Borehole Drilling

Part of Gunpowder and Explosives

Drilling boreholes in rock for charge placement using hand tools, star drills, and simple mechanized systems.

Why This Matters

An explosive charge placed on the surface of a rock wastes most of its energy blasting into the air. The same charge placed inside a drilled borehole directs energy into the rock mass, fragmenting it far more effectively while using a fraction of the powder. The difference is enormous — a charge in a properly drilled and stemmed hole can be five to ten times more effective than the same amount of explosive placed on the surface.

This means borehole drilling is not optional — it is the fundamental enabling skill for practical blasting. Without the ability to drill holes in rock, explosive operations consume impractical amounts of scarce black powder and produce unpredictable, dangerous results. With proper drilling, every gram of explosive does useful work.

The challenge is that drilling rock by hand is slow, exhausting work. A team of experienced drillers might advance 30-50 centimeters per hour in hard rock using hand-held steel tools. Understanding the techniques, tool design, and drilling patterns that maximize efficiency saves enormous labor and makes the overall quarrying or mining operation practical rather than merely possible.

Hand Drilling: The Star Drill Method

The Basic Technique

Hand drilling — also called “single-jack” or “double-jack” drilling — is the oldest and most accessible method. It requires only a hardened steel drill bit and a hammer.

Single-jack drilling (one person):

1. The driller holds the drill steel in one hand and swings a short-handled hammer (1.5-2 kg) with the other
2. Between each strike, rotate the drill 15-20 degrees
3. The rotation ensures the cutting edge strikes fresh rock with each blow
4. Periodically remove rock dust from the hole using water or by blowing

Double-jack drilling (two or three people):

1. One person (the “holder” or “turner”) sits or kneels, holding and rotating the drill steel
2. One or two others swing heavy sledgehammers (3-5 kg) with both hands
3. The holder rotates the drill between each strike
4. Striking rhythm must be consistent — the holder’s hands are at risk with every blow

Trust Is Essential

Double-jack drilling requires absolute trust between partners. The holder’s hands are centimeters from where a 5 kg hammer strikes. A miss means crushed fingers or a broken wrist. Partners must establish a rhythm, maintain eye contact, and never rush. A tired striker must stop immediately — fatigue causes missed blows.

Drilling Rate

Expected penetration rates in various rock types:

Rock Type	Single-Jack Rate	Double-Jack Rate
Soft limestone	30-50 cm/hour	60-90 cm/hour
Hard limestone	15-30 cm/hour	40-60 cm/hour
Sandstone	20-40 cm/hour	50-80 cm/hour
Granite	8-15 cm/hour	20-40 cm/hour
Basalt	5-12 cm/hour	15-30 cm/hour
Quartzite	3-8 cm/hour	10-20 cm/hour

These rates assume sharp drill bits. Dull bits can reduce penetration to near zero, making frequent resharpening essential.

Drill Steel Design and Maintenance

Drill Bit Types

Star drill (cross-bit): The most common design. Four cutting edges arranged in a cross pattern when viewed from the end. Good for medium to hard rock. The cross pattern prevents the drill from jamming in the hole.

Chisel bit (flat bit): A single flat cutting edge. Simpler to make but tends to jam in holes and doesn’t drill as round. Adequate for soft rock.

Button bit (improvised): For extremely hard rock, some smiths forge a flat face with raised bumps or points. The bumps crush rock through point loading rather than shearing. More resistant to chipping than flat edges in very hard material.

Drill Steel Specifications

Parameter	Recommended	Notes
Steel type	High-carbon or tool steel	Must be hardenable
Shaft diameter	20-25 mm (octagonal preferred)	Octagonal prevents rolling
Bit diameter	30-50 mm	Larger than shaft to prevent jamming
Length	Multiple sizes: 30, 60, 90, 120 cm	Start short, switch to longer as hole deepens
Cutting edge angle	90-110 degrees	Wider angle for harder rock

Making Drill Steel

If you have a forge and basic blacksmithing skills:

1. Start with high-carbon steel — Old vehicle springs, coil springs, or tool steel are ideal. Mild steel won’t hold an edge.
2. Forge the shank — Draw out to the desired octagonal cross-section. Octagonal (8-sided) is preferred over round because it doesn’t roll when set down and gives the holder a better grip for rotation.
3. Forge the bit — Upset (thicken) the working end to create a wider cutting head. For a star drill, forge a cross shape by alternating 90-degree rotations during flattening.
4. Heat treat — Heat the bit end to cherry red (approximately 800°C) and quench in oil or water. This hardens the cutting edges. Then temper by reheating to 250-300°C (straw to brown color on the polished edge) to reduce brittleness while maintaining hardness.

