While I like Don's solution, I have another idea. Use the auger to dig
an 18-inch diameter hole about 3 feet deep. Put about 6 inches of mixed
concrete in the bottom (I prefer that you mix it ahead of time rather
than pour water over the dry mix in the bottom of the hole). I
recommend adding a shovel full of portland cement to each bag of premix
to get a little higher strength.
When the concrete is hard enough to bear the weight of a post -- usually
the next day -- set some 6x6 pressure treated posts in the holes and
prop up to they are plumb and square.
Now, here's where my method radically differs from Don's: the gravel
has a high clay content so it should be ideal for tamping and
compacting. Using the material you dug out of the holes, put it back in
around the posts tamping it down as you go. Put no more than 6 inches
of material in the hole at one time. You will need to rent or buy a
tamper. These suckers weigh about 20 pounds, which doesn't seem like
much until you've lifted it a few hundred times.
As for the lumber, get material that is especially treated for embedded
post use. The normal stuff you buy for exterior decks won't do. Here's
Embedded poles shall be pressure treated per AWPA C23 for round poles to
a retention of 0.60 pcf and AWPA C24 for sawn poles to a retention of
0.80 pcf. Cuts and Holes in all pressure treated members shall be
treated after fabrication with 2% Copper Napthenate in accordance with
Once the holes are dug you should be able to spread this work over
several weeks or do it in one long weekend.
Bob Morrison, PE, SE
R L Morrison Engineering Co
I would consider an alternative to pressure treated wood as there is some
question of the arsenic content leeching out. Perhaps there is a process
that does not use arsenic or you might consider using steel posts? This is
rendered moot if there is sufficient airflow around these posts to dilute
the arsenic. Just my two cents.
LOL, there are many documented cases of stupidity in the construction field
as you are well aware of. I recall my proffesor telling how he hired his
not so bright cousin to treat the lumber in a large pit of solution because
everyone else had health concerns :)
Why is that? Are there other health concerns or practicality issues?
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