Subsurface Geologic Considerations in Siting an Underground Pumped-Hydro Project

Abstract

Geologic criteria for underground pumped-hydro (UPH) facilities require massive homogeneous rock, with minimal stratigraphic or structural weakness, low permeability, superior stability, and acceptable in situ stress relationships. In northern Illinois, only Precambrian granite fulfills these qualifications. A variety of geologic and geophysical methods was utilized to identify and confirm areas of shallowest Precambrian basement, where construction difficulties in the overlying water-bearing sedimentary strata would be minimized. Three deep core holes were extensively tested. The results show that the granite meets the criteria required for siting a UPH project. Additional core research and down-hole testing were performed subsequently by the academic com- munity. NDERGROUND pumped-hydro (UPH) storage utilizes an upper reservoir at the ground surface and a lower reservoir and power station underground (Fig. 1). Although the concept of UPH storage has been discussed since the 1960's, and several utilities have studied the concept, no project of this type has yet been constructed. In fact, the siting studies described in this paper, and those undertaken by one other utility, have carried the geologic investigations for this type of project to their furthest extent to date. Siting a UPH project in an area with favorable subsurface geologic conditions is critical to the technical and economic feasibility of the project. The subsurface conditions are especially important, as the largest percentage of the cost of the project is associated with the construction, support requirements, and maintenance of the shafts, underground reservoir, and power station. The geologic conditions of the site also directly affect the design of the project, including the depth, size, configuration, and orientation of the under- ground structures. Criteria for siting the surface reservoir are less critical to the overall siting of the project than are those for the under- ground structures. This is because the cost of constructing the surface reservoir is considerably lower than the cost of under- ground excavation and because remedial treatment of sur- ficial defects are simpler and less costly than those in the large underground structures. However, the soil or rock underlying the surface reservoir must be capable of supporting the im- pounding structures and must have low permeability. Close proximity to a surface water supply also is desirable, but the use of groundwater for initial reservoir filling and for makeup water also may be feasible.