Principles of Rock DrillingDrilling for Excavation by BlastingThis reference edition deals with surface rock drilling used for the purpose of excavatingrock 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 wellas an introduction to blasting technique, and the interrelation of drilling and blasting.Also discussed are the main parameters involved when planning and executing blast holedrilling 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 CopcoInternet home pages, are presented and discussed by comparing their suitability andexpected productivity related to various applications. Up-dated case stories from differentwork-sites in the world should prove interesting and beneficial, when planning andselecting 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, whosestrength must be overcome by the drilling tool. Depending on rock properties, there areseveral ways to accomplish this.Rotary DrillingRotary drilling can be subdivided into rotary cutting and rotary crushing.Rotary cutting creates the hole by shear forces, breaking the rocks tensile strength. Thedrill bit is furnished with cutter inserts of hard metal alloys, and the energy for breakingrock is provided by rotation torque in the drill rod. This technique is limited to rock withlow tensile strength such as salt, silt and soft limestone not containing abrasive quartzminerals.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 fittedwith tungsten carbide buttons, is simultaneously rotated, and drill cuttings are removedfrom the hole bottom by blowing compressed air through the bit. Drill rigs used for rotarydrilling are large and heavy. The downwards thrust is achieved by utilising the weight ofthe drill rig itself, and the rotation, via a hydraulic or electric motor, applied at the end ofthe 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 longmasts and pipes to accommodate single-pass drilling of maximum 20 m (65 ft). Electricpower is usually chosen for the large rigs, whereas smaller rigs are often powered bydiesel engines.Rotation rates vary from 50 to 120 rpm, and the weight applied to the bit varies from 0.5t/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 andadjustment of optimum feed force and rotation speed to prevailing geology and bit typeand diameter; and incorporation of latest technology in electric and hydraulic drivesystems.Rotary drilling, which is still the dominant method in large open pits, has limitations inthat the rigs cannot drill holes off the vertical line. As blasting theories and practice have
proved, it is generally beneficial to design, drill and blast the bench slopes at an angle ofapproximately 18 degrees off vertical.Many rotary rig masts have pinning capabilities permitting drilling at angles as much as30 degrees out of the vertical. However, the inclined hole drilling capabilities in rotarydrilling are limited by the heavy feed force required, since part of this force is directedbackwards. This causes rig stability problems, reduced penetration, and shorter life ofdrilling consumables. Consequently, most blast hole drilling using rotary drillrigs is invertical holes.Percussive DrillingPercussive drilling breaks the rock by hammering impacts transferred from the rock drillto the drill bit at the hole bottom. The energy required to break the rock is generated by apneumatic or hydraulic rock drill. A pressure is built up, which, when released, drives thepiston forwards.The piston strikes on the shank adapter, and the kinetic energy of the piston is convertedinto a stress wave travelling through the drill string to the hole bottom. In order to obtainthe best drilling economy, the entire system, rock drill to drill steel to rock, mustharmonise.