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Shieldhall shoring takes shape

Preparing for launch of the tunnel boring machine (TBM) on Scottish water’s Shieldhall Tunnel is Glasgow has called for Groundforce to mobilise a massive hire fleet for the scheme to aid faster construction.

Joint venture contractors Costain and Vinci Construction Grands Projets are using the props to support construction of the cut and cover section of tunnel that leads up to the launch chamber for the TBM that will drive the 5km tunnel.

The contiguous bored piles for the 190m long cut and cover section were constructed by Bachy Soletanche last year and the Groundforce props have provided temporary support during the excavation process.

The support system designed by Groundforce comprised two levels of Mega Brace waling, one approximately 0.75m from the top of the contiguous piles, the other at the bottom of the excavation, approximately 4.5m below.

“The original plan was to excavate in sections, casting the tunnel invert before striking the temporary supports and ‘leap-frogging’ along to the next section,” said Costain Vinci JV site manager Ian Bowley. “But we re-phased the operation so that the excavation progressed more quickly than originally planned.”

The new arrangement meant that instead of the excavation being opened in sections, the entire 190m-long trench was excavated at once. This accelerated the programme, but in order for this method to work, Groundforce had to ensure that it could supply enough equipment to support the entire 190m excavation. The original plan was to supply sufficient support for no more than 60% of the excavation at any one time.

Work on site is now progressing well and the first sections of the TBM are now being assembled in the launch chamber.

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