Design and verification challenges of the Limmern caverns

A 1,000 MW pumped-storage plant is under construction at altitudes between 1,600 and 2,400m in the Glarner Alps of Switzerland, around 90 km south-east of Zurich. The machine and the transformer caverns are situated in Quintner Limestone with an overburden of 400 to 500 m. The 150 m long machine cavern has a height of 53 m and a width of approximately 30 m. The distance between both caverns was increased to 59 m to avoid overlapping plastic zones following the evaluation of geological conditions of the pilot tunnel. During construction it was found that geological conditions are more complex than expected, resulting in displacements varying to a large degree. That was why the design was reviewed by comparing the prognosis with the actual behaviour during excavation of the heading. The rock bolting system with regard to pattern and length was adjusted based on the results of numerical back analyses including sensitivity studies of the rock mass parameters. Additional 3D computations were performed to take account of the numerous intersecting galleries. During excavation of benches, additional rock bolting was provided in specific areas, where deformations continued over some time.