During the construction of the large caverns of the Rogun hydropower station in Tadzhikistan, large displacements in the upper part of the major cavern were observed. The caverns are situated in a rock mass consisting of up to 85% jointed sandstones with high initial stresses. The construction process has been interrupted by financial and political considerations. When the construction process can once again proceed, engineering solutions that will provide for the stability of cavern walls are needed. The back analysis system, designed to cope with changing geo-mechanical situations during construction and based on the computer code STATAS and stochastic as well as deterministic models of rock mass, is considered as an important tool to achieve this.
Beim Bau der groβen Kavernen des Rogun-Wasserkraftwerks in Tatschikistan wurden betrachtliche Verformungen der Kavernenwande festgestellt. Die Kavernen liegen zu mehr als 85% in dickbankigem, festem, geklueftetem Sandstein mit hohen Primarspannungen. Der Bau des Anlagenkomplexes ist gegenwartig aus finanziellen Gruenden unterbrochen. Fuer die Wiederaufnahme des Baus muessen die notwendigen Ingenieurlösungen zur Begrenzung der Gebirgsverformungen, sowie zur Konstruktion und Ausbruchstechnologie ueberarbeitet werden. In diesem Zusammehang wurde eine Reihe von Erst- und Rueckrechnungen unter Verwendung des FE-Programmsystems STATAS durchgefuehrt. Das verwendete Rueckrechnungsverfahren basiert auf einer Kombination aus stochastischen Modellen zur Verteilung der Kennwerte des Gebirges, Vergleichsbetrachtungen mitanderen Projekten mit Hilfe einer Datenbank und einer systematischen Variation der Steifigkeitseigenschaften des Gesteins, der Trennflacheneigenschaften und des Primarspannungszustandes.
Pendant la construction des larges cavernes de la station hydro-electrique de Rogun dans le Tadzhikistan, de larges deplacements dans la partie superieure de la caverne principale ont ete observes. Les cavernes sont situees dnas une masse rocheuse composee jusqu'à 85 % de gres fissures avec des contraintes initiales elevees. Le processus de construction a ete interrompu à cause de considerations politiques et economiques. Quand le processus de construction pourra recommencer, des solutions d'ingenierie pour la stabilite des murs de la caverne seront requises. Le système d'analyse inverse, conçu pour les situations geomechaniques qui changent pendant la construction et base sur le code de calcul STATAS ainsi que sur des modèles stochastiques et deterministiques de la masse rocheuse, est considere comme un outil important pour cet achèvement.
Experience and knowledge have, throughout the history of man, been gained by an analysis of past events, decisions and mistakes, either viewed from the sidelines or described by those who were active participants. Rock engineering is not an exception to this general process of human development; it is notably a discipline in which previous practice is the most secure basis for structural design and for the selection of excavation and support methods. Following generally adopted terminology, in this paper we will call "back analysis" the set of procedures which provide for the given period of time the best possible description of the geotechnical situation around the structure under construction/or already constructed. We will describe "back calculation" as the numerical procedure which enables the verification of an updated structure design with respect to modified rock mass parameters obtained during the back calculations. The whole set of such procedures during construction is usually termed as "monitoring". Considering possible procedures for back analysis, one should be aware of the principal limitations inherent in such approaches. First, the definition of the deformation parameters of rock mass based on the known displacements usually measured in situ even for the linearly elastic material is a nonlinear problem because there are more unknowns (Young's modulus, Poisson's ratio, stress) than constitutive equations. On the other hand, most rocks show distinct non-linear deformation behaviour. In direct calculations, there are no difficulties in accounting for known, non-linear properties of the given material.
