The characterization of geological hazards is of great importance in underground space and, in particular, tunnelling projects because it plays a crucial part in the selection of the tunnel route, the type and characteristics of excavation, and the technology adopted. It is also a fundamental stage in the design and construction of tunnelling-related structures. Tunnelling projects encounter various geological hazards, including those associated with the ground conditions, the groundwater level and the in situ stress regime. The aim of this study was to identify the probable hazards affecting the alignment of the Gelas water conveyance tunnel in Iran and the magnitude of risk in various sections along the tunnel route. Following a literature review of the geological hazards found in tunnelling projects with similar geological conditions, several potentially hazardous parameters were identified for assessment, including squeezing and groundwater inflow. The geotechnical characteristics of the rock masses were evaluated based on laboratory and field investigations to assess the geological hazards along the tunnel route. Seven sections along the tunnel route were selected to study the squeezing and water inflow hazards and their degree of occurrence using various previously reported techniques. Empirical, semi-empirical and theoretical–analytical methods were used to assess the squeezing potential. Analytical methods were applied to the prediction of groundwater inflow and then site groundwater rating and tunnel inflow classification methods were adopted to classify the route of the tunnel from the perspective of the groundwater inflow hazard. The results indicated that areas where faults and crush zones are located are highly susceptible to groundwater in-rush and squeezing.