There is a significant increase in tunneling operations in most of the metropolitan cities of India for developing rapid transit systems. Provision of tunnels proves to be beneficial in densely crowded areas where availability of space is a major concern. Tunneling is a costly affair owing to the high number of uncertainties involved as compared to other infrastructure projects. The delay and cost escalations during construction of tunnel can be mitigated by identifying the uncertainties in the planning stage and taking appropriate steps during execution. This requires a robust model incorporating the most significant risk factors affecting the projects. In the present work, an exhaustive list of risk factors affecting tunnel projects is extracted from comprehensive literature review on tunnel construction. The list is augmented through interaction with stakeholders and collection of field data. This exercise provided an extensive pool of factors, which can't be completely incorporated while formulating the model to reduce delays. This work attempts to identify most critical risk factors from the pool. Prioritization of risk factors from the pool is done with a framework using RII [Relative Importance Index]. The pool is classified in a 3-level hierarchical structure. Expert opinion regarding relative importance of each factor is captured through a questionnaire survey and relative weights are assigned. This work is an attempt to compile and prioritize predictable risk factor database by merging a comprehensive literature review of scientific papers on tunnel construction projects and opinion of experts working in tunnel construction. The identification of critical factors in planning phase, may reduce complexity during execution. The proposed framework is expected to help researchers and decision makers to rank the risk factors and focus only on the most critical ones for minimizing the risks and reducing delays in project completion. © 2022