Classification framework for engineering geological challenges in fault zones during TBM tunneling: Insights from mechanized projects in Iran

Abstract

Most mountainous long tunnels encounter fault zones where phenomena such as uncontrolled water inflow, excessive ground convergence, rock mass failure and tunnel collapse, face instability, and rock block release are common. In mechanized tunneling projects, these hazards threaten TBM performance, especially at great depths and high groundwater pressure. Despite advances in passing through difficult geology, TBMs are still trapped in fault zones, causing delays in machine advance. This paper draws on experiences from mechanized tunneling projects in Iran, focusing on the challenges encountered while tunneling through fault zones. By considering key influencing factors (including fault characteristics, lithology, overburden, groundwater pressure, rock mass quality, tunnel diameter, and the machine type) and utilizing the Rock Engineering System (RES), it presents a comprehensive classification framework for rating geological hazards in these zones. The proposed classification system categorizes fault zones into five classes—Non-Problematic, Slightly Problematic, Problematic, Very Problematic, and Extremely Problematic/Critical—reflecting tunneling challenges during construction. This classification offers a framework to pre-evaluate potential hazardous events in fault zones, select suitable machinery and construction methods, and predict the performance of machines in these zones. The accuracy of the classification has been validated by information gathered from construction phase reports and through direct observation and study of fault zones encountered in major Iranian mechanized tunneling projects. The results show that the proposed classification effectively identifies critical conditions. The correlation between documented events and calculated geological hazards indices supports the predictive capability of this classification. © 2025 Elsevier B.V.