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Problem Texas Soils
• Description - Expansive or swelling soils, as their name implies, are soils that swell when subjected to moisture. These swelling soils
typically contain clay minerals that attract and absorb water. Another category of expansive soil known as swelling bedrock contains a special
type of mineral called claystone.
When water is added to these expansive clays, the water molecules are pulled into gaps between the clay plates. As more water is
absorbed, the plates are forced further apart, leading to an increase in soil pressure or an expansion of the soil's volume.
• Appearance - Soils containing expansive clays become very sticky when wet and usually are characterized by surface cracks or a
"popcorn" texture when dry. Therefore, the presence of surface cracks is usually an indication of an expansive soil.
Expansive soil showing cracks Expansive soil with "popcorn" texture
• Drilling and Laboratory Analysis - In many cases, expansive soils are buried under a layer of topsoil or dense vegetation and cannot be
identified at the surface. Therefore, collecting soil samples from various test holes each several feet deep is required. These test holes can
be drilled by geotechnical and civil engineering firms or by some construction companies. After the samples are taken, they are sent to a
laboratory where the swelling potential is determined. In areas where there is a high concentration of swelling soils, laboratory analysis of the
soil is required by law. Consult the real estate agency to find out if a swelling soil report is available for your property.
• Inspection of Existing Structures - If construction has already occurred at the site, inspection of the existing structures may help identify
the presence of reactive soils. The photos below illustrate some common problems that may occur as a result of expansive soils.
Driveways, sidewalks, and streets: If concrete slabs were used, look to see if the slab joints are at the same level. If not, heaving caused by
swelling soils has probably occurred. Wavy, "roller-coaster" surfaces may indicate swelling at certain layers or an uneven distribution of
swelling soils. Excessive patching or cracking of the asphalt is also a sign of swelling soils.
Exterior walls: Check for any cracks larger than 1/16 of an inch. Check the angle of the cracks. If they are straight, it is usually a sign of poor
construction. However, if they are angled diagonally, they are most likely the result of a significant movement of the foundation caused by
Cracks in exterior walls, as a result of upward soil expansion
Interior walls, floors, and ceilings: Check for any cracks larger than 1/16 of an inch. Check the angle of the cracks, as above.
Severe damages in a house basement in the Ken Caryl area of South Denver
Doors and windows: Make sure all doors and windows open and close properly and that there are no distorted glass panes.
cracks in exterior walls at doors and windows
• Location - Although expansive soils exist nationwide, certain areas are more severely affected than others. For instance, Colorado, Texas,
North Dakota, South Dakota and Montana, to name a few, have extremely high concentrations of swelling soils. You can view maps of each
state from the menu item "State Soil Maps" in the Main Menu. In this menu, states that contain areas of abundant clay (thus having high
swelling potential) are highlighted.
For further information, consult the state Geological Survey, the state Geology Department, or your local city or county building department to
obtain maps of swelling soil distribution in your area. The phone numbers of these agencies will be listed in the government blue pages of the
So what can you do to help control foundation movement with these expansive soils?
If a homeowner wishes to stop seasonal house and foundation damage, the first course of action should be to follow a controlled watering
program. By keeping the moisture content of the soil under the house foundation constant, foundation movement can often be stopped. This
has been written to assist the homeowner in performing a simple foundation repair preventive maintenance program.
The goal of a foundation repair preventive maintenance watering program is to maintain a constant level of moisture in the soil under the
house and foundation. The best way to water a foundation is to place a soaker hose from one to two feet from the edge of the foundation.
Placing the hose a short distance from the foundation allows the water to soak into the soil evenly.
The hose should not be placed against the foundation. When soil has dried and cracked, water can travel along the cracks for several feet in
all directions. If the soil around your foundation is dried and cracked, then water placed next to the foundation will run through the cracks and
accumulate at the bottom of the grade beam (the thick portion of the foundation that is under the exterior walls). In some cases, an
accumulation of water in the soil at the base of a foundation can cause the soil to lose some of its load-bearing capacity. If the soil loses
enough load-bearing capacity, the house will sink into the ground.
Obviously, it is necessary to water more during hot, dry weather and less during cold, damp weather. The amount of water required to keep a
foundation stable during the summer can be surprisingly large. A single large tree can remove as much as 150 gallons of water, or almost 20
cubic feet of water, from the soil each day. Shrubs and other plants can also remove large quantities of water. During persistent hot dry
weather, it may be necessary to water a foundation daily. Watering should supply enough water to keep the moisture content in the soil under
the foundation constant. If the amount of water applied is only enough to keep the surface damp, the watering program will not work.
Obviously, the homeowner is the only one who can weigh the benefits of controlling foundation movement versus the increased size of the
A sample of an expansive soil
with moderate swell potential.
The same soil sample after a
small amount of water has
been added. Notice the
sample has expanded
The same sample 48 hours
later, after the sample has had
time to shrink to a smaller
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