What Are Expansive Soils?
So-called “swelling” soils or “expansive soils” contain clay minerals that attract and absorb water. As a result, these soils expand when they get wet and shrink when they dry. Other names used to describe these types of soils include expansive, shrinking and swelling, “bentonite” (named for a particular type of clay), heaving, or unstable, causing all types of expansive soils problems.
In areas where expansive soils are anticipated, the law may require testing of soil prior to construction on a particular site. A geotechnical engineer typically does this by taking soil samples and subjecting them to laboratory testing. Such testing yields two primary measurements used to describe the degree to which soil may be described as “swelling.” First, “swelling potential” indicates the degree to which the soil may expand when wetted and may be described as none, low, moderate, high, or very high. Second, “swelling pressure” indicates the amount of pressure the soil exerts, in pounds per square foot, against a neighboring solid object when wetted.
Construction activity can exacerbate the effect of expansive soils. For example, artificial irrigation causes more water to infiltrate the ground, while at the same time less water evaporates due to there being more roadways, parking lots, driveways, sidewalks, and buildings. This results in an increase in subsurface moisture, and this excess water in the soils is the most significant and direct cause of expansive soils damage.
What Expansive Soils Problems Can I expect?
When expansive soils become wet and expand, the resulting expanding pressure can cause uplift against concrete slabs and foundation footings, causing a wide variety of damages to buildings and surrounding areas. These damages may include:
• Cracking and heaving of concrete, including garage slabs, driveways, sidewalks, patios, foundations, basement walls, and basement floor slabs
• Water leaking into basements
• Broken pipes and water lines
• Cracks in interior drywall
• Sticking doors and windows
These damages may occur slowly over time, and may affect individual homes, buildings or units in a neighborhood at different times. Further, the costs associated with expansive soils damage total several billion dollars annually, more than all other natural hazards combined.
How Can The Effects Of Expansive Soils Be Mitigated?
The risks associated with expansive soils can be significantly reduced through proper construction practices. These practices include proper ground preparation, proper surface and subsurface drainage, and proper landscaping.
Ground Preparation
Proper preparation of the site before construction begins can significantly reduce the tendency of soils to swell when wet, as well as the effects expansive soils may have on nearby structures. The use of engineered fills provides one way to do this. This process replaces some of the native soil with an engineered mixture, which may contain low- or non-swelling materials such as sand. These fill soils are spread on the surface in thin layers, then water is added and each layer is compacted to the density recommended by a geotechnical engineer. Because the resulting fill has a final moisture content and density higher than that of the native soils in their natural condition, it is less prone to swell.
Over-excavation provides a second way in which the effects of swelling soils may be mitigated through proper ground preparation. Here, native soils are cut and removed to a depth below the lowest foundation or road level, and the cut is then filled with layers of soil under controlled moisture and density conditions. This creates a buffer between the surface and the underlying swelling soils.
The use of chemical treatments provides a third way in which the site may be prepared for construction. Here, chemicals are added to the soil which change the clay chemistry and mineralogy to make it less expansive. However, the treatment may not penetrate the soil very deeply, and this method is less commonly used in residential construction.
Drainage
As noted above, excess moisture in the soils serves as the primary cause of damage due to expanding. Consequently, the proper drainage of rainfall, snowmelt, and irrigation is a crucial aspect of reducing the risk of these damages. This includes managing the drainage of surface moisture to prevent infiltration into the soil, as well as the drainage of subsurface moisture that has already penetrated the surface.
An effective surface drainage plan consists of managing roof and slope drainage, as well as the proper use of ditches and swales. Roof drainage can be managed by the proper use of gutters, downspouts, and splash blocks to direct rainfall and snowmelt away from building foundations. Water which infiltrates the ground near the foundation can not only leak into basements, but can also cause the soil around the foundation to swell, potentially damaging the slabs and foundation walls. Generally, geotechnical reports call for a proper surface drainage system to carry surface water 10 feet away from buildings at a slope of 10 percent.
Another element of proper surface drainage is the use of ditches (shallow trenches) and swales (depressions) built into the landscaping, to ensure that water is channeled away from the buildings but not directed toward neighboring structures. Once collected, this runoff may drain into commonly shared gutters, storm sewers, and/or culvert pipes.
In addition to effectively managing surface drainage, it is also necessary to properly manage drainage of the water which does infiltrate the soil. Good subsurface drainage will ensure that the soils do not retain an excess of this moisture. Drainage of subsurface moisture can be managed by the use of sumps, perimeter drains, interceptor drains, and area drains.
A sump consists of an enclosed pit or low area that collects subsurface water. A submersible pump in the bottom of the pit pumps water out and to the surface. A perimeter drain consists of a perforated drain pipe installed around the base of a building’s foundation. The drain pipe collects subsurface water and is sloped to direct it away from the foundation and into a sump, area drain, or gravity outlet. Again, this is an important means of preventing damage to the building by keeping excess water away from the soils near the foundation. An interceptor drain consists of a gravel- or sand-filled trench with or without a drain pipe, and is often used to intercept subsurface water coming from a source uphill from a home, such as an irrigation canal. Finally, an area drain, similar in construction to an interceptor drain, runs beneath streets and gathers water from perimeter drains and other sources. An area drain has the advantage of being able to collect water from numerous sources in a relatively large area.
Landscaping
In addition to proper ground preparation and drainage planning, proper landscaping is important in minimizing the impact of expansive soils, primarily by limiting the amount of water introduced near building foundations. Proper landscaping consists of following guidelines regarding the placement and watering of plants near buildings. Flowers and shrubs should not be planted closer than 5 feet from foundations, unless they require very little water and are hand- or drip-line watered. Similarly, sprinkler systems should not spray water closer than 5 feet from foundations. Finally, trees should not be planted closer than 15 feet from foundations, as they tend to take up large amounts of water in dry weather, which can cause expansive soils to settle and disrupt the slope necessary for proper surface drainage.
What Should I Do If I Suspect Problems Associated With Expansive Soils?
Damages due to expansive soils can be complex and expensive to repair. Many so-called repairs, especially to damaged foundations, serve as mere band-aid fixes which mask symptoms and fail to address the cause of the problem.
Affected homeowners have legal recourse in the face of these expensive repairs. Claims may exist against the developer, builder, general contractor, and/or subcontractors for damages resulting from expansive soils. Homeowners who suspect they suffer from problems associated with expansive soils should seek legal counsel immediately, as any potential claims will be subject to the applicable statute of limitations.
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