Limitations in Tunnel Portal Design: An Evaluation Using Numerical Models and Line Surveys

In this study, the engineering geology and the geotechnical design studies of the Aslankayası Tunnel Project are explained. Owing to the low overburden thickness, the tunnel in question, which is located near a slope face, will be exposed to asymmetrical loading after commencement of excavation. The asymmetrical loadings will especially affect the right tube, in the direction of increasing kilometer markings. Furthermore, the thickness between the right tube’s right wall and the slope face in this section has decreased down to 6 m. Moreover, as the tunnel is passing under a 1st degree protected archeological area. Some of the site investigation studies, such as geotechnical drilling and site laboratory works, could not be performed. The excavation support system of the tunnel was determined using empirical studies and numerical models with the help of line surveys, local sampling, and internationally accepted rock mass classification studies (RMR, Q, GSI). These studies were performed on rock mass outcrops. Rock mass engineering properties were determined through the utilization of empirical equations that incorporate data derived from site investigation studies and laboratory test results as input. By using geotechnical properties obtained from line surveys and engineering geology studies, a numerical model was generated. The numerical model results corroborated the asymmetrical loading predictions obtained from line surveys and engineering geology studies. The main aim of this study is to emphasize the importance of interpretation of the geological units and their post excavation behaviors on the excavation stability.