DEPTH OF post holes AND HOW IT AFFECTS TIME AND PRICE

One question everyone asks, is, "how many post holes can you drill in one day?  The simple answer is "we average appx 40 post holes in a day of drilling".  The complex answer is that "it is dependent on other factors".  There are many factors that affect the time it takes to dig post holes in the Texas hill country, but the 2 most critical factors are - hardness of earth, and depth of the hole.

In general, most people who need post holes drilled, need them to be anywhere from 18" to 36" deep.  In very soft earth, we can drill 36" deep almost as fast as we can drill 18" deep.  Really!  Its that quick!  3 to 7 minutes per hole.

But, very soft earth in the hill country is about as rare as finding diamonds as we drill.

A hard fact of post hole drilling is that we lose down pressure the deeper down we go.  So, we start off with lots of power to push the auger into the earth, but as we go down we are losing more and more of that power to push down.  So there is a rate of speed difference in in the first 10" of drilling vs. the next 10" of drilling vs. the next 10" of drilling and so on.  It just keeps getting tougher, and slower. 

Years of experience has led me to classify 4 different types of earth situations out here.  1)very soft earth.  2)moderately hard earth.  3)very hard earth.  4)extremely hard earth.

1)very soft earth. - This is typical river bottom earth, and it is rare out here.  It is soft, may contain a few loose rocks, and drills in just a few minutes.  It is rare that anyone would be drilling post holes in this earth unless planting an orchard.
2)moderately hard earth. - This is what 1/3 of you have in the hill country, and you are the lucky 1/3.  It is hard packed caliche and drills in 10 to 15 minutes down to 20" or 24".  Most people who want fences on their property in a subdivision have this type soil, but not necessarily!
3)very hard earth. - This is what1/3 of you have, probably solid rock, and it is hard to drill post holes. With luck, we can drill it in 15 to 30 minutes down to 18" to 22".  This type earth may be rock right at the top of the ground, and it makes it difficult to keep the tip of the auger where we want it, and the post holes end up being off a little bit.  It is slow going, and very wearing on the drilling equipment.  It may be solid limestone, but usually a less dense limestone.  Lighter density, or thinner layers of rock is what allows post holes to be drilled in 30 minutes or so.
4)extremely hard earth. - This is what 1/3 of you have, it is solid rock but it is very dense rock, and may not even be limestone.  This rock does not grind away easily, not even with all the thousands of pounds of pressure my rig applies, and not even with the ferocious carbide tipped rock teeth ripping and grinding away at it.  I have found this type earth especially in places like Canyon Lake, GardenRidge, and Bandera.  But the bad news - it can be found just about any place at all!  I have found it in Driftwood, Canyon Lake, Bandera, GardenRidge, Austin, Bulverde and even in the farmlands north of Austin and Round rock where dense and thick slab rock is 2 ft. down under rich black farmland soil.  Literally, all we can do is sit there, grinding away at it.  And, it takes anywhere from an hour to an hour and half to get down to 20" or 24" of depth, but this is "if".  If we can get down that deep at all in this type of earth condition.  In some cases like GardenRidge, there are microwave oven, and washing machine size rocks lurking underground, and they grab the auger and try to yank and twist the 5000lb drilling rig right off the ground.   Those rocks like that may not move my tractor and rig, but they sure can create stress cracks in the thick plate steel of the drilling platform, and they can snap augers in half, or break solid steel drill stems as if they were pixie sticks.

The bad new is that type 4 earth conditions can be anywhere in the hill country, from the pretty green lawn in Austin, to the soft, sandy 'looking' soils in Bulverde, and everywhere in between and all around!  And even worse news is that you can not tell by the look of the surface.  Not even I can tell.

That being said - now imagine if you place a stipulation on depth.  For example you are building a fence that could be built using 18" or 20" post holes, but you want 30" post holes, or that fresh out of college engineer tells you that 30" post holes are a requirement! (this is happening more and more by the way!)  Maybe you are in an area where I can drill a 20" post hole in 15 or 20 minutes.  But that last 10 inches to get you your full 30" may take the same amount of time, another 15 or 20 minutes.  Multiply that times your number of post holes, and you see how hardness of earth and depth of hole affect price.  I hate to say it, but I refuse to ever guarantee more than 24" of depth.  Yes, we routinely get 30" plus, but I can not guarantee that.  If you don't mind me parking over a hole and grinding away at it for half a day, let me know, and I may quote you a price to do that - but it's no fun, and it wrecks my equipment.  There are lots of drillers out there who say "oh yeah, we can get you 36" of depth".  But at the end of the day, they will tell you, "oh, well, we said our auger is 36" long, but that doesn't mean we can always get that deep".  

Well, I'm telling you now, honestly, and up front so you know the facts.  My auger is 36" long, and I can not always drill post holes that deep, and when I can, it may use up lots of precious time, and time is your money.

Because the ground is so solid and stable out here in the Texas Hill Country, you really don't need much more than 18" to 20" of hole depth to end up with a very solid post when building a fence, or other general post needs.  The width of the hole does not seem to be as much of a factor in the time it takes to produce a hole, until we get to the wider augers like the 12" and 14" augers, and larger.

Hopefully, your earth digging needs won't be this extreme!

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