(continuation...)
After I have managed to get through the layer of clay at 4 feet, the soil composition becomes a mixture of brown sand and clay, with the percentage of sand increasing as I go deeper. Here the electric auger works just fine, and has saved me loads of hard manual work.
I've tried lots of different bit designs on the electric auger, but the one that works the best is the bit from a manual post hole auger bit.
I make a temporary mark on the electric auger's vertical tube to keep track of drilling progress and let the auger do the work. I have found that after it has dug about 4 inches, the auger bucket is full, so I use the shop vac down the hole to vacuum out the debris and then drill down another 4 inches, etc.
Sometimes when working in this mode, the auger is turning but stops progressing, so I have found that rocking the auger stand back and forth seems to concentrate the force on an edge of the auger blade, and the auger digs on down again.
I can usually get from 5 feet to 8 feet pretty fast, maybe an hour's time.
6 FEET TO 9 FEET
At about eight feet, I start seeing black sand grains mixed in with the brown sand, so I know that the wet black sand layer is not far away.
After every five or ten buckets, I try removing the auger from the hole and try using the shop vac alone. If it works poorly, I continue with the auger, if it works well, I switch methods and use the shop vac alone. I put the 45 degree tip on and rotate the PVC pipe back and forth. This way the pipe acts both as a drill and as a debris remover, in one operation. The shop vac method is easy, fast and much safer than the electric auger.
I have used two shop vacs, one is from Lowe's and is rated at 4 HP. The other is a Rigid rated at 5.5 HP. Using a shop vac for drilling a borehole into the earth is at the very least 'cruel and unusual punishment'. As I said before, I estimated that the dirt removed from each 17 foot deep hole weighs about a half ton of dirt and rocks. The Rigid is up to the task, the Lowe's shop vac is not. This is not necessarily an endorsement for Rigid shop vacs, rather advice on choosing the right tool for the job.
I have found that the 2 inch PVC pipe works best. It sucks up great volumes of dirt and a surprising amount of stones get drawn up as well. I finally wised up and added a larger-radius PVC 90 degree turn (NOTE: large radius turns have less friction loss than sharp 90 degree elbows) to the top of the PVC pipe, and use a 2 inch rubber plumbing adapter to connect the shop vac hose to the PVC.
I use the hose clamp on the shop vac side, and use the rubber side loose on the PVC side. When the shop vac is running, the suction keeps it all together very well. The surprise benefit is that the 90 degree bend gives me a handle for rotating the pipe. And best of all, no more vacuum-hose kinks.
Blockage of the suction line is a very common occurrence. It usually occurs from stones, but can be a glop of wet sand, a leaf fallen into the hole and wet sand can do it to.
At first I thought blockage was a setback, but now I see that every blockage removed is just that much more material that is not in the hole any more.
The rubber slip-fit mentioned above, allows me to quickly disconnect the shop vac from the PVC to determine the location of blockage. I can tell when things get blocked because the shop vac will increase its RPMs dramatically. If the RPMs reduce to normal when the slip-fit is separated, it means that the vertical pipe (see photo on left above) or the PVC 90 degree bend is plugged (see photo on right above), usually but not always at the tip.
If the shop vac is still reving very high, it means that the hose is blocked or a stone has gotten lodged near the opening of the vac unit.
I have become so attuned to this process, that I can tell if the blockage is a round stone completely blocking things, or a flat stone causing a partial blockage.
9 TO 12 FEET
In my situation, more black sand means less clay and very fast removal, but it also means more and larger stones, which present unique problems.
William_Hackerson suggested that we needed some kind of down-the-hole claw to remove rocks. Here's what I came up with:
Here's another shot with a carpenter's tape for scale, along with a rock I hauled out of the hole using the claw:
If part of the claw looks like a wrecking bar, there's a very good reason.
(* Drilling Report Aug 31, 2009 Part Three ... coming soon)
Best Regards,
AC_Hacker