Percussion drilling, also called cable-tool drilling, makes a well by raising a heavy steel chisel bit 1-2 m on a rope and dropping it, so that its impact breaks the ground, then bailing out the loosened cuttings every 30-50 drops. It reaches 30 m or more at about 1-3 m a day in soft ground and much less in rock, and its main safety advantage over a hand-dug well is that nobody has to go down the hole.
Part of Deep Wells
Percussion drilling is a way of boring a narrow well by repeated impact from a heavy bit hung on a rope, with the rig usually a tripod and pulley worked by hand. It suits soil, gravel and soft or fractured rock where a hand-dug well would be too deep or too dangerous.
A Falling Bit Does The Work A Digger Would Do
Each drop delivers the bit’s weight times its fall height as energy. The energy per drop is m × 9.81 × h:
- Light end: 30 kg raised 1 m: 30 × 9.81 × 1 = 294 J
- Heavy end: 50 kg raised 2 m: 50 × 9.81 × 2 = 981 J
That is a few hundred joules to about a kilojoule per blow, enough to crack soil and soft rock and break up gravel, and not enough to go quickly through hard rock, which is why rates there fall to a fraction of the soft-ground figure. The rate in soft ground is about 1-3 m a day, so a 20 m well takes about 20 ÷ 3 = 7 days to 20 ÷ 1 = 20 days: one to three weeks.
What You Need
- A rig — a tripod of 3 poles, each 5-6 m long, lashed at the top, with a pulley at the apex (Drill Rig Construction)
- A rope — strong manila or synthetic, 6-10 mm thick, whose strength you have tested (Rope Strength Testing)
- A bit — a heavy steel chisel point, 30-50 kg
- A bailer — a pipe with a check valve at the bottom (Bailer Operation)
- Casing, sized before the bit — see below
- Water — to add to dry holes
Method: Lift, Drop, Turn, Bail, Case
Step 1: Raise the bit. Lift the bit 1-2 m with the rope over the pulley.
Step 2: Drop it. Let it fall freely; the impact fractures the ground.
Step 3: Turn the rope a little between drops. Turning the rope a little each time brings the cutting edge to a new place, so the hole stays round and the bit does not wedge. This is the usual cable-tool practice.
Step 4: Bail every 30-50 drops. Pull the bit, lower the bailer to the bottom, and scoop out the loosened material. If the hole is dry, add water; it carries the cuttings into the bailer.
Step 5: Log what comes up. Lay out each bailer load in order. The layers show you where the water-bearing sand or gravel lies, which is where the screen will go (Screen Section).
Step 6: Case unstable layers at once. When you pass loose sand or gravel that caves in, insert casing immediately to prevent collapse (Well Casing).
Worked Example: Choose The Casing Before The Bit
The Deep Wells guide gives a narrow bit of 10-15 cm and PVC casing of 110-160 mm. These do not fit together. A casing of 110 mm outside diameter in a hole drilled with a 100 mm bit does not go in at all; with a 150 mm bit it has (150 − 110) ÷ 2 = 20 mm clear on each side, little room for a gravel pack of 3-6 mm stone; a 160 mm casing cannot go into a 150 mm hole. A gravel pack needs clearance of at least 25 mm on each side (an assumed figure from general drilling practice, not from the Deep Wells guide), so a 110 mm casing needs a hole of about 110 + 2 × 25 = 160 mm. Decide the casing first, then make or buy a bit about 50 mm wider (Gravel Pack).
Common Mistakes
- Bit too narrow for the casing. The hole must take the casing and its gravel pack.
- Skipping the rope turn. The hole goes triangular and the bit jams.
- Waiting too long between bailings. Cuttings build up and cushion the blows.
- Casing late. A hole in sand or gravel collapses behind the bit and buries it.
Safety
A heavy bit and a loaded rope can crush hands and break legs
A dropped, swinging or stuck bit and a rope under load can crush hands and break legs. Keep people clear of the tripod legs and the line of the rope; never wrap the rope round a hand, wrist or body; keep feet away from the hole and the bit; and do not pull on a stuck bit with more than the rope and tripod are known to carry (Drill Rig Construction). An open hole is a hazard to children and animals: cover it whenever work stops. No one enters a drilled hole; if you ever have to enter a larger shaft, use Safety Protocol.
What’s Next
- Learn next: Drill Rig Construction — building the tripod and checking its loads
- Combine with: Well Casing — lining the finished hole