The Hida Tunnel is being constructed along Highway 2, which connects Nagoya, on the Pacific coast in central Japan, and Toyama, on the Japan Sea coast. It has been decided that TBMs of 4.5m and 12.8m diameter would be used for the emergency tunnel and the main tunnel respectively, excavation being carried out in one direction from the portal on the Toyama side. The emergency tunnel TBM has already been installed on site, and excavation is under way. The TBM for the main tunnel is now being manufactured.

The regions around the portals experience heavy snowfall and low temperatures in the winter season. Maximum snowfall is 3m and lowest temperatures between -7 and -10°C in January. It is especially difficult to provide measures against snowfall and avalanches during winter in the region near the portal on the Nagoya side and to maintain the construction roads.

Geological conditions

The tunnel runs through the extensive mountain range around Mominuka Mountain (elevation: 1744m). Maximum overburden is 1000m. Starting from the Toyama side, the geology consists of 18% Shirakawa granite (from the Cretaceous period); 34% Nohi rhyolites (welded tuff from the Cretaceous); and 48% Hida metamorphic rocks (Jurassic to Triassic). In addition, intrusive rocks of the Tertiary period are present.

Geological conditions are generally good, with over 90% in medium hard rock (rock mass classification B to CII). However, the following problems may arise due to some peculiar geological features in the region:

  • Large scale, high-pressure seepage in the vicinity of rock mass classification D or deep overlying layer
  • Contraction of the tunnel section at soft rocks along faults and intrusive rocks
  • Collapse of crack type rock mass
  • Rock burst at regions with deep overburden

    It is assumed that there will be normal water flow of approximately 22m³/min at the portal on the Toyama side, and spontaneous seepage is anticipated during construction.

    • Rock mass strength varies, ranging from 160MPa to 360MPa for the Nohi rhyolites to 85MPa for the Hida metamorphic rocks. The quartz content of the rock mass, which would greatly affect excavation efficiency (especially cutter consumption), is approximately 50% for the Shirakawa granite, approximately 37% for the Nohi rhyolites and between 23% and 50% for the Hida metamorphic rocks.

    The Ogimachi historic village of Shirakawa-go, located near the portal on the Toyama side, was registered as a World Heritage site in December 1995, so the alignment of the Tokai-Hokuriku Highway had to be almost invisible from the village. Likewise, during construction, the service tunnels and the construction base were located behind a ridge out of sight of the village.

    Choice of excavation method

    If the tunnel were to be driven from both portals on the Toyama and Nagoya sides it would require costly measures against avalanches and other phenomena on the Nagoya side. It was therefore decided that excavation would be carried out from the Toyama side only. NATM was considered to pose problems in terms of the construction process and schedules. TBMs seemed more suitable, for the following reasons:

  • Geological conditions are fairly stable, and they are therefore suitable for the use of TBMs
  • Excavating the emergency tunnel first would allow for an overall geological survey of the site and would provide the means for drainage at the time of the main tunnel excavation
  • Since the tunnel cross section is circular, the lower half of the tunnel could be used for ventilation ducts, thereby eliminating the need for ventilation facilities and towers – the first example in the world to use the lower half of the tunnel section for its ventilation system.
  • The construction period would be shorter and the construction cost lower compared to those if NATM were used.

    • The excavation radius of the main tunnel was fixed at 12.8m. A reinforced concrete lining was employed after various options had been studied.

    Continuous belt conveyors were used for mucking out in order to dispose efficiently of the large amount of spoil generated during rapid excavation by the TBM.

    Support structures

    Support structures are shown in Fig 3 and were designed with the following aims:

  • Support construction and other supplementary methods should not hinder rapid excavation to make full use of the TBMs’ most significant feature
  • Support structures should be able to cope with various emergency situations during tunnel construction while maintaining safety, construction feasibility and quality of the structure. They should also be economically feasible.

