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Soils and Building Stability

Few building problems are as significant and expensive to correct as foundation soil problems.

It is often said that a building will not stand on a "weak foundation."

However, even more basic than the built foundation is the earth foundation upon which the building rests. It is important to realize that, as well engineered as a structure may be, the propensity for the soil underneath it to move must also be evaluated. Thus, when inspecting any building for structural integrity, it is vital to also consider the behavior of the soil upon which it rests.

Few building problems are as significant and expensive to correct as foundation soil problems. They can literally cause the building to fall down. However, not all movement is dangerous and some is even normal. Recognizing the difference is the job of the engineer.

Types of Soils

Soils are generally categorized as gravel, sand, silt and clay. Sand consists of particles from near 0.5mm to 5.0mm (about 1/4 inch) in diameter. Gravel ranges from near 5.0mm to 75mm (about 3 inches) in diameter. Anything larger is considered either a cobble or boulder. Silt and clay are fine grained. The difference between the two is that silt consists of very small particles of disintegrated rock (the same material as sand and gravel) whereas clay results from changes or alteration of the original rock material. Soils are further differentiated by the shape of the grains (spherical, flat, etc.) which has a further effect on their stability.

Different soil types predominate in different regions of the country. However, in all regions, samples of all types can be found - even at the same site.

What Makes Soils Unstable?

The ability of the soil to support the foundation is measured by its compressibility or consolidation potential as well as bearing strength. Ideally, a built foundation should not move once put in place. However certain conditions in the soil can cause foundation movement and consequent foundation or structural damage.

In granular soils (sand, gravel), movement often results from a condition in which there are too many voids between particles. A good mixture of particle size normally will increase stability.

Some types of silt deposits termed "loess" are held together because of particle bonding or cementing minerals. These soils may be susceptible to collapse if exposed to excessive amounts of water.

A dramatic type of failure occurs in areas of so-called expansive clays. These soils are prone to large volume changes related to moisture content. They shrink in dry seasons; swell in wet seasons or when otherwise in the presence of water.

Ground freezing is responsible for soil expansion due to "frost heave." Soil deposits containing silts are very prone to severe frost heave. Expansion caused by frost heave is great enough to lift even heavily loaded foundations and slabs.

Some causes of soil movement (e.g. landslides where steep slopes exist) are predictable. Other causes of soil movement are less predictable. Earthquakes have an obvious effect on the building system. Foundations typically are designed to support downward vertical load. However, ground shaking during an earthquake produces horizontal forces. Designers in earthquake prone zones must account for these as well. Deposits of loose sand in these areas can also create further problems as the intense shaking may cause the soil to "liquify."

Certain types of movement are entirely preventable. Before a building foundation is constructed, any fill soil should be properly placed and compacted. There should be no organic material (e.g tree stumps or construction debris) in the soil. These will decay over time, leaving voids that result in unstable conditions.

Underground water has been known to cause severe problems. When a foundation is built on a site, it may interfere with the natural flow of groundwater. Buildings may even be built over aquifers or seasonal flows. The erosive nature of moving water is well known, and voids or excessive pressure build-ups are possible.

Basement walls can shift, crack, and possibly collapse as a result of poor foundation soils, or because of lateral pressures caused by soil and groundwater on the wall exterior. Concrete floor slabs can crack if the supporting soil settles, expands or lifts because of soil expansion or ground freezing.

What Can Be Done to Correct Problems?

The first step in correcting foundation problems caused by soil instability is to determine the cause of the problem. This may be readily apparent to the engineer, based on the area in which the building is located and observable conditions.

In other cases, the cause of the problem involves uncertainty. Investigation and testing will be required. The testing may consist of boring holes and removing "cores" or samples of the soil. The engineer examines the strata (layers) which reveal the composition of the soil. From that, he can determine with a reasonable degree of accuracy what is causing the problem.

The choice of repair technique is a function of the foundation type (spread footings, piles, caissons, or floating foundations) and the type of problem. Sometimes, the repair is not so much to the foundation soil as to the surrounding soil conditions (e.g. construction of retaining walls, drainage improvement, etc.).

Common foundation repairs include:

  • Underpinning whereby the built foundation is extended downward to competent strata.
  • Injection grouting in which grout (a cementitious material) is forced under the slab to level and stabilize the structure.
  • Anchors of various types are installed around the building and either driven into or screwed into the ground.

At the same time, it may also be necessary to replace some existing foundation soil.

Conclusion

The problems that soils can pose for built foundations and the structural integrity of a building can be the most severe faced by any building owner. For protection of the public, state laws typically indicate that only a licensed Professional Engineer (P.E.) can provide an opinion on the structural integrity of a building and scope of structural repairs that may be necessary. Such an opinion is well worth obtaining before any building acquisition.

The source reference for this issue of Engineering Advisor on soils was the engineering text Essentials of Soil Mechanics and Foundations by David F. McCarthy, P.E., owner of Criterium - McCarthy Engineers.

The Engineering Advisor is intended to enhance your knowledge of technical issues relating to buildings. For additional information on any subject, please feel free to call us. Our commitment is to provide you with timely, accurate information

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