Geomechanical influences on shaft construction for hydropower projects, a case study of the PSW Grimsel 3

Hydropower plants are of great relevance for European energy production. Inclined or vertical shafts serve as a cost alternative to inclined steel-lined shafts in case of favourable ground conditions. Vertical shafts have a higher ground coverage compared to inclined shafts oriented parallel to the surface. The increased ground coverage and the resulting higher in-situ stress magnitudes may allow for unlined solutions instead of cost-intensive steel-lining. Depending on the strength and thickness of the concrete lining, unreinforced concrete tends to crack intensely upon exceeding a critical effective internal water pressure. The structural integrity is associated with the number of induced cracks and the crack width at a given internal water pressure. The design of the concrete lining depends on the bedding of the lining (e. g. deformability of the rock mass), the hydraulic conductivity of the rock mass in relation to the concrete lining, the external and internal water pressure, and the in-situ stress state. The paper presents a procedure to determine the stress state within a concrete lining and the surrounding rock mass.