In-situ stress plays an important role in the design of underground cavities and tunnels. The design concept of sufficient rock cover may not always be adequate for the design of pressure tunnels. The knowledge of in-situ stresses, rock mass behaviour and hydrogeological conditions will have to be acquired and suitably considered in the design. The paper presents a case history of failure of a power tunnel in a hydroelectric project in the Himalayas. Ignorance of complete stress condition was one of the prime factors of failure. Experience of the authors in measurement of in-situ stress in another hydroelectric project site in the Himalayas has also been presented.
La contrainte in situ joue un rôle important dans la construction des cavites et des tunnels souterrains. La conception d"un converture de roche resistant n"est pas toujour suffisante pour la construction des galeries en pression. On doit avoir la connaissance des contraintes in situ, du comportement de la mass rocheuse et des etats hydrogeologiques dont it faut tenir compte dans la construction. Cet article fait un expose pour un projet hydroelectrique en Himalaya de la situation amenant à l"echec d"un tunnel de generation. L"ignoranc. de l"etat de contrainte etait un des causes principaux de l"insuccès. Les experiences des auteurs relative à la mesure de contrainte in situ sur le site d"un autre projet hydroelectrique en Himalaya ont ete presentees.
In-situ -Spannung spielt eine wichtige rolle in der Konstruktion der Untergrundhohlraeumenund Tunneln. Die Konstruktionskonzeption der ausreichenden Gesteinbedeckung mag nicht immer fuer die Konstruktion der Orucktunneln genuegen. Man muss sich die Kenntnisse der in-situ-Spannungen, Gesteinmasseverhalten und hydrogeologischen Verhaeltnissen erwerben und diese in die Konstruktion anwenden. Der Bericht schildert die Umstaende des Versagens des Krafttunnels in einem hydroelektrischen Projekt im Himalaya. Die Unwissenheit der suemthicchen stoannungrumstaende war einer der Hauptfaktoren des Versagens. Die Erfahrungen der Verfassern in der Messung der in-situ- Spannung in einer anderen hydroelektrischen projektbaustelle im Himalaya sind auch darfestellt.
In-situ stresses are essentially required for the design of underground openings. Knowledge of principal stresses and their orientation in space is also required to determine the extent to which steel liners in the pressure tunnels are required. This is linked with the concept of hydrualic fracturing or rock jacking (uplift) in the rock mass. This effect can develop if water pressures imposed within a rock mass are greater than the in-situ stress. Depending upon the deformability characteristics of the rock mass, and the area over which the hydraulic pressures act, existing joints can be opened. This may result in jacking of a large mass of rock away from the tunnel with excessive leakage and large scale landslides or instability. This effect has occured many times in the history of civil engineering.
When such failures occur, it often takes several months to complete the repairs. New adits and approaches have to be opened. Steel liners have to be carried around bends and placed inside already concreted tunnel sections. Lack of space makes matters worse. The cost of repairs and loss of revenue from power sales, may be tens of thousands of Rupees.
