CHAPTER 9

ROCK DRILLING

This is a water well drilling rig in the late 20th century AD:



First you have to level the rig or the hole that's drilled won't go straight down. This rig is just about to be levelled.

The tall mast is a gantry. It has ten drill stems or drill pipes, each 20 feet long:



They're stored in a circle, a carousel that turns around so each drill pipe in turn can be moved out to be lowered down and screwed into the top end above the drill bit that's always at the bottom next to what's being drilled into underground. If more are needed, they're stored along the sides of the truck. The rig uses an air compressor for power, also mounted on the truck, as is a water tank. Here's the first drill bit to be used. It cuts out an oversize hole:



You can see what are called 'buttons' on the end. They're made of tungsten carbide, which is harder than the rock it will drill down into.

First you have to get through top soil. That's the black earth made up of decomposed plants, trees and so on. It's usually only a foot or so deep, much less right here. Under that is subsoil for a few feet. Then you can get down into old gravel or clay or shale or hardpan, and below that you hit solid rock. This is the Canadian Shield under here, granite.

Somewhere between the various layers of rock will be porous, water-bearing rock. It may draw from an underground stream or even a river, or a trapped aquifer. Aqua is just the Latin word for water, so it means trapped water. That water may be 5, 10, 20 or even 30,0000 years old.

Now the hammer is screwed into the bit. The coupling is well greased before screwing down:





The hammer follows down the hole immediately behind the bit. The very loud noise of the hammer is like that of a jackhammer in road construction or a machine gun in warfare. It's very rapid, driven by compressed air. Water also is pumped down the tube and hammer to the bit, to flush out the rock and other debris and lubricate the bit. The return air and water flows force the debris to the surface, which gradually collects in a pile around the drill hole and has to be shovelled away. Now everything's in place, ready to go:



You're seeing gravel, not topsoil here, that's because the gravel was dumped when the house was built, to level the ground and provide some backfill. This bit drilled down the first twenty five feet very quickly. It's an oversize bit because as you'll see a steel casing has to be put down for the first twenty feet. That's a government requirement because there could be contamination with surface water down to thirteen feet or so. The casing also stops softer material falling into the hole before you get down to hard rock. The hammer is driving the casing down that's protected by a shop-made metal cover to save it from physical damage by the bit striking it directly:



First you see the bit turning, then it starts to jump up and down, hammering it's called, at the same time as it's turning. This is a rotary percussive hammer and bit. The bit is so hard it has no trouble hammering the steel casing down under pressure.

With the casing in place we've replaced the oversize bit and we're starting to go down the hole, drilling with a 6" rotary percussive bit.

Sometimes bits break down the hole and you have to fish them out. It's a slow job, but you don't want to waste two or three hundred feet of drilling if you can help it. But this bit doesn't break. It can drill 100 feet an hour through solid granite.

As you watch you can see how the pipe is going down, and how the drill pipe is being marked to count how many are underground already. The method is simple but effective. As the pipe turns the grease brush is held against it for each 20 foot pipe added:



This particular well came in with water at 300 feet.

You'll soon see why I've led you through all this information about drilling rigs and down-the-hole drilling. We'll need to remember it in chapter 10.

TO INDEX PAGE