Seminar - Giovanna Zanardo - Tunneling Induced Effects on Existing Bridges and Mitigation Measures Within The CTRL Project
Friday 15 October 2004, 3.00 - 4.30 pm
Civil Engineering Lecture Room 3
The Channel Tunnel Rail Link (CTRL) is Britain’s first highspeed railway and will connect the Channel Tunnel to London St Pancras. The last phase of the project, Section 2, represents the completion of the link, as it brings the CTRL Downline and Upline tunnels from east London into the heart of London, through a series of overland and tunnel sections of railway infrastructure, terminating in the new international station at St Pancras. In Section 2, the tunnels have been constructed at a depth of 30-35m from the surface level.
Empirical data derived from previous projects and from CTRL tunnels constructed to date, confirm that the shape of the initial surface settlement trough developed across and behind the tunnel face (longitudinal wave) follows that of an inverted normal distribution curve for a variety of ground conditions and tunnelling methods. The lateral extent of the trough (transverse wave) is primarily related to the depth of the tunnel and the properties of the soil.
Tunnelling effects on bridge structures mainly result in support movements which occur due to inelastic deformation of the foundation. Inelastic deformations are movements that tend to be permanent and create locked-in permanent actions. In particular, because of the longitudinal settlement wave, the piers are subjected to differential rotations, resulting in the bridge superstructure undergoing warping/twisting deformations, which are however transient in nature. On the other hand, the transverse settlement trough induces permanent distortions in the bridge, mainly in terms of superstructure bending. Further, as a consequence of the differential movements and rotations of piers, the span length may change resulting in a seating-length reduction, which could affect the functionality of the bearings.
In this talk, the assessment findings of the tunnelling induced effects on different bridge typologies are presented, as predicted and observed, as a part of the mitigation work performed within the CTRL project.