The Importance of Longitudinal Stress Effects on the Static Conditions of the Final Lining of a Tunnel

The effects of stresses acting in the longitudinal direction (parallel to the tunnel axis) in certain cases take on an important role in the definition of the static conditions of the final lining, and if they are not foreseen, can lead to instabilities that often require difficult and expensive repair interventions. The case in which the final lining shows a marked irregularity of thickness along the tunnel axis is typical, for example, when the preliminary supports are of a truncated-cone type placed in advancement ahead of the excavation face. The produced anomalous stresses are basically due to the difference in transverse stiffness (and, therefore, in the radial displacements) between the adjoining lining sections. A simple calculation procedure has been set up in this work which permits the estimation of the three-dimensional stress state inside the concrete in the presence of a thickness anomaly and of a homogeneous and uniform radial load at the lining extrados. The approach followed is able to furnish the trend of the safety factor regarding the rupture of concrete, in function of the distance from the thickness anomaly, in such a way as to be able to design the lining thickness and any necessary steel reinforcements. © 2002 Elsevier Science Ltd. All rights reserved.