Posted on Feb 11, 2009 in newsletter temp

It was a significant and somewhat poignant moment when Gautrain’s Tunnel Boring Machine (TBM) completed its last meter for the project before being powered down at the end of January.

Named Imbokodo, the TBM successfully completed a three-kilometre length of tunnel that runs between Rosebank Station and Emergency Shaft 2, situated at The Wilds in Houghton.

It will take three months to dismantle and remove Gautrain’s 325t TBM and its 560t backup equipment. The TBM started its drive in January 2008 at Rosebank Station below Oxford Road and progressed towards Killarney till now.

The geology along the TBM route consisted primarily of very variable granites – in places completely weathered and unstable, and in others extremely hard rock (up to 230 MPa, ten times stronger than concrete), all below the water table.

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The TBM cutter head was driven by seven motors and could be operated in “earth pressure balance” mode where pressure was applied the face of the excavation to balance the pressure of loose soil and water to prevent mud rushes and minimise surface settlements.

TBM’s are used on many projects around the world. Gautrain’s custom designed mixed face Earth Pressure Balance Shield TBM was built over a 12-month period in Germany. It was shipped to South Africa in the last quarter of 2007 before commencing its journey.

Having reached its planned final destination, dismantling of the TBM will take place systematically in phases between February and April this year. Firstly the 145m long back up system comprising 13 gantry trailers will be removed. Locomotives will tow the trailers out of the three-kilometre tunnel back towards Rosebank Station where the TBM started.

When most of the back up have been removed, dismantling of the front-end of the machine (the main body) and cutter head can start inside the tunnel. Some of the more durable dismantled components such as electric motors will also be taken out of the tunnel for reconditioning and use on other tunnelling projects.

The 6.8m diameter cutter head, which by now is well worn and which housed the 48 disc cutters, will be dismantled and scrapped. The cutter head was used for boring a 6.8m outside diameter tunnel for a single-track rail line which was systematically lined with a ring of pre-cast concrete tunnel lining segments erected behind the cutter head. It left behind a watertight and smooth lining.

The 10m long outer shield of the machine will be the only part of the TBM left behind. Over this short section the shield will be covered with shotcrete to match the internal diameter of the precast concrete lining.

The remainder of the 15km tunnel between Johannesburg Park Station and Marlboro Portal is being excavated using conventional drilling and blasting methods.

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