Choosing a Foundation

Choosing a Foundation

FOUNDATIONS

One of the most common questions we are asked here in North Texas is, “What type of foundation do you use?” Foundation repairs are quite common in this region on older homes and everyone wants to make sure they are taking the proper preventative measures up front to ensure they do not have to deal with a foundation repair down the road.

I feel it is important to take a moment and discuss the foundation options available and a few strengths and weaknesses of each.   Foundation is the single most important part of the build.  The success of everything else hinges upon the structural integrity of this single aspect of the home. If you are building a custom home it is in your best interest to have your homes foundation specifically engineered based on your lots soil sample results and your architectural plans.

Soil and Foundations

In North Texas the soils are very rich with clay resulting in a very high level of plasticity, meaning their ability to shrink and swell. Because of this, much more engineering is required to build a proper foundation than in a city like Phoenix, AZ where there is very little movement in ground.  When it rains, the soils in North Texas absorb water and expand pushing outward and upward.  When there is a drought or dry periods of time the clay contracts, shrinking and leaving cracks in the ground.

As the clay goes through this cycle of expansion and contraction the foundation will rise and fall and eventually the foundation will settle more deeply into the ground.  Most of the time, this settling does not occur evenly and one part of the foundation will sink further into the soil than the other.  This can result in a slab failure.

Another problem with the clay content in Texas soils is the fact that they tend to hold water.  When a heavy rain comes the clay in the soil does not drain as well as other types of soil.  When the soil becomes super saturated with water it can cause the foundation to heave due to swelling or it can sink as saturated soil cannot support the heavy structure that is bearing down on it.  This too can result in slab failure.

So how do we put our best foot forward and work to prevent this?  The first step is to obtain a soil sample from your lot.  Generally two cores will be taken from your lot.  One core will be from the back of the lot and one in the front.  These core samples will then be tested to create a soil report that will give your engineer the information they need to ensure a proper foundation is designed for the soil that is on your lot.

Once your engineer has your soil sample and the plan for your new home they can begin designing the foundation.  These two parts of the puzzle are critical to the design of your home’s foundation.  By analyzing your lots soil conditions and the architecture of your home the engineer is able to integrate solutions such as grade beams and piers to give your new custom home the strength it needs to last for generations to come.

So what are the most common types of foundations in North Texas?

There are 3 common foundations that are seen throughout North Texas.  They are 1) Slab on grade; 2) Pier and Beam; and 3) Piered slab foundations.

Slab on Grade

These foundations are solid concrete that is reinforced with either solid steel (rebar) or post tension cables directly on top of the soil.  The slab on grade foundation is very common amongst production homes – especially those with post-tension cables.  Over the years most builders have moved away from rebar reinforced concrete on foundations and elect to use post tension cables instead.

The post tension cables are draped in the concrete.  Once the concrete has hardened for 7 to 10 days, the cables are stressed creating tension in the concrete.  Concrete is naturally very strong when it comes to compaction, but not so when it comes to tension.  The stressed cables put tension on the concrete when they are tightened making the concrete stronger and less prone to cracking.  The cables are generally pulled to a tension of 25,000 lbs per square inch.

Using the post tension cables allows for thinner foundations to be poured making them more economical.  Even with post tension cables, your slab on grade foundation must be engineered with the proper grade beams to carry the load of your home.  The grade beams will generally run between 24” to 36” deep and be about 10” wide.  (This is just a general average.  Your engineer plans will state exactly what the grade beams should be set at.) These beams create a grid underneath your slab and help distribute the weight of the building over areas with unstable soil.

The advantage of a slab on grade foundation is that if done properly it will last forever and will always be solid – there is no chance of the floor getting “spongy” or squeaky like there is with the next type of foundation.  If done properly there is little to no chance of water pooling under the foundation causing problems down the road.  One disadvantage that is commonly stated with slab foundations is that since you are constantly walking on a hard concrete floor they can cause stress on your joints. Another disadvantage with a slab on grade foundation is that future remodels can be more costly due to the fact that any plumbing changes you wish to make will result in the foundation needing to be jackhammered.  While this can safely be done to the foundation it can add to the cost due to added labor.

