Abstract
During tunnel excavation, convergence, i.e., a tendency for reduction of the section area of the void formed, is observed due to the stress redistribution around the excavation free surface. This deformation accumulates with a gradually decreasing rate until an equilibrium level is obtained, its amplitude depends on the rock mass conditions, the support system and the excavation procedure, as well as on the state of stress, and is usually recognized as a two dimensional effect; i.e., a deformation confined to a plane normal to the tunnel axis, not affecting neighboring sections. In the last years, electronic theodolites, a new generation of geodetic instruments which can describe in detail the 3-D changes in the shape of tunnels during their excavation stage relative to fixed reference frames, became available, making possible the understanding of the detailed pattern of such changes; especially the effect of rejuvenation of deformation in previously stabilized sections while tunnel excavation is progressing. Analysis of geodetic monitoring records of certain recently constructed tunnels indicates that, occasionally, large deformation from specific ‘source’ sections is transferred along the tunnel axis to “host”, neighboring sections, causing additional time-delayed deformation of previously stabilized sections and necking of the tunnel. This “induced” deformation is not due to effects such as nearby excavations, changes in the hydrological conditions, etc., and to tertiary creep; its distribution along the tunnel axis seems to depend on the potential of “host” sections to accommodate additional stresses from neighboring deformation “source” sections, and it may lead to a progressive, domino-type failure. The overall process is reminiscent of the triggering of new earthquakes by Coulomb static stress changes caused by a certain earthquake, on the condition that the stress level in the epicentral areas of the new events was close to the rupture limit.
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