Loss displacement estimation of Azad pumped storage powerhouse cavern using combination of numerical modeling and instrumentation results

Instrumentation and monitoring are common methods to control the stability of large underground spaces. During monitoring underground excavations, it is usually impossible for the instrument to fully record the displacements taking place ahead of the tunnel. This occurs due to the fact that there is always a distance between the instrument and the excavation face, so some displacement are lost before the installation of the instruments. In this research, efforts have been made to estimate the loss displacement of the Azad Pumped Storage Powerhouse cavern in Iran, using combination of numerical method and instrumentation results. For this purpose, according to the excavated sequence (pilot tunnel, downstream slashing, and upstream slashing) the roof of the powerhouse cavern was modeled, using two-dimensional finite element software, verified by the results obtained from extensometer installed in the cavern's roof. The comparison of modeling and monitoring results indicates that the displacement occurrence recorded via the instrument is less than the modeling values. The most obvious reason is the displacement loss and impossibility in recording all displacements via the instrumentation. Then, employing the results of the adjacent instruments and displacements recorded by the cavern roof's instrument and utilizing a combination of modeling results, the amount of loss displacements in the cavern's roof was evaluated to be 40% of the total displacements. Afterward, employing analytical methods and fitting the curve on instrument's results, the outcomes indicate that the longitudinal deformation profiles obtained by Hoek equation has the best fit, confirming the 40% of the displacements that occurred before the installation of the instrument.