After a series of difficult ground conditions to overcome over four and a half years, breakthrough was achieved on the Alborz road tunnel in Iran early this month.
A 5.2m diameter open gripper machine met a string of tough conditions that delayed completion of the 6.3km drive: methane, H2S and COx gases: major fault zones; anhydrite with karst sections that required numerous bypasses; squeezing ground that blocked the cutterhead or shield; material and water inrush of up to 800 l/s.
Three parallel tubes are eventually to form the Alborz tunnels on the 121km long Tehran Shomal Freeway to link the capital to the Caspian Sea.
The service tube was first to be bored by the Wirth TBM with the plan for the shield to then drive the bores either side for the main tunnels, which would subsequently be widened to approximately 14m.
However, following completion of the service tunnel by contractor Jäger, the commencement of excavation of the main tunnels has not yet been determined.
The freeway is being designed and built in lots. Swiss firm Amberg Engineering has provided consultancy, design and site supervision services to the client during the TBM excavation. The TB 520E machine was delivered to site in late 2003 along with auxiliary equipment supplied by Wirth, and excavation began in the second half of the following year.
Aside from the fault zones, geology anticipated along the alignment comprised sandstone, tuff, andesite, basalt and limestone with strength up to 150MPa. Tunnel support used rockbolts, mesh and shotcrete but a ring beam lining device was fitted to the shield should poorer ground conditions be met.
