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Utah water tunnel complete

AN UPGRADED Robbins TBM has completed a 1,584m long tunnel for part of a water diversion project in Utah, USA, bringing to an end the troubled contract.

An earlier Robbins machine had to be abandoned underground when joint venture contractor Obayashi and WW Clyde ran into problems on the Upper Diamond Fork project.

The tunnel and pipeline will divert 1bn hectolitres of water from the Provo River to be used in Salt Lake, Wasatch and Utah Counties, as part of client Central Utah Water Conservancy District's plans to meet the water need of all 10 counties in Utah.

The system will also carry 760M hectolitres a year to the Spanish Fork River for agriculture in South Utah County.

Originally the project comprised two, 2.9m diameter tunnels at Tanner Ridge and Red Mountain but cost and environmental considerations cut this to the single 3.81m diameter tunnel.

The $53M contract was awarded to Obayashi/WW Clyde JV in February 2000 and was due to be finished in June 2004.

However with tunnelling 90% complete, major ingress of water carrying almost lethal levels of hydrogen sulfide gas and a collapse behind the Robbins TBM meant almost half of the tunnel had to be plugged and the machine abandoned at the face.

Back at the drawing board, Phase 2 was devised. This reverted to the original plan of a tunnel under Tanner Ridge, but at 3.81m diameter. This feeds a pipeline alongside the abandoned tunnel section before rejoining the completed section.

To complete the job, Obayashi and WW Clyde leased another TBM from Robbins. The 86t machine's 3.81m diameter cutterhead was fitted with 28, 432mm cutters and maximum cutterhead thrust was 616t.

It began work in December 2002 from the western side of Tanner Ridge back towards the 6th Water Shaft.

Most of the tunnel was in the Upper Red Narrows conglomerate which contained some 'awkward' voids. Despite this, boring was completed in only 66 days, three days ahead of schedule.

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