The Tempering Balance

Too hard and the bit chips or shatters on impact. Too soft and it dulls instantly. Aim for a straw-yellow temper color on the cutting edges. This gives the best compromise between edge retention and toughness.

Sharpening

Drill bits dull continuously during use. A dull bit bounces rather than cutting, wasting energy and slowing progress dramatically.

Resharpening procedure:

1. Heat the bit end in a forge to yellow heat
2. Hammer the cutting edges back to the original profile — restore the cross shape and edge angle
3. Maintain the correct gauge (diameter) — bits that become too narrow will produce undersized holes that jam on the shaft
4. Re-harden and temper after resharpening

Frequency: In hard rock, bits need resharpening every 15-30 minutes of drilling. Have multiple bits of each length ready so drilling can continue while dull bits are at the forge. A three-person team (two drillers, one sharpener) is the most efficient arrangement.

Drilling Procedure Step by Step

Starting the Hole

Starting a borehole on a flat rock surface is the most difficult phase — the bit tends to skitter and slip.

1. Mark the location — Scratch or chisel a small X at the exact drill point
2. Create a starter dimple — Using a short, heavy drill or a pointed chisel, hammer a small depression (1-2 cm deep) at the mark. This guides the drill bit.
3. Begin with the shortest drill — Start with a 30 cm drill steel. Short drills are more controllable and less likely to deflect.
4. Light blows initially — Use controlled, moderate strikes until the hole is 5-10 cm deep and the bit is tracking straight.
5. Check alignment frequently — Sight down the hole to ensure it’s going where intended. Correcting alignment is easy in the first 10 cm, nearly impossible after 30 cm.

Deepening the Hole

As the hole deepens, switch to progressively longer drill steels:

Hole Depth	Drill Steel Length
0-25 cm	30 cm starter
25-50 cm	60 cm
50-80 cm	90 cm
80-110 cm	120 cm
110-150 cm	150 cm

Each successive drill should have a slightly smaller bit diameter (2-3 mm per step) so it passes freely through the hole drilled by the previous, larger bit. This stepped approach prevents the drill from jamming.

Clearing Cuttings

Rock dust and chips must be removed from the hole regularly, or they cushion the bit and prevent cutting.

Dry clearing:

1. Pull the drill out every 5-10 cm of advance
2. Tip the hole downward (if possible) and shake out cuttings
3. For upward or horizontal holes, use a scraper — a bent wire or thin stick with a hook — to pull cuttings out
4. Blow into the hole to clear fine dust (turn your head and close your eyes after blowing — dust rebounds)

Wet drilling (preferred):

1. Pour a small amount of water into the hole before drilling
2. The water mixes with cuttings to form a slurry
3. The drill action pumps the slurry up and out of the hole
4. Replenish water every few minutes

Wet drilling is faster because the slurry acts as a cutting lubricant and carries chips out of the hole continuously. It also reduces dust inhalation — a serious health concern in prolonged drilling operations.

Silicosis Risk

Drilling rock, especially quartz-bearing rock (granite, sandstone, quartzite), produces fine silica dust. Inhaling this dust over months or years causes silicosis — irreversible lung scarring. Always use wet drilling when possible, and wrap a damp cloth over your nose and mouth when dry drilling.

Horizontal and Upward Holes

Horizontal holes (bench blasting) and upward holes (underground mining) present special challenges:

Horizontal holes:

• Water drains naturally — pour water in periodically and let gravity assist
• Support long drill steels with one hand near the bit and the other at the striking end
• Keep the holder’s body to the side of the hole, not directly behind the drill

Upward holes:

• Water and cuttings fall back down, making clearing difficult
• Use a cup-shaped scraper or spoon to scoop out accumulated material
• Work with goggles or eye protection — falling debris is constant
• These holes are the most physically demanding to drill

Hole Patterns for Different Applications

Single Row (Bench Blasting)

For quarrying a rock face:

Face -->  O    O    O    O    O
          |----S----|
          |---B---|

• O = borehole
• S = spacing (60-120 cm depending on rock and charge)
• B = burden (distance from face to hole row, 50-100 cm)

Grid Pattern (Flat Surface Removal)

For excavating a flat area of rock (foundation, well, cistern):

O    O    O    O
  O    O    O
O    O    O    O
  O    O    O

Staggered grid provides more even fragmentation than a square grid. Spacing: 50-80 cm between holes.

