With completion of the draft geotechnical study into five possible routes to including a tunnel option for the 4.5-mile (7.2-km) ‘missing link’ of the SR 710, in the freeway network north of Los Angeles, consultations are underway to brief the public on what’s been learned to date and to debate the alternatives.
The feedback is being pulled together for the final report, due for completion shortly, following which a decision will be made on how to proceed, says California Department of Transportation (Caltrans), which is working with Los Angeles County Metropolitan Transportation Authority (Metro) on the studies.
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Traffic congestion is the main driver for completion of network. At the moment the existing 710, or Long Beach Freeway runs north and just stops. Vehicles are diverted onto two stub roads which exist to tie-in the missing section of 710, and so bypass the 4.5-mile (7.2-km) long gap that has remained a problem for almost half a century as various concepts have been proposed, evaluated and deferred.
The problem with the previous project proposals – all surface roads – was that each failed to meet regional mobility needs and also satisfy both community and environmental concerns. No-one was happy about plans for a road to plough through their neighbourhoods, no matter how carefully and sensitively it would be engineered. But the traffic problem remains, and grows worse every year.
Tunnel Studies
The Southern California Association of Governments (SCAG) has had the problem listed in its regional transport plans for the last 20 years. Then, about five years ago, Metro took the initiative with Caltrans to explore the merits of completing the missing link underground.
A report was produced for Metro by Parsons Brinckerhoff in 2006 and focused on the north-south running corridor which held slightly different horizontal tunnel alignments. The key aim of the technical feasibility assessment was to determine if a tunnel was physically, environmentally and financially viable, and could deliver the benefits sought for traffic, and whether more detailed examination was recommended.
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By GlobalDataThe study focused on deep tunnelling by either TBM or sequential excavation method (SEM) for most of the route of the twin-tube project. Each bore would hold four lanes, and in the study the TBM-driven outside diameters were estimated at 48ft- 57ft (14.6m-17.4m). SEM drives would be mined to widths of 60ft- 72ft (18.3m-22m). Major ventilation structures would be required to be excavated along the route.
The report concluded that the scheme was both physically and environmentally feasible, and placed the estimated cost at USD 2.3bn-3.6bn (2006 prices). It was estimated that the construction period, from mobilization of project resources to opening of the new tunnel to traffic, could be about nine years for the TBM build methods and 11 years for sequential excavation alternatives.
The most recent push to explore the tunnel option began in mid-2008 when Metro and Caltrans launched a programme to examine five potential routes that fan out from the southern stub road, including the north-running corridor from Los Angeles towards the Foothill Freeway (Interstate-210), in Pasadena, this time called the Zone 3 option.
The approach to the alternatives has been ‘route neutral’, they said, but add that as well as relief of traffic congestion a secondary benefit is to improve air quality.
Site Investigation
With five corridors being examined – Zones 1-5 – the SR 710 Tunnel technical study covers a large area and has involved significant site investigation work, though no significant environmental assessments. The study area encompassed the cities of Alhambra, Glendale, La Canada-Flintridge, Los Angeles, Monterey Park, San Marino, South Pasadena and Pasadena.
The site investigation work was undertaken by Caltrans’ geotechnical services unit in conjunction with a jv of consultants comprising CH2M Hill, Earth Mechanics, Jacobs Associates and ILF.
The range of field works included mainly borehole sampling and tests, and geophysical surveys, for the route of a hypothetical tunnel of 50 FT (15.2m) width and an invert level that runs approximately 200 ft (61m) below the surface, though there is consideration of tunnel excavation taking place at depths of more than 250 ft (76.2m).
The aim of the analyses was to determine geotechnical factors – strata, groundwater and seismicity – that would affect tunnel design and construction, and also means of comparing the data.
Field Exploration
Along the corridor studied previously, in the 2005 study, and named Zone 3 this time, both igneous and sedimentary formations had been found but volcanic rock was unexpectedly found in one borehole, at a shallower depth than expected for a tunnel. The geological structures had bedrock units folded into a series of synclines and anticlines, plus there were a few major faults. There was only limited information on deep groundwater.
In the latest site investigation, carried out in the first half of 2009 with analyses in the following months, the work was carried out again in Zone 3 and the four new zones. All five are set out as trapezoidal-shapes with three (Zones 1-3) having of a similar average length of approximately 5 miles (8 km). The other two are longer; one slightly and the other very much – Zone 4 is almost 7 miles (11.2 km) long on average, and Zone 5, running to the east, is approximately 10.5 miles (16.8 km) long.
To supplement previous site investigation data the team could draw upon, the latest field exploration undertook a further 25 core borings, mostly in Zones 1-3, plus 17 seismic reflection lines and 78 surface wave lines, the geophysical surveys being spread reasonably evenly over the zones. Previous data called upon 286 borings, and in each of the zones these broke down as 74 in Zone 1, 61 in Zone 2, 40 in Zone 3, 34 in Zone 4 and 77 in Zone 5. The least amount of data was available for Zone 3, and hence the site investigation work focused more in that area. Literature reviews show that the five zones straddle five separate groundwater basins. There is no correspondence of zone limits to those of the basins.
