Keywords: gravity masonry dam, storage capacity increase, finite element model, composite structure, seepage, uplift.

This paper illustrates the principal aspects of a study for the heightening of a masonry gravity dam built in 1930. So far the dam has been utilized for flood control with the reservoir normally empty. Now operation modification is required in order to employ the structure for both flood control and seasonal storage. Heightening of the maximum water level is thus required. This produces both the additional water thrust on the upstream face and a rise of the pore water pressure in the foundation rock due to seepage.

Three principal aspects connected to the structural intervention have been analysed: (i) present dam status, both when the reservoir is empty and in a "full" condition (i.e. water level at the top); (ii) investigation and selection of a proper solution for the heightening intervention; (iii) assessment of the elastic behaviour of the new composite structure by means of both mechanical and hydraulic numerical models.

Concerning the actual dam condition, the results of the analysis performed show inadequate stability; furthermore the comparison between the results obtained from the employment of both numerical and analytical models reveals that the former can simulate the geometrical and mechanical discontinuity at the interface between the dam and the foundation rock. In order to adopt remedial measures, the suggested solution consists of a concrete wall close to the upstream face [1,5]. The new structure becomes complex [4] and the study of the coupling between the wall and the dam requires the adoption of the finite element method [3,6,8], to carry out both mechanical and hydraulic models, including a portion of the foundation rock [2]. The adoption of the hydraulic model permits the evaluation of the influence of pore water pressure on the dam stability: the numerical solution of the filtration net reveals an uplift distribution less severe than that imposed according to Italian Standards [9].

Two coupling modalities have been considered: wall-dam fixity and frictionless contact. The same model simulates both conditions by means of an interface layer, confined between the old and the new structure, with variable mechanical characteristics, i.e. a homogeneous isotropic material having the same properties as the wall's concrete for the first coupling modality and orthotropic material with a very small ratio between the Young's modulus in the vertical and horizontal direction for simulating the absence of frictional resistance.

Initial analyses have been carried out in order to define an adequate extension of the foundation rock to be included in the finite element model, thus minimizing the boundary effects and reducing computational effort. The significant part of the foundation rock has been determined by means of an analysis carried out through subsequent steps in which the displacement components of a fixed number of nodes have been computed and their variations monitored while changing the rock extension.

The meshing criteria and element type have been selected through the numerical analysis of an analogous and simple problem for which the analytical solution exists: by comparing both results, the influence of the aforesaid factors in the solution process has been examined [7].

1

Berga L., Clemente P. "Heightening of Riudecanyes dam" Icold Congress, Durban 1994, Q.70/R.21.

2

Gell K. "The adaptation of existing old masonry dams with respect to rock mechanical and geometric boundary conditions" in Icold Congress, Durban 1994, Q.68/R.8.

3

Icold-Cigb "Computational procedures for dam engineering", Bulletin no 122, 2001.

4

Lazzarini P., Bourdarot E., Bonnet P., Frossard E., Lurin P. "Problèmes specifiques aux surélévation de barrages en béton au en maçonnerie" Icold Congress, Durban 1994, Q.70/R.26.

5

Serrando M. "Strengthening of Estagendo dam" in Icold Congress, Lausanne 1985, Q.59/R.22.

6

Zienkiewicz O.C. et al. "Criteria and assumptions for numerical analysis of dams" in International Symposium held at Swansea U.K. 8-11 September 1975.

7

Zienkiewicz O.C., Taylor R.L. "The Finite Element Method", Volume I fourth edition (McGraw-Hill, 1989).

8

http://www.ansys.com - ANSYS Inc. Corporate.

9

DM LL PP 24/3/82 "Norme tecniche per la progettazione e la costruzione delle dighe di sbarramento", (in Italian).

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