SUMMARY:

The objectives of large rock cavern design are to establish the most suitable location, the optimum shape and size, the best orientation, and the necessary support or reinforcement. Most of these design objectives can be better accomplished if the virgin state of stress is known and its effects properly considered. The stress regime, however, is generally unknown and has to be determined at each project site. In North America it has become quite common to use the deep-hole hydro fracturing method in the preliminary stages of site exploration in order to determine the preexcavation stress condition in the rock at the depth of the planned cavern. In particular, hydrofracturing has been used in many underground hydro projects. Three such projects, namely Helms, Kerckhoff 2 and Balsam Meadow are all situated in the Sierra Nevada Mountains just east of Fresno, California. The preexcavation stress measuements in each case are described· together with the specific use of the results in the design process of the underground powerhouses and other components. It is shown that knowledge of the state of stress was critical in making important design decisions, and that site specific stress measurements were necessary in each case despite the relative proximity of the three projects.

ZUSAMMENFASSUNG:

Die Ziele in der Planung grosser Steinhölen sind gute Lage, Grösse, und Orientierung, sowohl wie die nötige Stuetzung oder Verstarkung. Diese Ziele sind einfacher to erreichen wenn der urspruengliche Belastungszustand bekannt ist und dessen Einfluss beruecksichtigt werden kann. Weil Belastungen aber im allgemeinen unbekannt sind, muessen sie fuer jede Lage einzelnd bestimmt werden. In Nord Amerika wird Ublicherweise die "deep-hole hydrofracturing" Methode benutzt urnden Belastungszustand im Stein an der geplanted Tiefe zu bestimmen. "Hydrofracturing" wurde an drei Lagen fuer unterirdische Wasserwerke benutzt (Helms, Kerchhoff, und Balsam Meadow), die aIle in den Sierra Nevada Bergen östlich von Fresno, California sind. Die Stressmessungen vevor Aushölung sind fuer alle drei Lagen beschrieben, zusammen mit dem Gebrauch der Resultate in der Planung unterirdischer Kraftwerke. Es wird gezeigt dass ein Kenntniss des Belastungszustandes kritisch ist um Beschluesse zu fassen ueber den Entwurf, und dass Messungen an allen drei Lagen nötigwaren, obwohl diese nahe zu einander liegen.

RESUME:

La connaissance des contraintes in-situ dans les roches facilite l''etude des projets de construction de grandes cavitees souterraines. Les contraintes ne sont generalement pas connues. La methode d''hydro-fracturation dans des trous profonds est souvent utilisee en Amerique du Nord pour explorer les sites de construction. Les sites de Helms, Kerchoff 2 et Balsam Meadow sont tous les trois situes dans les Sierra Nevadas a l''est de Fresno en Californie. Les mesures de contraintes et leurs utilisations dans les etudes de ces sites sont presentees. Dans certains cas, ces mesures ant eu une influence determinante sur les decisions prises. Malgre la proximite des trois sites, les mesures des contraintes a chacun des sites ont ete necessaires.

ROCK STRESSES AND THE DESIGN OF UNDERGROUND HYDRO PROJECTS

A hydro project in which the powerhouse is located underground will typically have in the subsurface all of its main components between the inlet and discharge reservoirs (such as headrace tunnel, pressure tunnel, penstock, surge chamber, transformer and machine halls, tailrace tunnel). The major objectives of the design process of these underground components are to establish for each of them:

  • the most suitable location

  • the optimum shape and size

  • the best orientation

  • the most efficient and safe sequence of excavation

  • the necessary lining, support and reiforcement.

Most, if not all, of these design objectives can be better accomplished if the state of stress in the area of the project is measured and its effect is properly considered. For example, the location of the underground tunnels and chambers is usually assumed to be a function of the site geography and topography (minimize volume of excavation between upper and lower reservoirs), and of the rock mass conditions (seek zones of high rock quality, avoid major shear zones or other large discontinuities). The state of stress becomes an important factor if the surface topography is varied over the distance between the two reservoirs.

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