The design of underground openings in soft rock requires an evaluation of the following: (1) stress conditions around the openings; (2) the in-situ stresses; (3) the strength of the rock; (4) condition of the rock; (5) ground-water conditions; and (6) an evaluation of the openings at existing projects which have similar rock strength to stress ratios. A study in which this approach is used was recently performed by the Omaha District, Corps of Engineers for the proposed Gregory County Hydroelectric Pumped Storage Project. This paper presents a synopsis of the studies performed and the results of these studies.
The proposed Gregory County Hydroelectric Pumped Storage Project is in the initial planning stages; however, studies have been carried out in enough detail to assure the proposed plan is constructable and has a positive cost-benefit ratio when compared to alternate sources of power. The proposed project consists of two stages. Each stage would have an installed capacity of approximately 1,180 MW. It is envisioned that Stage I would be basically completed before Stage II is started. The proposed project would be located in south central South Dakota in Gregory County, on the west side of Lake Francis Case, approximately 56 km. (35 miles) north of the Fort Randall Dam. The project configuration contains the following major facility components: -An embankment forebay (upper reservoir) (50xl08m3 (40,000 acre-feet) Stage I; 10.6x108m 3 (80,000 acre-feet) Stage I and II) constructed on top of the relatively flat Missouri Plateau which is about 213 meters (700 feet) above Lake Francis Case. - A below-ground powerhouse containing three 3930 MW reversible pump turbines with surge chamber located on the tailrace tunnel. -An intake structure and orm vertical shaft (244 m (800 feet) long by 9.14 m (30 feet) dia.) and tunnel (278 m (800 feet) long by 9.14 m (30 feet) dim) for each stage to provide water passage between the forebay storage area and powerhouse. - A tailrace tunnel (2487 m (8160 feet) long by 10.2 m (33.5 feet) dia.) for each stage for water passage from the powerhouse to the discharge channel. -A discharge channel including trashrack structures to provide water passage from the tailrace tunnel to Lake Francis Case.
The rock formations at the project are indurated and compacted sedimentary marine deposits. The stratigraphic sequence in ascending order is the Carlile Shale, Niobrara Chalk and Pierre The underground powerplant will be located in the Niobrara chalk formation which is approximately 52 m (170 feet) thick at the powerplant location. The chalk is overlain with about 214 m (700 feet) of Pierre shale. The top of the powerplant is situated so that 12 m (40 feet) of Niobrara chalk would be between the top of the powerhouse and the Pierre shale. The Pierre shale is weaker than the Niobrara chalk and it is desirable to contain the higher stresses from the opening within the Niobrara chalk. Figure 1 shows a geologic cross-section at the site.