The Zones
Zone 1
A tunnel in Zone 1 would be bored mostly through weak to moderately strong formations mainly of sandstone and thin siltstone interbeds but with some other sedimentary strata plus some marine deposits. The potential for methane gas in the sedimentary strata presents concerns. Groundwater may have some toxic contamination due to the presence of a dump (San Fernando Valley Superfund site). Alluvium might be found in the portal areas, and groundwater level within the formation is shallow. There are several inactive faults.
Findings were that the low strength and uniform nature of the formations in this zone suit TBMs, and higher progress rates would be expected, as compared to most of the other zones. Alluvium at the portal areas would require ground treatment and groundwater control. When crossing the Los Angeles River, lower cover to the tunnel could increase the potential for significant groundwater inflows, although the formations in general would not be expected to have much ingress. There is also the concern related to the presence of methane in the formations, as has been experienced with other tunneling projects in the region. A further concern relates to whether groundwater may have some toxic contamination due to the presence of dumps, and so might present a chemical attack risk for the tunnel lining.
Zone 2
This zone would see the bore pass through similar formations to Zone 1, including shale, but older. There is also the possibility of local volcanic intrusions. The potential for methane gas exists, like Zone 1. There is an active fault and several inactive faults. The steeper terrain is expected to mean less possibility of alluvium at the portal areas. Groundwater is shallow in the alluvial valleys.
Similar to Zone 1, there are weak sedimentary strata which are expected to be favourable for efficient TBM excavation. While there are no river or toxic groundwater concerns along this alignment, there is some potential for encountering methane and also there is an active fault to cross – the Raymond Fault. Such a difficult tunnel design has been undertaken previously, at the Hollywood Hills tunnel of the LA metro.
Zone 3
Data shows this zone to be the most geologically varied with formations ranging from weak sedimentary layers, and alluvium, to strong granitic rocks. There is less potential for methane gas. There is an active fault (Raymond), which is also a groundwater barrier. There are also several inactive faults. Groundwater depth varies significantly. As the earlier study also identified, there are folds.
The more varied geology of this zone is considered to present a greater challenge to tunnelling with lower progress rates, and even more specialized approach to driving through alluvium is suggested to be needed. While the potential for methane is less due to proportionally less sedimentary deposits, the route is like Zone 2 as it would have to content with the Raymond Fault but at a deeper location, farther from the portals.
Zone 4
Mainly alluvium (possibly with cobbles and boulders) and unconsolidated soil deposits lie in this zone, although there are some sedimentary formations, like Zone 1. The risk of methane gas is estimated to be the least of Zones 1-4. There are two active faults (Raymond, Alhambra Wash). Depth to groundwater varies a good deal, but most of the tunnel would be below groundwater level. The possibility of toxic contamination of groundwater due to a dump (San Gabriel Valley Superfund sites) is a concern.
Ground stability presents a key risk for tunnelling in this zone. This zone would be expected to see active groundwater control, closed mode drives and the slowest progress rates of the alternative excavations of Zones 1-4 as effort is made to also minimize the risk of ground movement in alluvium, and consequent surface settlement. The methane risk is viewed as the least among the Zones 1-4 alternatives. Like Zone 2, however, there is the challenge of crossing two active faults – the Raymond and the Alhambra Wash, the former being the greater risk.
Zone 5
Neighbouring Zone 4, this zone has similar formations of alluvium or unconsolidated soil deposits. Methane gas risk is similar to Zone 4, and so is much less than the other zones. There is only one active fault (Alhambra Wash) but the risk of toxic contamination of groundwater is viewed as greater due to Superfund sites (San Gabriel Valley). Like Zone 4, most of the tunnel would be below groundwater, the depth of which varies.
Being the longest zone and requiring the longest tunnel, this presents a proportionally greater challenge than for a bore in the similar geology of Zone 4, in terms of ground stability, groundwater control, close mode drives and methane risk. Potential for groundwater contamination is greater, too, due to more Superfund sites in the zone, and this increases the risk of chemical attack over a greater length of tunnel. However, there is only one active fault to be crossed, the Alhambra Wash.
Further Steps
The study concluded that from a geotechnical perspective, based on previous data and new information gathered and analyzed, tunnelling is feasible in a five of the alternative routes.
Following the public consultations and finalizing the geotechnical report, the project investigations for the twin-bore tunnel would progress to studies that will result in an Environmental Impact Report.
At that stage, in addition to environmental assessments, the analyses will also address further design matters, including risk mitigation, seismic design, fault/rupture displacement, cross passages, shafts and ventilation, constructability and, of course, a review of the cost estimates.
