest potential to damage a foundation. Expansive clay soil can cause foundations to crack, heave and shift. When clay soils dry out, they shrink and crack, leaving gaps around the house, allowing water from the next storm to penetrate easily and repeat the expansion cycle. Clay-rich soils typically cause more foundation damage by expanding than by contracting.
Characteristics: Silt particles are medium-sized and have properties in between those of sand and clay. Silt retains water for a prolonged period and turns into mud when very wet.
How it affects foundations: The main issue with foundations built on soils containing silt is shifting and expanding, which can put prolonged stress on a home’s foundation. This can eventually lead to structural damage or foundation failure.
Characteristics: Loam is composed of almost equal amounts of sand, silt and clay. While true loam is ideal for construction, most types of loamy soils have more of another type of soil mixed in — although the soil is still considered “loamy.” For example, clay loams are mostly clay, making them heavier and slower to drain. Sandy loams are mostly sand and tend to dry out faster than clay loams.
Because loam soils often consist of a range of textures and structures, they are more versatile than other types of soil. Their mix of particle sizes gives loamy soils the stability and strength that make them ideal soils on which to build a foundation.
How it affects foundations: A balance of silt, sand and clay make loam a good soil type for supporting a foundation. A potential concern with building on loamy soil is the possibility of undecomposed vegetation that can cause the soil to shift as it decomposes. When loamy soils shift underneath the foundation, they are less able to support the weight of the foundation, which can lead to settlement and cracking.
The “Active Zone” of Soil and Foundation Problems
A soil’s active zone, located directly under and around your foundation, is most affected by changes in moisture and temperatures.
Your home is resting on many different layers of soil, each with different thicknesses and characteristics that can affect a house foundation. These soils have formed or been deposited over thousands of years — by water, wind, glaciers and or by the contractor who built your home.
Typically, soil layers become more stable and have a higher load-bearing capacity the deeper down they are. Deep below these layers is bedrock — a layer composed of rock or stable, densely packed soils. The soil immediately under and surrounding your home is known as the active zone. The active zone may vary from a few feet below the surface to more than 30 feet below grade.
The active zone of soil is most affected by changes in moisture levels and temperatures. When the ground under and around the home absorbs too much water, it expands and swells. This upward movement of the ground as a result of soil expansion is known as “heave.” When soil shrinks, expands or heaves, this can cause foundation damage, including shifting, sinking and cracking.
Soil Conditions and Foundation Settlement
Foundation settlement occurs when the soil can’t properly support the weight of your home. Three of the most common reasons for foundation settlement include drying and shrinking soil, wetting and softening soil and poorly compacted fill soil.
Drying and Shrinking
Foundation soils experience most of their drying and shrinking from two common causes:
- Drought: Prolonged dry periods cause the soil to dry out. As we know, when clay dries out, it shrinks. Soil shrinkage beneath a foundation has the same effect as soil settling: It usually causes a section of the foundation to crack and settle into the void or hollow area where settlement has occurred.
- Tree Roots: If a tree's branches extend over your home, there's a good chance that its roots extend under or against it as well. Tree roots can drain high volumes of water from the soil, contributing to a significant change in moisture levels that can cause shrinkage and foundation damage.
Wetting and Softening of Soil
The soils around your foundation experience wetting and softening primarily for two reasons:
- Heavy Rain and Flood Conditions: As clay soil gets wet, it holds on to water and becomes very soft. This soft soil can be weak, causing the home to shift.
- Poor Drainage: If water is allowed to stand or "pool" next to your home, the soil will absorb the water and swell. As it does, it can lead to bowing walls and cracks in the foundation. Make sure your gutters and downspouts are carrying water far enough away from your home and foundation. Check that the ground around your home slopes away from, and not toward, your foundation.
Poorly Compacted Fill Soil
To level a site where a foundation will be built, builders sometimes bring in loose soil from another location to fill depressed or hollow areas. This "fill" soil can be looser than the dense, hard-packed virgin soils at the site that haven't been disturbed — possibly for millennia! The fill soil brought in by the builder has to be compacted thoroughly before a foundation is built on top of it. If the soil is not compacted well, it may begin to compress underneath the weight of your home, creating settlement problems that can damage your foundation.
Determining the Type of Soil You Have
Since soils can differ in composition from one property to another, it’s best to send a sample to a soils lab to identify the composition of the soil beneath and around your home. If you’d like to do some investigating yourself, check the texture of the soil. Here’s how to use the “hand method” to determine the texture of your soil:
- Dig about 6-8 inches beneath the top layer of your soil.
- Take a handful of soil and form it into a ball.
- Add water, if necessary, so the soil can be kneaded easily. If the soil can be formed into a ribbon, it contains a high amount of clay.
- Add more water until the soil becomes very wet. If the soil feels gritty, then you have sandy soil.
Steps to Prevent Foundation Damage
Ideally, you want to build a foundation on good soil to avoid potential foundation damage caused by soil movement. However, this isn’t always possible, especially if you’re buying an existing home. A qualified foundation specialist can inspect your home to identify problems before they begin and provide effective foundation repair solutions. Here are our top tips for preventing foundation problems:
- Get a home inspection: If you’re in the process of buying a home, don’t skip the inspection. The home inspection can tell you about the condition of the foundation and help you avoid potential foundation problems in the future.
- Maintain your soil: When your soil gets too dry or too wet, it puts stress on your home’s foundation. If you live in a drought-prone area, you may consider adding moisture by spraying the soil periodically with water or planting shrubs and flowers around the base of your home to help hold moisture in the soil.
Caution should be taken to avoid oversaturating the soil around the foundation, which can contribute to foundation cracks and leaks. An arborist or horticulturist can help you determine a safe planting distance from your home. If you live in an area with a lot of moisture, make sure the soil around your home properly drains water away from your foundation.
- Check your grading: Water should drain away from your home when it rains. Generally, the correct slope when grading away from a foundation is about 6 inches for the first 10 feet.
- Maintain your trees: Keep an eye on the trees close to your home to prevent their roots from spreading too close to your foundation. Relocate trees, if needed and, when planting new trees, make sure to leave plenty of room around your foundation.
- Clean your gutters: Clogged gutters can lead to water overflowing and saturating the soil around your foundation. Clean your gutters at least twice a year or add gutter guards to keep out leaves and debris that can cause clogs.
- Fix foundation cracks: If you notice foundation cracks, don’t wait to have the damage assessed by a professional. Acting promptly can help prevent further damage and potentially save you money on the cost of major repairs in the future.
Fix and Stabilize Your Foundation With Help From a Professional
Home foundations that have been built on bad or improperly compacted soil can cause significant damage to the home. As the foundation settles and shifts, it can cause cracks in walls and doors that stick. Ignoring foundation damage can lead to dangerous structural problems for your home.
Every situation and foundation is different, so be sure to talk to your local foundation repair professional about what type of repair is best for the type of soil that’s underneath your home.