What Will Your Soil Support?

Shandy Heil, Attaway, Denise, Baker, Eugene  |  10/30/2007 10:47:37 PM

The load-bearing capacity of Louisiana soils varies widely. It is important to know the soil type in the area where you plan to build. Often you can determine if you want to select a certain location for your new home or look further by knowing as much as you can about the area. It is wise to know the flood zone, availability of utilities, and believe it or not, the soil type.
 
The site soil bearing capacity must meet or exceed the required soil bearing capacity for the planned design. Testing is to be performed by a qualified geotechnical engineering representative. Listings for these representatives can be found in your telephone book, or online.

Load-bearing capacities for various soil types are listed in the Soil-bearing Capacities table below. The soil types listed are broad descriptions of the common combinations seen in our soils. Many localities require an on-site soil-bearing test before a permit is issued. This permit is always the best way to determine the value of the soil.

The NRCS (National Resources Conservation Service) has some accurate data available to the public at http://websoilsurvey.nrcs.usda.gov/app/. This database contains extensive information on the soils of our country, including soil types at specific locations. The data provided on this site give more specific soil type information than the table above, but most of the soil types can be generalized into one of the categories in the table to determine the soil-bearing capacity except for those soils very high in organic matter, and the general recommendation is those soils are not acceptable to build on unless you provide support piling and take other steps. Soils high in organics require special engineered designs that can add cost to the building project.

As you will see, the NRCS site is user-friendly. When you go to the front page, you will find a list of options in the system. Go to the site and click the green b START WSS button. You will now be able to define your AOI (area of interest). The best way to do this is to enter the address are interested in. If you do not know the address, go to the map and click on the plus (+) button in the upper left corner to increase the scale. Then, select the area of interest. This will bring you to the desired location quickly.

An AOI must be defined. If you use the address, the AOI is a point. If you want to define the AOI as an area, click on one of the two AOI buttons in the upper right. Then move the curser to outline on the map the boundaries of your interest.

Next, click on the Soil Map tab at the top of the page. A table listing the soil types within the AOI boundary appears to the left of the map. You can stop here if you want and relate the soil type to the table shown earlier in this document. Or, you can click on the Soil Data Explorer tab again at the top of the page and get additional information about the soil at your site. The way that is done is to identify the information you want from the list in the table that shows up to the left of the map. The requested information appears to the left of the map. This tab contains many types of information that may be useful to you.

Finally, click on the tab titled Shopping Cart and print the information you have retrieved from the site. The information you obtain from the NRCS site is detailed but does not give the direct value of load-bearing capacity of the soil. It does, however, give the soil type. You can use in the Load-bearing Capacity table to estimate that number for your location.

If you're wondering why you should put so much effort in to determining the soil type on which your house will be resting, Bob Vila says it very well on his Web site:

Foundations rest on soil, soil pushes against their sides, and wet soil pushes water and humidity against them, so it's hard to plan for a foundation without a basic understanding of soils. The average person thinks of soil as dirt. For engineers, soil is a complex material worthy of a lot of study. In fact, there are thousands of soil varieties, but the main categories are gravel, sand, silt, and clay. What separates them is basically the size of the particles. Gravel is made of big chunks; sand consists of grains as small as the width of a human hair; silt is made of still smaller particles that are nearly microscopic in size; clay has particles too small to see. Most soils are blends of these main types, with names like "clayey sand" or "sandy silt." Soil also has air and water mixed into it, so compacting the soil with rollers, pounding or vibrating equipment densifies and strengthens it.

To be absolutely sure of your soil, you have to send a sample to a soils lab. If they find more than 12 percent clay, the clay will be analyzed for its behavior when wet. This is because clay can turn to liquid, reduce the soil's bearing strength, and cause the soil to exert pressure on the foundation. On a large commercial project, soil "borings" are taken vertically in two-foot increments. On a residential project, builders often rely on instinct and rule of thumb, because some building departments don't insist on a soils report. Unfortunately, it can be hard to identify a soil by eye, or to predict its behavior by guesswork. A soil that seems to have a lot of gravel or sand in it could still contain 20 to 30 percent clay. If it does, it's going to act like clay, which can give your project poor drainage and plenty of problems.

Soil-bearing Capacities

Type of Soil

Load-
bearing
Pressure
(lbs/sq.ft.)

Crystalline Bedrock

12,000

Sedimentary Rock

6,000

Sandy Gravel and Gravel

5,000

Sand, Silty Sand, Clayey Sand, Silty Gravel, Clayey Gravel

3,000

Clay, Sandy Clay, Silty Clay, Clayey Silt

2,000

Source: Table 401.4.1; CABO one- and two-family dwelling code: 1995

 
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