Ask Phoebe: Waterview tunnel rock to be used in Wiri Quarry development

The rock is so hard that it is causing damage to some of the machines. Photo / Martin Sykes

I am a Mount Albert resident, living approximately half a kilometre away from the Waterview Connection construction area, so I'm not directly affected, but I do enjoy reading the newsletter updates which arrive in our mailbox.

My question is, what becomes of the huge amount of rock which has to extracted to complete the project?

Bob Simons, Mt Albert.

About 100,000cu m of hard basalt rock is being removed from the Alan Wood Reserve in Owairaka. The basalt is from a lava flow and is up to 14m thick in places. It needs to be removed so that a trench can be excavated in time for the arrival in mid-2013 of the project's tunnel-boring machine, which is coming from Germany. The trench will ultimately be used as part of the motorway to and from the southern portals of the Waterview tunnels.

The rock is so hard that it is causing damage to some of the machines. Explosives are being used to break through the volcanic rock to the softer material below. The work at the Alan Wood Reserve is preparation for a section of the Waterview Connection that will run underneath what will become a bridge at Richardson Rd. By building the bridge early, trucks removing rock from the site will be able to access the motorway earlier, and thus avoid using local roads.

The basalt is taken to the Wiri Quarry in South Auckland where the project has established a crushing plant. It would seem logical to have established a crushing plant at the site itself, but read on.

Some of the crushed rock will be trucked back to Waterview to be reused on the project and the surplus is being sold as metal for gabions, road foundations and backfill. Gabions are those wire cages filled with rock that are used for erosion control and trendy garden walls, among other things. The project has also established a landfill at the quarry for disposal of the estimated 800,000cu m of material that will be removed from the tunnels themselves. This means that over the life of the Waterview project, the old quarry will be rehabilitated ready for redevelopment as a commercial area. This would seem to be why the crushing plant is at Wiri.

The tunnelling machine, otherwise known as an "earth pressure balance machine" (EPBM), will bore twin 14m-diameter tunnels of total length 2.5km, at a maximum depth of 45m, just part of the new 4.8km, six lane motorway. Tunnelling is expensive, and the cost of the custom-built EPBM plus the associated work accounts for over two-thirds of the project's budget. The tunnels it creates will be the largest ever built in Australasia.

Tunnelling is expected to start in early 2014, with the Western Ring Route, of which the Waterview Connection is part, completed and open by 2017.