Shani Wallis, Editor

Wednesday 20 August 2008: Final breakthrough of the 6km long (19,890ft) West Tunnel for the Arrowhead project in Southern California closes out a long five years of gruelling TBM excavation through extraordinarily difficult ground conditions.

Final breakthrough celebrated by the entire crew

Breakthrough by the 5.8m (19ft) o.d. Herrenknecht TBM follows hole through in May of the 6.8km (22,443ft) Arrowhead East Tunnel and after passing under 630m of the San Bernardino Mountains and against high ground water inflows and potential hydrostatic pressures of up to 30 bar.

As well as long periods of standstill to complete comprehensive cycles of pre-excavation grouting to control high-volume, often high-pressure, ground water inflows, the two high specification Herrenknecht tunnelling machines coped with rock qualities ranging from massive, hard marble/granite to crushed squeezing fault-zone gouge with a highly abrasive quartz content that caused excessive ware to cutter head tools and machine components. After completing all the tunnel excavation work, the Shea-Kenny JV now completes the contract by installing an inner precast concrete steel-skinned carrier pipe.

Despite major setbacks including environmental objections and legal actions; termination of an earlier contract to complete the tunnels; redesign, rebid, and remobilization of a new contract; events of 'force majeur'; contract adjustment from stalemate; major TBM and mechanical modifications in mid-drive; and considerable extension of the contract's original construction schedule; John Bednarski, Program Manager for the Metropolitan Water District (MWD) of Southern California, told TunnelTalk in March last year (2007) that the overall Inland Feeder Project, of which the Arrowhead Tunnels are a vital element (Fig 1), will be completed under the original $1.2 billion budget. "We will not have to go back to the Board for extra funds to finish the job," he said. Speaking of the final West Tunnel breakthrough, Bednarski said, "it is a great and memorable day".

Turnout cost of this extraordinary project will have to wait until the Shea-Kenny JV contract is closed out and final account is reported.

References

Clawing success from the extreme at Arrowhead - TunnelTalk, Dec 2007

Technological breakthrough and progress milestone at Arrowhead - TunnelTalk, May 2008