Drilling and Blasting

Drilling and blasting are all about putting the right amount of energy in the right place at the right time at minimum cost to achieve maximum control over the shot rock volume and the resulting particle size distribution in the muck pile

Mineral/Ore Mined using the Hardrock Blasthole Drill

Heavy duty Rotary and Down-The-Hole rigs for Blasthole drilling

Open Pit Blast Pattern

Rules of thumb

  • Burden
    • 25 to 35 hole diameters
  • Spacing
    • Hole size greater than 140 mm (5½”)
      • 1.2 to 1.5 burden
    • Hole size smaller than 140 mm (5½”)
      • 1.5 to 1.8 burden
  • Stemming
    • 0.7 to 1.3 burden.
  • Powder factor
    • Governed by:
      • Budget
      • Rock hardness
      • Mine to mill process
  • Hole diameter
    • Governed by:
      • Required material size which is influenced by the loading and hauling equipment


  • Quarry
    • Intent is to break and move the rock
    • Proper fragmentation reduces post – blast crushing
    • Proper muck pile dimensions make digging easier
  • Surface Metal
    • Intent  is to break and move the rock
    • Proper fragmentation makes digging easier and reduces post – blast crushing
    • Movement should be kept to a minimum to optimize ore control
  • Cast Blasting
    • Intent  is to break and move the rock
    • Intent  is to break and move the rock
    • Explosives are the cheapest way to move material
    • Design for maximum movement

Drill Types and Technology

Drill Types

Hammer Technology

  • Top – Hammer Drilling
    • Normally good to depth of 45 feet, other wise it is easy to get drill rod bent and get borehole deviation
  • DTH(down the hole) Drilling
    • Normally good to depth of 120 feet, because the percussion move happens on the bottom of drill string, it can drill deeper than Top hammer drilling

Top Hammer Blasthole Drilling

  • Benefit
    • Highest penetration rate in hard material and small to medium hole size
    • Highest overall drilling efficiency
    • Small maneuverable drill mountings, rapid setup
  • Drawback
    • Higher drill string cost
    • Energy loss at depth
    • Highest chance of hole deviation

Down the Hole Drilling

  • Benefit
    • Best method for loose and fractured material
    • High penetration rate in hard material and large hole size combination
    • Good productivity at Depth
    • Best control of hole deviation
  • Drawback
    • High rig operating cost
    • Low efficiency
    • Limited economic hole range

Rotary Drilling

  • Rotary Drilling Fundamental
    • Rotary drilling is a versatile method for blasthole drilling.
    • It is the dominant method for holes larger than 203 mm (8”) diameter. In smaller holes, it is most common in softer rock formations.
    • The drilling principle is to use a high pull down force (weight-on-bit), rotate the drill bit, and blow the rock cuttings to the surface with compressed air.
    • Hardrock drills typically use the Rotary method to drill blastholes from 200 mm (7 7/8”) to 311 mm (12 ¼”) diameter.
  • Hardware
    • The critical drill features are pull down force, rotation power, and compressor capacity.
    • For optimum hole cleaning, the outside diameter of the drill pipe must be balanced with the hole size and air compressor.
    • All actual drilling is done by the drill bit and choosing the right bit has significant impact on productivity and total cost.
    • There is a variety of rotary drill bit cutting structures to select from for different rock formations.
    • Visit Hardrockdrills.com for the newest technology and learn more about proper bit selection.
  • Drilling Parameters
    • The pull down force must be sufficient to fracture the rock. Excessive force causes bit wear and vibration damage to the drill.
    • The optimum pull down depends on rock strength and bit size.
    • For example, in a typical coal mine with a rock strength of 10 000 psi (69 MPa), the optimum pull down is about 67 kN (15 000 lb ).
    • The compressor output should be matched with the drill pipe and bit to give an up-hole air velocity of 30-40 m/s (6000-8000 ft/min).
    • An over sized compressor causes excessive steel wear and fuel consumption.
    • Theoretically, once enough force has been applied to overcome the strength of the rock, increased rotation speed should result in higher penetration rates. In reality, vibrations and bit bearing life limit the maximum rotation.

Three Main Components in Drilling

  • Feed
  • Rotation
  • Flushing Air

Feeding System

  • Hydraulic Cylinder
  • Feeding Chain
  • Timing Sprocket
    • PLANITARY Gearbox

Air Compressor Flushing Air

Two stage air compressor in left picture
Single stage air compressor in right picture


  • Flushing Air Based On:
    • Penetration rate
    • Cutting fragmentation
    • Material density
    • Cutting size
    • Cutting shape (roughness, roundness)
    • Available annulus space
    • Presence of water
    • Hole inclination

Blasthole bit selector

Blasthole Drilling Ground Concerns

  • Trip and Fall Hazards
  • Broken rock
  • Uneven terrain
  • Slippery surface
  • Highwall hazards
  • Falling rock debris
  • Slides −Errosion
  • Weathered rock
  • Snow and ice

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