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California high speed rail designs 19 Apr 2018

Ofelia Alcantara, Noopur Jain and Randy Anderson, California High-Speed Rail Authority
Steve Dubnewych and Steve Klein, Parsons Brinckerhoff (now WSP USA)
The California high-speed rail system could require 45 miles to 50 miles (72km to 80km) of tunnels ranging in length individually from several thousand feet to more than 20 miles (32km) under a cover exceeding 2,000ft (610m) at certain locations. Geologic, hydrogeologic and seismic conditions along proposed routes through the California Coast Range in northern California and the Tehachapi and the San Gabriel Mountains in the south will be challenging and will include highly variable geotechnical conditions, groundwater pressures exceeding 50 bar, active fault crossings and formations with high potential for methane gas. Members of the California High-Speed Rail Authority and of consulting engineers Parsons Brinckerhoff (now WSP) describe the tunneling challenges ahead.
Fig 1. Dedicated high-speed rail route
Fig 1. Dedicated high-speed rail route

The California high speed rail (HSR) system will be the first constructed in the United States and will include more than 800 miles (1,287km) of service between up to 24 stations. Trains will be able to travel at more than 200 miles/hr (322km/hr) with expected transit time between Los Angeles and San Francisco at less than three hours. It will connect Sacramento, the San Francisco Bay Area, the San Joaquin Valley, Los Angeles, the Inland Empire, Orange County and San Diego (Fig 1).

Construction of the project has already started on the 119 mile (191km) Central Valley section between Fresno and Bakersfield. For the northern and southern California routes, eight project sections are being evaluated, three of which require all the extensive tunneling for the project. Silicon Valley in northern California to Central Valley will be the initial operating segment1, providing service between San Francisco and Bakersfield. Following the start made in the Central Valley, the San Jose to Merced segment of about 113 miles (182km) and crossing the rugged California Coast Range, will be the next to go into construction.

San Jose to Merced through the California Coast Range

The San Jose to Merced section extends south from San Jose along the Santa Clara Valley to Gilroy where it turns to the east and cuts through the California Coast Range at the Pacheco Pass to the San Joaquin Valley (Fig 2). The 24-mile (39km) section across Pacheco Pass will include about 15 miles or 24km of tunnels.

Several alternatives are being considered for the underground sections and environmental studies, and conceptual engineering work and a comprehensive geotechnical investigation of the alternative routes are underway. Presently there will be three to four tunnels ranging from 1.7 miles to 10.5 miles (2.7km to 17km) in length passing under a cover ranging from about 200ft to more than 1,000ft (61m to 305m). Tunnels of about 28ft (8.5m) i.d. will be required for a twin tube configuration.

Fig 2. San Jose to Merced with Pacheco Pass tunnels
Fig 2. San Jose to Merced with Pacheco Pass tunnels

Poor rock mass conditions, consisting of weak, highly fractured and sheared rock, are challenges for the underground sections. Previous tunnels constructed by the US Bureau of Reclamation in the area encountered thick-bedded, moderately to widely fractured sandstone and very-intensely to intensely fractured siltstone and shale as well as a Franciscan Complex mélange and greywacke, with some siltstone and chert2,3.

Ground instability, raveling conditions, overbreak and squeezing ground are predicted conditions and precautions to deal with methane gas will be required in the Franciscan Complex. Groundwater inflows are likely not to be an issue, although deeper alignments could encounter significant groundwater pressures.

Bakersfield to Palmdale through the Tehachapi Mountains

In the southern part of the project, the Bakersfield to Palmdale section of approximately 80 miles (130km) travels south and southeast through the Tehachapi Mountains and descends into the Antelope Valley, traversing valley, mountain and high desert terrain as well as urban, rural and agricultural lands.

Tunnels are required to meet both grade and horizontal curvature limitations and to minimize impact on existing facilities. Nine tunnels have been identified along the alignment through the Tehachapi Mountains (Fig 3) with configurations of either single bore double-track tunnels or single-track twin-tube bores ranging from 0.3 miles to 2.5 miles (483m to 4km) long with a total length of 8.8 miles to 9.7 miles (14km to 16km) depending on the selected alignment.

A wide range of conditions will be encountered during excavation, which will occur through short lengths of weak sedimentary material containing large hard-rock boulders. The longer hard-rock tunnels will likely contain zones of heavily fractured, weathered and altered rock. Evaluations of surface outcrops indicate a wide range of geological strength index values4 for each rock unit ranging from poor to very good conditions with the lower bound of values anticipated near fault zones5.

An oil and gas field is present to the southeast of Bakersfield and oil and gas wells are present near the proposed tunnels. Rock units to be encountered during excavation may be connected by fractures to underlying sedimentary rocks that yield oil and gas.

Based on available information, the three most northerly tunnels will likely be situated below the groundwater table where water inflow, tunnel stability and tunnel dewatering will be issues to manage. Remaining tunnels located to the south are likely to be above the groundwater table.

