When a round shaped object touches
another object, it does so at one small point. A flexible round
object, like a tire or a water balloon, then conforms to the shape of
whatever it is touching, which is why an under-inflated tire has a
flat-looking spot where it touches the road.
A rigid round object then presents a
single point of contact and maintains that single point until forces
overwhelm the rigidity.
Now look at the way most fence posts
are set. You bore a round hole into the ground, three, four, maybe
even six feet, then you put your post in the hole with some concrete.
The concrete hardens. Now, whenever there is a side load on that
post, the round shape of the concrete plug you created presses into
the surrounding earth at a single point, directly opposite the line
of force applied.
This concentrates that load onto a very
small point of earth, which can cause it to compact, or to shear, or
to simply slide past the post and its anchor, as the force causes the
post to move away from the applied force. Think of a rolling pin forcing pie dough into a flat shape as it rolls.
One way to counter that and achieve a
side load bearing capacity much higher than what a round concrete
post foundation offers is to dig a square hole. You might even start
with an auger and shape the hole after the majority of the dirt has
been removed. What does this accomplish?
Once the concrete is poured around a
post in a square hole, the resulting block of concrete is contacting
the earth over several square feet, instead of a single point, or
line opposite the application of force. Take for example an 18”
hole, four feet deep. If the hole is round, the surface area
transmitting a force to surrounding earth is pretty much just a four
foot long straight line.
Same 18” hole, made square, affords
six square feet of surface area to spread the load to surrounding
earth. That flat face is much less likely to compress the earth on
the “downhill” side of a force. This means the post stays more
stable, right where you want it.