V-Cut (Tunnel Advance)

For starting a tunnel in solid rock, the first blast must create an initial void that subsequent holes can break toward:

      O     O
    O    V    O
      O     O

V = cut holes (angled toward each other to meet at depth). These fire first, creating a wedge-shaped void. Surrounding production holes then break rock inward toward this void.

The cut holes are angled at 60-70 degrees toward each other, meeting 50-80 cm behind the tunnel face. This is the most critical part of underground blast design — if the cut fails, the entire round fails.

Mechanized Drilling

Hand-Cranked Drill

A step up from hammer-and-steel:

1. Mount a drill bit in a chuck at the bottom of a vertical shaft
2. A horizontal handle at the top allows one or two people to rotate the shaft
3. Weight on top of the shaft provides downward pressure
4. Rotation speed: 30-60 rpm manually
5. This is faster than hammer drilling in soft to medium rock and requires less skill

Bow Drill for Rock

The same principle as a fire-starting bow drill, scaled up:

1. A heavy drill bit is mounted in a vertical spindle
2. A bow string wraps around the spindle
3. Pushing and pulling the bow rotates the drill alternately clockwise and counterclockwise
4. A weighted cap on top provides downward force
5. Sand or crushed quartz can be added to the hole as an abrasive

This method is slow compared to hammer drilling in hard rock but works well in soft stone (limestone, sandstone) and is less physically demanding.

Water-Powered Drill

If a stream is available near the work site:

1. Build a water wheel connected to a cam mechanism
2. The cam raises and drops a heavy drill steel repeatedly
3. A spring or lever mechanism rotates the drill between drops
4. Water can also be channeled to the hole for continuous wet drilling

This automation transforms drilling from exhausting manual labor into a supervised process. One person monitors the drill while the water wheel does the work. Penetration rates can match or exceed double-jack rates with far less human effort.

Planning Drill Patterns

Calculating Number of Holes

For a quarry bench blast:

1. Measure the face: Height (H) and length (L) in meters
2. Determine burden (B): Typically 0.8-1.2 m for hand-drilled holes
3. Determine spacing (S): Typically 1.0-1.5 times the burden
4. Number of holes = L / S (rounded up)
5. Hole depth = H + subdrill (subdrill = 0.3 × B, to ensure the blast breaks cleanly to grade)
6. Total drilling = Number of holes × hole depth

Example: A face 5 m long, 2 m high, with 1 m burden and 1.2 m spacing:

• Number of holes: 5 / 1.2 = 5 holes (rounded up)
• Hole depth: 2 + 0.3 = 2.3 m
• Total drilling: 5 × 2.3 = 11.5 meters

At 40 cm/hour in limestone (double-jack), this is approximately 29 hours of drilling — roughly 4 days of work for a two-person drilling team working 7-hour shifts.

Minimizing Drilling

Drilling is the bottleneck in any blasting operation. Strategies to reduce total drilling:

1. Maximize the rock broken per hole — Use the largest burden and spacing the powder will effectively break
2. Choose free faces wisely — Always blast toward an open face. This requires less energy (fewer holes, less powder) than blasting confined rock.
3. Exploit natural fractures — Place holes to take advantage of existing joints and cracks in the rock
4. Maintain sharp bits — A sharp bit drills twice as fast as a dull one. The time spent resharpening is always repaid in faster drilling.
5. Use wet drilling — 20-30% faster than dry drilling in most conditions

Safety Considerations

Hole Inspection Before Loading

Before placing explosive in any borehole:

1. Measure the depth — Use a wooden measuring rod (never metal near loaded holes). Confirm the hole reached the designed depth.
2. Check for water — Water in the hole can deteriorate black powder. If the hole is wet, line it with a watertight cartridge or waterproof wrapping.
3. Clear all cuttings — Debris at the bottom prevents the charge from seating properly. Flush with water or use a scraper.
4. Check for cracks — If the drill encountered a natural crack that connects to the rock face, blast gases may vent through it rather than building pressure. Such holes may need extra powder or should be abandoned.

Drill Steel Safety

• Never use drill steel with mushroomed heads (the flared end that results from repeated hammer strikes). Fragments can break off and become projectiles. Trim mushroomed ends on the forge.
• Never drill into an unexploded charge or into a hole that previously contained a charge. Drill new holes at least 60 cm from any suspected misfired hole.
• Wear eye protection when possible — steel chips and rock fragments routinely fly during drilling.
• Take breaks every 30-45 minutes. Fatigued drillers miss blows, injure holders, and produce poor-quality holes.