INTRODUCTION

The principal type of tunnelling machine currently used in Great Britain is the boom header. These machines consist of a moveable boom with a rotating cutting head, which is equipped with an array of tungsten carbide tipped cutting tools. The cutting boom assembly can either be mounted on crawler tracks or in a tunnelling shield, as shown in Plate 1. A fuller description of the types and sizes of machine available has been given elsewhere (McFeat-Smith and Fowell, 1979). Since the introduction of this type of machine into British mines and, later, into civil engineering, there have been problems in assessing suitable applications and in predicting excavation rates. The design and specifications of the machines have continued to be improved, and with the introduction of a new generation of heavier, higher powered machines and the consequent increase in capital costs of such machines, it is becoming increasingly important to determine at an early stage in any tunnelling project whether the rock materials to be encountered are amenable to excavation at an economic rate by a boom tunnelling machine, and to ensure that the optimum machine design is chosen.

ROCK PROPERTIES INFLUENCING MACHINABILITY

For many years the most common means of assessing the likely performance of boom tunnelling machines has been the use of uniaxial compressive strength; however, it has now been shown that in many cases compressive strength alone does not give a good indication of rock machinability. Recent examples of rock types encountered that have proved difficult to excavate include: a dolomitic conglomerate, where, despite the relatively low overall compressive strength, the tunnelling machine was forced to cut through cobbles of high strength limestone which were strongly bound into the matrix material; the second example is that of a massive anhydrite which, due to the ductile nature of the material and the high coefficient of friction between the rock and cutting tool material, proved to be a more difficult cutting proposition than its compressive strength alone would indicate. These examples are described more fully in the case studies presented later.

A good correlation has been found to exist between in situ machine cutting rate and laboratory measured specific energy (S.E.). The specific energy is measured using an instrumented cutting test developed in the Department (Roxborough and Phillips, 1974) which simulates the action of a full scale drag type cutting tool on a sample of rock. Under the standard conditions of the test (described in Appendix 1) the specific energy of a rock material may be defined as the amount of energy required to excavate a unit volume of rock. This measure of S.E. provides the best indication of likely machine performance and the relationship between laboratory measured S.E. and in situ machine cutting rate for two classes of machine is shown in Fig.1. The lines shown are meant to represent the lower boundary of an envelope of results. The lines shown represent the relationship between S.E. and in situ cutting rate for massive rock conditions.

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