SUMMARY:

On the basis of a few simple rock mechanics tests a tentative system is developed which enables one to evaluate the economic utilization range of a tunnel boring machine (TBM). The following rock indices are considered as necessary for this purpose: point-load strength index, rebound hardness, abrasion hardness and content of minerals with equal or higher hardness then quartz. These four parameters allow one to determine a new boreability index bq, as defined in this paper, which is important for the fundamental decision of whether or not a TBM can be employed. It may even allow one to estimate the penetration rate, respectively the economic range of TBM utilization. The test methods are briefly described and discussed, and test results are given.

RESUME:

On à tente, sur la base de quelques essais simples de mecanique des roches, de developper un système de pronostic d'utilisation d'un tunnelier (TBM). A cet effet, on considère que les paramètres suivants sont necessaires: indice de resistance ponctuelle, durete au rebond, resistance à l'abrasion et teneur en mineraux de même ou plus grande durete que le quartz. Sur la base de ces quatre paramètres on peut determiner un nouvel indice de forabilite bq, qui est important pour la decision fondamentale d'utilisation d'un TBM et qui permet eventuellement même d'estimer la vitesse de penetration et le domaine pratique d'utilisation du TBM. Les methodes d'essai sont brièvement decrites et discutees, et des resultats presentes.

ZUSAMMENFASSUNG:

Es ist versucht worden, aufgrund einiger weniger einfacher geotechnischer Versuche ein Klassifizierungssystem zu entwickeln, das erlaubt, den wirtschaftlichen Anwendungsbereich von Tunnelfrasmaschinen (TBM) zu bestimmen. Folgende Versuche werden dafuer als notwendig erachtet: Zugfestigkeit bei punktförmiger Belastung, Prellharte, Abrasionsharte und Gehalt an Mineralien mit gleicher oder grösserer Harte als Quarz. Diese vier Parameter erlauben die Ermittlung eines hier neu eingefuehrten Bohrbarkeitsindex bq, welcher wichtig fuer den grundsatzlichen Entscheid ist, ob eine TBM eingesetzt werden kann. Der index erlaubt möglicherweise auch die Abschatzung der zu erwartenden Vortriebsgeschwindigkeit, bzw. den wirtschaftlich guenstigsten Anwendungsbereich.

1. INTRODUCTION

The decision of whether a tunnel boring machine (TBM) can be used or not for the realization of a tunnel project depends principally on the cutter costs and on the rate of expected heading advance (ROBBINS. 1970; TARKOY, 1974). The latter is on the one hand a function of pure technical factors, such as type of machine selected, handling and maintenance of TBM, but on the other hand it is also a function of the geological conditions such as rock type, fracturing, stability of excavation and water inflow. The technical factors can be well controlled by adequate organization, so that generally the geological conditions constitute the critical factor. Of the different geological parameters of importance for tunnel boring, only one can be determined approximately by physical testing methods, namely the bore ability of the rock. As all the tests are made on samples, the bore ability is a characteristic of rock which is valid only for the sample and not for the rock mass. Since the penetration rate (bored length per machine hour) and the cutter costs depend on the bore ability, numerous methods have been developed to quantify the term bore ability which is a function of the rock's strength. hardness and abrasive properties but also of its mineralogical composition and its texture. Unfortunately, there is no method which would permit one to establish the bore ability of rock in a quick, cheap and simple manner. Many of the proposed methods require sophisticated testing equipment or difficult techniques which make it impossible to test a large number of samples under the simple conditions. which often occur in non-industrialized countries. Confronted with the above problem, during the geotechnical investigation for the 26 km long power tunnel of the Pueblo Viejo Project, Guatemalas biggest hydro-electric scheme now under construction, the authors attempted to develop a system which qualifies the boreability of rock in a similar manner as the methods proposed by TARKOY (1974] and RUTSCHMANN (1974], namely on the basis of 4 parameters which can be determined easily be means of well introduced instruments or methods;

  1. The tensile strength, determined with the point-load strength apparatus

  2. The rebound hardness, with the so called Schmidt rebound hammer

  3. The abrasion hardness, with the Los Angeles rattler test

  4. The abrasiveness (content of minerals with equal or higher hardness than quartz), by chemical, optical or X-ray methods.

These 4 parameters allow to determine a bore ability index bq, which classifies the rock with reference to its aptitude to be bored by a full-face tunneling machine and to estimate the penetration rate of a TBM in a given rock type.

2. DESCRIPTION OF PERFORMED TESTS
2.1 Point-load strength index

This test consists of compressing a cylindrical (core sample) or any shaped specimen (irregular lumps) between two peaks with standard dimensions.

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