ABSTRACT:

The cavern housing the machine hall of the S. Giacomo al Vomano power plant was excavated at a depth of 650m in a strongly fractured limestone formation. After extensive in situ exploration, structural analyses by means of numerical models were carried out in the design stage aiming at estimating the mechanical behaviour of the rock mass surrounding the cavern and at assesing the final deformations. An extensive monitoring system was installed in order to adjust, during construction, the initial support scheme. Comparison between calculated and measured deformations made it possible to evaluate the reliability of the geomechanical parameters utilized in the design and supplied information for improved modeling.

RESUME:

La caverne de I'usine hydro-electrique de S. Giacomo al Vomano a ete excavee a une profondeur de 650 m dans une formation calcaire très fracturee. Après avoir complete la reconnaissance geotechnique, on a utilise dans I'etude du projet des modèles numeriques pour evaluer Ie comportement du rocher autour de la caverne et les deformations finales. La conception initiale à ete complete en cours de travaux en utilisant les indications de la strumentation mise en place. Une comparaison entre deformations calculees et mesurees a permis d'evaluer la fiabiliite des paramètres geomecaniques utilisees dans Ie projet et a fourni des informations pour ameliorer Ie modele à adopter.

ZUSAMMENFASSUNG:

Die Krafthauskaveme von S. Giacomo aI Vomano wurde in einer gekluefteten Kalksteinformation bis zu einer Tiefe von 650m gebaut. Nach geologischen und felsmechanischen Untersuchungen, Wurden eine Vordimensionerung der Felsbefestigung und ein Voraussicht des Verformungen mit Hilfe der numerischen F. E. Modellen durchgefuerht. Kontrollmessgeraete wurden istalliert, um die Anfangsauskleidungsproject zu aendem. Berechnete und gemessene Verformungen wurden verglichen; die Zuverlaessigkeit der in dem Entwurf angewandten Felskennwerte wurde dadurch bewertet und Daten zur Modellentwicklung wurden erteilt.

1. INTRODUCTION

The new S. Giacomo underground power plant is located at some 120 km east of Rome at the foothill of the Gran Sasso, the highest peak in the Appennini range; it is a dual purpose scheme with a total installed power of 375 MW built to fully exploit the hydroelectric potential of the Vomano river and to obtain a pumped storage capacity. The machine hall is positioned at about 1600 m inside the mountain slope, at the depth of 650 m below the ground surface; it is 72 m long, 24 m wide and 35 m high (Fig. 1). Its cross section has a typical mushroom shape required for the construction of a concrete arch as final lining system of the vault; therefore the total excavation width at the base of the crown is about 30 m. A 50 m deep shaft houses the pump-turbine unit. A number of structural analyses were performed in the design stage in order to select the most appropriate support scheme and to evaluate the final deformations. Because of the uncertainties about the mechanical parameters of the rock mass, an observational method of construction was adopted which required the installation of an efficient and automated monitoring system.

2. GEOLOGICAL AND STRUCTURAL FEATURES

The cavern is located in the "Bisciaro" formation of Miocene age. The rock is fine-grained limestone The structural pattern of the region is characterized by many important over thrusts generated during the Alpine orogenesis (Late Miocene-Pliocene). The formation was subjected to intense shear deformation both along the bedding planes and along other surfaces, so that the rock material frequently presents pervasive fissuring. The general structure of the rock mass, although very complex in detail, can be approximately represented by a polar symmetry where the equatorial plane dips W ο WSW and has an inclination of 35°÷45°.

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