DRILLING, BLASTING AND CUTTING - IS IT POSSIBLE TO QUANTIFY GEOLOGICAL PARAMETERS RELATING TO EXCAVATABILITY?

Determining tunnel stability is a key issue during preliminary site investigation. In contrast, problems of excavatability have been largely ignored. While the choice of an economic tunnelling method is admittedly a clear priority in the planning stage, special investigations focussing on rock fragmentation (e.g. drilling or cutting performance, rock mass blastability or tool wear) are rarely carried out. This paper ex- plores possibilities to quantify key parameters for rock mass excavatability in drilling, blasting and cutting by TBMs and roadheaders. Although rock mechanical properties play a key role, geological parameters are rarely fully included in most projects. In some cases, the influence of geological features on rock fragmenta- tion can be more significant than varying rock properties. Geological difficulties can have a high impact on the economics of an underground construction project, especially when the chosen excavation system turns out to be unsuitable for the conditions encountered. Thus it can be argued that the geological and petrological characteristics of the rock mass should be evaluated with the same degree of effort as that for the geotechni- cal prognosis. RESUME: Dordinaire lors d´etudes preliminaires aux grands projets de percement de tunnel, les pronostics sur la stabilite de lexcavation se trouvent au premier plein dinteret. Ce pendant ces dernieres annees les difficultes de prevoir correctement la resistence des roches lors de percement mechaniques et de forage. Encore des problems revenaient en connexion avec la consommation des ciseaux et avec le progres de couper de bas avec des machines de percement et de forage. Dans cet bulletin, sont exposees les correlations fondamentales entre quelques proprietes geologiques, le progres de couper et lusage des ciseaux, utiliser lassistance une etude allemande dans granites. EXCAVATABILITY - A DEFINITION Is it possible to put numbers to geological parameters concerning excavatability? Are only rock mechani- cal properties quantifiable? Is a classification system possible for excavation by drilling & blasting or cut- ting, including all necessary geological and geotechnical parameters? These questions arise when the be- haviour of rock mass material has to be considered during underground excavation. Excavatability is a term used in underground construction to describe the influence of a number of parameters on the drilling, blast- ing or cutting rate (excavation performance) and the tool wear of a drilling rig, roadheader or TBM (wear or usage criterion). The interaction of the main factors involved is illustrated in Figure 1. These terms are used in underground as well as in surface construction. In this paper, only the aspects relating to tunnelling are discussed.