Let’s look at the three main types of soil tests.
The plasticity test, which is sometimes called the pencil test, is used to determine if the soil is cohesive.
This test is performed by rolling a moist soil sample into a thread that’s one-eighth of an inch thick and
two inches long, resembling a short, slim pencil. If the sample can be held at one end without breaking, it
The thumb penetration test is used to quickly estimate the compressive strength of a cohesive soil
sample. To perform the thumb penetration test, simply press the end of your thumb into a fresh clump of
soil. If the soil sample is Type A, your thumb will only make an indentation in the soil with great effort,
as you can see demonstrated here. If the soil sample is Type B, your thumb will sink into the soil up to the
end of your thumbnail, just like this. If the soil sample is Type C, your thumb will sink all the way into
the soil clump, as you can see here. Your results for this test will probably be somewhere in between
For a more numeric measurement, the pocket penetrometer test can be used. A soil’s compressive
strength can be given a numeric value by using the pocket penetrometer test. There can be some
variability in these results, so it’s a good idea to run this test on a few soil samples from the same part of
the excavation, just to make sure your results are consistent. A pocket penetrometer works much like a
tire pressure gauge. A thin, metal piston is pushed into a soil sample, and the penetrometer records the
compressive strength of the soil. Be sure that the scale indicator is inserted into the penetrometer body
until only the “zero” mark is showing. To conduct the test, push the piston into the soil until it reaches the
engraved line. Then, simply take the reading from the scale indicator. It’s important to recognize that a
penetrometer may give false results if the soil contains rocks or pebbles, which won’t compress.
As you can see, Type A soil will measure at least 1.5 tons per square foot. Be aware, however, that you
can’t classify a soil as Type A if the excavation site didn’t meet all the conditions of the visual test: if it is
granular, near a source of vibration, or there are signs of previously disturbed soil, water seepage, or
fissured soil. For Type B Soil, the reading will be between 0.5 and 1.5 tons per square foot. Type C soils
are equal to or less than 0.5 tons per square foot.
Determining the type of soil on a site will help a competent person decide which methods of sloping,
benching, or shoring are needed to prevent cave-ins and keep workers safe.
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