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Post Hole Digging in Clay and Rock

Updated on January 10, 2014
Post hole auger operated by 3 point hitch on John Deere Tractor
Post hole auger operated by 3 point hitch on John Deere Tractor | Source

Power Auger Drilling

I thought drilling post holes with a power auger would be really easy,
but as I sat on the tractor with the bit going nowhere, I learned
otherwise. In soft black dirt, yes, a power auger will bore a hole in
less than a minute. Other types of soil, however, can be a challenge.
This article will focus mostly on hard clays but I'll give some tips
for rocky soil too. Auger bits are generally made of sheet metal with
two opposing flutes and radial cutting edges and some sort of cast tip
in the center. The bits hold replaceable cutting inserts. Mine holds
two per bit.

Auger Bit Configuration and Operation

First off, for the hard clay and probably for rocky soil too, it
seemed that two cutting inserts per bit provided too much surface area
of the bit to touch the ground therefore reducing contact pressure at
the cutting edge. To combat this, I removed the two inserts closest
to the center of the bit leaving just the two outer ones. This
effectively doubled the contact pressure and I noticed a lot more
aggression when digging.

When digging, the tractor uses the weight of the auger attachment to
feed the bit into the ground. The tractor routes power from the
engine to the PTO drive to turn the bit. Be careful to get a feel for
what the bit is doing in the ground. Running the engine at full
throttle could cause the shear bolts to fail. This is a safety
measure designed into the auger where something cheap and easy to
replace (bolts) will fail before something more critical (like the
gearbox) does. It is a pain if this happens, but better than
something else failing. Have some spares on hand and be sure to use
Grade 2 to ensure they work properly. Running at minimum throttle can
be unproductive but less likely to be damaging if the bit catches on a
root or a rock. I found a sweet spot of about 1/4-1/3 throttle where
the bit dug most aggressively

Digging in Clay and Rock

Dry clay can still be tough; removing the cutting inserts was only
marginally helpful. I found that if I could get the bit to dig in a
couple inches, I could fill the pilot hole with a bucket of water.
Letting the water sit for a couple hours would soften it enough to
productively continue. The water can stay in the hole when digging as
it will continue to soften and lubricate as the bit digs deeper.

Rocks are a whole other world of frustration. Rocking the tractor
back and forth a little bit while digging can help, just don’t rock
too far or you will stress the auger in ways it wasn’t designed for.
Stubborn ground may need to be broken up periodically with a shovel.
Keep extra cutting inserts and replace as they dull. They can be
re-sharpened with a bench file.

When digging, some of the swarf will fall back into the hole. I've
had the most success with emptying the hole by stopping the bit while
it is in the hole. I then slowly raise it straight up and pull the
tractor forward once the bit is clear of the hole. Once away from the
hole, the bit can be cleared of all the dirt it pulled from the hole
by running it in the air for one or two revolutions.

If you are having trouble digging in clay or rocky soil, It is nice to
know you aren’t alone or crazy. This is a lot of work, even for power
tools. Use these tips to your advantage and hopefully you will have
an easier time of it.

Guest Author: Capt. Casey Coolich


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