The evaluation of the degree of fracturing of a rock mass is a major starting point for any rock engineering design. There are different methods available in literature for the evaluation of the rock mass degree of fracture. Among the most popular we have considered: the Jv estimation by Palmström (1996) and the P32 estimation by Dershowitz (1985). The first one considers the total number of discontinuities in a certain volume weighed by the joint persistence whilst the second method consider the joint area computed by a statistical analysis of the traces under the hypothesis of different joint shape.

A comparison between the two different approaches firstly developed on simplified synthetic schemes and then on real rock mass data has been the topic of this work.

In particular a rock slope of the Mount Blanc massif and the excavation faces of the Marta-Giulia tunnel have been analysed to highlight the influence of scale effects on the rock mass degree of fracturing evaluation.

1 Introduction

A rock mass is a discontinuous medium made of rock blocks delimited by discontinuities, which generally are the subjects of in situ surveys. Different characteristics, defined by the International Society of Rock Mechanics (ISRM, 1978), have to be included in the description of discontinuities: orientation, spacing, continuity or persistence, roughness, wall strength, opening, filling and infiltrations. The first three types of characteristics are geometrical magnitudes that can be measured using the compass or non-contact techniques. From these it is possible to obtain evaluations on the degree of fracturing of a rock mass and on the dimensions of the blocks that can be formed. Such evaluations can be made in either a deterministic or statistical manner, whenever the collected data sample is sufficiently representative.

The key point of the correct evaluation of the degree of fracturing and the characterisation of the rock mass is the correct identification of the effects of the observation window that could significantly influence results and lead to a totally different quality rate. The size of the observation window is sometime influenced by external boundaries such as the dimension of the excavation front in a tunnel that cannot be enlarged. In these cases is particularly important to quantify the scale effect in the evaluation of the degree of fracturing.

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