The effect of topography on stability of shallow tunnels case study: The diversion and conveyance tunnels of Safa Dam

Due to ignoring the role of real topography in the numerical modeling carried out for back analysis and considering that in the tunnel longitudinal profile the distance of tunnel roof to the ground surface is the maximum, some results inaccuracy may occur. This issue is especially critical for cases when two tunnels or underground structures are constructed next to each other. The purpose of the present research is to study the role of real topography on the obtained results of back analysis and determine the accurate geomechanical parameters of the rock mass using back analysis based on convergencemeter data. For this aim, 8 instrumentation sections were modeled using FLAC3D 5.0 and the real topography was determined. Next, another numerical model was built to study the role of real topography and the obtained modeling results were compared for cases with and without real topography; where a significant difference was obtained between the results. In the case of ignoring real topography, the error function obtained from monitoring and numerical method results are between 1.5 and 10 times more than the case of considering real topography, suggesting the significant role of real topography. Furthermore, stability analysis and optimization of tunnels support system performed in this study using the real topography data. The results showed that removing the designed rock bolts the tunnel was still stable through other designed supports.