Interpretation of deformational behaviour and support mechanism of a 35 year old tunnel subject to extensional excavation.

The cross-section of the 1.4 km long Tennozan road tunnels has been extended by removing the existing lining and excavating the required amount prior to installing a new lining. The initial construction had experienced large deformation and high pressure on the concrete linings, so special care was taken in the planning and selection of construction method. Monitoring was undertaken of deformation during construction and other field investigations included bore-hole loading tests, vertical seismic profiling and vacuum permeability tests. Only slight deformation occurred, and results of monitoring were used in numerical simulation to trace the history of the mechanical behaviour of the tunnel. A 3-zone structure was found to have formed since original construction, consisting of an undamaged zone, a zone of loosened rock and a zone of once damaged but now re-stiffened rock.
The cross-section of the 1.4 km long Tennozan road tunnels has been extended by removing the existing lining and excavating the required amount prior to installing a new lining. The initial construction had experienced large deformation and high pressure on the concrete linings, so special care was taken in the planning and selection of construction method. Monitoring was undertaken of deformation during construction and other field investigations included bore-hole loading tests, vertical seismic profiling and vacuum permeability tests. Only slight deformation occurred, and results of monitoring were used in numerical simulation to trace the history of the mechanical behaviour of the tunnel. A 3-zone structure was found to have formed since original construction, consisting of an undamaged zone, a zone of loosened rock and a zone of once damaged but now re-stiffened rock.