What are Expansive Soils?
Expansive soils shrink and swell in response to soil moisture levels. Moving soil exerts tremendous pressure on paving, footings, and foundations.
How To Identify Expansive Soils
Landscape architects can design and build on sites with expansive soil if they get the right information and engineering guidance.
First, identify if the soil on your client’s site is expansive. There are a couple of preliminary methods you can use when you inventory a site.
Identify the soil type and expansion characteristics in the USDA Soil Survey. The maps included with the survey indicate what soil type or types exist in the vicinity of your project. Read the soil’s detailed description in the survey guide. If the soil survey indicates moderate to extreme swelling potential, consult with a licensed civil or geotechnical engineer to test the soil for expansion potential.
You can also get a general idea if there is a known problem with expansive soils based on a map published in Swelling clays map of the conterminous United States by W.W. Olive, A.F. Chleborad, C.W. Frahme, Julius Schlocker, R.R. Schneider, and R.L Shuster; 1989.
Clay soil showing a network of cracks caused by drying. Photo courtesy of Yuna Mika.
Look for deeply cracked soils when you walk the site. Sometimes, expansive soils exhibit deep cracking during the dry season if the have not been irrigated.
Observe walls and paving for cracking and heaving. These may indicate damage caused by soil movement.
Ask The Building Department
Municipal building departments can be a wealth of information about conditions within their jurisdiction. Call or stop by the building department and ask them about soil conditions in the area. They may have established requirements or procedures for mitigating damage caused by expansive soils. For example, the city of Chula Vista has regulations governing soil tests and construction methods on expansive soils.
Get the Right Consultants
Once you have identified expansive soils on your client’s site, it is time to bring a consultant on board. Both geotechnical and civil engineers are qualified to test soul for expansion. A geotechnical report contains (at a minimum) site preparation guidelines, allowable soil bearing values, soil expansion index value, and slab and foundation design recommendations.
Construction Design on Expansive Soils
There are several techniques that can be used to reduce the negative aspects of expansive soils.
Replace Expansive Soils
Expansive substrates can be removed and replaced with non-expansive material. Another alternative is to mix expansive and non-expansive soils together to reduce the expansion index to a lower, non-threatening level. Consult with a civil or geotechnical engineer to determine recommended soil depths and mixes.
Reinforce Slabs And Footings
Smaller slabs can be strengthened with wire mesh in the concrete. The wire mesh provides tensile strength to the concrete which helps slabs or paving resist soil movement.
Another reinforcement technique involves using steel reinforcing bars to post-tension a concrete slab on grade.
For example, the city of Chula Vista recommends a minimum footing depth of 18 inches for all footings and additional rebar at both the top and bottom. Additionally, the recommend wetting the sub-grade 24 hours prior to pouring concrete to get the soil to recommended moisture levels to minimize swelling. Test the moisture content prior to pouring to ensure the soil moisture level matches the engineer’s recommendations.
Add Gravel Under Slabs
Adding crushed aggregate under paving can help reduce soil movement. As soil swells, soil particles infiltrate the gravel which reduces heaving pressure on pavement. Install between four to eight inches of compacted crushed aggregate under slabs and paving to reduce the affects of soil movement.
Expansive Soils Can Be Overcome
Although expansive soils can pose problems for sit developers, most issues can be resolved through proper design and engineering.
For more information on designing and constructing with expansive soils, check out the book Expansive Soils: Problems and Practice in Foundation and Pavement Engineering (Wiley Professional).