Numerical investigation on the responses of existing single piles due to adjacent twin tunneling considering the lagging distance

This paper presents an assessment of the influence of the lagging distance between two horizontal tunnel faces of the side-by-side twin tunnels on the responses of the adjacent existing single pile by a series of three-dimensional numerical analyses. Two different relative positions between the pile tip and the tunnel are considered to cover the short and long pile behaviors. The responses of the existing pile in terms of pile head settlement, axial force, lateral movement and bending moment are considered and discussed. The numerical results indicate that the lagging distance between twin tunnel faces significantly affects not only the soil movements but also the responses of the existing single pile. The critical case that produces unsatisfactory pile responses due to twin tunneling is when the lagging distance between the second tunnel and the preceding tunnel equals to the shield length. It is recommended that the lagging distance be not less than three times of shield length when the two tunnels need to be concurrently excavated.