Three-dimensional simulation of the influence of the oblique segmental joints and of the lagging distance between twin tunnel's faces on the tunnel behavior - case study of the second line of Shiraz subway

Summary : The case of the Shiraz (Iran) subway twin tunnel construction is an example of transport network extension. In this work, a three dimensional model of the shielded mechanized tunneling of the Shiraz subway was simulated using FLAC3D software to study the excavation influence of a new tunnel on an existing one. Also, the joint pattern in segmental lining is studied.
Due to the fast development of Iranian metropolises, a wide urban underground transport network is necessary. Thus, the prediction of the influence of mechanized tunneling method on the surface settlement is important in terms of design and construction. In recent decades, extensive studies around the world have focused on considering the effect of mechanized shield tunneling on the surrounding ground.
Methodology and Approaches : In the present paper, a 3D numerical model was developed to investigate the tunnel's lining behavior and the ground displacement surrounding the second line of the Shiraz subway tunnels. All the excavation phases of EPB-TBM (Earth pressure balanced, Tunneling Boring Machine) were simulated using the Finite Difference Method (FLAC3D). Furthermore, a new simple method were proposed to take account of the TBM shield conical shape. Also, to achieve an accurate simulation, the influence of the geometry of the longitudinal joints had been investigated on the lining behaviour.
Results and The numerical results indicated the necessity of using segmental lining with oblique joints pattern to obtain an accurate evaluation of the structural forces applied in segmental lining. In this work, the influence of the longitudinal distances between the tunnel faces on the surrounding ground displacement and the induced lining forces was investigated. The excavation of a new tunnel near to an existing tunnel with high lagging distance between twin tunnels leads to increase in the normal and the longitudinal displacements, in the internal lining forces and to a decrease in the ground surface settlement above the tunnels.