With the end of winter in sight, it is hoped that ongoing site preparations will allow excavation to start on the three 6.3km Alborz tunnels in Iran by the middle of this year.
The tunnels are being driven at an altitude of 2,450m asl and when the TBM arrived on site at the beginning of November 2003, a heated tent and cover was provided at the portal to enable assembly and commissioning during adverse weather conditions. The 5.2m diameter Wirth TB 520E gripper TBM that will be used to excavate all three tunnels on the project is hoped to be fully commissioned and in operation by August.
Boring of a twin tunnelÕs central service tunnel will give a better understanding of the ground conditions, which are expected to comprise of a variegated succession of sandstone, tuff, andesite, basalt and limestone with a strength of up to 150MPa, together with localised fault zones. The TBM will then bore 5.2m diameter pilot tunnels for the two 6.3km highway tunnels. The pilot tunnels will subsequently be widened to their final diameter of approximately 14m; consultant Amberg Engineering AG will advise on the tunnelling excavation, the widening technique is yet to be agreed.
The service tunnel will be supported with rockbolts, mesh and shotcrete. The TBM is also equipped with a ring beam lining device that allows a more robust lining to be placed if ground conditions are particularly poor.
The project is the longest twin tunnel on the 121km long Shomal freeway project, linking the Iranian capital of Tehran and the city of Chalous on the Caspian Sea. The new road should cut the journey time between the two cities from 4.5 to 1.5 hours.
