Stability of Shield-Bored Tunnel for the Challenge of Nile Crossing

Abstract

Underwater tunnel structures are vital in urban areas, and their use is continuously increasing. The engineering assessments for these impressive phenomena have to be performed from both geotechnical and structural engineering perspectives. The designer should take account of the requirements of the underwater structures to adequately withstand different applied loads. The underwater tunnel might be particularly vulnerable at locations where the geological conditions are uncertain. This study will cover all aspects concerning the stability of the tunnel crossing of the River Nile. The protentional exists for the tunnel failure during construction due to insufficient cover thickness and after construction due to a combination of long-term degradation and local scour. There were no imposed constraints on the alignment and the original design with a cover between the tunnel crown and the riverbed of at least one tunnel diameter. The tunnel stability was analyzed based on the most critical underwater section with a minimum cover thickness of the Greater Cairo Metro, Line 2 as the case study. Then, three-dimensional (3D) numerical analysis based on the Finite Element (F.E.) models was employed to explore soil–tunnel interactions. Comparison between numerical models’ results indicated that the safe cover thickness was equal to the tunnel diameter. The minimum cover thickness can be used to verify the required factor of safety calculated by theoretical analysis. The safety factor of the tunnel stability should not be less than 1.5 for construction and service stages and 1.3 for the exceptional case; scour accrues.