The design and analysis of a circular, domed powerhouse for Pacific Gas and Electric (PG&E) Company's Kerckhoff 2 Project is described. Exploration, geologic stress determinations, mechanical properties, reinforcement and instrumentation are addressed briefly. The analyses of the circular, domed cavern and the comparison of predicted and field deformations are presented in somewhat more detail. The behavior of the completed powerhouse cavern has been found to be substantially as predicted by the analysis.
The Kerckhoff 2 Hydroelectric Project is located on the San Joaquin River about 48 Km (30 miles) northeast of Fresno, California. Water will be conveyed to an underground powerhouse through a 6.4 Km (4 mile) long, machine bored, 7.3 m (24 ft.) diameter tunnel from PG&E's existing Kerckhoff Lake. A single 140 MW unit will be housed in a circular, 26 m (85 ft) diameter cavern topped by a 28.3 m (93 ft) diameter dome; one of the worlds largest. The main features are shown on Figure 1.
The Project is located in the granitic rock of the western foothills of the Central Sierra Nevada Mountains. The Powerhouse Cavern is located in fresh granodiorite with core recovery and Rock Quality Designation (RQD) generally at about 90%. Exploration for the cavern consisted of a series of cored 76 mm (3 in.) borings. A set of borings was made at a prospective site and severe artesian conditions were encountered. A new location was selected and a second set of borings was made. The second s•te was more favorable and therefore selected. A 213 m (700 ft.) horizontal boring was also made to monitor the ground water. All rock cores were logged, sketched, photographed in color, bound in a formal report and made available to the contractors. Surface geology and joint patterns were thoroughly mapped. A model of the Powerhouse was made and projected joint patterns and borings were shown on it to aid in design. Young's Modulus (E) of 48,263 MPa (7,000,000 psi) and Poisson's Ratio of 0.19 were established from seismic refraction survey data. Laboratory tests on solid rock cores gave an average "E" of 55,158 MPa (8,000,000 psi) and an average Poisson's Ratio of 0.17. An "E" of 48,263 MPa (7,000,000 psi) and a Poisson's Ratio of 0.17 were chosen for use in the Finite Element Method (FEM) analysis. However, the elements that would be within 6.1 m (20 ft.) of a blasted surface were assigned an "E" of 34,474 MPa (5,000,000 psi) to compensate for the disturbance caused by excavation. In-situ stresses of the site were determined by borehole "hydrofracturing" (Haimson, 1977). These stresses were found to increase with depth, as expected, but the major principal horizontal stresses were 3 to 6 times larger than the vertical stresses. The minor principal horizontal stresses were only 2 to 3 times larger than the vertical stresses.
An underground powerhouse was chosen rather than a conventional surface design for environmental as well as economic reasons.