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  Invert Construction of an Expressway Tunnel in Service with Significant Heaving of Road Bed

  In recent years, cases have been reported in which road beds in in-service tunnels have heaved, necessitating suspension of expressway traffic. In Toyasan tunnel, which is a two-lane roadway 2600 m long from Aizubange to Nishi-Aizu along the Banetsu expressway, heaving of the road bed was found along a 126-m section two years after it was opened for traffic.Though the section where the road bed had heaved had been repaired by cutting and overlaying for 20 years, the height of the heave had reached about 190mm. During the construction phase, there was a lot of water flowing into this tunnel. However, the cutting faces were stable and there was little displacement of tunnel supports. The rock mass was recognized as hard, therefore the tunnel support pattern did not include an invert in this section.After the road bed heaved, the road surface was inspected and the geology examined. The cause of the heaving was also investigated by simulation analysis. New inverts were constructed as a countermeasure along the 126-m section with severe heaving.This was a rush job including removal of the accompanying structures before invertconstruction and reconstruction after the invert construction. This was the first attempt in Japan that expressway traffic had been suspended for cutting full face during continuous day and night construction. The countermeasures were safely completed within the designed construction period. In addition, there was almost no displacement of the lining concrete during construction. This report describes the road surface inspections, continuous day and night invert construction by haltingexpressway service, and verification of the countermeasures.

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