Influence of Forepoling Umbrella on the Settlements Induced by Shallow Urban Tunneling

In this study, the influence of a forepoling umbrella system to control ground surface settlements induced by shallow tunneling in urban areas under dynamic railway traffic loads was investigated. 2D and 3D numerical models of the Zarbalizadeh-Maddah underpass tunnel were developed to evaluate excavation face stability and ground deformations. The 2D modeling results indicate stable conditions of the tunnel due to its well-designed support systems. Based on a sensitivity analysis performed in the 2D modeling, cohesive strength and elastic modulus of the equivalent element to the forepoling zone, show the greatest effect in controlling ground surface settlement. In spite of the stable condition of the tunnel, surface settlement obtained from the 2D numerical modeling results in a relatively higher value than instrumentation data (around 43%), recorded during tunnel excavation. In order to more accurately investigate the effect of forepoling on surface settlement, a 3D simulation was performed, with consideration of the Longitudinal Deformation Profile of the tunnel face to better estimate stress relief for different stages of excavation. The results displayed a lower value of surface settlement compared to the 2D model, and in good agreement with the instrumentation data. Therefore, a forepoling pre-support system could have a pivotal role on the control of ground surface settlement by developing the radial and longitudinal arching around the excavated opening of a tunnel in shallow soft ground under high dynamic loading.