The Way Water Moves: A Case Study in Expansive Soil

A case study in expansive soil

Could something be moving in that dark, quiet basement? Of everything that could lurk down there, the sneakiest and creepiest is water. Boogeyman aside, the last thing a customer wants in a basement is musty smells, damp carpet, damaged finishes, or standing water. Moisture in the basement can cause permanent damage to both the home and the builder’s reputation.

Water should never have a chance to enter a basement, so the best move against it is to be proactive. That takes a plan and the gumption to follow it through. Set a course of action for moisture control and make sure those methods are used on the construction site. If customers call back to question the quality of workmanship, prepared builders will be able to defend themselves by explaining their approach. So, what’s a good plan for below-grade moisture control? First, it’s important to understand the ways water can move into the basement. Gravity, splash back, and hydrostatic pressure are frequent ways that water enters over or through foundation walls. Capillary action is another more subtle way it moves though the wall and slab. A solid plan for moisture control should have protection strategies from each of these types of movement.

Attacking from Above

Bulk water, such as rain or melting snow, is the usual suspect in the case of a wet basement, and the dripping and sloshing that come along with it can infuriate a homeowner. Gravity will always take bulk water along the path of least resistance. An appropriately sized guttering and downspout system will capture all the water that falls on a house, collect it, and remove it to a surface or storm drainage system. By letting water simply drip off the roof, a phenomenon called splash back can occur, where roof drainage falls to the ground and splashes the lowest foot or so of the house—generally the exposed foundation. This moisture saturates the foundation and can lead to damp basements.

In addition, surface drainage is critical. All the ground around the foundation should slope away from the house at a minimum of 5%, after any post construction settlement. This may require additional fill to be delivered to the site during the first year of occupancy, or the initial post construction grade may need to be increased to allow for a settled grade of 5%. These two measures work with gravity to direct the water down a home’s sides and away, so it doesn’t have the chance to test the waterproofing on the basement system.

Wriggling through the Cracks

Below grade, the best practice is to install a high quality dampproofing or waterproofing system, which includes a coating on the exterior of concrete walls and a drainage material to allow for any ground water to drain to the bottom of the foundation and away. At the base, install an interior and exterior drain tile encased in uniformly sized crushed stone, fully wrapped with a filter fabric that will prevent soil from working its way into the drains over time. This system is commonly referred to as a French drain.

This drainage system also controls hydrostatic pressure, where water pressure in the soil around a house pushes water right through cracks and pores in the foundation wall. Remember there are two kinds of concrete: concrete with cracks and concrete that will crack. Installing deliberate control joints with additional waterproofing can help to prevent customers’ concerns about moisture and random cracking in basement walls.

What kind of system should be below-grade? It depends on how much rain your climate gets and the soil properties. If you backfill with a free draining soil like sand, and you don’t get much rain (20” per year or less), then a modified bitumen dampproofing is usually adequate. If you build in climates with more rain, then an elastomeric on cementitious waterproofing with a dedicated drainage material (fiber glass mat, dimpled sheet, grooved foam, etc.) should be used. There are many systems on the market; it’s important to verify that qualified technicians install them according to manufacturer’s specifications.

Once bulk water, splash back, and hydrostatic pressure have been taken care of, the next issue to deal with is capillarity—the characteristic of water to rise into a pore, just like the way an old-fashioned feather pen will suck up and hold ink. The pores in a foundation wall soak up ground moisture like a sponge and can release it inside the basement either by evaporating into the air or, in severe cases, showing up as water. To effectively stop capillary flow, seal the top of the footing with polyethylene, polymer-enhanced asphalt, or other spray- or brush-applied masonry sealants. It’s also important to put a capillary break under the slab, typically by installing a 4” layer of uniformly sized crushed stone beneath a plastic vapor retarding sheet, which creates a void large enough to prevent water from wicking up.

Swelling From Below

After learning how water moves, you understand why it’s important to block any opportunity it could have to get in. David Romero, of Fort Collins, Colorado, understands how easily water can move into a home. He recently asked IBACOS whether building methods in areas of expansive soil require greater care to prevent the entry of water into the foundation.

If you haven’t heard of expansive soil, it’s not surprising because it’s only a problem in certain regions. Expansive soil is a type of soil that expands and contracts based on its moisture level, which can be damaging to the structural integrity of homes. The wetter the soil, the more it expands. In Colorado, expansive soil is attributed to the presence of clay and shale, while in other areas the soil can expand and contract due to seasonal and climate conditions. Various kinds of expansive soils exist in the Rocky Mountain area, Texas, southern California, the Great Lakes area, and the Gulf states.

As Romero pointed out, builders in regions of expansive soil use a method called pier-and-grade-beam foundations. In pier-and-grade-beam foundations, builders set the foundation on caissons drilled deep into the ground. Between the piers, under the foundation wall, builders construct a void space. It’s this void space that helps to prevent shifting in the basement walls when the soil expands, but as Romero sees it, this void space is also an entry point for moisture. “This strategy works great [to prevent the foundation from shifting], but since a waterproofing barrier is never installed beneath this portion of the concrete, water can wick into the wall,” said Romero.

IBACOS’ Senior Building Scientist Duncan Prahl has been working with builders in Colorado. To address Romero’s question, Prahl says to think about the way water moves. As it turns out, the void space not only plays a part in preventing shifting but also in keeping things dry. To wick water, the concrete has to be in contact with the soil. With expansive soils, builders are diligent to keep bulk water away from the house to keep the soil from swelling around the foundation and potentially buckling the wall inwards. In addition, French drains installed on the inside of the foundation at the base of the wall should get rid of any rising ground water.

“With a void space, you don’t have direct contact between the home and the soil, so you won’t need a capillary break to prevent water moving through capillarity into the home,” says Prahl.

Sometimes when the ground swells, there may be some contact, but by installing proper above-grade drainage and French drains, this swelling should be minimal. Check the engineering reports or talk to your geotechnical engineer regarding the amount of potential swell in the soil, and install a void form that exceeds the swell potential to ensure the ground never touches the bottom of the wall.

While expansive soil may cause structural problems, the same gap that protects a home’s structural integrity also helps to control capillary moisture flow. Water is a sly intruder, but it’s also lazy—always taking the path of least resistance. Understanding the way water moves will help keep it from creeping into the basement.

For more information about preventing wet basements, including technical details, see previous QualityHome® newsletter articles on basement moisture control, crawlspace moisture control, and foundation moisture control.

The Moisture Management Task Force also provides a guide at http://www.e-star.com/pdf/Moisture-Management-Task-Force-Guide_Version1.0.pdf.

For more information on expansive soil, see “Trouble-Free Foundations for Expansive Soil,” in the November 2002 issue of The Journal of Light Construction.

For online training in preventing mold growth in foundations, see www.BuildIQ.com.