FAQ

This page attempts to answer the frequently asked questions (FAQ) we receive at Atkinson Engineering, Inc., about residential foundations and foundation repair atop expansive soil. It is not meant to provide professional engineering services, so please call your engineer to discuss your particular concern. It is not meant to provide legal services, so please call your lawyer to discuss legal matters.

Please feel free to e-mail or call us with your comments about this page. It is divided into three sections: concrete slab-on-ground foundations, pier-and-beam foundations, and other questions.
Concrete Slab-on-Ground Foundations

My slab is cracked. What’s the best thing to do? Usually, nothing. Concrete cracks. That is why concrete is reinforced: to control cracking so that it is not objectionable.

What’s important about a foundation is not whether it has one crack or one hundred cracks, but how the foundation performs its intended purpose. (Engineers call this serviceability.) The things that are important about a foundation include the following:

    Is it safe, so that it can support design loads?
    Did it bend so much that doors won’t work or structural joints separated?
    Do conditions make the house unlivable?
    Did it bend so much that masonry veneer and wallboard have cracked?
    Do repaired masonry and wallboard cracks recrack?
    Has a slab crack reflected into ceramic floor tile?
    Do the conditions make the house unattractive?
    Do the conditions affect the value of my house?

How does expansive clay make my floor out of level? Expansive clay shrinks when it is dry, and swells when it is wet. The potential vertical rise (PVR) of the clay soil in Central Texas can be as high as several inches over a drought cycle. If the soil moisture content is kept constant, then the foundation will not move, but it is impossible to keep the soil moisture content constant, because the maximum depth of seasonal moisture change can be as deep as 15 to 20 feet.

Why do Central Texas foundations deform concave down? Foundations in Central Texas look as though their center has lifted (the center-lift condition) because of differences in soil moisture content. Before the foundation was constructed, the soil moisture content values should have been constant across the footprint of the foundation. When the foundation covered the soil there, then the soil under the house was no longer subject to evapotranspiration. Because Central Texas is arid, over time the soil under the middle of a house will become wetter than the soil at the perimeter. Because clay swells when it is wet and shrinks when it is dry, the foundation heaves in the middle. You can’t tell by appearance whether the center lifted or the edge subsided.

In Central Texas, it can take five or more years for the initial moisture dome to stabilize. The establishment of the initial moisture dome usually causes the worst of the damage from foundation deflection. Afterward, the foundation is subject to cyclic perimeter lifting and lowering from seasonal changes in soil moisture content. For example, because Austin normally gets most of its annual rainfall in May and October, since the summers can be quite dry, and since evapotranspiration is less in the winter, most homeowners with moving foundations find that cracks widen in the summer and close in the winter.

What if I support the foundation perimeter atop deep pilings, to jack the perimeter level with the interior?This is not the logical solution you would expect: you will have a temporary repair, and the repair could ultimately make conditions worse. This is because one day we will have a drought that will dry the soil under the middle of your house, and the soil then will shrink. When the soil shrinks, the slab interior will subside, but the foundation perimeter will remain supported wholly by the perimeter underpinning piers. This causes a lot of damage. Door frames can widen so much that the doors will not latch, walls can pull from the ceiling, and walls can lift from the floor. [I wrote a paper about this very topic. See “The Myth of Residential Foundation Repair by Perimeter Underpinning,” Proceedings of the Texas Section Meeting of the American Society of Civil Engineers, El Paso, Texas (October 2010).]

What if I support the foundation perimeter atop deep piers, jack the perimeter level with the interior, and fill the resulting sub-slab void with grout? Then the slab will remain fully in contact with the soil, so that if the soil swells, the slab will heave. Most foundation repair warranties do not cover swelling or heaving. The soil-supported interior still will subside when the soil dries.

How can I relevel my house without using piers? Use either mud-jacking, or else a combination of temporary footings and mud-jacking. Mud-jacking alone is capable of releveling a foundation by the hydraulic pressure of the pumped grout, but temporary footings can assist in setting key areas to the desired elevation until continuous support can be provided by the grout. The releveling is not a permament remedy, because the foundation still will be subject to the shrink-swell behavior of the soil.

How can I repair the cracks in my foundation? Assuming no vertical displacement across the cracks, epoxy repair cement can be injected fully into the cracks to restore shear capacity across the cracks and to provide flexural continuity across the cracks. If the foundation is post-tensioned, the tendons can be restressed after the epoxy has cured. These steps may be sufficient to restore the foundation to its original integrity.

My house has cracks in gypsumboard and masonry, and doors stick in their frames. What do I do? Probably in most cases the cost effective choice is to perform comprehensive cosmetic repairs of cracked components and to adjust doors as needed. (See the questions below about crack repair.) This might be required every few years because of the cyclic deformation of the foundation, but you can fix a lot of cracks for the price of one underpinning pier.

Can sub-slab plumbing leaks cause this type of damage? Yes. This is because leakage contributes to the sources of moisture that cause foundation deflection. Insurance companies tell us that half of the houses that have symptoms of excessive foundation deflection also have openings in their sanitary sewer system.

How can I see whether my sewer pipes have a sub-slab opening? Get a licensed plumber to conduct a simple hydrostatic test of your sanitary sewer. In this test, the sewer pipe is plugged outside the house, and the sewer pipes are filled with water to the top of the floor slab. If the water level holds, then the pipes must be water tight. If the water level drops, then the pipes have one or more openings in them. Not all sewer openings leak under actual use, however: just because a sewer pipe has an opening in it does not mean that the pipe leaks effluent under normal use.

