A novel 3D engineering geological zonation method for major transportation corridors in mountainous regions

Major transportation corridors in mountainous terrain often cross faulted rock masses, steep slopes and deeply buried tunnel sections, making assessments based only on surface conditions inadequate for route planning. This study proposes a three-dimensional (3D) engineering geological zonation (EGZ) method that combines ground-surface and underground-profile information along a proposed alignment. Surface indicators cover topography, geological structures, rock groups and geohazards, whereas the underground assessment considers fault proximity, geostress, rock formations and stratum temperature. Factor analysis is used to reduce redundancy among surface indicators, while entropy weighting determines the contributions of the evaluation factors. Surface and underground zonation indices are then combined using a burial-depth-dependent function. The method is applied to a selected section of the Sichuan–Tibet corridor on the eastern Tibetan Plateau. The resulting 3D zonation index ranges from 0.13 to 0.76. Good, second-good, medium and second-poor conditions account for 15.6 %, 46.4 %, 34.8 % and 3.2 % of the alignment, respectively. Less favourable sections are mainly found in the western part of the corridor, where tectonic mélange is widespread. Decision-tree and SHAP analyses indicate that steep slopes, high relief, relatively poor rock-mass quality, higher peak ground acceleration and high geohazard density are associated with the less favourable conditions in mélange areas. The method provides a basis for screening corridor alignments and prioritizing ground investigations at the feasibility stage.