Abstarct: Due to the population growth and the urban transport problems, the need for the use of rail transport is growing rapidly. Today, underground structures, particularly tunnels play an important role in the development of urban transport. Tunnel boring machines (TBMs) are widely utilized in urban areas that are usually composed of alluvial lands. The efficiency of TBM is one of the most important factors in feasibility studies of tunneling projects. TBM utilization factor is defined as the percentage of time that the system spends for drilling. There are issues affecting the soft ground tunneling operation including geotechnical parameters, design parameters etc. The current paper mainly discusses the TBM utilization in soft ground tunneling with a case study in East-West part of Tehran Metro Line 7. The rock engineering systems (RES) approach is utilized in this study to interpret the interactions between the pairs of parameters influencing the whole system. In order to do this, a probabilistic coding method (PESQ) is employed and finally a utilization index is developed to calculate the utilization along the tunnel path of studied tunnel. The results were compared with real utilization values of the project, which showed a good logarithmic correlation with coefficient of 0.724