The west coast tunneling sector is full of activity. Demands such as the need for additional commuter rail infrastructure in cities such as Los Angeles, San Francisco, Seattle and Vancouver, highway tunnel improvements, as well as water and wastewater convenience projects continue to fuel the tunneling industry.
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"In highest demand is financing," says Randy Essex, executive vice president, Hatch Mott MacDonald. "Many projects remain at the conceptual stages of development because there is no federal, state, or private funding for construction."
Indeed, there are a few projects in the pipeline that have not yet entered the construction phase. In Vancouver, the Evergreen Line, estimated to cost CAD 1.4bn (USD 1.34bn), will run for 11km connecting Coquitlam to Vancouver via Port Moody and Burnaby, with 2km underground. Construction was originally expected to start in 2011, once regional funding had been set in place. However, the request for proposals (RFP) is now believed to be awarded this year with an estimated construction period of four years.
Difficult ground conditions
It is also the region’s drastically varied terrain that makes many projects uniquely challenging. "The west coast of the US and Canada is a seismically active region," says Essex. "Offshore projects must cross active and potentially active faults. In Vancouver, liquefaction is believed to be able to occur to depths of 197 to 213ft (60 to 65m); the Port Mann Water Tunnel will need to cross beneath these deep liquefaction zones, requiring hyperbaric interventions at up to six bar pressures and 230ft (70m) deep slurry wall shafts."
Seismic areas and seismic stability issues need to be taken into consideration during design. Essex provides examples: "For the Port Mann water main tunnel that will cross the Frazer River in Vancouver, the tunnel alignment has been lowered in order to pass below the potential zone of liquefaction. This will result in an exit shaft in excess of 65m depth, and thus very high groundwater pressures will need to be managed if interventions are required."
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By GlobalDataEssex explains that conditions such as highly plastic clays and claystones, presenting sticky behaviour issues for pressurised face tunneling machines, as well as the presence of methane and hydrogen sulfide gas, continue to present challenges for tunneling projects in the region. "On the New Irvington water tunnel, the occurrence of gas has caused the rating of the project to be increased from potentially gassy to gassy," Essex says. "On the Brightwater tunnels in Seattle, the central contract involved slurry pressure machines that had to engage clays with a high stickiness/clogging potential. The Alaskan Way Viaduct replacement tunnel will also encounter large portions of plastic clays with a high stickiness potential."
In Los Angeles, the relatively young sedimentary deposits include claystones and mudstones, explains Essex: "These weak materials can break down during the mining process and in doing so their moisture content will increase.
"Given the mineralogy of the particles comprising these weak rock materials, sticky behavior can manifest itself as the water content increases."
Essex adds that pressurized face TBMs are giving contractors the opportunity to tunnel through increasingly challenging subsurface conditions. "Despite the sophisticated machinery, these projects carry higher risks associated with the difficult ground and groundwater conditions. We need to revisit how these risks are identified, allocated, and managed. For example, mining through abrasive ground under high groundwater pressures is also becoming more popular."
Environmental considerations
West coast projects must comply with stringent environmental laws. California is one of three states that require private projects to comply with its own environmental law, the 40-year-old California Environmental Quality Act (CEQA). All developers must go through the CEQA process of project environmental impacts and how they will be mitigated.
For the contractors behind the five-mile (8km) Bay Tunnel in San Francisco, the first bored tunnel beneath the bay, the strict environmental laws have been a particular challenge. The alignment passes through environmentally sensitive salt ponds, marshlands and mudflats and the team had to be cautious choreographing the borings so as not to get too close to the wildlife refuge area.
"Environmental constraints will always present challenges to the tunneling industry," says Essex. "The use of biodegradable soil conditioning agents in conjunction with pressurized face tunneling applications is a common requirement. Tunnel construction in congested areas such as downtown LA has had to deal with permit restrictions on noise and diesel exhaust discharge, which has limited the number of operating diesel equipment at any one time. The use of noise mitigation measures such as shaft wall insulating panels is also becoming more commonplace."
Seattle
The Alaskan Way Viaduct replacement tunnel in Seattle is one of the most highlyprofiled projects on the West Coast. The 1.7 mile (2.7km) bored tunnel will have an excavated diameter of 57ft (17.4m), and the cost estimate for the tunnel is USD 1.96bn. Currently, south of downtown construction, mobilization for TBM launch pit construction has begun. "This is the project that has the attention of many," says Essex. "The project will see a double deck highway tunnel under downtown Seattle with a 17.4m diameter EPBM – the largest in the world,"
Meanwhile, other projects in Seattle include the Brightwater Conveyance Tunnel, a 13-mile (21km) conveyance tunnel that is part of the county’s project to build a new regional wastewater treatment plant – Brightwater, and a number of rail transit tunnel projects that will extend Seattle’s Light Rail system to the north and east of downtown. Other projects underway are the University Link, the North Link, and the East Link.
San Francisco
In San Francisco and the Bay area billions of dollars are being spent on infrastructure projects for transportation and water conveyance that are driving a strong demand for tunneling. Programs include California High Speed Rail and the downtown Transbay Terminal.
There is also the San Francisco Central Subway. The San Francisco Municipal Transportation Agency awarded a USD 233M joint contract to SA Healy and Barnard in June 2011 that includes two single-track tunnels of 2.7km, excavated by two TBMs of 6.4m diameter.
There are three underground station contracts yet to bid.
Meanwhile, the San Francsico Public Utilities Commission’s program to upgrade and repair the Hetch Hetchy Regional Water System continues. Totaling 86 construction projects including three new tunnels, this work allows the commission to continue providing drinking water to its 2.5 million customers in the area in the event of an earthquake. The Bay Tunnel has reached an interesting stage – the team has just started to construct the retrieval shaft at the eastern end of the alignment, which is a more restrictive site than the western end.
Los Angeles
"Los Angeles is looking at a number of rail transit programs," says Essex. "Hatch Mott MacDonald is currently completing the reference design documents for the 16km Crenshaw Line which will extend from Wilshire Boulevard to Aviation Boulevard near LAX Airport. About one third of the alignment will be underground, the remainder being at grade and elevated."
There are several other rail transit projects also under development, including the Westside Subway Extension project, which is one of Metropolitan Transportation Authority’s priorities. The nine-mile (14km) extension will include seven new stations and be built primarily underground using TBMs, and the Los Angeles County Metropolitan Transportation Authority estimates it would cost USD 5.1bn if completed by 2022.
In addition, the Regional Connector, a USD 1.25bn program, which is a two mile (3km) fully underground light rail line connecting the Gold, Blue and future Expo Lines through downtown LA. Construction could begin in 2014 and be completed by 2019, again depending on funding.
A potential highway project, the 710 Freeway Tunnels, is also being studied. At the moment the existing 710, or Long Beach Freeway runs north and just stops. A tunnel option for the 4.5-mile (7km) ‘missing link’ of the highway is being discussed to address traffic relief.
