In recent years, the increasing demand for constructing long and deep tunnels has heightened the likelihood of encountering fault zones and their associated geological challenges. Identifying these areas, especially in mechanized tunneling projects, is crucial for reducing risks, minimizing execution delays, and controlling project costs. This study evaluates the performance of Tunnel Boring Machines (TBMs) and the boreability of rock masses within fault zones using the Field Penetration Index (FPI). The results indicate that the FPI values in fault zones differ significantly from other parts of the tunnel alignment, suggesting more favorable excavation conditions in these areas. Despite the suitable boreability, most geological issues that cause delays in tunneling projects occur within these zones. Drawing on experience from mechanized tunneling projects in Iran and considering key factors influencing common geological hazards in fault zones, such as water inflow, squeezing, collapse, and face instability, a classification system is proposed to assess tunneling risks in these zones. To validate this system, key lessons learned from case studies encountering fault zones are presented. The alignment between documented events and the calculated hazard indices supports the accuracy of the proposed classification in predicting major hazards. According to this classification, fault zones can be categorized into five classes, ranging from “Non-problematic” to “Extremely problematic,” reflecting the potential for geological issues during both construction and post-construction phases.