ABSTRACT
The Ritom hydropower plant is located in Switzerland south of the Gotthard massif. Ritom SA, a partner company of the Swiss Federal Railways (75%) and the Canton of Ticino (25%), represented by Azienda Elettrica Ticinese, holds the hydroelectric concession until 2094. The capacity of the over 100-year-old power plant is increased and converted into a pumped storage plant by building a new steel-lined headrace tunnel. The sub-horizontal section (length = 750 m, cross-section = 18.4 m²), excavated in granite by drilling and blasting, encountered a highly fractured section with high water inflows requiring special grouting measures. The following inclined pressure shaft (length = 1.4 km, Outer Diameter = 3.23 m) was excavated by an open hard rock tunnel boring machine specifically configured for this project. The excavation of the shaft with gradients of 42% in the lower section and 90% in the upper section has proven to be one of the biggest challenges of the entire project. This paper focuses on the unforeseen geological challenges encountered during the inclined shaft excavation, and the TBM configurations that allowed them to be safely overcome.
