SYNOPSIS:

Stability analysis of large openings may be carried out using 3-D numerical models, namely those based on finite elements and boundary element methods. Studies are made for an underground powerhouse by using finite element models, and the results are compared with those obtained on an experimental model and with the monitored displacements.

RESUME:

La stabilite des grandes cavernes peut être analysee par des modèles numeriques tridimensionels, en particulier en employant la methode des element finis et celle des elements de frontière. Des etudes sont effectuees pour une centrale souterraine et les resultats sont compares avec ceux d'un modèle experimental et avec des deplacements mesures in situ.

ZUSAMMENFASSUNG:

Die Stabilitatsanalyse großer Hohlraume kann mit Hilfe dreidimensionaler numerischer Modelle durchgefuehrt werden, namentlich jener der Methoden der Finiten Elemente und der Grenzelemente. Es werden Untersuchungen fuer ein in Mozambique gelegenes unterirdisches Kraftwerk aufgefuehrt, und die Ergebnisse mit denjenigen, die an einem experimentellen Modell erhalten wurden, und mit den in situ gemessenen Verschiebungen verglichen.

1. INTRODUCTION

In the development of underground power plants, the construction of large underground openings plays an important role in the design of such undertakings. Concerning stability and safety problems, the conceptual models used for this underground structures, based on the approach of continuous or discontinuous media will be analyzed. As the static calculations for the stability analysis are carried out generally with the help of numerical models, especial emphasis will be given to the use of three-dimensional numerical models based on finite element methods and also on boundary element methods. A 3-D finite element model was developed, with which an analysis can be made of the influence of the excavation sequence and of the introduction of supports. For a continuous approach, it assumes an anisotropic elastic body, while for a discontinuous approach, it assumes inability to sustain tension and non-linearbehaviour during shear deformations. Studies are made with this 3-D model for an underground hydroelectric powerhouse. The characteristics of the main cavern are 216.7 m in length, 28.9 in width and 57.0 m in maximum height. In the numerical model, a block was considered, which contains part of the machine hall and one of the surge chambers. Calculations of the excavation sequence were carried out and the results are compared with those obtained on a 3-D experimental model, developed et LNEC, and on a 2-D non linear finite element model. The monitored displacements obtained are also presented.

2. DESIGN CONSIDERATIONS FOR LARGE UNDERGROUND STRUCTURES
2.1. General

In the development of underground power plants the construction of large underground openings performs a relevant part in the design of such undertaking. The design stages of these underground structures imply the knowledge of the geological and geotechnical conditions of the rock masses where the underground space should be located. It should include the different type of rocks end their mechanical properties, the geometry and nature of the discontinuities the weakness zones, the initial state of stress and the hydraulic and thermal properties of the rock masses. To the design of such underground openings, a procedure may consist of the following stages (Olsen and Broch, 1977): i) selection of the location which shows optimal conditions from a stability point of view; ii) definition of the orientation of the openings axis that will be suitable with respect to the in situ stress field and to major discontinuities; iii) shaping of the under ground openings taking into account mechanical properties of rock masses, in situ stresses and jointing; iv) dimensioning of the different underground structures followed by detailed static calculations concerning stability and safety problems. Dimensioning of large underground structures, as underground power plants, is essentially based on empirical methods, using the precedent practice and observation results of other works, on stability studies using results of stress analysis on assuming continuous and discontinuous approaches and on analysis with limit equilibrium techniques applied to specific failure mechanisms(Richards et al, 1977). The shaping of supports should generally be a function of the average rock mass block size, of the span dimension and of the space between rock bolts. Moreover, as regards the calculation methods for analysing underground works, one question is frequently put: what kind of idealization or idealizations of the rock masses are adequate to the different situations to be found in practice, that are reproduced in the conceptual models, and particularly which is the meaning of continuous and discontinuous approaches (Rocha; 1976). It should be emphasized that-the insufficient knowledge of the properties of rock masses does not, in general, justify complex idealizations. The continuous approach is justified in the case of sound rock masses, without relevant discontinuities, and also in the case of jointed rock masses with small ratio between the distance of joints and dimensions of the openings. In the latter case, the ubiquitous joint method is generally used.

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