Evaluating rock mass parameters is a critical and complex aspect of rock mechanics. Although methods such as the Geological Strength Index (GSI) and conventional back analysis exist, their reliability can be compromised by potential errors. Integrating instrumentation results with back analysis for structures under construction, like caverns and tunnels, offers an efficient and accurate means of determining rock mass parameters. However, due to the numerous factors influencing rock mass behavior, conventional back analysis methods often fall short of delivering accurate results. In this research, we estimated the actual values of rock mass parameters using the Taguchi method, a systematic approach that minimizes the number of numerical analyses required. The Taguchi method was selected for its effectiveness in addressing the complex geometry, variable geological formations, and diverse geotechnical properties of the rocks at the Siah bisheh pumped storage project. Located on the Chalous River in northern Iran, the project comprises a dam and powerhouse, with the underground complex featuring three main caverns in close proximity. The powerhouse cavern measures 131.4 meters in length, 46 meters in height, and 24.5 meters in width. Various types of instrumentation, including extensometers, load cells, and convergence meters, are utilized in this project. Moreover, considering the favorable outcomes of the small strain model (HSS) in real-world projects where unloading plays a significant role, this study determines the parameters of this model and the stress ratio for the selected case using continuum numerical modeling with Plaxis 2D software. The displacements obtained from numerical modeling showed good agreement with the measured displacements, validating the accuracy of both the numerical modeling and the results obtained through the Taguchi method.