Integrated three-dimensional modelling of underground structures and engineering geological bodies using the CDIT–TEN framework

Integrated three-dimensional modelling of underground structures and engineering geological bodies is important for representing complex subsurface environments. Existing workflows commonly construct geological and structural models independently and integrate them through post-processing, which may introduce boundary and topological inconsistencies. This study proposes a constrained Delaunay irregular triangulation–tetrahedral network (CDIT–TEN) framework for native structure–geology integration. Stratigraphic interfaces, structural boundaries, and structure–stratigraphy intersection lines are organised as unified geometric constraints during surface triangulation and tetrahedral network generation. The framework was evaluated using an urban underground-space area in Nantong, China, including regular block structures, tunnels, and pipelines. Spatial block cross-validation of 587 boreholes yielded weighted mean errors of 3.34 m for stratigraphic-top depth and 2.58 m for stratigraphic thickness. Independent coupling validation produced an overall RMSE of 0.18 m, MNPE of 2.8%, and Pearson mean correlation coefficient of 0.92. Compared with mesh-clipping-based and Boolean-operation-based post-processing workflows, CDIT–TEN showed smaller boundary gaps and overlap/interpenetration volumes, fewer detected non-manifold edges, higher minimum tetrahedral quality, and lower computational time under the tested configuration. The resulting unified subsurface representation provides a spatial foundation for underground-space digital twins and subsequent engineering analysis.