Cutting edge

  • The Gautrain tunnel stretches off into the distance towards Rosebank station.
  • Concrete segments each weighing about four tons are slotted into place like a giant jigsaw to line the tunnel behind the cutter head.
  • Image credit: Gautrain
Date:30 November 2008 Author: Anthony Doman Tags:, , ,

Only an elite few are qualifi ed to steer the giant borers that create tunnels around the world. One of them’s South African

It’s midnight, and progress has been good – a metre and a half – since clocking in at 9 pm. Time to grab a quick coffee. In the brief lull after the borer’s steady rumble is stilled, an intermittent hooting and a fl ashing red light indicate the winch operator outside our cabin manoeuvring a 4-ton concrete segment into position.Two colleagues guide his movements, checking alignment with a straightedge.

We’re deep under sleeping Rosebank, with seven hours to go before the next shift takes over. When the team emerges back into daylight, the Gautrain tunnel will have progressed another 3,5 metres.

Most of Gautrain’s tunnel length is being mined by conventional drill and blast methods. (See “The big dig”, May 2008.) For Rosebank, a tunnel borer was built and imported, at huge expense. Only a few people are qualifi ed to operate these complex beasts. One of them is a Free State boy, Izak “Sakkie” van Zyl.

Trained as an electrician on the gold mines, Van Zyl fi rst became acquainted with tunnel borers while working on the Lesotho Highlands water project. He decided he quite liked operating them, and his work took him on a long spell away from South Africa. The work was good, but it could be extremely tough – and dangerous. “I did lots of tunnelling in Hong Kong at Chingi island, under the sea. We did gas and sewage lines. I was working as an electrician and operator. There, you always had water seeping through.”

He quickly began making an impact in the small digger community. “It’s mostly Singaporeans, Filipinos, Indians, and Thais. You know many of the people.” After several years working abroad, he jumped at the prospect of moving back home.

He’s now one of three “pilots” on the Gautrain tunnel borer.

I reached the rear end of the borer after a trek down stairs into the tunnel mouth at Rosebank Station and a oneand- a-half kilometre trip on the shuttle train, which ferries supplies and the team of about a dozen men. Reaching the cutter head entails another walk and plenty of clambering along catwalks. In the air-conditioned control cabin, which is the width of outstretched arms and a couple of paces long, an array of video displays records the borer’s performance. The phlegmatic Van Zyl moves at an almost leisurely pace, but his eyes seldom leave the closed-circuit television screen that shows the front of the gigantic machine.

Among the data he keeps track of are speed, of course, in millimetres per minute, and the number of concrete liner rings installed to date (1 028 on my visit). And power output: “Here, you can see we push 11 800 kW.” We can also see that the cutter head is turning more or less optimum at just three revolutions a minute, and that torque output is a staggering 2,5 meganewton metres.

Real-time information from the head itself can be interpreted to show which of the cutters needs replacing.

Because of the need to balance the pressure between the front of the machine and the working area, access to the front (for cutter head maintenance) entails passing through a decompression lock. It’s like going deep-sea diving and, depending on the pressures required, can be just as likely to result in the crippling affliction known to divers as “the bends”.

Steering the cutter is done incrementally – not by joystick or steering wheel! – by making small adjustments in pressure and angle at the cutter head. “Most of the time I monitor the steering,” says Van Zyl. “I adjust pitch and yaw apply a bit more pressure to create instability so the machine goes in the right direction.”

At the same time, it’s important to condition the excavated earth so that it passes out smoothly. “If it is too dry, you can’t get it out. Your torque will go very high and your screw could be damaged.”

Teams work 10-hour days, with two shifts a day and a four-hour maintenance shift. Their working period is 6 days on, 3 off.

What’s the hardest part of the job? “Having to concentrate. Especially when you use Earth Pressure Balance. You are always busy fighting with the ground.” Rosebank’s weathered granite soil breaks up easily, making it easier for the cutter but also holding potential complications because of its instability.

“Another tricky part of the job is when we change cutters. Every day there is a cutter inspection. They will be measured once or twice a week. If you see the cutter head doesn’t wear any more, you know it’s flat in the front.”

When it’s time for farewells, and I make the walk back to catch the shuttle, I pause and take a look towards the front. Up there, at the sharp end, it seems pretty laid-back it’s almost as if they’re standing still. It’s hard to believe that, by the time I emerge into the night air, they’ll have moved further away – and quite likely, as you read this, they’ll still be moving.

What went wrong?

It was the moment they’d been dreading. Midway through the Tuesday lunch hour, Oxford Road was even more clogged than usual because an extra section had been closed, aggravating ongoing diversions caused by excavations for Gautrain’s Rosebank Station.

Finally, it happened. As horrified bystanders looked on, the ground shivered briefly, then slid from sight. Moments before, the stretch of tar between North Road and 8th Avenue (above, arrowed) had been solid ground. As water from broken water and sewer mains gushed in to fill the hole, the crater stretched to a gaping 12 metres across.

Below the road surface, the tunnel borer Imbokodo, about to get moving after a maintenance shift, was shut down.

A slushy mess of water and earth threatened to add safety to the list of concerns that had plagued Gauteng’s citizens since the multibillion rand Gautrain project was first mooted.

Fortunately, the alarm had been sounded several hours earlier. What damage and delay there were had been anticipated and contained. Nobody was hurt and there was no damage to either the borer or the section of completed tunnel.

Initially about 3 metres deep, the hole resulted from what Gautrain safety personnel referred to as a subsidence resulting from “ground loss”.

Earlier, during routine monitoring, the borer’s technical team noticed surplus material coming through the machine. That’s a sure sign of a void being formed above the borer. Investigation confirmed that suspicion: they identified water seepage in the borer chamber, and a void above.

A decision was taken to change the borer’s mode from air-pressurised to Earth Pressure Balanced Mode. By mid-morning, part of Oxford Road had been closed off as a precaution and extra monitoring points were put in place. When the ground eventually did subside, it led to a break in the water and sewer mains, worsening the situation.

Preliminary investigation by the contractor, Bombela, suggested that it was in fact water seepage from the water line above the tunnel that weakened the soil structure. After the water had been pumped out and concrete had been poured in, work was expected to resume once safety authorities had given the all-clear.

Borers are expensive but, particularly in cities, are more convenient – and less intrusive – for tunnelling than drilling and blasting. At the front of the Gautrain borer, behind the cutter head, the excavated soil is taken out by a giant auger to be removed on a conveyer. Jacks brace and push the cutter area forward.

In the unstable geology around Rosebank, with large areas of soft ground, tunnel boring needs to be done under pressure to avoid caving in. A positive force is needed to compensate for the pressure imbalance caused by soil having been taken out. This is done by means of compressed air or earth. Alternatively, bentonite slurry can be used to stabilise the tunnel wall at the cutting head. As the cutter head and its shields pass through, concrete tunnel liners are locked in place immediately.

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