This paper describes the structures for the repowering of the Miranda hydroelectric scheme that consisted in construction the of a new hydraulic circuit and a semi-buried powerhouse. Geomechanical tests carried out for the mechanical, hydromechanical and in situ state of stress characterization are described. Monitoring plans were established for controlling the safety of the structures during constructing and first use phases. Some significant results observed are presented and compared with numerical results.


Cette communication decrit les structures pour le nouveau projet de Miranda, qui sont composees par un nouveau circuit hydraulique et par une usine en puis. Des essais geotechniques ont ete conduis pour la caracterisation mecanique, hydromecanique et de l'etat de contraintedu massif rocheux. Plans d'Observation ont ete etablis pour le contrôle de securite des structures pendant la construction et I' entree en service des nouvelles structures. On presente quelques resultats significatifs relatifs awe observations et qui sont compares avec des resultats numeriques.


Die vorliegende Arbeit bescbreibt die Strukturen der Kapazitaetserweiterung des hydroelektrischen Damms in Miranda, wo ein neuer hydraulischer Kreislauf und ein halb unterirdisches Kraftwerk gebaut wurden. Es folgt eine Bescbreibung geomechanischer Tests, welche fuer die mechanische, hydromechanische und in-situ Stressbestimmung durchgefuehrt wurden. Ueberwachungsplaene zur Sicherheitskontrolle der Strukturen waehrend des Baus und der ersten Benutzungsphasen wurden resell. Es folgt eine Darstellung der wichtigsten beobachteten Ergebnisse und ihr Vergleich mit numerischen Resultaten.


This paper describes the structures for the power increase of the Miranda scheme. The geomechanical tests carried out for the mechanical and hydromechanical characterization of the rock mass are presented, as well as the in situ state of stress measurements performed. Descriptions are made of the monitoring plans for controlling the safety of the new structures and the older ones during construction and first use phases. Some significant results and tests performed are referred to. For the interpretation of the monitoring measurements 2D and 3D numerical models and analytical solutions were developed, numerical results being compared with corresponding results observed.

Existing structures

The initial Miranda hydroelectric scheme in operation since 1960 consisted in a diamond-shaped massive head buttress dam with a maximum height of 80m, incorporating four waterways with radial gates designed for a maximum flow of 11000m3/s, The plant included an auxiliary surface spillway on the right bank with 500m3/s capacity. The dam spillways nowadays carry out the control of the reservoir water level control. Three independent hydraulic circuits were constructed for each set (Figure 2). The penstocks have- an average length of 95m with 28.2m2 circular cross section. An underground powerhouse (I) was built with 80m long, 19.6m width and 42.7m maximum height with a traditional horse shaped profile. The generated plant consisted of three 52MW vertical Francis units with an average capacity of 870GWh (Azevedo and Martins 1961;Aguiar1993).

Power increase structures

The power increase structures of the Miranda hydroelectric scheme consisted of converting the inlet and the draw off tunnel of the auxiliary spillway in a water intake and a penstock to feed drilled bench-cut method a set installed in a semi-buried shaft, designated as powerhouse II. A short tailrace tunnel was constructed. Two cofferdams were, constructed upstream and downstream which were demolished stale subsequently (Figure 1). The upstream cofferdam develops in plan like an arch with a 35m radius; the upstream wall is vertical and its maximum height is 42m The downstream cofferdam is smaller, its maximum height being 15m. These cofferdams made it possible for the Miranda reservoir to be normally operated during the construction works. The powerhouse II has an almost circular shape due to the generating set and the need of vertical communications for access of people and installation of cables, bars and piping. At several levels the section extends rectangularly to downstream in order to accommodate equipment. The shaft is about 66m deep and was excavated by open-cut methods, and its section has an approximately 25m radius (Figure 3).

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