Tunnel possible for 710 Freeway in Los Angeles - TunnelTalk
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Tunnel possible for 710 Freeway in Los Angeles Nov 2009
Paula Wallis, TunnelTalk
All of the five tunnel options under review for the proposed 710 Freeway extension in Los Angeles have issues of concern, including contaminated soil and active fault lines, but a newly released study says a tunnel is feasible in any of the possible corridors.
Fig 1

Fig 1. Plans to close the freeway missing link

The California Transportation Department (Caltrans) study is an initial step in the long planning process to extend the 710 Freeway that has been a contentious issue for decades. Fifty years ago the State first proposed a surface freeway to close the 4.5 mile (7.2km) gap from the end of the 710 Freeway in Alhambra to where it starts again just south of the 210 Freeway in Pasadena (Fig 1). Cities impacted by the proposal used the court system to effectively kill the surface option. While not as controversial, there is still strong opposition to a tunnel. Many fear it will become a shortcut for freight traffic between the Ports of Long Beach and Los Angeles to the major freight routes that traverse the State.
Preliminary geotechnical investigations began in January 2009. The goal of the study by Caltrans Geotechnical Services and a team lead by CH2M HILL and including Earth Mechanics, Jacobs Associates, and ILF, was not to recommend a preferred alignment, but to investigate the geologic, ground water, and seismic conditions within the selected route-neutral study zones that would impact design and building of the proposed tunnel.
Zones 1 and 2 run northwest from the 710 Freeway to the 2 Freeway in the city of Los Angeles. Zone 3 is the most direct route north to the 210 Freeway passing under the cities of South Pasadena and Pasadena. Zone 4 travels northeast through the city of Arcadia to the 210 Freeway and Zone 5 runs east to the 605 Freeway (Fig 2).
The geotechnical investigation assumed the proposed tunnel’s invert would be 200ft (61m) below ground surface with a 50ft (15m) diameter. Along with historical data, the investigation included 25 core borings, 17 seismic reflection lines, and 78 multi-channel analyses of surface wave (MASW) tests to characterize ground conditions within the tunnel zones.
Fig 1

Fig 2. Five zones of extension study

All of the zones pose challenges. Zone 1 has one ‘Superfund’ site that could be a source of contaminated soil that needs special clean up along with groundwater in the portal zone and approach excavations for the tunnel. The active Redmond Fault, capable of generating earthquakes of 6 to 6.7 magnitude and producing displacement at tunnel level of about 2ft to 4ft (.6m - 1.2m), crosses Zone 2. Zone 3 has the most varied ground conditions and also includes crosses of the active Raymond Fault and also of the potentially active San Rafael and Eagle Rock Faults. The active Raymond and Alhambra Wash Faults, plus a Superfund site and six other sites with various levels of soil contamination, close enough to impact the tunnel, pose challenges in Zone 4. Zone 5 includes the active Alhambra Wash Fault plus one Superfund site and seven other sites with various levels of soil and groundwater contamination.

The study is still in draft form and will not be finalized until early 2010, until after a series of public meetings are held.
References:
Highway 710 tunnel study

           

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