The drilling progress in a rock mass is determined by various geological and mechanical parameters. In this report the major correlations of specific rock properties with measured drilling rates are shown. Apart from conventional mechanical rock properties, a new property for toughness refering to drillability has been introduced: the specific destruction work Wz. This new method makes it possible to understand better the connection between drilling rate and the main mechanical rock character.


La vitesse de forage dans une masse rocheuse est determinee par differents paramètres geologiques et geomecaniques. Dans ce bulletin sont exposees les correlations fondamentales entre les proprietes mecaniques caracteristiques des roches et la vitesse net de forage. Outre les proprietes mecaniques conventionelles des roches une nouvelle mesure de la resistance des roches au forage est introduite: Ie travail destructeur specifique Wz. Cette nouvelle methode permet de mieux comprendre la correlation causaIe entre la vitesse de forage net et les proprietes mecaniques d'une roche.


Die Bohrgeschwindigkeit in einem Gebirge wird durch unterschiedliche geologische und felsmechanische Parameter bestimmt. In diesem Beitrag werden die wesentlichen Abhangigkeiten zwischen den felsmechanischen Gesteinskennwerten und gemessenen Bohrgeschwindigkeiten aufgezeigt. Neben den konventionellen felsmechanischen Kennwerten wurde ein neues Maß fuer die Zahigkeit bezueglich der Bohrbarkeit von Gesteinen eingefuehrt: die spezifische Zerstörungsarbeit Wz. Die neue Auswertemethode ermöglicht es, den Zusammenhang zwischen Bohrgeschwindigkeit und felsmechanischen Eigenschaften besser zu verstehen.


Usually the main subject in preliminary site investigations prior to tunneling projects is the prediction of tunnel stability. During the last years in conventional drill- and blast tunneling, problems have occured also connected with the accurate prediction of drillability in hard rock. The last decade has demonstrated a massive changeover to heavy drill hammers and so the need for drillability prediction for such equipment has increased. Drillability is not only decisive for the wear of tools and equipment but is - along with the drilling rate - a standard factor for the progress of excavation works. It is clear, that the correlation of net drilling rate and total drilling capacity only will be fundamental in good quality hard rock. But in this case estimation of drillability in predicted rock conditions might bear an extensive risk of costs.


Drillability is a term used in construction to describe the influence of a number of parameters on the drilling rate (drilling velocity) and the tool wear of the drilling rig. The interaction of the main factors is illustrated in fig. 1. First of all, the geological parameters generate the specific characteristics of rock material and rock mass. These characteristics may be at least partly put into figures with the help of mechanical rock properties (fig. 2). But rock mass conditions also depend on the geological history, containing weathering, hydrothermal decomposition and the structure of discontinuities. Together they build the basic parameters for drillability. According to rock conditions the corresponding drilling rig will be choosen. The machine parameters are depending on the drilling method: In underground excavation the rotary percussive drilling is standard, providing maximum performance under most circumstances. Parameters are the technical specifications of the drill hammer, flushing system and the design of the drilling bit (fig. 3). Typical tunneling rigs consist of a diesel hydraulic tramming carrier, carrying up to three booms with hydraulic drifter feeds and rock drills. For example the COP 1238 (15 kW impact power) and the COP 1440 (20 kW impact power, both made by Atlas Copco) are the most popular hydraulic rock drills in use on the marked today.

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