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Arrowhead Inland Feeder Project

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Herrenknecht TBM, which was used in the Arrowhead Inland Feeder Project
The Arrowhead Inland Feeder Project is a water transfer project consisting of a 44-mile (71-km) conveyance system of tunnels and pipeline designed to transport water to the growing communities of Southern California. The project, managed by the Metropolitan Water District (MWD) of Southern California, will have the capacity to transport up to 650 gallons (2,461 L) of water per day beyond its current capacity. Starting at the San Bernandino Mountains, the system extends to the Colorado River Aqueduct in the Riverside county of San Jacinto.[1] The idea for the project was conceived in the late 1980s with planning beginning shortly thereafter; construction began in 1997. The plan incorporated three very large tunneling projects: the Riverside Badlands Tunnel, the Arrowhead East Tunnel, and the Arrowhead West Tunnel.

With a completion date targeted for early 2010, the Inland Feeder has not only been costly at US$1.2 billion, but also extremely challenging to execute.[2] The largest hurdle to overcome was the mining of the Arrowhead East Tunnel and the Arrowhead West Tunnel through a hard rock composite made up of granite, gneiss, sandstone, and marble. Contractors also had to deal with excessive groundwater levels during tunnel advancement, eventually shutting the project down until another solution could be figured out. In addition, no tunneling project had ever before excavated so close to an earthquake fault zone such those found in the the San Andreas region. Natural disasters would also create complications.

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[edit] Construction History

Planning for the Inland Feeder Project began in the early 1990s and was in response to the growing need of Southern California and Orange County to store and better take advantage of the water supply provided by the winter rainfalls of Northern California. The old system was not capable of harnessing the excess water supply fast enough generated by winter rainfalls. Reservoirs in Southern California were also dropping to lower levels, suggesting to water agency officials that a water supply crisis was occurring.[3]

[edit] Riverside Badland Tunnel

In 2001 the Riverside Badland Tunnel, the longest of the three tunnels, became the first to be mined and completed. The project included eight miles (13 km) of 12-foot (3.7-m) diameter tunnel mined through diverse ground conditions and fault zones. Excavation of the tunnel also ran below the groundwater table, and therefore incorporated 1,500 feet (457 m) of additional 12-foot (3.7-m) diameter pipeline.[4] The tunnel was bored using a hard rock tunnel boring machine in one drive with tunnel depths ranging from 200 to 300 feet (61 to 91 m).[5] It was then stabilized using a segmental precast liner. Steel-welded 11.5-foot (3.5-m) diameter pipe was then installed inside the tunnel using cellular concrete backfill.[6]

The project was considered a success given that an accurate analysis of the ground conditions was carried out before excavation on the tunnel resumed. This helped to alleviate any major potential problems from surfacing once mining got underway.

A number of types of ground control and ground treatments were also used during the excavation process, including chemical grouting, pre-excavation grouting with micro-fine cements, and dewatering alluvium canyons, as well as ground support techniques such as steel ribs, shotcrete, and concrete segments. For example, pre-excavation grouting was needed to stabilize ground conditions and reduce the inflow of groundwater during boring. Chemical grouting and dewatering helped control ground flow conditions.

Herrenknecht TBM cutterhead

[edit] Arrowhead East Tunnel

Challenges began to surface in the excavation of the Arrowhead East Tunnel and the Arrowhead West Tunnel through the San Bernardino Mountains. Two 19-foot (5.8-m) diameter tunnel boring machines (TBMs) were used to advance both tunnels. Excavation on the 5.8-mile (9.3-km) East Tunnel[7] used to transport water from the  Strawberry Portal east toward the City Creek portal began in 1997 under the original joint venture contract with Shank/Balfour Beatty. The company had mined about 8,000 feet (2,438 m) of the East Arrowhead Tunnel when the entire project was shut down by the Metropolitan Water District as groundwater flows increasesd. Disaster struck when groundwater levels in the tunnels began to exceed permit levels set out by the US Forest Service. Grouting the tunnels only proved to be an impractical and laborious solution. Mining on the tunnel was halted and a 12-diameter pipe liner was installed to seal the portion of the tunnel already excavated.[8] In 2000, The Metropolitan Water District terminated the contract with Shank/Balfour Beatty.

The MWD collaborated with the US Forest Service to determine the best way to complete the tunnel's construction. A redesign of the tunnel—drawing upon concepts executed in the construction of the English Channel Tunnel to deal with high water pressures—was devised by Jacobs and Bechtel Engineering. One key component of the redesign was using bolted and gasketed precast concrete tunneling segments designed to not only provide critical structural support, but also to serve as a watertight seal inside the tunnel to prevent the inflow of groundwater while mining.

