The best practice is to plan for the worst case scenario in the engineering stage and carefully oversee the foundation construction to combat the inevitable stresses that Mother Nature will use to test the integrity of your structure.
The question is not whether soil will move. The key is to make sure a home foundation moves as a single unit, otherwise the walls anchored to it can crack and separate. Even when proper steps are taken, over time, slabs can shift, buckle, and crack due to the ground conditions beneath the slab, extreme weather, or other geological events.
When using a poured concrete slab-on-grade foundation, savvy builders consider it essential to start with a geotechnical engineering analysis. The Texas State Historical Association’s division of land resources recognizes over 1,300 different types of soil in our state, each with its own distinct attributes and characteristics. The soils analysis begins with drilling holes into the ground surface to provide a comprehensive report that evaluates the percentage of clay, sand and water content. From there, a specific type of foundation will be specified. Recommendations for conditioning the soil may also be included.
In Texas, dry soil conditions often require digging a few feet of dirt from underneath the area where the foundation will be laid and replacing it with moisture conditioned soil to minimize swelling. When a foundation is poured on dry soil, any amount of potential water that runs under the home will cause the soil to swell and put upward rising pressure on the foundation.
To combat soil with clay platelets, commonly found in Texas, soil injections may also be recommended. Environmentally safe chemical soil stabilizers minimize the soil’s basic swell/shrink characteristics to increase its compressive strength and support value by changing the soil’s electrochemical nature to inhibit its ability to associate with water.
Where helical piers are specified, they work by transferring the weight of a structure and transferring it to load-bearing soil or rock deep below the surface. But it is important to note that piers will not provide relief if moisture gets under the foundation and thus are not an alternative for soil preparation recommendations.
A licensed professional engineer and technical inspector will next prep the site for the pouring of the slab. The house size will determine the footing depth and width of the footings, while the foundation thickness is determined by building code requirements. Your engineer’s knowledge and understanding the concrete mix’s compressive strength is very important.
For typical slab-on-grade construction, form boards are set in place and a layer of gravel, several inches thick, is introduced inside to help with drainage and then topped with plastic sheeting as a moisture barrier. Next, the area is reinforced with steel reinforcing bars (rebar) and welded wire mesh. The final step before pouring is integrating plumbing and electrical underground.
A floating foundation may be used for house expansions or garages, involving the building of trench around that perimeter that is reinforced with rebar and filled with concrete to create the foundation footing. A stem wall is poured on top of the footing. “Floating” refers to the fact that the concrete slab poured inside isn’t physically attached to the stem wall. Care must be taken so that a long straight stretch of foundation doesn’t cause the stem wall to tilt out. Since the floor slab isn’t really part of the structural part of the home, ceiling and roof loads must be routed to outside walls using truss framing.
Debates vary on the pros and cons of traditionally reinforcing the concrete with rebar or implementing post-tensioning. The later uses steel cables arranged in grid fashion inside the forms as indicated by the engineer, which are tightened after the concrete hardens. The typical process mandates digging concrete trenches around the perimeter of the house where the external walls will be placed. Additional trenches (footings) are dug between the perimeters. Concrete is poured into the trenches and then topped off with a slab of concrete that is typically thinner than a concrete and rebar slab, allowing lighter and longer slabs with less steel required.
Post-tension cables are slid inside a plastic duct to prevent them from touching the concrete when it is poured. Care must be taken to ensure cables remain perfectly straight during the pour and are not displaced by the weight of the concrete. When the concrete has sufficiently cured, the cables are stretched. Stretching the post-tensioned cables applies force to the concrete system, lifting the slab into a compressed state. The cables, in theory, can achieve a minimum of 50 psi (pounds per square inch) compression throughout the slab. The principle behind prestressed concrete is that compressive stresses induced by high-strength steel tendons in a concrete member before loads are applied will balance the tensile stresses.
Critics of post-tension foundations claim that because the cables are not tensioned until at least 7-10 days after the concrete is poured, they cannot provide control of cracks that occur in between. More serious are claims that post-tension slabs are inherently weaker and failure may occur due to non-uniform weight distributions of the house causing uneven loads on the foundation and non-uniform thickness of soil underneath the foundation.
So why build on top of the problematic soil in the first place you ask? Pier and beam foundations were the most common type of foundation prior to the 1950s and remain the second most common type of foundation used in Texas. They may, indeed, be a wise choice for areas with substantial soil expansion and contraction, where a home is built on an uneven grade or a hillside or when the home is in an area prone to flooding. A home’s treated floor is elevated about 18 inches from the ground, resting on a series of concrete piers. The piers are connected with a series of pressure treated wooden beams, connected with wooden joists, creating a subfloor for the residence. Plumbing and electrical systems are placed in the crawl space between the ground and subfloor. One of the main disadvantages with pier and beam construction is deterioration. Even when pressure-treated lumber is used in the home’s construction, wood decomposition occurs over time, and termite infestation is a constant threat. Freezing temperatures can also have a detrimental effect on exposed pipes.
The Bottom Line: Whatever type of foundation is recommended by the engineering team, careful observation and inspection are necessary to ensure a foundation ready for all that will stand to challenge it over time.