    • Based on these criteria, it was decided to install an invert liner made of reinforced concrete, covering the entire length of the invert. Arch supports for the sections in favourable ground (classifications B to CII) would be of shotcrete combined with rockbolts, and for those with unfavourable ground (classification D), liners made of reinforced concrete would be used for the arched portion.

    It was anticipated that flaking would be more conspicuous behind the TBM cutterheads and that debris would pile up on the machine. In such a situation, it was decided that intermediate support would be necessary; therefore, simple liners made of steel are being considered.

    Basic concepts for the main tunnel TBM

    It is a fundamental fact in TBM excavation that the cutting face can support itself. The machine’s main grippers thrust against the surrounding rock mass so that a reaction force is maintained which enables rapid excavation to take place. Where the cutting face cannot support itself, or geological conditions are unfavourable and the reaction force is insufficient, ground improvement measures should be carried out beforehand, and the TBM’s equipment and capacity should be adjusted so as to minimise the reduction in excavation speed.

    There may be cases where the supplementary methods provided would not have the planned effect, or where unfavourable ground is encountered. The basic specifications of the TBM should make the machine capable of coping with these situations so that an extension of the construction period and the consequent increase in cost is kept to the minimum.

    TBM types and specifications

    Single shielded, double shielded, open and improved open types of TBM were studied for their support structures, excavation radius, length, measures against undesirable geological conditions, excavation speed, etc.) The large cross section of the tunnel and the long machine length were not conducive to quick reactions when ground conditions changed. Therefore, the following points have been noted:

  • A shorter machine length would be desirable

  • The machine type should allow for selection of economic support structures and should enable the use of shotcrete and rockbolts for rapid construction of support under favourable geological conditions and the assembly of reinforced concrete liners inside the shield under unfavourable geological conditions. It should also be possible for simple steel liners with intermediate features as described above to be selected.
  • The length of rock mass divisions should be considered

    • Based on these points, the improved open type TBM, which combines many features of the open and shield type machines, has been chosen as the most appropriate type. The basic specifications of the TBM are shown in Table 2

    Measures against unfavourable ground

    Table1 summarises the anticipated geological conditions that may cause problems during tunnel construction. Also included in the table are the measures to be taken against these situations and ways to predict them. Excavation of the emergency tunnel will precede that of the main tunnel by 3000- 4000m to gain information about ground conditions both for the main and the emergency tunnels.

    TBM assembly

    Assembly of the TBMs outside the tunnels would offer no advantages for the following reasons:

  • The service yards outside the tunnels are small. The angle of the service tunnels connecting with the main tunnel through which the TBMs would have to pass would be too small for TBM assembly. There would therefore be the requirement for an assembly chamber with a large cross section
  • Excavation of the emergency tunnel would have to be stopped during the time taken for transporting the TBM into the main tunnel. For this reason, it was decided to expand a part of the main tunnel to make it into an assembly yard, which will have a cross sectional area of approximately 385m².

    • Ventilation ducts under the invert

    Since the tunnel has a circular cross section, it is planned to use the lower half for ventilation ducts. In order to secure the space for this purpose, the cross section of the lower half of the tunnel is divided into sidewalls, slabs and a central wall. The reinforcing bars and forms are assembled under mobile pedestals and concrete will be cast in place. These structures will be built separately from the excavation process by the TBM but will be carried out simultaneously to meet construction schedules.

    Power supply

    Out of the total power supply necessary for the construction of Hida Tunnel, 3500kW (2500kW for the emergency tunnel and 1000kW for the main tunnel) will be supplied by the utilities. The remainder will be provided through private power generation.

    Conclusion

    The possibility of high pressure, large volume seepage must always be borne in mind for the Hida Tunnel project because it is the first tunnel to be excavated in the rock mass of a mountain having l000m of cover. Since it is likely that it will take a long time for the groundwater to be lowered sufficiently, considerations must be given to faster excavation of the emergency tunnel and the enhancement of the resultant drainage capacity it would provide for the main tunnel.



    Related Files
    Support Structures
    Hida Tunnel
    Geological Conditions