The Soil under Your Foundation

Soil types vary from region to region throughout the United States.  Some soils are more prone to swelling and shrinking than other types of soil.  It is these soils that are more likely to cause foundation problems than any other type of soil.  This is particularly true in homes with shallow foundations.

Houses are built in areas where there is a strong demand.  That does not necessarily mean that the soil in that particular area is optimal for house building.  In fact, in most cases, it is not. So, how can you find out what kind of soil is under your house’s foundation?  There are a few things you can do to find out.

First of all, there is the possibility of a Soils Report available to you in many housing developments.   In some areas, it is mandatory and others it is not. Inquire to see if there is one available for your area.

Also, there is a publication called a ‘soil survey’ that has been published for over 100 years.  It includes explanations of every major soil type in each particular area surveyed along with maps.  It is available through the USDA Natural Resources Conservation Service.

The most excellent way to find out about the soil under your home is to call on a Geotechnical Engineer.  A skilled Geotechnical Engineer will come to your house and take samples of the soil via soil borings around your property.  The samples of soil will be tested to see exactly how expansive the soil is. The tests will also determine at what depth the soil expansive occurs.  

You will be given a written report with the test results prepared for you by the Geotechnical Engineer.  One of the things the report will tell you is what type of soil you have under your house. Different soil types include sand, rock, select fill and expansive clay.

Sand does not change when moisture levels change.  Still yet, if soil cracks due to dryness, sand can get into the cracks and eventually cause the foundation to shift horizontally.  Also, improper drainage that allows an abundance of surface water under the house can cause the sand to erode, which can then cause foundational issues.

Rock soil can have faults or fracture in it, especially on slopes.  This increases the chances of the foundation sliding. To avoid this, the foundation must have tie back anchors securely pinned to it.  Mostly though, only rock with a low shale density will expand or erode. And this would be very minimal.

Select fill is a type of soil that has a sandy loam that is experiences virtually little to no change in moisture levels.  However, in the event of erosion, the potential for the bearing capacity of the soil to be altered is a possible threat.

Soils with expansive clay tend to shrink when dry and heave (or swell) when wet.  If the foundation is shallow, it is most likely to be inclined to move when water levels rise and fall in the active zone.  

The ‘active zone’ is the area approximately 18 foot deep from the ground surface downward, where water moisture levels within the expansive soils are most affected as a result of seasonal changes.  

Some structures have more than one soil condition underneath them.  If the foundation of a house is not properly designed to suit the multiple soil types, differential movement of the foundation can occur and cause damage to the foundation and the house.

It is important to know as much as possible about the soil upon which your house is built.  This will help you properly maintain your foundation and prevent costly repairs in the future.