ABSTRACT:

The report describes semi empirical method of designing of rock supports of tunnels. Results of longstanding geophysical studies of destressed zone around tunnels are used. Curves were constructed allowing the thickness of destressed zone depending on tunnel diameter and other acting factors to be predicted. The example of use of the semi empirical method of rock support designing of the particular project is offered.

RÉSUMÉ:

Le rapport traite d'une methode semi empirique pour le projet d'un soutènement des tunnels. Des resultats de recherches geophysiques de plusieurs annees pour les zones decompri mees autour de tunnels, ont ete utilises. Des courbes sont etablies pour permettre la prevision de l'epaisseur de fa zone decomprimees, en fonction du diametre de tunnel et d'autres facteurs. Un exemple d'application de la methode semi-empirique est donne pour la projet d'un soutènement Pour un ouvrege reel donne.

ZUSAMMENFASSUNG:

Der halbempirischer Verfahren der Projektierung des Tunnelsausbaus ist beschreibt. Die Ergebnisse der vieljarigen geophysischen Untersuchungen der Entlastungszone um die Tunnels sind benutzt. Die Kurven sind gebaut, sie gestatten die Dicke der Entlastungszone je nach den Umstanden vom Tunnelsdurchmesser und von anderen einwirkenden Faktoren zu prognozieren. Das Beispiel der Verwendung das halbempirischen Verfahrens der Projektierung des Ausbaus fuer ein konkretes Objekt ist angefuehrt.

1. INTRODUCTION

The stability analysis and designing of undergrowld excavations and tunnels in particular are characterized at the present time by a great variety of methods. It is explained not only by different natural conditions in wich tunnels are constructed but by absence the sufficiently rigorous physically substandiated theory of behaviour of the rock mass around underground excavations. The currently available numerical methods in the nonlinear formulation including the distinct element method allow the behaviour of rock masses to be mathematically modelled to a maximum extent. However these methods come across difficulties of adequate determination of stress state, physical and mechanical properties and jointing of rock masses. Among existing methods of designing the tunnel rock supports empirical methods have found a wide utility. Generally it should be noted that any designing even at most detail calculations to this or that extent is based on empiricism i.e. it is performed with looking back at analogues. However in this case we are dealing with purely empirical methods which do not practically use calculation techniques. These methods of stability analysis and designing of tunnel supports include, for example, the methods proposed by (Barton 1974), (Bieniawski 1984) and other specialists. They are based on a large number of precedents and selection of parameters of supports depending on rating and other quality indexes of the rock mass. Many tunnels were designed and constructed on the basis of empirical methods. The avantage of empirical methods consists in the fact that they take into account the real experience of construction. However the use of the rating system which to a certain degree is matter of convention may result in a wide scatter of parameters of supports because of subjective character of rating of rock mass by geomechanical engineers and because of non uniformity of the structure and properties typical for any rock mass and scatter in engineering-geological indexes producing an effect on this rating. In our opinion the more objective would be the empirical or semi empirical method in which parameters of supports are defined according to factors messured directly in the process of tunnelling and operation of tunnels and having a direct effect on parameters of supports: thickness of zone of destressed and loosened rocks around undeground excavations, value of rock pressure, deformation of excavation walls, etc. With availability of sufficient number of such experimental data it is possible to develop different empirical relationships and to establish regularities of variation of mentioned data in conformity with tunnel diameter, strength of rocks, stress state of the surrounding rock mass and some other factors.

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