Use of a Pocket Penetrometer
There are many ways to identify the load bearing
capacity of soil. You can take soil samples and send
them to a laboratory. You can ask an engineer who
has been specially trained in the analysis of soil.
Or, you can do a very good job with a tool called
a pocket penetrometer. If you follow a few simple
directions, you can get a very good idea of the load
bearing capacity of soil. And it won't take long.
First, the scale of a pocket penetrometer is graduated
in tons per square foot. To use the charts in a factory
manual, we'll have to convert that to pounds per square
foot, so we'll multiply that by 2,000 pounds. For
instance, the number 1 on the pocket penetrometer
translates to 2,000 pounds or 1 ton. 1.5 means 3,000
pounds or 1.5 tons, and so on. The charts in most
manuals go to 4,000 pounds. Any values you find above
that will still be considered 4,000 pounds.
Notice that there is a spring in the handle of the
pocket penetrometer and that the body of the mechanism
slides into the handle. The tip of the probe has a
little grove around it exactly 1/4" up from the
bottom. This is the part that is inserted into undisturbed
soil that has the same moisture content as the soil
under the home. What this means is that you can't
test sunbaked soil or loose fill. Never place a home
on loose fill. Any fill needed should be compacted
to the original density of the soil prior to its being
Finding the same moisture content may require that
you dig into the ground 6" 8", though in
some areas your footings will not be down that deep.
In other areas of deep frost you will have to test
the soil at the bottom of the footings how ever deep
they are. What you are looking for is ground that
has not been affected by the sun, just like the ground
under the home will be after the home is installed.
That's where you should take your readings. Insert
the probe to the little grove on the bottom then read
the placement of the red ring up the barrel. If you
take five readings in the same general area, throw
out the high and low readings, average the remaining
three readings, this gives you a fairly accurate reading
of the load bearing capacity of the soil.
There are a few things to watch out for. Pocket penetrometers
do not work on sand or gravel. If you have sand use
a 1500 lb. load bearing capacity. If you have a layer
of gravel test the soil under the gravel. Don't put
the Penetrometer on stones larger than it's tip, this
will cause an inaccurate reading.
The time it takes to accomplish this task is certainly
less than a couple of minutes and will ensure the
success of the rest of the installation. We didn't
have to do this 40 years ago in the trailer business,
but we're not in that business anymore.