The excavation of the Power House Cavern (PHC) for the Siah Bishe Pump Storage Project was accompanied by an extensive monitoring program. The gathered monitoring data was evaluated and used as a basis for the back analysis—special emphasis was put on the extensometer measurements. Firstly, an FE-model with a simplified geological setup of the PHC, which was initially used before the cavern was excavated (Design Model), was adapted to the as-built conditions, i.e. the excavation sequence and as-built support measures were implemented. At this stage of the back analysis it was already noticed that the initially (before excavation) presumed rock mass parameters and shear zone parameters had to be reduced to achieve a match between measured and calculated deformations. Furthermore, the analysis revealed that a comparatively small increase of the horizontal in-situ stress leads to similar results as a reduction of the rock mass parameters.

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