To determine the stability of the roof and walls of the excavation for the Morrow Point underground powerplant the Bureau of Reclamation made 4 different types of measurements. These measurements were: wall convergence, absolute and relative deformations of the roof arch, and rock bolt loads. A description is given of the instrumentation, installation techniques, and the methods followed in making the measurements. Also, presented is a short discussion on the trends of the results.
Pour determiner la stabilite du toit et des parois de l'excavation pour la centrale souterraine de Morrow Point, le Bureau of Reclamation a effectue quatre types differents de mesures. Ces mesures comprenaient la convergence, absolue et relative, des parois; les deformations de la voûte; et les charges sur les boulons d'ancrage aux roches. La presente communication decrit l'instrumentation, les installations techniques, et les methodes employees à faire les mesures. Les tendances des resultats aussi sont discutees.
Das Bureau of Reclamation hat vier (4) verschiedene Messungen vorgenommen, um die Standfestigkeit des Firstgewölbes und der Wande der Austunnellung fuer das unterirdische Wasserkraftwerk Morrow Point zu bestimmen. Die folgenden Messungen wurden vorgenommen: Konvergenz der Wande, absolute und relative Deformationen des Gewölbes, und die Belastungen der Felsanker. Eine Beschreibung der Messgerate, deren Einbauweise, und die angewandten Messungsmethoden, wird gegeben. Ausserdem wird eine kurze Diskussion der Tendenzen der Messergebnisse geboten.
A rock mass, in its natural state, is subject to a three dimensional stress field which is produced mainly by the weight of the rock above the mass and by deformations of the earth's crust. When an underground opening is excavated within the mass, a redistribution of stress takes place in the rock adjacent to the opening. The manner in which this redistribution occurs depends on the geometry and orientation of the opening, blasting techniques, and to a minor extent on artificial supports. In wet ground, the release of pore water can also influence the redistribution of stress in rock surrounding an underground excavation. The magnitudes and directions of principal stresses in the rock adjacent to subsurface excavations cannot be estimated with any degree of certainty. Consequently, unanticipated local stresses may be produced which can cause rockfalls, particularly in formations which have random jointing. To locate zones of unstable surface rock, measurements are required. This paper discusses some of the measurements made in the subsurface excavation for Morrow Point Power plant. Morrow Point Dam and Power plant are presently under construction on the Gunnison River in southwestern Colorado (Figure 1). When completed, the dam will be a 460-foot-high (140 meters), double-curvature, thin-arch structure. The power plant, which is located about 225 feet (69 meters) downstream from the dam will be housed in a subsurface chamber having a width of 57 feet (17.4 meters), a length of 206 feet (62.8 meters), and a maximum height of 125 feet (38.1 meters) [l]. The vertical depth of the powerplant below the natural ground surface is about 400 feet (122 meters). The powerplant site is located in a metamorphic formation which, for the most part, is composed of mica or quartz- mica schist [2]. The rock has variable-thickness irregular bedding, with principal jointing having a strike approximately normal to the powerplant axis and a dip of about 35° upstream. Two major shear zones intersect the boundaries of the exacavation, one in the rock arch and the other along the left (south) wall. Plans for the excavation do not include a reinforced concrete roof arch, as is generally constructed in European underground machine halls. Instead, the design provides for stabilization of the rock arch by extensive bolting (Figure 2). This practice has been followed at the Poatin Powerplant in Tasmania [3], and also at the Oroville Powerplant which is presently under construction in California. To obtain geologic and other data needed to design the chamber, several types of field investigations were made. The quality and structure of the rock mass at the site were determined by drilling 8,000 lineal feet (2,438 meters) of cored holes, by examining the side of some of the cored holes with a borehole television camera, and by visual examination of the walls of the exploratory tunnel driven into the rock mass in the vicinity of the arch crowd. In addition to laboratory tests on rock cores, in situ tests were performed to gather data on the physical properties of the rock. These included: geophysical tests to obtain seismic moduli of elasticity, jacking tests to obtain deformation moduli, and borehole and surface stress relief tests to obtain information on the magnitudes and directions of the natural rock stresses. Stress relief investigations were performed in the rock adjacent to the exploratory tunnel. Many of the basic techniques for detecting unstable ground in subsurface excavations were developed for use in mines where rockfalls and rockbursts are a continual hazard.
