Abstract. Georgia Power Company's Rocky Mountain Project, located approximately ten miles northwest of Rome, Georgia, in the Valley and Ridge Physiographic Province, is a pumped-storage facility rated at approximately 800 megawatts of peaking energy. Geological field explorations and borings enabled the division of the rocks along the tunnel and shaft alignment into distinct lithologic units and defined the ground-water regime. Field explorations included a detailed analysis of the rock mass discontinuities at available rock unit exposures. Testing of rock strengths was performed in several phases in both the field and the laboratory. The field testing consisted of point-load testing, geophysical testing, in situ load testing, and hydrofracture testing. Laboratory testing determined the Brazilian split-tensile strength, the unconfined compressive strength, the triaxial compressive strength, the joint-direct shear strength, and the pulse velocity. The rock mass along the tunnel and shaft was also characterized by empirical classification systems. The classification systems included the Rock Mass Rating (RMR) and the Rock Mass Strength Determination (RMSD). The results of the field explorations, field testing, laboratory testing, and rock classifications were used to divide the rocks into two structural groups. These rock groups were defined as volumes of rock wherein the rock mass behaves similarly in response to the excavation. Each group, therefore, required separate rock engineering consideration.
INTRODUCTION
Georgia Power Company's Rocky Mountain Project, located approximately ten miles northwest of Rome, Georgia, in the Valley and Ridge Physiographic Province, is a pumped-storage facility rated at approximately 800 megawatts of peaking energy. Construction of the project required the excavation of a 12-m (39.5-ft) diameter shaft, 212.1-m (696-ft) deep; a 12-m diameter, 27.4-m (90-ft) radius elbow; a 12-m diameter tunnel, 640.1-m (2,100-ft) long; two successive bifurcations approximately 45.7-m (150-ft) long; and three 7.0-m (23-.ft) diameter penstocks, approximately 121.9-m (400-ft) long. Construction also included the placement of a 10.7-m (35-ft) diameter reinforced concrete liner in the shaft, elbow, and main tunnel, two reinforced concrete bifurcations, and a 5.7-m (19-ft) diameter steel liner in the penstocks. The construction of the water tunnel and shaft has been completed. Siting, design, and construction of the Rocky Mountain Power Tunnel required extensive interaction between the engineering geologists and the engineers of the design team to determine the most economical design and location for the structures.
EXPLORATIONS
Initial investigations began in 1965 and final design investigations were completed in 1980. During this period, detailed geological work was performed to determine the geological conditions which would affect the design and construction of the project. During 1965, two deep borings were drilled along a proposed tunnel alignment to determine the general geology. From 1977 through 1979, a series of borings (H-7 through H-5B) were drilled along the tunnel alignment (Figure 1) to determine the geological conditions and for use in geophysical data acquisition. Geophysical data acquisition included crosshole velocity measurements, an uphole velocity survey at boring H--7, and downhole measurements (sonic velocity, gamma radiation and borehole verticality). Water pressure testing was conducted an borings H-.5, H-6, and H-7.
