Stability assessment of tunnel portal using DFN-DEM modeling and photogrammetry: A case study of Tehran-Shomal freeway in Iran

Abstract

Assessment of tunnel portal stability is a crucial consideration due to the portal's proximity to the ground surface and the non-uniform stress distribution around it. Rock stability depends significantly on the frequency and orientation of discontinuities. Recent advancements in photogrammetry and the integration of discrete fracture networks (DFN) into distinct element models (DEM) now allow for more realistic rock mass modeling and stability assessment. In this research, a combination of Structure-from-Motion (SfM) aerial photogammetry, laser scanning, digital joint mapping, and DFN-DEM was utilized to analyze the stability of tunnel portals. This proposed methodology was applied to both portals of twin tunnels No.39 of the Tehran-Shomal freeway, to examine two conditions with and without support installations. The findings showed acceptable agreement between manual and digital joint mapping methods. DFN-DEM modeling results demonstrate that this approach is a highly suitable solution to evaluate block stability around tunnels and accurately simulate lining installations. The analysis of multiple DFN model generations revealed that in the worst case, the displacement of the blocks decreased substantially from 1.91 to 0.098 m after the installation of linings. Furthermore, the safety factor analysis for both portals indicated that the installed support systems are sufficiently strong to ensure safety and prevent potential rock falls.