Spotlight on our monster machines

The Karangahape tunnels will be dug out using two road headers (one pictured above)

Karangahape Station is one of the most interesting and technically diverse builds in the entire City Rail Link project – and it requires some seriously cool machines to get the job done right.

But before we touch on the machines, however, it helps to know why we need them.

At 32 metres below street level, Karangahape Station will be New Zealand’s deepest underground station. It sits roughly halfway between Mt Eden and Aotea Stations, with a twin platform of 203 metres.

With such a long concourse, there will be two entrances to the station, one next to the Mercury Theatre on Mercury Lane and the other a couple of blocks away on the north side of Karangahape Road in Beresford Square. Several heavy mining machines will be lowered into a temporary shaft on the site of the old Mercury Plaza to dig the tunnel caverns before the TBM gets to the station late 2021 at the end of the first leg of its journey from Mt Eden.

When the TBM does arrive, it can be pushed through the pre-made station tunnels before starting to dig again on its way to Aotea.

We had a chat to Cam Butcher - Link Alliance’s construction manager of mined tunnels at Karangahape Station - to get the lowdown on the machines that will make all this possible.

Road headers

Excavation of Karangahape Station’s platforms and tunnels is courtesy of two road header machines; which essentially are large vehicles with boom-mounted cutting heads, with conveyers that move rock to the rear of the machine.

The road headers will be dismantled and lowered down Karangahape’s temporary access shaft in pieces. They’ll then be reassembled below ground before mining can begin.

During active phases, they will dig about 1.5 – 2.5 lineal metres of dirt and rock a time, depending on ground conditions. After they’ve done one section, the robodrill and shotcrete machines (mentioned below) will go in and reinforce the area before the road headers can move forward and repeat the process. 

The larger of the two road headers has a sit-in cabin, which is air pressurised – a lot of dust is generated while mining so this allows the operator to breath clean air in the cab. The smaller one is remote controlled from nearby and the operator will wear a pressurised face mask.

Robodrill

This photo is indicative of what a robodrill rig looks like only.

Following behind the road header is the robodrill, which essentially is a manned vehicle with a robotic ‘arm’ that stabilises the freshly dug cavern by installing rock bolts, or giant metal pins, into the roof and sides of the tunnel to protect against collapse.

The robodrill can also probe ahead for water and is less commonly used to install spiral bars and canopy tubes. 

Once the rock bolts are drilled into the exposed rock, nuts are wound up and sit against the tunnel wall, then grout is then pumped around the bolt

Shotcrete

This photo is indicative of what a shotcrete rig looks like only.

After the robodrill has done its thing, the shotcrete spray rig will come in and spray concrete over everything for extra tunnel stabilisation. The machine is remotely controlled by an operator standing about five – six metres away.

It can spray about six cubic metres of shotcrete in an hour – and there will be about one or two sprays a day – depending on ground conditions. 

Butcher said the advancement in technology has made it safer to excavate the tunnel. “Our tunnel machinery enables workers not to be working under unsupported ground.”