Powerhouse cavern Obervermuntwerk II - Analysis and remediation of the shotcrete spalling

The project Obervermuntwerk II of the Vorarlberger Illwerke AG is located in Vorarlberg, Austria. In May 2014, the construction work started with the first pre-cuts of the access tunnels and at peak times, four excavations ran simultaneously. For the preliminary design, 2D finite element calculations of the machine cavern were carried out to determine the initial stress distribution. During the excavation works of the machine cavern 3D Finite Element calculations followed. The high deviatoric stresses caused shotcrete spalling in the right half of the calotte, which caused severe restrictions of the excavation works. In order to establish a feasible remediation of the shotcrete, detailed numerical analyses in combination with measurements were required to identify the reason for the shotcrete spalling. In this article both, a general overview of the project and the decision-making process for determining the remediation works is presented.