A major landslide occured on 19 July 1991 on the slope behind the Leresti power plant, affecting the last 25 m of the pressure tunnel, the valve chamber and the penstock. The investigations conducted after the accident have rendered evident that the failure took place in two successive phases. In the first phase the pressure forces acting on the fault faces have induced a sudden movement of a large rock block. The relative movement broke the pressure tunnel lining and the inner water outflow along the fault faces. The large flow out coming from the rock slope has carried away the deluvial cover and in the second phase a succession of landslides developed into a major accident. The paper presents the investigations and back analyses which elucidate the main cause of the failure.


Un terrible glissement de terrain s'est produit Ie 19 Juillet 1991 sur un versant derriere la centrale hydraulique de Leresti, en affectant les derniers 25 m de la galerie forcee, de la chambre des vannes et de la conduite forcee. Les etudes effectuees apres l'accident ont prouve que Ie glissement s'est produit en deux phases successives. Dans la premiere phase, la pression agissant sur les faces d'un certain bloc ont produit brusquement le deplacement d'un important bloc de rocher. Le deplacement relatif a sectionne le traje de la galerie force, l'eau de l'interieur a commence a s'ecouler au long des failles. Le jet d'eau jaissant du versant a enleve Ie materiau deluvial et, dans une deuxième phase, a determine une succession de glissements conduisant à un accident de proportions. L'article presente les investigations et le retablissement des causes qui ont determine l'accident.


Ein Erdsutsch grossen Ausmasses begann am 19 Juli 1991 am Hang an dessen Fuss die Hydrozentrale Leresti liegt. Die letzten 25 m der Druckgalerie, das Schieberhaus und die Druckrolerleitung wurden beschadigt. Die Untersuchungen nach dem Unfall ergeben dass der Erdrutsch in zwei aufeinauderfolgenden Phasen stattfand. In der ersten Phase, haben die Druckkrafte, welche auf die Gesteinsspalten einwirkten eine plötzliche Vershiebung eines beteutenden Felsblockes bewirkt. Die Druckgalerie wurde lerch die relative Veschiebung Zerbrochen was einen Wasseraustritt entlang der Gescheinsspalten zur Folge hatte. Die starken Wasseraustritte an der Hangoberflache bewirkten das Wegspuelen des Hangdeluviums und veran-lassten in der zweiten Phase eine Reihe von Hangrutschen von besonderem Ausmass. Dieser Beitrag behandelt die Untersuchungen und Hypothesen, welche die Hauptursachen des Unfalls aufdecken.


Leresti powerplant, with an installed power of 19 MW and a total head of 180 m, is supplied with 15 cum/s from the Riusor dam through a 5780m long power tunnel with an internal diameter of 2.6 m. Downstream the surge tank the water circuit is provided by a 175 m-long pressure tunnel. At the exit of the tunnel a valve chamber houses two butterfly valves and from there the 188m-long penstock extends to the powerhouse (see fig. 1). The rock mass in which the surge tank and the pressure tunnel were performed is made of quarzosemicaceous shists and is divided into rock blocks by several fault systems. Due to faulty performance of the concrete lining extensive seepage has occurred from the shaft and the lower chambers of the surge tank, the water being stored in the rock mass. Large pressure forces were acting on the fault faces which were true impervious curtains due to their clayey breccia. The rise of the reservoir water level amplified the pressure forces leading to a large and sudden movement of a rock block.

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