Principles of Rock Drilling

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 Principles of Rock Drilling



  • At the end of this chapter, Participants will be able to achieve:
  • Understanding principles of drilling
  • Understanding of equipment characteristics
  • Rock properties and geological structure that affect drilling
  • Selection of drilling machines


  • Drilling & blasting planning
  • Purpose of rock drilling
  • System of rock drilling
  • Types of drilling operations used in rock breakage
  • Drilling methods
  • Drilling Pattern
  • Rock properties and geological structure that affect drilling
  • Selection of drilling machines

Drilling &  Blasting

  • Drilling & blasting still continue to be the favored combination for breaking out & fragmenting rocks
  • The interaction between drilling and blasting is very effective.
  • Any shortcoming in one operation would certainly jeopardize the other.

Importance of Drilling and Blasting

Example of cost breakdown

crushing method
crushing method

General Drilling Requirements

(What is Needed to Drill a Hole?)

  • Energy Source
  • Cutting Mechanism
  • Energy Transmission
  • Feed System
  • Rotation System
  • Hole Flushing Medium


General Drilling Requirement
General Drilling Requirement

Mechanism of rock Drilling

Mechanism of rock Drilling
Mechanism of rock Drilling

Components of Drill Machine

Drilling Machine
Drilling Machine


Drilling machine parts
Drilling machine parts

Dynamics of surface blasthole drill

Dynamic blast hole
Dynamic blast hole

Systems of Rock Drilling:




Rotary Percussion 








Laser Ray

Types of Drilling Operations

  • Manual Drilling

    By light & hand held equipment

    Hand held Equipment


Mechanized Drilling

By equipment mounted upon rigs “Crawler or wheel”

Blasting Equipment


Rock Drilling Principles/Methods


Drilling Methods

Rotary Percussive

Down the Hole Hammer – Top Hammer – COPROD


Rotary Drilling

Drilling methods
Drilling methods

Energy Utilization:

top hammer - DTH
top hammer – DTH

Rotary Drilling

  • Bit load must overcome rock strength
  • Bit loading via hydraulic cylinder and drill pipe attached to Rotary Head
  • Air compressor flushes cuttings from hole
  • Rotary head indexes bit to fresh material

  • Economic all hole sizes with soft material
  • Medium and hard material hole size 7-7/8” and up
  • Requires high bit load and rotary torque


Rotary Drilling benefits : 

  • Easiest drilling method
  • Low operating cost in nonabrasive material
  • Good productivity at depth
  • Single-pass capability due to large platform

Rotary Drilling Drawbacks 

  • Large platform to support pulldown and torque requirements
  • Limited setup options
  • Limited in small hole hard rock applications
  • Mid range in efficiency

  • Used for large diameter holes or for deep hole drilling, such as water and oil wells, geothermal research and drilling operations in large open-pit coal and metallic mineral mine
  • Depth > 30 m and hole diameter > 200 mm

Drifter Drilling (Top Hammer)

  • Economic in all strength material to ~5” hole diameter
  • Low feed and torque requirements
  • Low (< 150 psi) pressure air required

  • Percussion used to overcome rock strength
  • Feed used only to keep bit in contact with rock and limit rebound
  • Rotation is supplied by drifter through drill steel
  • Compressive shock wave transmitted to bit by drill steel
  • Depth < 15 m
  • Low energy cost (7-12 m3/min ,7-12 bars)
  • Lowest cost/t because of small diameter holes (65 to 102 mm)
  • Good penetration rate (70 –120 cm/min) for compact or partially fractured rocks
  • Successful for hole diameter up to 127 mm (5”)
  • When depth> 20 m, drilling results in slower drill penetration rate due to loss of percussive energy through the drill rods and couplings. Hole deviation, in-hole cleaning, and explosive loading also cause a lower overall production rate.

Down The Hole Hammer

  • Economic in medium to hard material 5-1/2” +
  • High pressure air required for max. productivity
  • Low feed and rotary torque requirements
  • Percussion used to overcome rock strength
  • Feed used only to keep bit in contact with rock and limit rebound
  • Air compressor drives DHD and flushes cuttings from hole
  • Rotary head indexes bit to fresh material

Down The Hole Hammer Drawbacks 

  • High rig operating cost
  • High accessory cost
  • Low efficiency
  • Limited economic hole range
  • DTH follows immediately the bit into hole. Therefore little percussion energy is dissipated in the pipe joints.
  • Penetration rate (20 –60 cm/min)is nearly constant regardless of the depth of hole.
  • Minimum hole deviation.
  • Drilling pipes conduct compressed air to the DTH hammer. The same air flushes cutting out of the hole.
  • Used for fractured rock, for large diameter holes 152mm (6”) or for hole depths exceeding 20m up to 50 m.
  • Fuel consumption ~ 50 to 100 % higher (20-30 m3/min; 20-25 bars)


  • Combines the speed of top hammer and with the precision and long service life of DTH.
  • Economic drill string life is high.
  • High penetration rate.
  • High production capacity.

