ABSTRACT:

Most of the supports for tunnels in rock masses are designed based on empirical classifications. Firstly, the authors present a brief description of the classification proposed by Rocha. After that and based upon the results obtained from 15 tunnel sections, the Wickham et al, (RSR), Bieniawski (RMR) and Rocha (MR) classification systems are correlated.

RESUME:

Les soutènements pour les tunnels en massifs rocheux sont choisis tres frequemment a partir de classifications empiriques. Les auteurs commence pour presenter la classification propose par Rocha. En suite ils presentent des correlation entre les classifications de Wickham et al, (RSR), Bieniawski (RMR) et Rocha (MR), en se baseant sur les resultats de 15 sections de tunnels etudies.

1 INTRODUCTION

Most of the supports for tunnels in rock masses are designed on the basis of empirical geotechnical classifications which have been developed for this purpose by several authors. The first of these type of classifications was introduced by Terzaghi (1946) and a number of others appeared more recently, particulary during the 70's, namely those proposed by Wickham et al.(19 74), Bieniawski (1974, 1976, 1979), Barton (1974) and Association Française de Travaux en Souterrain - AFTES (1976). In this kind of classifications, the quality of a rock mass is "quantified" on the basis of a number of geological and geotechnical features and parameters in order to antecipate the behaviour of the rock mass to be tunnelled and to produce some recommendations on the support to be used. Selecting the most appropriate and relevant features and parameters that control the behaviour of the rock masses as well as their relative importance in order to build up a classification lies, to a great extent, upon the experience of each author. Consequently, it is important to investigate if there is a correlation between the different classification systems. If this correlation exists, one may apply the easiest system according with the parameters available in the situation under study and derive the result that would be obtained by using other. At the end, this would enable a comparison between the recommendations about the support established by the different classification systems. Also during the 70's, Rocha (1976) proposed his own empirical classification to "quantify" the quality of the rock masses in order to assist the design of tunnel SUE ports. Although frequently used in Portugal together with others, namely the ones from Wickham et al., Bieniawski and Barton, Rocha's classification is almost unknown abroad. It was introduced in 1976 during the lessons delivered by that author to the M.Sc. courses in Engineering Geology and Soil Mechanics at the Universidade Nova de Lisboa. By that time Rocha prepared, for their students, a set of written notes under the litle "Underground Structures" where the basis of this classification are explained. Rocha, certainly wanted to carefully - check the results of his own classification and this is certainly the main reason why it was never published before his death in 1981. Firstly, in this paper, the authors summarize the classification developed by Rocha. After that, this classification together with the ones proposed by Wickham et al, and Bieniawski are correlated by using the results obtained from 15 tunnel sections.

2 THE ROCHA'S CLASSIFICATION (MR)

Rocha assumes that the pressure on the support is imposed by a volume of rock that may detach (come off) from the rock mass after the excavation, this being ruled mainly by the characteristics of the existing joints. That volume has the shape presented in Fig. 1 where hc and hn are the widths of rock that may come off from the roof and from the walls of the excavation.

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