In the past decade there have been over 300 significant tunnel projects in North America representing over 800km of tunnel and $10bn in construction value. To provide some shape to the review Lee Abramson divided up the projects according to their end use.

The first group considered was highway tunnels. There have been 10 recent major projects in North America, most of them conventional drill and blast rock tunnels, plus two immersed tubes and two soft ground drives.

First was the H-3 Interstate project in Hawaii, where the twin H-3 tunnels are 15m wide, 12m high and 1.6km long. They were constructed using the “North American tunnelling method” through weathered and unweathered basalt, with shotcrete and rockbolt support. An extensive test programme was carried out on various types of support including Swellex, split sets and grouted anchors, with Swellex eventually chosen for the drives. A full membrane was installed around the tunnels because of concerns about contaminants from traffic entering the Honolulu water supply.

The I-90 Mount Baker Ridge project represents probably the largest “stacked drift” tunnel ever constructed. The 25m diameter soft ground tunnel passes through hard glacial soils. After driving the portals through deposits prone to landslides, the tunnel perimeter was formed from 24, 4m diameter, overlapping drifts excavated with a shield and lined with precast concrete. These were then filled with concrete before the central core of the tunnel was started.

On the other coast is the famous – or infamous – Boston Central Artery Project. About half this scheme is underground and is described as removing 13M.m¬≥ of soil and placing 4M.m³ of concrete, at a cost between $15bn and $20bn.

One section of highway passes under several commuter rail tracks, and tunnel jacking was the construction method selected. Each box to be jacked is about 100m long, 25m wide and 12m high. To assist construction in the Boston Blue Clay, jet grouting was carried out below the boxes, and the soil to the sides was frozen. One of the many innovations on the project is the drilling of relief holes in the frozen ground to relieve side pressures on the boxes and shoring.

Some 20,000 freeze pipes were installed, and these were then excavated, along with a number of timber piles and other obstructions, by roadheaders within the box.

The final highway tunnel described was the Downtown Seattle bus tunnel. Twin 2km tunnels were shield driven through glacial soils which contained over 500 tiebacks from previous adjacent basement construction. These tunnels are used for buses but are to be converted to be part of the Seattle subway.

Rail and transit

Over the last two decades there have been over a dozen major rail and transit programmes in North America. The Washington DC metro project has spanned this period, using almost every type of tunnelling method, including 35km of cut-and-cover, 23km of soft ground EPB and NATM tunnel and 25km of rock tunnel. Washington pioneered disclosure of geotechnical data to the contractor, risk sharing and the use of geotechnical baseline reports.

The Atlanta metro included Peachtree Centre Station, which was excavated in hard gneiss with very little rock support. The rock was so good that it was left exposed in the side walls.

A more current project is the Los Angeles metro extension between Hollywood and the Santa Monica mountains. This consists of twin tunnels,10km long, with three cut-and-cover stations. A particular feature of the project is the section across the active Hollywood fault. This fault may move up to 2m in a magnitude 6.5 earthquake, and a special wide section of tunnel was constructed over a 700m length to allow for this.

For the most recent section of the Toronto subway, the owner procured the two EPB TBMs and one-pass concrete lining segments to save time on the programme. The tunnels are 4.4km long through glacial soils. TBM performance was monitored extensively to document productivity. This is one of the first and best documented projects using foam in North America, with 10% foam required to protect the machines, provide face support and suitable muck discharge.

Mount MacDonald tunnel in Rogers Pass Canada is the longest railroad tunnel in North America. Located in Glacier National Park, the tunnel is 14km long and was constructed by two contractors. One used drill and blast for the 9m by 6m tunnel, while the other drove the top with a TBM and then took out the invert with drill and blast. For the winner was an extra 1,000m of tunnel. Unfortunately the TBM had bearing problems and ended up the loser.

The largest rail tunnel in North America is the St Clair river tunnel, 2,000m long and 10m diameter. The size was needed to accommodate double decker container freight cars which are becoming increasingly popular in the US. The tunnel was constructed using an earth pressure balance TBM through extremely soft clay. The machine passed successfully under a sensitive petrochemical plant using hydrofracture grouting to compensate for ground settlement; but just before crossing under the river it was realised that the bearing system lubrication had become contaminated. An emergency shaft was put down to repair the machine.

