Twin tunnel configuration for Greater Cairo metro line No. 4

Construction of twin tunnels in crowded urban cities compels the designer to select a suitable configuration and the associated railway alignment. Therefore, the estimation of optimum clear distance between tunnels, then vertical and horizontal curves of rails is significant for twin tunnel projects to obtain the optimum design of tunnels, safety and passenger comfort. A case study is the Greater Cairo metro line No. 4, Phase 1. The configuration of twin tunnels includes vertical, horizontal and diagonal alignment. Two cases for diagonal alignment, one of them tunnels pass under the River Nile. Also, two cases for horizontal alignment, one of them tunnels running above bottom clayey soil. Proper clear distance between twin tunnels has been identified according to internal forces and displacement in tunnels. Vertical and horizontal displacements have been presented for twin tunnels and for the surrounding soil. Moreover, induced internal forces in tunnel lining have been computed. Two-dimensional numerical models are employed by using PLAXIS program to perform the analysis. Also, the procedures of construction of the twin tunnels have been discussed. Based on the calculated result, the clear distance between tunnels has a major effect on soil movement and internal forces in tunnel lining. It shows that the construction procedure affects the soil displacement and internal forces. Construction of the bottom tunnel at first reduces bending moment in top tunnel lining and results in small changes in bending moment in the bottom tunnel lining after the construction of top tunnel. On the other hand, the results have also shown the influence of twin tunnel configuration on railway track alignment.