nm

1. Principles of Rock Drilling
Drilling for Excavation by Blasting
This reference edition deals with surface rock drilling used for the purpose of excavating
rock by means of blasting. Other types of drilling, such as drilling for oil and water,
mineral exploration, and grouting are excluded.
The reader is given a brief explanation of the various prevailing drilling methods, as well
as an introduction to blasting technique, and the interrelation of drilling and blasting.
Also discussed are the main parameters involved when planning and executing blast hole
drilling at quarries, open pit mines and various types of civil engineering projects.
The range of Atlas Copco products, where references can be found to the Atlas Copco
Internet home pages, are presented and discussed by comparing their suitability and
expected productivity related to various applications. Up-dated case stories from different
work-sites in the world should prove interesting and beneficial, when planning and
selecting methods and equipment for blast hole drilling applications.
Blast holes have certain unique and important characteristics. These are: hole diameter,
hole depth, direction and straightness. Drilling produces a circular hole in the rock, whose
strength must be overcome by the drilling tool. Depending on rock properties, there are
several ways to accomplish this.
Rotary Drilling
Rotary drilling can be subdivided into rotary cutting and rotary crushing.
Rotary cutting creates the hole by shear forces, breaking the rock's tensile strength. The
drill bit is furnished with cutter inserts of hard metal alloys, and the energy for breaking
rock is provided by rotation torque in the drill rod. This technique is limited to rock with
low tensile strength such as salt, silt and soft limestone not containing abrasive quartz
minerals.
Rotary crushing breaks the rock by high point load, accomplished by a toothed drill bit,
which is pushed downwards with high force. The bit, being of tricone roller type fitted
with tungsten carbide buttons, is simultaneously rotated, and drill cuttings are removed
from the hole bottom by blowing compressed air through the bit. Drill rigs used for rotary
drilling are large and heavy. The downwards thrust is achieved by utilising the weight of
the drill rig itself, and the rotation, via a hydraulic or electric motor, applied at the end of
the drill pipe. Common hole diameters range from 8 to 17½ in (200-440 mm) and,
because adding the heavy drill pipes is cumbersome, most blasthole drillrigs use long
masts and pipes to accommodate single-pass drilling of maximum 20 m (65 ft). Electric
power is usually chosen for the large rigs, whereas smaller rigs are often powered by
diesel engines.
Rotation rates vary from 50 to 120 rpm, and the weight applied to the bit varies from 0.5
t/in of bit diameter in soft rock, to as much as 4 t/in of bit diameter in hard rock.
Recent technical advances include: improved operator cab comfort; automatic control and
adjustment of optimum feed force and rotation speed to prevailing geology and bit type
and diameter; and incorporation of latest technology in electric and hydraulic drive
systems.
Rotary drilling, which is still the dominant method in large open pits, has limitations in
that the rigs cannot drill holes off the vertical line. As blasting theories and practice have

2. proved, it is generally beneficial to design, drill and blast the bench slopes at an angle of
approximately 18 degrees off vertical.
Many rotary rig masts have pinning capabilities permitting drilling at angles as much as
30 degrees out of the vertical. However, the inclined hole drilling capabilities in rotary
drilling are limited by the heavy feed force required, since part of this force is directed
backwards. This causes rig stability problems, reduced penetration, and shorter life of
drilling consumables. Consequently, most blast hole drilling using rotary drillrigs is in
vertical holes.
Percussive Drilling
Percussive drilling breaks the rock by hammering impacts transferred from the rock drill
to the drill bit at the hole bottom. The energy required to break the rock is generated by a
pneumatic or hydraulic rock drill. A pressure is built up, which, when released, drives the
piston forwards.
The piston strikes on the shank adapter, and the kinetic energy of the piston is converted
into a stress wave travelling through the drill string to the hole bottom. In order to obtain
the best drilling economy, the entire system, rock drill to drill steel to rock, must
harmonise.