Soil types Strength and Practical Methods for Measuring Soil Strength and Bearing Capacity

Baker Fred "Gene"  |  9/14/2007 8:24:57 PM

For an average person, soil might just be nothing but dirt on which you walk and never pay attention. But for engineers it is a complex material that must be studied.

Soil is very important for any structure because the foundation carries the structure and the soil under the foundation carries both. Therefore, knowing the type of the soil you have on your lot before building your house is very crucial.

There are thousands types of soils in the world. They can be combined under four main categories: gravel, sand, silt and clay. The difference between them is basically their size. Most soils are mixtures of these main types and named accordingly such as “clayey sand,” “sandy silt,” etc.

The type of soil is important because it gives us a indication of how the soil will react under the load. This soil strength is called bearing capacity. Bearing capacity of soil is the ability of the soil to carry the loads applied to the ground without failing.

For the stability of your structure, you would like to have soil that has good bearing capacity. Gravel and sand are the soils with higher bearing capacity while silts and clays typically have lower capacities. In order to make sure of your soil and its strength, you need professional help. The best and most accurate option is sending a sample to a soil lab to be analyzed. This is required for commercial projects; however, some building departments do not necessitate on a “costly” soils report for residential projects. Soil bearing capacities are often determined on site with special equipment. This is required in some cases and is very accurate but also costly.

It is almost impossible to visually identify soil. It might look like it contains a lot of gravel and sand but still could contain clay. If it has more than 20% of clay, it would behave like clay and it can give your structure lots of problems. Still there are some practical ways to learn more about your soil type. You can dig a small hole in your lot and take a handful of soil from the bottom of the hole and make a ball in your hands. If it crumbles after releasing, it means it has lots of sand and gravel in it. If it holds, it is silt. If it still stays in a ball shape after you drop it from 2-3 feet, it is more than likely clay.

If you don’t mind playing with the soil little more, you can try the worm test. Roll the ball of soil with your hands in to a shape of a worm or a noodle. If it becomes a pencil shape without getting crumbled, it is very likely to be clay, which is not very good news. You need professional help if it is the case.

Another practical way to measure your soil’s bearing capacity is using a hand penetrometer. It is a small and relatively inexpensive device. It gives decent results. Still, the results measured by a penetrometer should only be used as a guide in estimating soil strengths. These values should not be used for design purposes.

Writer: Emre Ergen, Graduate Student, University of New Orleans
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