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View of the Arrowhead tunnels from one end to the other Workman inspecting the Arrowhead tunnel Workman welding on part of the inland feeder project

Innovative solutions to safeguard Southern California’s water supply

The Arrowhead Tunnels project was completed $14 million under its revised budget, and one year ahead of the schedule set out in the modified contract.

ExpertiseConstruction management, resident engineering inspection, contract administration, constructability reviews, contract reviews, geotechnical, hydrogeological, risk analysis
Awards
  • 2010 Construction Management Project Achievement Award, Construction Management Association of America (CMAA)
  • 2010 National Recognition Award, American Council of Engineering Companies (ACEC)
  • 2010 National Outstanding Civil Engineering Achievement Award Finalist, American Society of Civil Engineers (ASCE)
  • 2010 Honor Award, American Council of Engineering Companies (California)

Opportunity

The Inland Feeder system was designed to help meet the growing water demands of Southern California. When large volumes of water are available from the north of the state, the Inland Feeder allows excess water to be stored for use during dry spells and emergencies. Northern California water can be blended with Colorado River water, which has a higher mineral content, to ensure better overall quality.

The Inland Feeder stretches from Silverwood Lake in the foothills of the San Bernardino Mountains 44 miles southeast to Diamond Valley Lake. It includes 26 miles of buried pipeline and three large tunnels totaling 18 miles: Arrowhead West, Arrowhead East, and the Riverside Badlands Tunnel.

The Arrowhead West tunnel was bored through water-bearing layers of granite and metamorphic rock, at depths of up to 1,100 feet and crossing branches of the San Andreas Fault and Arrowhead Springs Fault. Both of the Arrowhead tunnels were required to meet stringent US Forest Service restrictions on the inflow of groundwater.

Solution

The Arrowhead tunnels were bored using twin tunnel boring machines (TBMs) customised using a hybrid design by the German manufacturer Herrenknecht. We designed and developed probe drilling projection computer models and an early warning detection system to detect strain on the shields of the TBMs.

The Arrowhead West tunnel, the more difficult of the two, took four years to bore. In faulted areas, blocks of rock were broken into fine debris and treated with grout before tunneling could safely proceed. At one point, a flash flood temporarily submerged the TBM.

Precast bolted gasketed segments were designed to keep the tunnels watertight. The segments were capable to handle more than 900 feet of external head, the highest head in the industry at the time.

During placement of the cellular concrete backfill around the final liner, which was 12 feet in internal diameter, the subcontractor achieved a world record by pumping the cellular grout 3.8 miles through a 5-inch steel slickline.

In 2008, the TBM crossed five significant fault and shear zones in the last 1,500 feet of the tunnel. Concrete and steel foundation pads were used to support the cutterhead while crossing a forty-foot fault zone of weak rock.

Outcome

The project was completed $14 million under its revised 2006 budget, and one year ahead of the schedule set out in the 2006 modified contract. About 90 days were saved by redesigning the height of the tunnel access vault at Waterman Portal, and by redesigning the grading and drainage at the three other portal sites.

By studying alternative routes, our construction management team avoided major problems posed by hauling pipe through the city of San Bernardino.

To reduce environmental impact, the contractor salvaged steel and other metal scraps from the tunneling and pipe installation. Timbers and planks used on the project were recycled into building stock for outdoor furniture. Tunnel muck was recycled in the local community as landfill cover, embankments in storm debris basins, and in foundations for roads, bridges, parking lots, and building developments.

Innovative technical solutions devised to cope with the project’s extreme conditions are now being used in the creation of new tunnels.

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