عنوان مقاله [English]
Back analysis is a practical engineering tool to evaluate geomechanical parameters of underground and surface structures, based on field measurements of some key variables such as displacements, strains and stresses. These parameters are necessary for the stability analysis and design of a support system for geostructures. The Siah Bisheh powerhouse cavern is located in a discontinuous media. Considering the block size, pattern and spacing of the discontinuities,a 3 dimensional distinct element analysis was performed. In this study, a displacement based direct back analysis of a powerhouse cavern, on the basis of a univariate optimization algorithm, were applied, and the optimized geomechanical parameters of the powerhouse confining rock masses were obtained. Numerical modeling results are in good agreement with measured displacements using extensometers, which confirm the numerical modeling and back analysis results. Then, the ordinary analysis of a powerhouse cavern under natural conditions, using back analysis results, was done. The results of the analysis show that a powerhouse cavern is stable under natural conditions and the existing support system has a suitable efficiency that could effectively control displacements. Finally, the powerhouse cavern long term stability under a saturated condition was analyzed. Results of the analysis show that, after lower dam impounding, considering the vicinity of the powerhouse cavern to the lower dam reservoir, the pore water pressure and uplift pressure in discontinuities around the powerhouse cavern will rise and tend to the local failure of the powerhouse cavern. To prevent powerhouse failing and guarantee long term stability, a cut off curtain was proposed.