Excavation methods for tunnels in the Bakersfield to Palmdale section will need to address:

  • A hazardous atmosphere with the potential for methane gas in sedimentary formations adjacent to oil/gas-producing rock units. Documented oil wells near project boundaries are to be avoided.
  • Boulder excavation for reaches within the conglomerate that may result in significant localized overexcavation and overbreak. Contractors will have to consider this impact on chosen excavation and ground support methods.
  • Several active fault zones that exist near the tunnel alignments will present conditions that are likely to be very poor and highly fractured.
  • Karst features have been identified in the marble limestone units during site reconnaissance and will be carefully evaluated during the design phase geotechnical investigations.

Palmdale to Burbank through the San Gabriel Mountains

For the 38-mile to 44-mile (61km to 71km) Palmdale to Burbank section, three alignment alternatives are being studied, each involving tunneling beneath the Los Angeles National Forest and within the San Gabriel Mountains National Monument (Fig 4).

Fig 3. Bakersfield to Palmdale through the Tehachapi Mountains, proposed tunnels in purple
Fig 3. Bakersfield to Palmdale through the Tehachapi Mountains, proposed tunnels in purple

The most westerly SR14 alignment consists of seven tunnels ranging in length from 0.5 miles to 9.5 miles (800m to 15.3km). The E1 and E2 easterly alignments have four and two tunnels, respectively, ranging up to 20 miles (32km) long. Ground cover is extremely variable and typically decreases to the west with an average of less than 400ft (122m) and reaching more than 1,000ft (305m) for the SR14 and Eastern Alignment Alternatives, respectively.

Geologic conditions range from massive, very strong granitic and gneissic rocks to weak alluvial soils. Several formation contacts, geologic contacts and fault crossings are anticipated for all the alignment alternatives. Contacts could be highly weathered and behave as conduits for groundwater flow.

It is anticipated that rock masses will be hydrothermally altered due to tectonic activity associated with formation of the San Gabriel Mountains. Crushed or sheared rock and high groundwater flow are expected within the three fault zones crossed by the tunnel alignment.

Potential hydrostatic groundwater pressures of more than 50 bar could be encountered beneath the National Forest. In most other areas, they are expected to be significantly less at potentially 3 to 9 bar. Groundwater control will be important in water-bearing soil and rock and especially where tunnel groundwater inflows may result in significant tunnel instability or unacceptable environmental impacts.

The entire study area crosses a complex geologic zone of structural folding and faulting in which some of the key considerations will be:

Fig 4. Longest tunnels are on the Palmdale to Burbank route through the San Gabriel Mountains, proposed tunnels in purple
Fig 4. Longest tunnels are on the Palmdale to Burbank route through the San Gabriel Mountains, proposed tunnels in purple
  • Long tunnel drives of up to 9.5 miles, 13.6 miles and 20.7 miles (15km, 22km, 33km).
  • High groundwater pressures will influence the tunnel lining system, wear of tunneling equipment, and the potential impacts of tunneling on the groundwater system must be considered.
  • Variable ground conditions with unstable ground conditions associated with cohesionless alluvium, as well as fault zones and fractured or weak weathered and altered rock are to be expected with instability magnified when these are exposed to high groundwater pressures.
  • Active fault crossings are to be negotiated to cross the San Gabriel and San Madre fault zones. A fault chamber or special tunnel lining design is needed for crossing active faults to avoid rupture and possible collapse due to displacement induced by a seismic event.

Delivery and program schedule

A design-build approach is expected to be adopted for procurement and construction of all elements of the project, similar to procurement of the construction packages already awarded for the Central Valley section. Completion of the valley-to-valley segment in northern California, which includes the Pacheco Pass tunnels, will be the next construction package procured1.

A geotechnical investigation of the San Jose to Merced project section is underway and will provide data required for preparation of a geotechnical baseline report (GBR) to support the procurement process.

The tunnels in Southern California will trail the northern valley-to-valley segment by several years, however it is expected that geotechnical investigations will continue to support the planning and environmental process and eventually procurement contracts.

Author references

  1. California High-Speed Rail Authority, 2016, Connecting and Transforming California Business Plan.
  2. USBR, 1986a. Construction Geology—Santa Clara Tunnel. US Department of the Interior, Bureau of Reclamation Mid-Pacific Region.
  3. USBR, 1986b. Construction Geology—Pacheco Tunnel Reach 2. US Department of the Interior, Bureau of Reclamation Mid-Pacific Region.
  4. Hoek, E, 2013, Quantification of the Geological Strength Index Chart, 47th US Rock Mechanics/Geomechanics Symposium, San Francisco, CA, US.
  5. California High-Speed Rail Authority, 2013. Bakersfield to Palmdale, Preliminary Geotechnical Design Report Tunnels.
  6. California High-Speed Rail Authority, 2016, San Jose to Merced Project Section, Draft Preliminary Geotechnical Data Report for Tunnel Subsection, September.

More details are available in the paper presented by the authors at the June 2017 RETC conference in San Diego, California.

References

Other tunnel projects completed in these areas of northern and southern California reported by TunnelTalk

           

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