Abstract

The Kuriko Tunnel is a long tunnel on the Tohoku-Chuo Expressway, spanning about 9,000 m. Before excavation of the main tunnel, an evacuation tunnel was constructed. The portion about 5,500 m of the evacuation tunnel was drilled with a TBM 4.5 m in diameter. The geological structure in the TBM-driven zone is generally complex, with Mesozoic granite and volcanic fractured strata produced by green tuff movement. The volcanic fractured strata include large-scale intrusions of andesite and rhyolite.

The geology was assumed to be adverse at faults anticipated to cross the tunnel at several locations, and also in the boundaries between sedimentary formation and intrusive rock. It was therefore essential to survey the geologically adverse areas in advance, to improve the ground ahead of the face to prevent the TBM from inadvertently plunging into such areas. To predict encounters with geologically adverse areas, boreholes 50 m long were driven with a rock drill to log locations where an advance survey predicted poor geological conditions and locations where the geological condition was supposed to change referring to the frequency of small collapses of the tunnel crown and the data of torque and thrust of the TBM.

Locations were classified as geologically adverse areas, if the fracture energy calculated from the logging results was below the management criterion. In these areas, ground was improved before excavation, with long steel forepiling. The management criterion was set on the basis of work record in the zone of initial excavation phase.

The process discussed above enabled regular construction without serious problems such as interruption of TBM advance in the geologically adverse areas. The monthly advance was 240 m on the average and 453 m at maximum, and the maximum daily advance was 31.5 m.

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