ABSTRACT:

A numerical model for the study of arch dams on jointed rock foundations is presented. A new module was developed for the three-dimensional discrete element code 3DEC, in which the concrete structure is represented by finite elements. Two applications are reported: a comparison with a physical model test, and a comparison with field monitoring data.

RESUME:

On presented un modèle numerique pour I'etude de barrages-voûtes sur fondations fractures. Un nouveau module a ete developed pour Ie 3DEC, avec une modelisation de la structure de beton par des elements finis, On relate deux applications du modèle numerique: une comparison avec un modèle physique, et une comparison avec des resultats de I' auscultation du comportement d'un barrage.

ZUSAMMENFASSUNG:

Ein numerisches Modell fuer die Untersuchung von Bogenstaudammen auf geklueftetem Felsuntergrund wird vorgestellt. Es wurde ein neuter Module fuer das drei-dimensionelle Diskrete Element Rechenprogramm 3DEC entwickelt, in dem die Betonstruktur durch Finite Elemente dargestellt wird. Zwei Anwendungen werden beschrieben: ein Vergleich mit einem physischen Modellversuch, und ein Vergleich mit Feldbeobachtungsdaten.

1 INTRODUCTION

The safety assessment of concrete arch dams demands a detailed study of the stability of the foundation rock mass. Numerical models, such as three-dimensional finite element models, are routinely used in the analysis of the global mechanical and hydraulic behaviour of dam foundations. However, stability analysis is often based on simplified limiting equilibrium procedures. These methods consider independent volumes of rock that may become kinematically unstable, and make simple assumptions about the forces acting on them, including water pressures. Since failure mechanisms are typically defined by natural rock discontinuities, these simple procedures may provide a more realistic approach than the use of continuum models. Discrete element techniques, however, have the ability to represent the discontinuous nature of rock, therefore allowing the analysis of the stability of rock blocks, within the frame of a global model of the dam-foundation system. No simplifying assumptions about stresses or water pressures acting on given discontinuities are required in this case, A limitation of most discrete element codes lies in the modelling of the concrete structure. Even when deformable blocks are used, the stress analysis of the arch may not be sufficiently accurate. The model presented in this paper combines a discrete element representation of the foundation rock, using deformable blocks, and a finite element representation of the concrete shell, by means of three-dimensional quadratic elements. This approach allows the consideration of failure scenarios involving the arch collapse, as well as, sliding and separation on the rock joints. The model was implemented in a new module included in the code 3DEC (Cundall 1988, Hart et al. 1988). Numerical models have to be validated against experimental data, as there are few analytical solutions for the behaviour of jointed or blocky media. Comparison with physical models provides a good way to assess the performance of numerical techniques in the simulation of the collapse of structures such as concrete dams (Pina & Costa 1993). The first application reported compares the performance of the proposed model with a physical experiment at LNEC involving a 3D blocky foundation (Lemos et al. 1995). The second example is part of an ongoing study of an arch dam on a very heterogeneous foundation, which recently had its first reservoir filling. A comparison with currently available monitoring data is presented.

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