By Shani Wallis, Editor

Four years and eight months after starting, hole-through of the Arrowhead East Tunnel was hailed on Friday, May 2, 2008, to complete the 6.8km (22,443ft) heading and junction with 2.4km (8,000ft) of the tunnel completed under an earlier contract. The hole-through ends at times, painfully slow progress with short spurts of TBM excavation interrupted by long cycles of pre- excavation grouting to control high-volume, often high-pressure, ground water inflows and zones of crushed, fractured rock.

The finish of Arrowhead East occurs as crews in the contract's 6km (19,890ft) Arrowhead West tunnel continue slogging through more difficult rock and water ingress conditions to complete the final 445m (1,450ft) towards an anticipated portal breakthrough sometime in September 2008, after starting in October 2003.

The story of Southern California's Arrowhead Tunnels is a long and frustrating saga of design and redesign; bid and rebid; contract termination and remobilization; events of 'force majeur'; contract adjustment from stalemate; of environmental objections and legal actions; of conditions never before encountered; of major TBM and mechanical modifications in mid-drive; and ultimately, of dogged determination by men, machines and management to get these two vital tunnel headings completed against tremendous odds.

The Arrowhead Tunnels, to transfer water from supply aqueducts to the Metropolitan Water District's new Diamond Valley Reservoir, have been described as being among the most technologically and geologically demanding attempted. The 5.8m (19ft) o.d. headings of more than 6km each pass up to 335m to 630m deep through the San Bernardino Mountains and are subject to ground water hydrostatic pressures of up to 30 bar. As well as the time-consuming water control problems, the tunnelling machines have had to cope with wide ranging rock qualities from massive, hard marble/granite to crushed squeezing fault-zone gouge and a quartz content that is highly abrasive to cutter head tools and machine components, particularly when water mixes fines to an abrasive paste.

The Shea/Kenny JV was awarded the redesigned contract to complete the Arrowhead Tunnels in May 2002 with a bid of $242 million. To meet the redesign specifications, the JV bought two new Herrenknecht hard rock shielded TBMs to erect a 5.8m (19ft) o.d. high- specification precast concrete segmental lining. The machines were said to be of the highest technical specification when ordered. They have since undergone significant Herrenknecht upgrading and modification to cope with conditions as encountered. Only the Herrenknecht machine delivered to the Hallands<8c>s rail tunnel project in Sweden is said to be of a higher specification and it incorporates many of the modifications and developments introduced initially for improvement of the Arrowhead machines.

The tunnelling challenge of the current Arrowhead experience was the topic of the British Tunnelling Society meeting on Thursday, May 15 at the Institution of Civil Engineers in London. Brian Fulcher, Project Director for the Shea/Kenny JV and Mike Bell, Resident Engineer for construction supervision engineers Hatch Mott MacDonald, presented details of the undertaking and answered questions from the audience. For BTS meeting details visit www.britishtunnelling.org.uk/meetings-2008.php.

The Arrowhead challenge has been the focus of many conference papers and presentations over the years as well as several magazine articles. Shani Wallis, Editor of TunnelTalk has visited the site several times and interviewed managers and engineers associated with the project to publish various project articles, the most recent in Tunnels & Tunnelling North America, Dec 2007.

When the West Tunnel finally breaks through, the Arrowhead experience will be a benchmark for technically demanding projects, the axiom being 'if similar or more onerous conditions were mastered at Arrowhead, they can be faced and overcome again'.

References

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