The criteria for the design of the shaft/tunnel system excavation at the Rocky Mountain Project was to provide a stable opening during and after construction to receive the concrete lining and to minimize the disturbance of the surrounding rock mass so that the rock mass resists the displacement of the tunnel lining upon internal pressurization. This paper will summarize the geotechnical design of the shaft, elbow, tunnel, bifurcations, and penstocks.
Georgia Power Company's Rocky Mountain Project, located approximately 16 kilometer (lO miles) northwest of Rome, Georgia, is a pumped-storage facility that, when completed, will be capable of producing approximately 800,000 kilowatts of peaking energy. The underground works consist of a vertical shaft, elbow, nearly horizontal tunnel, two bifurcations, and three horizontal penstocks which will conduct water from the upper reservoir intake structure to the powerhouse. There is a drainage adit over the bifurcations. The shaft, elbow, and tunnel are 12 m (39.5 ft) excavated diameter. The penstocks are 7 m (23 ft) excavated diameter. The shaft is 212.1 m (696 ft) deep. The elbow has a radius of 27.4 m (90 ft). The tunnel is 640.1 m (2100 ft) long. The two successive bifurcations are approximately 45.7 m (150 ft) long and the 3 penstocks are approximately 121.9 m (400 ft) long.
FACTORS AFFECTING DESIGN
The criteria for the design of the shaft/tunnel system excavation at the Rocky Mountain Project were to provide a stable opening during and after construction in which to place the concrete lining and to minimize the disturbance of the surrounding rock mass so that it will be able to resist excessive deformation of the tunnel lining upon internal pressurization.
The excavation was stabilized for loadings due to gravity, blasting vibrations, hydrostatic forces due to groundwater in joints and bedding planes, and additional loads caused by destressing the rock as the excavation proceeded. The rock stabilization designs take these conditions into account, while retaining the flexibility required to handle unexpected rock conditions.
The rock properties are discussed by Grainger et al., of these proceedings.
Based on hydraulic design and flow characteristics, the finished inside diameters of the shaft, elbow, and tunnel have been set at lO.l m (35 ft) and the penstock inside diameters at 5.8 m (19 ft.) Given a concrete lining thickness of 0.6 m (2 ft) including shotcrete, this gives excavation diameters of 12 m (39.5 ft) for the shaft, elbow, and tunnel. Similarly, the 0.6 m (2 ft) lining specified for the penstocks gives an excavation diameter of 7 m (23 ft).
Access to the tunnel system during construction was obtained through one or more the penstocks. The shaft was accessed from either the tunnel or the mountain top. Excavation and Construction Sequence. Initial access was through penstock No. 1, with that penstock being driven until the bifurcatic area was reached. The bifurcations were then excavated. Once complete, the main tunnel and remaining penstocks were excavated.