Keywords: dams, roller compacted concrete, finite elements, optimised shape, preliminary design, Ribeiradio RCC dam.

This work details a preliminary design for a gravity dam to be built with roller compacted concrete (RCC), located in central Portugal. Although a preliminary analysis for the initial geometry proved structurally safe, a plane stress finite element analysis - performed for the evaluation of the generalized stresses and displacements in the dam core, interface and foundation medium - indicated the need for geometric refining insuring safety under a specific seismic load combination. An improved section for the dam was achieved, inducing safe detailed patterns of stresses and displacements for the dam and foundation.

Mankind's constant need of water for its survival motivated, throughout the ages, the construction of dams. Moreover, the RCC techniques made also possible to use RCC gravity dams as an economically competitive alternative to conventional concrete or embankment dams. The possibility of rapid construction at low cost and the continuous placement of RCC, minimising cold joints between horizontal layers, allow contractors bids at lower unit prices for a dam construction with little risk. The preliminary design calculations and subsequent finite element analysis, partially shown in the present paper, resulted from consultancy services permitting the attendance of a participating partner in the international public tendering promoted by the Instituto da Água in 2000, associated with the construction of the above-mentioned RCC dam [1]. In order to permit a competitive bid for the construction costs of the dam infrastructure an alternative more economic solution was searched minimising the construction costs per unit length, whose structural strength integrity and safety had to be assessed. Conceptually the potential economic gains would be associated with the creation of a large gallery at the dam core.

The safety of the proposed RCC dam with an arc-rectangle gallery is then ascertained with respect to its strength, stiffness, base stability and global stability, as performed earlier by Barros [2].

Under the preliminary strength study, safe values for the normal stresses at the upstream and downstream dam feet are calculated, under several combinations of load cases. The stiffness study is performed estimating the dam crest total lateral displacement, according to the practical methodologies proposed by Herzog [3], taking into account flexure and shear deformations of the concrete and also of the foundation, upstream rotation of the valley bottom and thermal gradient between upstream and downstream faces of the dam. Since RCC is often placed in 250-400mm thickness layers and subsequently compacted, the global stability of the RCC dam is also ascertained at each dam elevation associated with successive lifts, calculating the sliding safety factor associated with the active and reactive forces.

The structural behaviour of the dam's critical cross section is modelled with an eight-node parabolic finite element mesh, in order to simulate the plain stress state installed in the concrete of the RCC dam, using the VIFEM software [4]. In this finite element analysis two geometries are ascertained, both satisfying geometric and operational restrictions: the original geometry refers to the arc-rectangle gallery proposed for preliminary design; the improved geometry corresponds to a decrease of its dimensions and a more downstream implantation in the dam core. In order to model the structural behaviour of the dam with acceptable accuracy, a portion of surrounding soil with appropriate boundary conditions was also meshed, minimising the boundary effects due to the finite character of the mesh.

The external loads applied to the RCC dam for this finite element analyses, under the seismic load case, are the gravity and seismic mass forces, and the hydrostatic and hydrodynamic pressures on the upstream face. The finite element analysis permits to determine distributions of generalized stresses and displacements in the distinct modelled media. With the values assumed by the principal stresses a verification of the safety of the dam is carried out at critical points, based on the CEB-FIP Model Code for biaxial stress state of concrete. The comparison of the yielding zones of the two geometries revealed to be an important tool to ascertain the safety of distinct solutions for the RCC dam.

1

COBA, "Aproveitamento de Ribeiradio", Solução de Referência realizada para o Instituto da Água (INAG), COBA-Consultores de Engenharia e Ambiente, Lisboa, 1999.

2

R.C. Barros, "Solução Estrutural Alternativa para a Barragem de Ribeiradio (Geometria Optimizada com Galeria Arco-Rectângulo)", Relatório Técnico Interno BAH-1/00, Engedec, Porto, 2000.

3

M.A.M. Herzog, "Practical Dam Analysis", Thomas Telford Publishing Limited, London, UK, 1999.

4

R.T. Teixeira, "Análise de Problemas de Grandes Deformações em Geotecnia", M.Sc. thesis in Structural Engineering, Dept of Civil Engineering, FEUP, Porto, Portugal, 2001.

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