The TBM Dame Whina Cooper, is slowly making its first steps. Progress over the first initial metres is cautious but steady while the crew bed down getting used to the machine and conditions underground.
The TBM can travel at a top speed of 32 m a day, it will likely average at slower speeds depending on soil conditions - and will operate 24 hours a day, 7 days a week when it’s at full capacity. There will be a crew of 12 working each shift on the TBM, supported by another team of around 12 above ground.
So tunnel excavations have started from City Rail Link’s Mt Eden site. Its first destination is the Karangahape Station, 830 m away. From there it bores on to the Aotea Station in central Auckland – a total journey for the TBM of 1.6 km - to join the section of CRL tunnels already built from the Britomart transport hub.
Mt Eden will be ‘base-camp’ for the TBM, and the equipment and systems required to operate it. These include the spoil enclosure and conveyor system, workshops, water treatment plant and a storage area for the pre-cast concrete segments that will line the interior of the tunnel.
Karangahape Station
Two roadheaders have been completed the top heading (crown) for the 223-m long southbound platform caverns. The excavation now runs the full length of the platform tunnel, from the bottom of Mercury Lane to the Pitt Street / Beresford Square intersection.
The machines made their way through the East Coast Bays sandstone, taking 12.5m cuts at a time. Following each cut, rock bolts and shotcrete are installed to provide safe support.
The 3-6m long bolts are used to stabilise the area, transferring loads from the less stable exterior to more compact, confined and much stronger interiors of the rock mass.
Once installed the bolts are filled with grout (flowable cementitious fluid) which sets to provide additional strength. Approximately 7,500 bags of grout will be used upon completion of the mined tunnels. Shotcrete, which is sprayed concrete, is then applied over the top to provide further support before the roadheader can continue to the next section. 5-10m³ of shotcrete is used per section during excavation, this is around 5-10,000 litres of shotcrete being used every 1-2.5m.
At the completion of both the 223-m platforms, over 1,000,000 litres of shotcrete will have been used during excavation of the tunnel caverns.
The focus now turns to removing the bottom bench (floor) of the tunnel, which requires fewer supports than the crown of the tunnel as the crown bears much of the load. This process is expected to take around four weeks.
Another milestone for the team has been the formation of adit 2 and the adjoining pilot tunnel that runs beneath the Mercury Lane Station’s footprint, enabling excavation on the platform tunnel to the south. (Adit means a passage or tunnel that provides access to and from underground workings in a tunnel). There are some challenging ground conditions in this area requiring additional support in the form of spiles, mesh and canopy tubes. The pilot tunnel will eventually be incorporated by the station, becoming part of its B7 level as the station builds down. In total five adit tunnels will be excavated as part of Karangahape Station.
Click here for a video, here and nz/15 for the tunnelbuilder archive, and also click this link https://www.cityraillink.co.nz/. 21/21.
