Commonly known as DTH, the Down-The-Hole hammer drill is a percussive device in tubular form with integral drill bit that is fitted directly onto the bottom-most part of a drill string. DTH is one of the most efficient ways of drilling rock, using a rapid hammer action to break the rock into sizeable chips which are then exiled from the bore hole by the blowing air. The method provides fast drilling of accurately aligned, straight and clean holes in almost all terrains.
It is significant to remember in rock extraction that blasting is the first phase of crushing. It is therefore vital that time and effort is spent on guaranteeing that the hole size,
overall blast design and explosive loading is appropriate for the conditions, in order to achieve good disintegration, optimum productivity, safe steady faces and level floors at minimal cost.
DTH is a comparatively recent technique, introduced in the 1950s when the choice of drilling system for quarrying was limited, that then included the hand-held jack hammer, chain tool, and rotary drill and the air-driven top hammer machines.
Jack hammers were used for springing, stoping and popping. Although they were lightweight, portable, easily upheld and inexpensive, they were essentially labour intensive, low volume producers, resulting in small holes and limited control, as well as presenting significant health and safety issues.
The cable tool was primeval and used a heavy bar with a chisel, suspended on a wire rope that was then automatically dropped on the rock to crush it within the hole, using water to bale clear the resultant cuttings and dust. Again easy to use, simple to maintain with low cost operation, minimal dust and effective in bad ground, but the cable tool was slow and partial to larger holes, with no alignment control. It required a regular water supply, two man operation and was really only of use for soft to medium rock formations.
Rotary drilling required large, heavy rigs to create weight behind the drill bit and provide fast rotational speeds with high torque capabilities to penetrate the rock. Although they were relatively fast, economical to run and maintain and good in poor ground, they required a high capital spending, were disposed to hole deviation and gave less imposing performance in hard rock.
Top hammer drill or drifter was a motorized version of the hand-held jack hammer with the air driven percussive unit mast mounted on a simple three wheeled carriage. It was fast, movable, easy to maintain and good in hard rock. However, it was subject to hole deviation, reduced drilling speed at depth and produced high noise levels and vibration. Hole sizes were restricted to 70 to 100 mm diameters
Essential Features
• Greater versatility in terms of hole sizes
• Low capital expenditure
• Less labour intensive
• Simple and easy to operate
For blast hole drilling DTH meant straighter, cleaner, better aligned holes for easier charging, improved blasting control, improved faces, better floors, improved disintegration and throughput as well as lower operating costs. Perhaps most significantly, DTH meant greater overall safety.
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