ASSESSING THE IMPACT OF GEOLOGICAL VARIABILITY ON TUNNEL STABILITY: SENSITIVITY ANALYSIS OF THE KOHAT TUNNEL, PAKISTAN

Abstract

This research aims to evaluate the influence of geological heterogeneity on the tunnel stability and to determine the most sensitive rock mass parameters affecting structural performance, using the Kohat Tunnel in Pakistan as a case study. This study establishes a quantitative relationship between geological variability and tunnel stability through the integration of finite element modeling and Grey Correlation analysis, providing a structured framework for evaluating parameter sensitivity in heterogeneous rock masses.  Finite element modeling was performed using RS2 software to stimulate the response of varying rock mass conditions along the tunnel alignment. Three geotechnical units -limestone ,sandy limestone, and shale -were modeled to analyze deformation behavior, stress redistribution, and Factor of Safety (FOS). A sensitivity analysis based on the Grey Correlation Method was conducted to quantify the relative influence of key geomechanical parameters on tunnel stability. The results indicate that the Geological Strength Index (GSI) are the dominant parameters controlling tunnel stability, with the correlation degree of 0.889 and 0.833, respectively. In contrast, Poisson’s ratio and Young’s modulus exhibit comparatively minor influence on the Factor of Safety. The analysis demonstrates that geological variability significantly alters stress distribution patterns and deformation characteristics within the tunnel surrounding rock mass. The findings support the development of more reliable and cost-effective tunnel support designs by prioritizing critical geomechanical parameters during  site investigation and modeling. The proposed approach can be applied to tunnel projects in complex geological environments to improve stability prediction and risk mitigation strategies.