SUMMARY

In this paper some methods utilized for studying the stability of large rock caverns are described, These methods were applied to a practical case, a fairly large cavern in Sweden With a width of the span just above 30 m. The cavern is under excavation at present.

It is emphasized that numerical analysis in rock mechanics must be treated with caution implying that a stepwise refinement of the numerical analysis should be advisable. Relatively simple methods based on manuals are often sufficient. The finite element method (FEM) offers, however, the possibility of describing the geometry of a cavern more accurately. Mostly, an elastic model of the rock material will yield enough guidance. If further refinements are desirable, joints can be introduced into the FEM model and adequate material properties for these joints can be defined.

A vast amount of work is associated with FEM modelling and the generation of the input data for the FEM program to be used in the calculations. For this work, however, a versa-tile. and flexible interactive graphics input data generator, called GEORGE, has been utilized. Besides considerable time saving the preprocessor, as it is designed, can imply further insight and understanding of the finite element method and its actual limitations In rock design,

ZUSAMMENFASSUNG

Dieser Beitrag beschreibt einige Verfahren zur Stabilitatspruefung grosser Felskavernen. sie.kamen in Schweden bei einem praktischen Fall, einer ziemlich grossen, z.Z. im Bau befindlichen Kaverne mit gut 30 m Spannweite, zur Verwendung.

Es wird betont, dass numerische Analyse in der Felsmechanik nur mit Vorsicht zu ver- wenden ist und tunlichst stufenweise verfeinert werden sollte. Oft genuegen auf Lehrbuecher. fussende, verhaltnismassig einfache Verfahren. Die Finite-Element-Methode (FEM) ermöglicht Jedoch, die Geometrie einer Kaverne genauer zu beschreiben. Meistens reicht es dabei, von einem elastischen Modell auszugehen. Sind weitere Feinheiten erwuenscht, lassen sich kluefte mit lns FEM-Modell einbauen und fuer die kluefte geeignete Materialeigenschaften festlegen.

Derartige FEM-Modelle zu entwickeln und die fuer Berechnungen mit dem FEM-Programm benötigen Ausgangsdaten einzugeben, erfordert viel Arbeit; die Aufgabe wurde jedoch mit einem flexiblen, interaktiven Graphik-Programm namens GEORGE bequem gelöst. Neben wesentcher Zeitersparnis gewahrt dieses Programm in seiner heutigen Form auch tieferen Einblick in die Finite-Element-Methode und Verstandnis fuer deren faktische Begrenzung bei felsmechanischen Entwuerfen.

RESUME

Ce document fait la description de quelques methodes utilisees dans 1''etude de stabilite des grandes chambres souterraines creusees en pleine roche. Ces methodes ont ete appliquees sur un chantier en Suede, consistant au creusement d''une grande chambre d''une portee legerement superleure à 30 m. L''excavation est actuellement en cours.

En mecanique des roches, il est à remarquer que 1 ''analyse numerique doit être traitee avec beaucoup d''attention pour pouvoir faire une approche successive devant mener à un resultat convenable. Des methodes relativement simples, basees sur des manuels, sont souvent suffisants, mais la methode des elements finis (MEF) permet de decrire plus exactement la geometrie d''une chambre. Dans la plupart des cas, un modele elastique de la roche fournira des renseignements assez consistants pour tracer une ligne de conduite. Pour celui qui desire ameliorer les performances, des joints pourront être introduits dans le modele MEF après avoir defini les proprietes du materiau adequat pour ces joints.

De tres nombreux ouvrages sont compatibles au modelage MEF et les donnees d''entree sont generables dans les calculs du programme MEF. A cet effet, un generateur graphique, appele GEORGE, a ete utilise pour introduire les donnees. Ce processeur, versatile et tres souple, a permis des gains de temps considerables et a, de par sa conception, abouti à une meilleure comprehension de la methode des elements finis et de ses limites actuelles en mecanique des roches.

INTRODUCTION

(Figure in full paper)

A cavern with a free span of 30 m is under excavation at present. The size and the shape of the cavern were determined from general functional and service requirements based on demands from defense authorities in Sweden. One of the main questions in this respect was whether the relatively large span and the shallow shape of the cavern would seriously affect its stability, also taking the actual site constraints into consideration. Originally, the cavern was designed as two separate smaller rooms. It was quite clear, however, that the total construction costs would be higher for a two-room-alternative than for a larger single-room-alternative, provided that the costs for additional reinforcement could be limited for the second case.

Consequently, the interest was focussed on the rock mechanical aspect during the design stage including investigations of the real conditions in the rock mass at the actual site.

Considering the parameters affecting the stability of the cavern, already a simple elastic analysis shows that the tangential stresses in the roof are not very dependent on the overall size of the cavern for ordinary ratios between the horizontal and vertical virgin stresses of the mass. For small horizontal stresses, however, the roof will be affected by tensile stresses which, in combination with unfavourable joint conditions, may cause stability problems and corresponding risks for substantial increase of necessary reinforcement.

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