Suitable for difficult drilling conditions

Drilling Method Guide

Drill MethodTop HammerDTHCOPROD
Penetration RateOOOOOOOOOOOO
Straight holesOOOOOOOOOOOO
Production capacityOOOOOOOOOOOO
Low fuel consumptionOOOOOOOOOOOOO
Economic drill string lifeOOOOOOOOOOOO
Low investment for drill stringOOOOOOOOOOOO
Suitable for difficult conditionsOOOOOOOOOOOO
Operator friendlyOOOOOOOOOOOO


Fields of Applications of Drilling Methods

Fields of Application of Drilling Methods

Comparison of Hydraulic and Pneumatic drills


  • Air Delivery Volume m³/min (CFM)
  • Maximum Operating Pressure
  • Generally the compressor should be roughly sized for economical operation when drilling a particular hole size.
  • A larger compressor will only give better performance if the hammer can use this air efficiently

Compressor Types

  • Piston compressor
  • Sliding-Van compressor
  • Helical (Rotary-Screw) compressor

Piston compressor

  • The piston compressor is single -acting when a single cylinder compresses the air on one side of the piston.
  • The piston compressor is a double acting when two single acting pistons are operating in parallel in one casting.
  • The piston compressor has both intake and discharge valves to control the air flow out of the cylinder.

Sliding-Van compressor

  • The rotary sliding-van compressor consists of a cylinder form , or casting, with slots about its circumference running a longitudinally that contain sliding strips , or vans.
  • The casting rotates about a longitudinal axis within a housing that is eccentric about the casting.
  • The van compressor has no valves. The inlet and discharge of the air depend on ports in the housing.

Helical (Rotary-Screw) compressor

  • The positive – displacement rotary unit compresses air between the intermeshing helical lobes, on the two screws, and the cambers of the housing.
  • The most common types of screw compressor uses a flood of oil; the screws are flooded with oil , which acts as a seal, a cooler, and and a lubricant.

Mobile compressors


Drilling Capacity

Drilling capacity is controlled by:

  • Rock structure
  • Percussion power level
  • Bit diameter, type and rotary speed
  • Flushing
  • Coupling loss percentage

Penetration  Rate

Penetration rate, drilled-meter/hour is mainly depends on:

  • Drill rig set up time
  • Collaring
  • Drill hole stabilization time
  • Rod handling and rod count length
  • Moving time (bench location)
  • Effective hour per shift

Tips for Better Drilling Efficiency

  • Measure and mark-up drill hole locations
  • Diameter in accordance with drilling pattern
  • Drill holes parallels with constant burden
  • Bottom of holes must be at the same level
  • Drill rig well positioning
  • Drill operator records rock properties  (fracture, karst, water, ..)


Mine to Mill Concept

Factors Influencing Drilling Accuracy

  • Training of The driller
  • Rock conditions
  • Drilling equipment
  • Drill rods, Drill bits, Guiding devices
  • Alignment , Accuracy in collaring
  • Overburden removal
  • Etc

Hole Deviation


Silenced Smart-Rig

  • Feed Enclosure
  • Lower fuel consumption
  • Smart Rig automation
  • Increased productivity


Bit Design and Rock TypeTop-hammer


Bit Design and Rock Type DTH


Rock Properties That Affect Drilling

3. Elasticity
4. Plasticity
5. Abrasiveness
6. Texture
7. Structure


  1. Hardness

Hardness and Compressive Strength

  1. Elasticity
    The majority of rock minerals have an elastic-fragile behavior, which obeys the Law of Hooke, and are destroyed when the strains exceed the limit of elasticity.
    4. Plasticity
    some rocks the plastic deformation precedes destruction. This begins when the stresses exceed the limit of elasticity.
    5. Abrasiveness
    Abrasiveness is the capacity of the rocks to wear away the contact surface of another body that is harder, in the rubbing or abrasive process during movement. This property has great influence upon the life of drill.
     6. Texture
    The texture of a rock refers to the structure of the grains or minerals that constitute it. The size of the grains are an indication, as well as their shape, porosity etc. All these aspects have significant influence on drilling performance.
     7. Structure
    Discontinuities, cracks, joints, and planes of weakness are important for a proper understanding of the real ability of a rock mass volume to withstand load, of how and why it fails, and of the resulting fragment size and shape.

Factors Affecting Bench  Orientation & Height

  • Factors affecting bench orientation

  • Dip and Strike
  • Thickness of layers and joints
  • Geochemical variability orientation
  • Factors affecting bench height


  • Vertical variability
  • Dipping of layers
  • Production scale
  • Drilling equipment and rods (If possible,bench heights should be multiplies of drill rod length to minimize maneuver time)


Knowledge of Geological Structure

What hole diameter should be used?

Selection of Drilling Machines

  • Geometrical design of quarry faces
  • Safety factors of operation
  • Mobile equipment used
  • Quarry production
  • Crusher capacity
  • Rock properties


Importance of Drilling and Blasting  Example of cost breakdown

Straightness of Blast-holes

  • Straight blast-holes:
  • Minimize the amount of drilling; holes will be drilled to their intended positions,
  • Increase drill steel life; the drill string is affected neither by bending nor by jamming of
  • Insufficient hole straightness causes two types of problems in blasting:
  • The burdens and spacing at the bottom of the bench are too large; the results are poor floor clearance and oversize boulders,
  • The burdens and spacing at the bottom of the bench are too short; results are excessive rock scatter and throw
  • With straight holes, the burden and spacing in a blast can be increased by up to 20%. This means that also rock yield will be up to 20% greater and cost can be down to 20% lower.


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