The Passante dam is a 70 m high concrete gravity-darn founded on gneiss and crystalline schists having poor mechanical characteristics. The behaviour of the foundation is controlled by means of inverted plumb-lines long-base extensometers (rock meters), electro-acoustic piezometers, and by measuring the rotations of the dam base. In this paper data collected during the dam construction and the first fillings of the reservoir are analysed by means of an FEM model. The rock deformability was identified and compared with the values provided by the in situ tests performed during the design stage. The presence of a superficial low-module layer was evidenced; a considerable rheological behaviour was also observed.
Le barrage du Passante est un barrage poids en beton de 70 m de hauteur. Sa fondation est constituee de gneiss et de schistes avec mediocres proprietes mecaniques. Le comportement du rocher de fondation est contrôle par des pendules inverses, des extensimètres à longue base, des piezomètres acoustiques et par la mesure des rotations de la base du barrage. Les mesures effectuees pendant la construction du barrage et pendant la mise en eau ont ete analysees en utilisant les methodes d'elements finis. Cette analyse a permis d'etablir la deformabilite du rocher et de la comparer avec les valeurs obtenus par des essais in situ realises au stade preparatoire. On a pu mettre en evidence la presence d'une couche superficielle plus deformable ainsi que de nettes deformations rheologiques pendant la construction et la mise en eau.
Die Passante Staumauer ist eine 70 m hohe Gewichtsstaumauer, die auf Gneis und kristallinen Schiefern mit schlechten mechanischen Eigenschaften gegruendet ist. Das Verhalten des Baugrundes wird durch inverse Pendel, Dehnungsmesser mit langer Basis, elektro-akustische Piezometer und durch die Messung der Rotationen der Dammbasis erfaßt. Im vorliegenden Beitrag werden die Daten, die wahrend des Dammbaues und der ersten Auffuellungen gesammelt wurden, mittels der Finite Element Methode analysiert. Durch diese Untersuchung ist es möglich gewesen, die Formanderung des Gesteins zu identifizieren und mit den Werten von in situ Versuchen wahrend des Bauentwurfes zu vergleichen. Das Vorhandensein einer Schicht stark verformbaren Gesteins und ein bemerkenswertes rheologisches Verhalten wahrend des Baues und der ersten Auffuellungen werden hervorgehoben.
The Passante reservoir is part of the hydroelectric scheme which exploits the waters flowing down the eastern slopes of the Sila massif in Calabria (Fig.1); The Passante dam was built across the upper course of the river bearing the same name at about 1100 metres elevation; the reservoir thus formed has a maximum capacity of 35 10 6m3, being fed by a catchment area of about 70 km2. It will supply water to two hydroelectric plants in series for an overall head of 900 m and a capacity of 75 MW. The dam is a gravity dam with triangular section and a central spillway; its features are summarized in Table 1 and illustrated in Fig. 2. Its construction was completed in 1977 however, due to delays in the completion of other structures and in particular of the nine-kilometre long diversion tunnel, the reservoir has so far never been completely filled. Monitoring devices were installed in the dam and in the foundation rock so as to follow the behaviour of the structure right from the initial stages of construction.
The reservoir is located in metamorphic rocks of the Calabride complex. In the gorge area two main units, known as "biotitic gneiss" and "white schists" are present (Fig.2). The latter one outcrops in the lower part of the valley slopes and is therefore the foundation rock of the higher blocks. Its petrographic characteristics vary considerably but the most common lithotype is a weakly schistous rock, composed of quartz and feldspar with a low percentage of phyllosilicates; its texture is characterized by the presence of large highly fissured crystals and of microgranular cataclastic layers (Fig. 3). Other less frequent lithotypes present a higher percentage of muscovite and marked schistosity.
The mechanical characteristics of the rock material are summarized in Table II and in the histograms of Fig.4. The tests relate to the most common weakly schistous, lithotype. The high scatter of the strength and deformability characteristics may be attributed to the variable conditions of microfissuring and, in some cases, to the presence of macroscopic weakness surfaces. The intensity of microfissuring is evidenced by the difference between seismic velocity in dry and saturated conditions, by the low values of the Young modulus and by its dependence in the applied stresses.
A seismic refraction survey (Fig.7) indicated the presence of a superficial layer of low quality rock characterized by a seismic velocity of 1.7- 2.5 km/s; this layer is virtually absent at the bottom of the valley, whereas its thickness increases towards the higher elevation along the slopes.
