After battling heavy water inflows of up to 4,000 gal/min and infiltration of hydrogen sulfide gas, conditions in the TBM heading for the Upper Diamond Fork tunnel project in Utah became too difficult and attempts to continue with the drive were abandoned in January 2002. Since then the Central Utah Water Conservancy District (CUWCD) has moved quickly and decisively.
With no reasonable prospect of resuming the drive the contract was reduced in scope and a new contract was bid and awarded to the Obayashi/W. W. Clyde JV, the same contractor as for the previous contract, to complete the project incorporating part of the project’s original design. “Excavation of the 6.9km long x 3.8m o.d. TBM tunnel was about 90% complete when it was abandoned,” said Michael Gowring of Obayashi and Project Director for the JV. “High volume water ingress was under pressures of up to 17 bar and the tunneling crews had been dealing with hydrogen sulfide concentrations exceeding 100ppm locally for several months.”
Obayashi/W. W. Clyde was awarded the original $53 million design-build contract in early 2000 to excavate its single deep level tunnel alternative to an earlier design of higher level tunnels and pipelines (T&TNA, June 2000). The deep level alternative, designed for the JV by Jacobs Associates with Camp Dresser McKee and Golder Associates, was driven from the down stream end and is stopped some 2,900ft (880m) from the shaft below the existing Sixth Water outlet in the CUWCD’s Central Utah Project. The new contract will incorporate the Tanner Ridge Tunnel that is within the previously approved easements of the original project designed for CUWCD by URS Greiner-Woodward Clyde.
With such a major change taking place on a largely federally funded project, it was decided to reduce the scope of the original contract and to request public bids for the completion work. New documents were prepared, released, and bid, all within a period of six weeks. Four invited groups returned bids on March 26, 2002 and of these, Obayashi/W. W. Clyde submitted the lowest bid at $29 million. The competing bids were $31.8 million from Clark Construction, $33.5 million from C. R. Fedrick, and $40.1 million from the McNally/PCL Constructors JV. The Engineer’s Estimate was $29.7 million.
To complete the project Obayashi/W. W. Clyde intends to use a refurbished 3.8m diameter Robbins TBM furnished by The Robbins Company. The machine is scheduled to arrive on site later this year to excavate the Tanner Ridge Tunnel towards the Sixth Water outlet. A 2,000m long, 2.4m diameter buried steel pipeline, part of the original design, connects the Tanner Ridge Tunnel to a new flow control structure on Diamond Fork Creek where a 45m deep shaft connects the discharge into the completed section of the Upper Diamond Fork Tunnel. Work to be completed on the original contract includes the drop shaft and stilling chamber, the in-situ concrete tunnel lining and the connection to the buried pipeline.
The refurbished TBM leased from Herrenknecht USA for the deep level tunnel has been abandoned at the face of the tunnel and three concrete plugs have been placed in the original deep level tunnel to abandoned tunnel 1,900m of the tunnel and contain groundwater contaminated with hydrogen sulfide in solution.
