Frequently Asked Questions

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. The homes foundation may be built to be strong, but the conditions of the soil beneath must be considered to be sure that the foundation is strong. Weather or not the soil will move over time or under certain conditions such as when water is added, when erosion occurs, etc. need to be evaluated. Thus, when building or buying a home, it is vital to consider the behavior of the soil upon which it rests. Few problems are as significant and expensive to correct as foundation soil problems. They can literally cause the building to fall down. Not all movement is dangerous and some is normal. Recognizing the difference is the job of an engineer.

All areas within Iron County and Washington County, including Cities and Towns, require soils reports for all new subdivisions and testing on individual lots.

In our fully equipped soils lab we can run the tests needed to determine the soil conditions and our engineers and geologists can give recommendations on site preparation and corrective measures.

Types of Soils

Here in this area of Southern Utah we have a wide variety of soil types. Different soil types can be found even at the same site.

Soils generally fall into the following categories:

•  Gravel
•  Sand
•  Silt
•  Clay

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.

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 are held together because of particle bonding or cementing minerals. These soils may be susceptible to collapse if exposed to excessive amounts of water. These soils are called Hydro-collapsible Soil.

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, thus causing massive damage to structures and pavements.

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 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. Designs for earthquake prone zones must be accounted for as well. Deposits of loose sand in these areas can also create further problems as the intense shaking may cause the soil to "liquefy."

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 (i.e. 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 builds-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.

What is Hydro-Collapsible Soil?

Soil that is susceptible to collapse when exposed to excessive amounts of water.

What is Compaction?

Densifying the soil to minimize settlement potential.

Do we have Hydro-collapsible soils and expansive clays in Southern Utah?

YES

Here in Cedar City?

YES

What Can Be Done to Correct Problems?

A Geotechnical Soils Report can be prepared for the site and the recommendations for site preparation and grading, foundation type and depth, soil corrosion, moisture protection, etc. can be implemented before and during construction.

If the structure has already experienced distress, there are corrective measures that can be taken. 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 soil type and the area in which the building is located and the observable conditions.

In other cases, when the cause of the problem is uncertain, investigation and testing will be required. The testing may consist of boring holes or test holes and removing “cores” or samples of the soil. The engineer will examine the layers to 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 determined by the type of foundation (spread footings, piles, caissons, or floating foundations) and the type of soil problem. Sometimes, the repair is not so much to the foundation soil as to the surrounding soil conditions (i.e. construction of retaining walls, drainage improvements, etc.)

Common Foundation Repairs

  • Underpinning whereby the foundation is extended downward to competent layers.
  • Injection grouting in which grout is forced under the slab to level and stabilize the structure.
  • Anchors/Piers of various types are installed around the building and either driven in or screwed into the ground.