Digging the Gautrain

By Anthony Doman
Images by Gautrain

Drill-and-blast works fine for excavating 80 percent of Gautrain's 15km of tunnels. For the really tough stuff, they called in Imbokodo.

Deep beneath Oxford Road in Johannesburg's Rosebank, things move even slower than the stop-go rush-hour traffic 60 metres above. At a maximum of 12 metres a day, progress is positively glacial.

The difference is that, down here, progress is all but unstoppable. And it proceeds 24/7.

In just three months, a R300-million high-tech borer will have chewed up and spat out three kilometres of waterlogged, rocky subsoil to produce a 6.8m-diameter tunnel. It's a hugely impressive part of a feat of engineering — and of political will — that is seldom far from the headlines.

From the outset, politics and red tape ensured Gautrain got off to a rocky ride. It's hard to put a brave face on a project that started near R4-billion, ballooned to R20-billion and currently stands at R25-billion.

But that was just a prelude to the engineering challenges of hoisting a track above ground, and guiding it below.

When Gautrain hits the tracks in 2011, South Africa will enter the era of high-speed trains. Its 80km of rapid transit track snakes across Gauteng's flatlands, vaults the valleys on viaducts, and burrows deep underground for a total of 15km.

Mechanical mole

On its southern section — between Marlboro portal in Midrand and Johannesburg Park Station, via Sandton and Roseban, the track is housed in a 15km tunnel. Gouging out all of that entailed mostly some good old-fashioned digging, drilling and blasting.

It was on the 3km Rosebank stretch that the massive mechanical mole came into play.

Tunnel borers are used where it's essential to disturb the ground surface as little as possible. A handy side-effect is that they produce a smooth, watertight tunnel wall.

A year in the making, the 325-ton Imbokodo arrived from Germany to start work in January 2008. It is due to crunch its last rock somewhere near Killarney early next year. Dismantling and removing the behemoth — and its 560 tons of backup equipment — will take three months. It will then be reconditioned, ready to go to work where it's needed.

Naming a tunnelling machine after a woman is thought to bring good luck. Imbokodo is, firstly, the grinding stone used by tribal African women to grind maize. However, it also echoes the women's chant of 9 August 1956, commemorated every year on Women's Day, "Wathinta abafazi, wathinta Imbokodo" — if you strike a woman, you strike a rock.

And does this machine ever strike a rock.

A cutting train

Officially known as a mixed face Earth Pressure Balance Shield tunnel boring machine, Imbokodo has been custom-designed to cope with the harsh and complex geology in its section of tunnel. Besides a high water table, there are differing degrees of hard rock, sand and soft soil.

Essentially a train set with a massive cutting head, the borer has a 145m-long backup system comprising 13 gantry trailers.

As Imbokodo cuts a hole wide enough for a single-track rail line, excavated material is taken out on a conveyor belt. For every 1.5m of forward progress, there's an hour-long halt while the tunnel is lined with precast concrete rings. Each ring comprises six interlocking segments; a standard segment weighs four tons and it's hauled in on flat cars drawn by a diesel locomotive riding on service tracks.

That concrete ring effectively shrinks the diameter, leaving the cutter head no way of getting out intact. The cutting wheel itself will be scrapped, but the hubs of its 48 cutters can be reused.

Every 24 hours, equipment and materials are restocked during a four-hour maintenance break. Fortunately, maintenance — even replacement — can be tackled from behind the cutter head. That work has to be done through an airlock: the cutter wheel chamber must be pressurised when passing through water-bearing ground.

Imbokodo's driver, who is really more of a pilot, monitors banks of computer screens in his "cockpit". Operations are controlled by a sophisticated electronic guidance system. The driver steers the machine, continuously monitoring its position (in 3D) in relation to the programmed tunnel centreline anywhere along the route.

Powering a beast that develops a total thrust against the ground of 4000 tons makes the kinds of demands that would cause Eskom executives to blanch. It requires a total of 3.2MW, and has a substation dedicated to its needs on site.

The cutter head is driven by seven motors and has 150 drag teeth for softer ground. For hard rock, there are also 40 single-disc cutters and four twin-disc cutters. The electric power rating of the cutting wheel is 2450kW.