To finish the tunnel, the MWD awarded a new joint construction contract worth US$242.2 million to J.F. Shea Co. and Kenny Construction Co. in the summer of 2003.[9] To control groundwater volume and pressure, they bought two tunnel boring machines from the German TBM manufacturer Herrenknecht AG. Both TBMs were designed for working in hard rock but outfitted with shields similar to those on soft ground models. This modification allowed the Herrenknecht TBM to work in earth balance pressure mode for short periods of time and also accommodate water pressures of up to 10 bar.[10] The TBM used for the advancing of the East Tunnel was also fitted with more elaborate dewatering equipment. With the new TBM in use, Shea/Kenny mined an additional 7,000 feet (2,134 m).[11]

The new TBMs allowed engineers and officials to see what lay ahead of the machines as they advanced through the ground. Ports fitted to the machine’s cutterheads allowed probes to drill from 100 to 150 feet (30.5 to 46 m) ahead of the bore. When the inflow of water exceeded 0.3 gallons (1.1 L) per minute, the TBM would automatically stop mining and grout would be injected through ports ahead of the machine. When the flow of water receded, mining would resume. Spoil from the mining process was removed with a train of muck cars.[12] As the TBM advanced, crews assembled a precast concrete segmental liner behind the cutterhead and then aggressively grouted the surrounding annular space. The Arrowhead East Tunnel was constructed using a cut-and-cover method and was mined up to 2,000 feet (610 m) below the ground in some places with more than 1,200 feet (366 m) of water pressure in several parts of the tunnel. Mining of the tunnel was completed in May of 2007.[13]

[edit] Arrowhead West Tunnel

The shortest of the three tunnels, the 4-mile (6.4-km) Arrowhead West[14] was driven from the Waterman Portal uphill to the west towards a portal located at Devil Canyon. After a five-year passage underground, the TBM broke through the dirt in August 2008. The excavation of Arrowhead West Tunnel was lengthy and extensive. The TBM faced many challenges as it bore through water-bearing strata of metamorphic and granitic rock. Crews also had to deal with water pressure in excess of 150 pounds per square inch (1,034 kPa).[15] Again, heavy precast concrete tunnel liner segments along with aggressive grouting and in-tunnel drainage supports were used to compensate for the pressure issue. The tunnel's path crossed several fault and shear zones where the rock had to be broken apart into fine debris and grouted before tunneling could be considered safe.

[edit] Natural Disasters Strike

During construction, the Arrowhead Inland Feeder Project was hit by a few natural disasters that slowed its progress. In October 2003 a 10,000-acre (4,047-ha) wildfire swept through Waterman Canyon near the Arrowhead West Tunnel portal.[16] Several pieces of construction equipment were destroyed in the fire. Two months later on Christmas day, a strong winter storm poured down and penetrated the charred mountains, resulting in a massive mudslide that flooded the same tunnel portal site and the tunnel boring machine. Various pieces of equipment were washed away and consequently destroyed.

[edit] Equipment Used

[edit] Unique Facts

  • The Arrowhead Inlander Feeder project consists of 26 miles (42 km) of buried pipeline and 18 miles of tunnels
  • The amount of water the Inland Feeder will be capable of transporting per day is enough to fill an Olympic-size swimming pool in 30 seconds[17]
  • The total project cost was US$1.2 billion 
  • The Herrenknecht TBMs were outfitted to advance through hard rock strata but were adapted with special soft ground shields that allowed them to work in earth balance pressure mode for short bursts of time to accommodate for water pressure during mining
  • The TBM was also equipped with on-board thrust rams that pushed against installed concrete segments to move the machine through the mountain with a motive force equivalent to that needed to launch the Atlantis space shuttle.[18]
  • During peak production the TBM advanced up to 80 feet (24 m) in a 24-hour pass. The average daily advance made by the TBM was 20 feet (6.1 m) a day[19]

[edit] References

  1. Inland Feeder System - Metropolitan Water District. EPC Consultants, Inc. 15-04-2009.
  2. Machine grinds path for water through mountain. The Orange County Register. 15-04-2009.
  3. Machine grinds path for water through mountain. The Orange County Register. 15-04-2009.
  4. Inland Feeder System - Metropolitan Water District. EPC Consultants, Inc. 15-04-2009.
  5. Inland Feeder System - Metropolitan Water District. EPC Consultants, Inc. 15-04-2009.
  6. California Inland Feeder Project. Mott MacDonald Corporate website. 15-04-2009.
  7. Inland Feeder. Arrowhead East Tunnel Breakthrough. Metropolitan Water District Brochure.
  8. Inland Feeder. Arrowhead East Tunnel Breakthrough. Metropolitan Water District Brochure.
  9. High Water Pressure Complicates Arrowhead Tunnels. Engineering News Record. 15-04-2009.
  10. High Water Pressure Complicates Arrowhead Tunnels. Engineering News Record. 15-04-2009.
  11. High Water Pressure Complicates Arrowhead Tunnels. Engineering News Record. 15-04-2009.
  12. High Water Pressure Complicates Arrowhead Tunnels. Engineering News Record. 15-04-2009.
  13. High Water Pressure Complicates Arrowhead Tunnels. Engineering News Record. 15-04-2009.
  14. Mechanical Mole Opens Water Tunnel to Southern California. 15-04-2009.
  15. Feeder Arrowhead West Tunnel Breakthrough. Metropolitan Water District Brochure. 15-04-2009.
  16. Mechanical Mole Surfaces from Nearly Five-Year Journey under Mountain. Reuters. 15-04-2009.
  17. Inland Feeder Arrowhead West Tunnel Breakthrough. Metropolitan Water District Brochure. 15-04-2009.
  18. Inland Feeder Arrowhead West Tunnel Breakthrough. Metropolitan Water District Brochure. 15-04-2009.
  19. Feeder Arrowhead West Tunnel Breakthrough. Metropolitan Water District Brochure. 15-04-2009.