How can I tell whether a sewer opening leaks? To tell whether a sewer opening leaks under normal use, you can perform a test that measures effluent loss under normal use. The test is called a hydrodynamic flow test. You will want to test both the suspect fixtures and pipes and a non-suspect reference fixture. See our Atkinson Engineering Standard for Sewer flow Test.

Do trees affect foundation behavior? Indeed. A nearby tree can have a profound effect on foundations, especially during drought, enough that a floor slab will tilt toward the tree. This phenomenon has been written about at length.

Does drainage affect foundation behavior? Yes. In general, because surface and ground water stack up on the uphill side of a foundation and flow away on the downhill side of a foundation, the tendency for slabs atop sloping expansive soil is to tilt downhill. In my experience, however, nearby trees trump the influence of drainage.

I want to repair my slab-on-ground foundation so that it never again is a problem. How do I do that? If your slab-on-ground foundation is performing poorly because of expansive soil, then the engineering consensus is to repair it according to the Texas Section of the American Society of Civil Engineers’ document, Guidelines for the Evaluation and Repair of Residential Foundations

I was one of the authors of that document. I wrote a commentary about it. See Texas ASCE Guidelines for the Evaluation and Repair of Residential Foundations.
Pier-and-Beam Foundations

As above, we will assume that the area of concern is Central Texas, and that the soil is an expansive clay.

Why do the cedar posts under my house appear to have settled? Cedar is not treated lumber, and with time it will rot. The rotting progresses from the bottom up and the outside in, so that the bottom of the post looks like it was placed in a giant pencil sharpener. The sharpened post pushes into the ground. Most houses in which we have seen this were fifty or more years old.

Why do the concrete piers move under my house? Probably because they were not founded deeply enough below the maximum depth of seasonal moisture change, and perhaps because they were constructed with too little reinforcement to resist uplift. In Central Texas, the design depth for concrete piers in deep expansive soil typically ranges from twelve to thirty feet, depending upon the geologic formation and the site slope.

How do I determine how to construct piers for replacement or new construction? You do this by first getting a comprehensive geotechnical report. This report should include pier design parameters such as founding depth, allowable end-bearing pressure, design skin friction, design uplift pressure, design settlement, and lateral load considerations. The report also may address the appropriateness of particular foundation types, and anticipated site difficulties such as ground water and caving soil.

Second, the engineer performs calculations to design the foundation components so that the structural dead and live loads have a direct path to the piers, and all can safely support the design loads with appropriate serviceability according to the requirements of the Building Code.

Does my settled pier-and-beam house need more piers? Usually not, unless the occupancy has increased the service loads. Engineering calculations can determine whether structural members are compliant with the Building Code, and whether additional foundation elements are needed.

Can my house be releveled by shimming atop the existing concrete piers? Usually, yes. Often the apparent settlement has resulted from soil moisture content changes caused by the drying effects of tree roots. The longevity of such a repair then depends upon the ability to minimize seasonal changes in soil moisture content.
Other Questions

How can I tell whether a plumbing leak has damaged my foundation? This may require a comprehensive engineering investigation. It includes examination of the house, topographic measurement of the floor, comprehensive plumbing testing including measuring leak loss rates, and evaluation of the soil moisture content values.

I have three bids from foundation repair contractors. Which is best? This question cannot be answered without knowledge of the geotechnical properties of the soil, and evaluation of the material and structural properties of the foundation. A better approach is first to retain an engineer to ascertain the need for foundation repair, to propose repair choices for your consideration, to design the chosen foundation repair plan (based upon a geotechnical investigation and the properties of the foundation); and then to solicit bids from repair contractors to perform the repairs according to the engineer’s plans.

What is a “threshold-type crack”? Threshold type cracks are as narrow as “at the threshold of detectability” and as wide as 3 mm. They occur at joints between either similar or dissimilar materials, but do not progress diagonally across the material.

What is a “minor-type crack”? Minor type cracks have thicknesses ranging from 3 to 6 mm if they occur at joints, or from hairline to 6 mm if they occur diagonally across the material.

How do I fix threshold-type cracks in gypsumboard? Threshold-type cracks at joints can be repaired by sanding the joint down to the paper surface, securing the panels with additional wallboard screws spaced 3 inches on center, and joint taping with fiberglass-reinforced repair tape, floating the taped joint with setting compounds appropriate for each layer, texturing, priming, and painting the surface. (Unless the separated panels are refastened with additional screws, they have no resistance to recracking.)

How do I fix minor-type cracks in gypsumboard? Minor-type diagonal cracks can be repaired by replacing as large a panel as is practical with new wallboard, securing the board with screws, (after installing backup boards at nonsecured joints), and finishing the straight joints as above. (Replacing panels with minor-type cracks is necessary, because panel loads cannot be transferred across a crack with just the joint tape. The new panel is resistant to cracking at the same area because the new panel is installed without stress; think of it as having been stress relieved.) Thorough attention to detail in the repair of wallboard cracks can reduce the likelihood of their return, but repaired areas can recrack if structural movement is sufficient.

What foundation repair choice is most often overlooked? The “null” choice: do nothing. I suspect that it is what most people do. This choice should be part of the matrix of costs and benefits in determining the appropriate foundation repair choice.

Give an example of why a cost-benefit analysis is advised. Assume that you are selling your house. In its present condition, because it “needs foundation repairs,” it is worth $100,000 and you have a qualified buyer. If the foundation were repaired, however, the house would have a market value of $120,000. The low bid for foundation repairs is $20,000, and the interior restoration thereafter is projected to cost another $10,000. Instead of taking the risk and the time for foundation repair, and then losing $10,000 on the investment, do nothing and save yourself $10,000.