Arrowhead Tunnel Finally Makes BreakthroughA tunnel boring machine that has spent nearly five years burrowing up to 460 metres beneath the San Bernardino mountain range in California surfaced on 20th August, 2008, leaving in its wake the 6.1 km-long Arrowhead west tunnel, between Waterman and Devil canyons, that is the last major piece of the USD1.2 billion Inland Feeder water project designed to boost Southern California's supply. The breakthrough marks a major step in completing a 71 km water line that will eventually serve thirsty Southern Californians.The Inland Feeder, built by the Metropolitan Water District of Southern California (MWD), is a high-capacity, gravity-fed water delivery system stretching from the foothills of the San Bernardino mountains to the Colorado river aqueduct in the Riverside county community of San Jacinto. It is expected to pipe water from the Sacramento-San Joaquin delta during the rainy season into the Diamond Valley Lake reservoir in Riverside county. The water will be distributed from northern Los Angeles county to the Mexican border.The Inland Feeder consists of 41.8 km of buried pipeline and three tunnels spanning more than 30 kilometres. Most of its tunnels and pipelines that weave through San Bernardino and Riverside counties are already constructed. The Arrowhead west tunnel is the last of three needed for the scheme. Mining on the other two project tunnels - the 6.9 km Arrowhead east tunnel and 12.9 km Badlands tunnel - was completed last May and in July 2001, respectively. The pipeline will eventually supply 28.3 cubic metres of water per second and will bring an additional 2.46 billion litres a day to 19 million Southern Californians. Once completed in 2010, the Inland Feeder will triple the amount of Northern California water that can be transported to Diamond Valley Lake.TBM excavationA 820-tonne 122 m-long hydraulically driven tunnel boring machine carved out the tunnel, scraping the rock with discs at up to 13 cm/min and collecting the pieces in buckets attached to the head. At a cost of USD9 million each, two Herrenknecht hard rock TBMs were used to dig the Arrowhead tunnels. The machines were designed to bore through anywhere from 3.05 to 24.4 m per day. Visit www.herrenknecht.comGPS and laser-guided survey system tracked its progress. Although laser technology helped guide the cutter face, a human operator applied the right amount of pressure to various locations on the rock face to keep the machine, and therefore the tunnel, on course.When bored out, the tunnel is 5.8 m in diameter. With concrete segments installed, it shrinks to 3.66 m. A bolt and gasket system was employed between the reinforced concrete tunnel segments. The segments have been produced at a Traylor/Shea/Ghazi plant and were fitted with Phoenix sealing gaskets. Visit www.phoenix-dt.de The project has faced fire, flood, regulatory disputes, difficult geology, grouting problems, earthquake considerations, a change of contractors and numerous water district managers. View photos here. Click us/23. Natural eventsConstruction of the Arrowhead west and east tunnels was challenging, not only due to the physical constraints of mining in such extremely difficult geologic conditions and so close to several earthquake faults, but also because of other hazards unique to Southern California that were encountered, like forest fires and flash floods. In October 2003, a wildfire engulfed the Arrowhead west tunnel portal in Waterman Canyon above San Bernardino, destroying numerous pieces of construction equipment. Two months later on Christmas Day 2003, a strong, sudden winter storm drenched the charred mountain area, releasing a torrent of mud, water and rock down the canyon and flooding the same portal site and tunnel boring machine, as well as washing away equipment. It took about five months to clear out the mud and to dewater the buried tunnel boring machine.A maze of earthquake faults - including the infamous San Andreas - made chewing through the granite and other rock below the mountain surface more difficult. There were fears of a collapse on the boring machines and miners.Water worries The first contract was awarded in January 1997 to Shank and Balfour Beatty for USD88.4 million. Heavy water inflows and squeezing rock halted the TBM's progress, revealing that the ground conditions were different than those described in the project's files. In 2000, after approx. 2.4 km of the east Arrowhead tunnel excavated, massive water ingress at pressures of up to 17 bar obliged to suspend all construction work. The contract was terminated in April 2001. Following a redesign and a rebid, MWD brought in a new contractor for the tunnelling project in April 2002. The new contractor, the Shea/Kenny JV, took over the project for USD242.2 million and ordered the two hard rock TBMs from Herrenknecht. The construction management services were awarded to Hatch Mott MacDonald in May 2002. Visit www.hatchmott.comMWD devised new techniques to stop the groundwater from seeping into the tunnel by using grout to stabilize the ground before the machine bore through it. Four ring-mounted drill rigs undertook systematic probing of the ground to control water inflow and improve mining conditions.Each TBM back-up was equipped with a Häny grout mixing and pumping device. A batching plant at each portal was supplied by Wiggert for the sand/cement grout mixing used to fill the annulus gap around the segments. Visit www.haeny.com and www.wiggert.com/enIn addition, there was the two-year delay in the late 1990s prompted by worries from the San Bernardino National Forest and the San Manuel Band of Mission Indians that the tunnelling would affect their groundwater. Forest officials were concerned that the groundwater seeping through the tunnel as it was being drilled could affect springs that keep streams running on the surface through the canyons. Monitoring of the springs and creeks by forest officials and the MWD has shown that the tunnelling technique was successful.Climate changeThe Metropolitan Water District of Southern California is a cooperative of 26 cities and water agencies serving nearly 19 million people in six counties (Los Angeles, Orange, San Diego, San Bernardino, Riverside and Ventura). The district imports water from the Colorado river and Northern California to supplement local supplies. Visit www.mwdh2o.comProjects like the Inland Feeder will help accommodate the inevitable changes in climate and weather that Southern California will experience. The pipeline will help Southern California cope with future weather pattern uncertainties and longer periods of drought. With warmer temperatures, faster-rushing rain rather than slowly melting snow will become more the norm, making capturing water in reservoir systems more difficult. First envisioned in the late 1980s, the project will deliver water to be stored in surface reservoirs, such as Diamond Valley Lake and groundwater basins for later use. 39/08.