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You've Got To Do The Dirt
By George Porter
In the last five years the installation business has changed
drastically. It is no longer just hard work and good intentions,
it is a science. The truth is it always was but it was not as
critical as it is today. Homes today weigh two or three times
as much as they did twenty years ago and it is necessary that
you "do the science" if you want them to "be all
they can be".
The easiest science is the load bearing capacity of soil. (When
you are talking about science you should call the dirt soil, it
makes you seem smarter) Some kinds of soil have a good perk rate
for septic systems and some kinds don't. Certain soils can grow
good tomatoes and other kinds can't. Not all dirt is the same
and one of the many differences is load bearing capacity. This
is a measure of how much weight the soil can hold per square foot
before it collapses and lets the weight sink into the ground.
"Before Science" is also known as "BS".
Back then you would simply drive your truck on the lot and see
what happened. If your truck got stuck then you knew the ground
was soft and you had to put more blocks under the home than usual
so it would not settle out of level too fast. With any luck it
might stay fairly level for the whole warranty period, if not
then the deal would be that you would re-level the home one time
only for free, this is BS.
Can you name another building industry that says such things?,
in writing no less! If you contracted for a new skyscraper would
you be happy with such a contract? You don't want to have to re-level
skyscrapers, or high schools, or bridges. Can you imagine how
much trouble that would be? Actually you would never have the
problem because no buyer would ever accept such a contract. The
builders plan to get it right the first time and never have to
go back and waste their money and time messing with it. They use
science and have done so for thousands of years, it is time for
us to do the same thing and it is easy.
Step one is to stop thinking in terms of BS and decide that
it is a new world and you are going to be part of it.
Step two is to get a Pocket Penetrometer. Nearly every factory
manual mentions one and how to use it and without it you have
no idea of the load bearing capacity of the soil. The device could
not be easier to use, just follow the directions that come with
it and stick it in the dirt. when you read the scale on the side
of the instrument it will tell you the load bearing capacity of
the soil. All this takes about five to ten seconds and can not
be called too much trouble, it's a lot easier than re-leveling
Of course you can do this wrong so here are a few things to
- 1. Make sure that the soil you are testing is in the same
condition that it will be under the home. Do not test soil that
is baked by the sun, the sun doesn't shine under the home. You
should dig down a few inches until there are some signs of a
stabilized moisture content. This is usually indicated by a slight
change in color and is probably no more than six inches under
- 2. You have to test the soil that is holding the home. If
you have a footing that is three feet deep then you have to test
the soil in the bottom of the three foot hole. If you test the
soil at the top then shovel it out you have accomplished nothing.
- 3. The soil must be undisturbed or compacted fill, not just
freshly dumped dirt with a few truck tracks in it. Testing five
feet of loose fill is a waste of time. The whole five feet will
settle and you are only testing the top 1/4 inch, you're not
supposed to put a home on that stuff anyway. This is an excellent
example of Before Science (BS).
- 4. You have to have site preparation with proper drainage.
If you don't do this you can forget the whole thing. Dirt that
gets flooded with water is called mud and it won't hold a thing.
- 5. The Pocket Penetrometer will not measure sand. Sand has
a load bearing capacity of 1500 pounds per square foot. The tool
only measures cohesive soils, this means dirt that will sort
of stick together, like clay. Most of the dirt in the world is
OK, so we followed the directions and we found out that the
dirt where we are going to put the home has a load bearing capacity
of 2000 lbs/ft, what does that mean?
It means that you can put 2000 pounds on one square foot of it
and it won't sink, unless something changes the dirt, like water.
Now here's the hard part; If you have to hold 4000 lbs. how
many square feet would you have to cover? If one square foot will
hold 2000 lbs. then you will need to have 2 of them to hold 4000
lbs. The answer is 2 sq. ft. or about 17 inches x 17 inches. Now
that didn't hurt did it?
Just to be sure that you understand that it really is very
simple, suppose you have a load bearing capacity of 1500 lbs./ft.
(sand) and you need to hold 4500 lbs. If one square foot can hold
1500 lbs. then divide 1500 into 4500 and you get 3, that's how
many square feet of footing you need. 3 sq/ft is a footing that
measures about 21 inches x 21 inches. Do this and the pier will
never settle, it is one of the laws of nature.
You have just performed the craft of the ancient Roman and
Greek engineers who built structures that are still standing today
after thousands of years. How do you know how much weight you
have to hold? That is the topic of another article, this time
we are just doing the dirt.