Sina lotfollahi - Department of Civil Engineering, University of Mohaghegh Ardabili, Ardabil, Iran.
Ehsan Khanahmadloo - Department of Civil Engineering, Islamic Azad University, Zanjan Branch, Zanjan, Iran.
Vahid Hoseini Toodashki - Department of Civil Engineering, Islamic Azad University, Zanjan Branch, Zanjan, Iran.

Tunnel excavation on soil lands may leads to horizontal and vertical displacements around the tunnel. The displacements can reach the ground surface and cause damages to existing structures on the ground. Hence it is so important to estimate the ground settlement induced by excavation, particularly in urban environments. In this study, the effect of longitudinal distance between two tunnel faces on ground surface settlement is examined during the excavation of twin tunnels. Accordingly, the ground settlement is measured for the states where the distance between tunnel faces is 0D, 0.5D, 1D, 1.5D and 2D. The most important results suggest that creating a longitudinal distance (lagging) between the faces of twin tunnels during excavation operations causes changes in surface ground settlement. The maximum surface ground settlement along the width and length of tunnels decreases as the distance between two tunnel faces increases.

twin tunnels, Ground settlement, tunnel face, Excavation, lagging distance