Major water projects

Turning to water and wastewater, there have been over 30 major schemes carried out recently. The 3km Lake Merced tunnel in San Francisco was excavated in dune sands next to the Pacific Ocean, very close to the San Andreas Fault. This was the first case in North America of a lining being designed for seismic effects by including the interaction between the initial precast lining and the cast-in-place lining.

The longest outfall tunnel in the US is the 15km, 8m diameter Boston outfall tunnel. This was excavated with a rock TBM and one-pass lining, connecting to 55 predrilled riser shafts at the end.

The San Diego south outfall tunnel is nearly 6km long and 3.3m diameter. A 12m diameter, 70m deep, access shaft was excavated through frozen sands adjacent to the Pacific Ocean. At 27m down the ground was found not to be frozen and the system had to be converted from brine to liquid nitrogen to achieve the required result. Despite this particular problem, freezing, which might have been considered “old fashioned” still has some significant uses on particular projects.

The tunnel from the shaft was excavated through sands and gravels under the ocean with an EPB machine using foam injection. Despite a groundwater pressure head of 70m the tunnel was completely dry during construction.

Another tunnel in California that is not so dry is the Inland Feeder project. This aqueduct includes 30km of 5.5m diameter tunnel, mainly between the San Bernardino mountains and the Colorado river. The excavation is through igneous and metamorphic rock, which contains numerous faults and shear zones. Of the three sections, the southern section is proceeding smoothly, but a second section has been stopped for redesign because construction has lowered the water table by about 70m, leaving a Native American Indian Reservation without adequate water supply.

The Denny Way/Lake Union combined sewer overflow (CSO) control project in Seattle includes the Mercer Street tunnel. This is 2,000m long and 5m diameter through glacial soils below the water table. Running sands warranted the use of an EPB machine, but there was also the likelihood of large boulders, which can be difficult to remove from the front of this type of machine. Therefore the risk analysis for the design included a Monte Carlo simulation to estimate levels and costs of potential risks. This concluded that the EPBM would still minimise risks, despite the potential boulder problem.

Special uses

As well as these main categories, tunnels have also been constructed for scientific and military use, and the storage of nuclear waste. The Super Collider project in Texas was a 4.2m diameter tunnel in shale and chalk with a planned length of 100km. The scheme was unfortunately cancelled with about half the tunnel constructed.

The Yucca mountain project in Nevada will be used to store nuclear waste underground. So far, about 10km of 5m diameter tunnel has been built through volcanic tuff. Extensive tests are being carried out prior to waste placement in the next 10 years.

Another major area is the rehabilitation and upgrading of existing tunnels. The Chambers Creek sewer tunnel near Seattle is less than 15 years old but has been severely damaged by hydrogen sulphide. Before repairs could be carried out, several new access shafts were required. As the as-built survey records were not sufficiently accurate, geophysical and tomography methods were used to locate the tunnel underground in critical areas.

The Whittier tunnel in Alaska was a 4km rail tunnel which has been converted to dual rail and road use to provide better access to the town of Whittier. It is now the longest road tunnel in North America. The design and build contract included the installation of precast invert units with embedded rails, so that rail traffic was uninterrupted during construction. As well as excavating vehicle turnouts extensive work was carried out on the electrical, mechanical and control systems for the tunnel.

Future projects

Over 20 major projects are coming up in the US. Among them is the East Side access project in New York which will connect Grand Central Station to new facilities in Queens and Long Island. Caverns about 25m high will be constructed in Manhattan schist to provide multiple cross connections for the rail tracks.

For the West Coast is a scheme to provide a high speed rail connection, possibly by Maglev, between Los Angeles and Bakersfield across the Tehachapi mountains. The 10m diameter tunnels pass through a large range of rock conditions including crossing the Garlock Fault which is predicted to be capable of 9m horizontal and 1m vertical rupture.

In the past 10 years the industry has much technical progress with exploration tools such as borehole cameras and tomography, new methods such as EPBMs, NATM, microtunnelling, and with one pass linings, shotcrete and ground improvement.

There has been significant progress also on contract mechanisms including geotechnical baseline reports, differing site condition clauses, dispute review boards and escrow bid documents. More recently in the US, design and build is becoming popular.

In North America the following areas still need work: reasonable environmental mitigation; standard tunnel design documents; fair low bid contracts; and more enlightened design and build contracts.

Related Files
Figure 1 – Train tunnels and station diagram
Figure 2 – East Side access section