Pier and Beam

The second type of foundation that is seen in North Texas – particularly in older homes, is a pier and beam foundation. Pier and Beam foundations use “anchors” or “piers” to carry the weight of the structure deep into the solid bedrock beneath the expansive soil that is closer to the surface.  A pier is a hole that is drilled down into the earth.  The hole is then filled with concrete and reinforced steel to give the column the strength it needs to help carry the load of the home.

The piers for your new home will be designed and located specifically by your engineer based on your soil sample report and the architectural design of your new home. The engineer will specify how deep the piers need to be drilled so that they are resting on bedrock to help prevent your home from rising and falling with the expansion and contraction of the soils closer to the surface.

Around the perimeter of your home there will be piers with a grade beam poured on top of them.  This grade beam will provide a solid perimeter to your home working to keep moisture and unwanted water from getting under your new custom home. The subfloor for your new home is then framed on top of this grade beam and the piers are strategically located throughout the foundation.

The framing of the subfloor on top of the piers results in a crawlspace between the earth and the floor.  One advantage of this is that your plumbing lines (mostly drain) can be placed above the soil.  The big advantage here is that both the floor of your home and the plumbing is less susceptible to movement in the soil thanks to the crawlspace. The piers and grade beams could still be affected, but there are steps that can be taken to limit this by using void boxes under the grade beams.

Void boxes are cardboard forms that sit at the bottom of the concrete beam.  The concrete is poured over these.  Eventually the concrete will rot and decay, leaving a gap between the soil and the concrete grade beam.  The piers under the grade beam will carry the full weight of the home and the gap between the soil and the actual concrete beam make the home less susceptible to soil movement.

Pier and beam foundations are tried and true in unstable soil conditions.  They perform very well and are a great choice for unstable soils, however, they are generally more expensive than the other alternatives and need to be built by experienced contractors to ensure they are done properly.

Pier and beam foundations also require more maintenance than slab on grade foundations, as they need to be vented during the winter to allow moisture to evaporate and prevent mold and mildew from accumulating in the crawl space.   Pier and beam foundations can also become spongy and sometimes squeaky over the years as the wood subfloor can begin to breakdown from moisture in the crawlspace.

Piered Slab

The third type of foundation is a piered slab.  This type of foundation combines the strengths of the slab on grade with the strengths of a pier and beam foundation. A piered slab will be designed in such a way that the grade beams bear down on the piers transferring the weight of the structure down to solid bedrock. While concrete piers have been used for years and they resist settling they can be susceptible to heaving.  Due to this, many piered slabs will use helical piers.  Helical piers are specifically designed to lock into the earth resisting sinking and heaving.  Helical piers look like giant screws.  The fins (or helices) on them are designed based on the soil conditions and they are hydraulically screwed into the earth until they reach a specific torque locking them firmly in place. Once they are torqued in place they are resistant to moving up or down.

Due to the fact that helical piers are anchored so securely most foundations that bear down on them will be poured entirely on top of void boxes.  Over time, when the void boxes break down, the slab and grade beams will effectively be suspended above the soil. This makes the foundation almost entirely resistant to soil movement since it is firmly locked in place deeply by the piers and is floating on top of the problematic soil at the surface.

Helical piers are a great alternative to both standard concrete piers in a pier and beam foundation as well.  It should be noted that helical piers are more expensive than standard piers.

Now that you are aware of some of the more common types of foundations that are available, the most important thing to remember is that the right type of foundation is the one that specifically designed for your home and your lot’s soil conditions.  Soil conditions can vary greatly from one lot to the next, and just because your neighbor or family member has one type of foundation, does not mean that is what is best for you.

When building a custom home, always get a soil report and have your foundation designed specifically for your home by a licensed engineer.  And remember; always ask questions if you are unsure of anything.   Taking these precautionary measures will ensure your home will be built on a foundation that will last for years to come.

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