Effect of Surcharge, Pillar Width and Excavation Pattern on the Behavior of Shallow Twin Tunnels

Realizing influencing factors in reducing tunneling-induced settlement, especially in urban and industrial environment, is important to underground construction because excessive ground displacements could trigger potential damage to existing structures. In this research, the impacts of different NATM patterns on the magnitude of induced surface settlements due to excavation of shallow twin-tunnel (such as urban metro tunnels) are investigated. To this aim, finite element method using ABAQUS commercial application was implemented to simulate NATM excavation famous patterns including top heading (TH), central diaphragm wall (CDW) and side wall drift (SWD). The effects of further parameters like excavation lagging, pillar width (horizontal distance between the tunnels) and various loading patterns on the lining stability and surface settlements were also investigated. Comparison of the results show that in the same conditions of loading and using SWD pattern, the deformations is decreased and the threshold of load-bearing can be increased up to two times. On the other hand, the response of ground-tunnel system and the interaction between tunnels are significantly affected by the status and amount of surcharge. In addition, the finite element analysis confirmed that the reduction of the pillar width tends to increase the ground settlements and cause more subsurface deformations due to increasing the tunnels interaction, regardless of surcharge condition. It is demonstrated that although the increase of excavation lagging does not have considerable influence on the surface settlements, it decreases the lining stability. Generally, it can be concluded that by using SWD pattern with zero step lag of excavation and when the pillar width is higher than three times the radii of the tunnel, the unfavorable impacts resulting from the construction of shallow twin tunnel in soft ground would be limited. Accuracy in the prediction of such tunnels behavior is strictly a function of the proper modeling of surface loading conditions.