In my last column, I wrote about bailers for cable-tool drilling. In this installment, I cover another basic part of the tool string: the drive clamps or drive blocks.

Just about every hole drilled, whatever the purpose, requires casing pipe. This pipe must be driven into the earth. We can make some hole out ahead of the casing but, by its own weight, it will only follow a few feet. In a few scattered locations, we might hit bedrock from the start. I know of areas just west of me with virtually no drift on top of the rock, perhaps as little as 6 inches. But, in most cases, we have to drive casing pipe through sand, clay and mixtures of both to get to the formation. We drive this pipe using drive clamps, a process where our drill rig performs like a pile driver.

Drive clamps or blocks are exactly what the name implies: heavy steel sections held together by heavy bolts. They attach to the upper square of the drill stem and drive our casing pipe. These blocks vary from as small as 3-by-3-by-10 inches weighing 65 pounds up to 6-by-6-by-20 inches weighing 475 pounds. The bolts that hold these clamps to the drill stem range from 1½ to 3 inches in diameter. The wrenches to tighten these bolts range from an opening of about 2½ inches, weighing 10 pounds, up to an opening of about 4¾-inches weighing 31 pounds. Most drillers would loosen the bolts and leave the clamps on the top of the casing while drilling, particularly when using clamps over 150 pounds. When the operator pulls the tools from the hole, they pick up the clamps with the top of the drill bit, leaving the full hole diameter available for bailing.

Driving pipe looks like a simple process and, in fact, it is. One installs the drive clamps, engages the spudder clutch and starts driving the casing pipe. However, know the risks associated with this part of the cable-tool operation. Never get hands anywhere near where the drive clamps strike the pipe. Many drillers have lost fingers this way. If you do have to guide the casing, make up a steel tool or use a piece of wood between the casing and your hand. Better yet, just keep your hand at your side and away from the casing as the drive clamp does its work.

If you do any kind of driving at all (other than merely tapping the casing), you need a drive head. Drive heads, which protect the top of the casing, come in several types. The most common type features a male screw connection. This drive head threads into the top coupling of the casing pipe to protect it as the driving takes place. A typical drive head of this type has a wall thickness of at least ½ inch. On larger casing, that thickness goes up to 2 inches. The driller can leave a drive head in place while the tools go down the hole to drill, but will have to unscrew it when removing the tools.

Other types of drive heads include the inside and outside drop types. The inside drop type slides into the coupling and has a shoulder that will drive on top of the coupling. The outside type goes over the coupling itself, also driving on the top. When the tools are removed, these drive heads will come out at the top of the drill bit with no threading or unthreading necessary. You can order these to drive on plain end pipe — especially common with the inside type when one uses welded steel casing. For casing larger than 12 inches, at least one manufacturer I know of only makes the inside type. Drive heads in these sizes can weigh up to 500 pounds or more.

For light-duty driving, I have used a short pipe nipple and a sacrificial coupling that I would thread into the top coupling of the permanent casing. This works, but eventually the nipple cracks since this device has nowhere near the sturdiness of a proper drive head.

Again, please remember that driving casing is the most dangerous part of a cable-tool operation. Use extra care when doing it. Beyond the driving, bolts on the drive clamps can break. When this happens, one wants to be far away from the rig. I once broke a bolt on my drive clamps. Part of the bolt and the nut fastened to it went flying off. This projectile landed safely on the ground, but could have posed a danger to anyone in the area reasonably close to the drill rig.

Next time, I’ll write about the tools we use to remove casing from the earth, and there are a number of these.

I write this in early May and, in southern Michigan, one would think it early March (although we have no snow). This morning it was 45 degrees Fahrenheit when I got up. My infamous lawn, while nice and green, has not grown much and I have yet to mow it. You would still need a light coat to go out and about. It will eventually get warm and humid but, now it is cool and damp. Thankfully, we do not have the drought many areas suffer under. Until next time, work safe and if you are running a cable-tool machine be very careful around those drive blocks.

For more John Schmitt columns, visit www.thedriller.com/schmitt.