Through It All

Jan. 1, 2006

Numerous tunnels have been constructed for the interstate highway system over the past 50 years. The U.S. has approximately 72 miles of road tunnels. About 29 miles of these tunnels are along the interstate highway system.

Numerous tunnels have been constructed for the interstate highway system over the past 50 years. The U.S. has approximately 72 miles of road tunnels. About 29 miles of these tunnels are along the interstate highway system.

The tunnels along the U.S. interstate highway system have been constructed over many years utilizing a variety of construction methods, equipment and contract mechanisms. Construction methods varied primarily according to the subsurface geologic conditions encountered, current tunneling methods and economic considerations. The majority of tunnels were constructed using the design-bid-build contracting methods. However, some were constructed using design-build methods, especially in the recent past. These tunnels were most often required to go under a number of natural obstacles, including rivers and mountains. Occasionally, depressing highways underground in congested urban areas has presented additional opportunities for surface reclamation or air rights developments.

At least half of the roadway tunnels in the U.S. are not on interstate highways. But it is important to note that most if not all roadway tunnels are constructed using the same methods and are designed to the same standards as interstate highway tunnels. There are approximately 33 tunnels constructed on the interstate highway system. The interstates that have tunnels include I-10; I-64; I-70; I-71; I-76; I-77; I-78; I-90; I-94; I-95; I-264; I-279; I-376; I-478; I-495; I-540; I-579; I-664; I-895; and H-3.

Tunnel types

Tunnels are constructed through soils (soft ground) and bedrock (rock). Typically, soft-ground tunnels used to be constructed underneath rivers using compressed air-mined tunneling methods like what was used for New York City’s vehicular crossings of the Hudson River. These methods took long construction times and were fairly hazardous to the workers. More modern methods for subaqueous crossings include immersed tube tunneling and earth pressure balance tunnel boring machines and precast concrete segmental lining methods. A very innovative multi-drift method was used for the Mount Baker Ridge Tunnel on I-90 in Washington state.

Cut-and-cover tunneling methods are often used for tunnels in soil as well. Often these methods are used in congested urban environments requiring pre-installation of the sidewalls (slurry walls) and roadway decking prior to underground excavation. Rock tunneling methods generally include drill-and-blast methods using explosives and rock reinforcement methods (rock bolts, straps, mesh, shotcrete, etc.) for initial support. Mechanized excavation methods of rock also exist, including the use of roadheaders and tunnel boring machines when geologic conditions permit and size/shape considerations are met. A final concrete lining with membrane waterproofing is commonly used after the excavation has been made.

The Interstate Route H-3 Project in Hawaii consisted of a 10-mile-long segment of new highway that traverses the Koolau mountain range on the island of Oahu. The project included tunneling through this mountain range as well as several long viaducts, hillside road cuts and a short cut-and-cover tunnel through pristine tropical conditions that had historical religious significance to local natives. The 1-mile-long twin-bore Trans-Koolau Tunnel has two lanes in each direction and a roadway width of 38 ft including shoulders.

The Central Artery project in Boston cured massive traffic congestion in the heart of the historic city by replacing the elevated Central Artery section (opened in the 1950s) with a new 8- to 10-lane highway running mainly underground. The I-90/I-93 interchange links the Central Artery to the Ted Williams immersed tube tunnel leading to Logan Airport. This multilevel interstate interchange is one of the most complex sections of the project and involves three crossings beneath eight active railway tracks providing Amtrak and MBTA service into South Station. Jacked tunnels form the deepest section of a multilevel interstate interchange that also includes an open boat section, at-grade roadways, several levels of viaduct that tie into the adjacent roadways and immersed tube tunnels.

Early design concepts would have made interruption of rail traffic inevitable. Alternatively, a tunnel-jacking solution was developed that allowed construction without interrupting train service, saving millions of dollars of railway operating revenues. This part of the project included three jacked tunnel sections, three jacking pits where the to-be-jacked tunnels were constructed onsite and jacked from, and 1,200 ft of cut-and-cover tunnels as approaches to the jacked tunnel segments.

The jacked tunnels are the largest and most complex ever constructed in the world. The largest of the three was 79 ft wide by 36 ft high by 370 ft long. The deepest tunnel section had 24 ft of cover. Combinations of dewatering and ground treatment methods including ground freezing and jet grouting were used to control and minimize settlement during excavation. Soil conditions consisted mostly of reclaimed land, Boston Blue Clay, thin layers of fine sand and glacial till overlying Cambridge Argillite bedrock.

Immersed tube tunneling consists of constructing huge steel and/or concrete sections that can be fabricated away from the project site, floated to the location by tugboats and then sunk into place by the addition of ballast. A trench is pre-excavated in the waterway bottom along the tunnel alignment and is backfilled after the tunnel segments are in place. After the tunnel segments are placed next to each other and sealed, the tubes are dewatered and outfitted for service. Notable U.S. immersed tube highway tunnel projects in the past 50 years include the I-64 Hampton Roads, I-95 Fort McHenry and the I-90/93 Central Artery tunnels.

Cut-and-cover tunnels are often built within an open-cut excavation. If sufficient room exists and soil and groundwater conditions permit, the excavation can be laid back to stable slopes and the reinforced concrete tunnel structure built from the base of the excavation up and then backfilled. If ground conditions or available space do not permit a sloped excavation, then some type of excavation support and bracing must be used. The excavation support may include soldier piles and lagging, steel sheet piles, shotcrete or slurry walls. The bracing may include struts, soil nails or tiebacks.

Ground improvement methods such as deep soil mixing can be used to enhance the strength of the ground so it can help support itself. When this type of construction is used in congested urban environments, decking is usually installed that rests on pre-installed shoring so that traffic is not impeded during excavation underneath the decking.

First of all

Most of the tunnels described were constructed using the common design-bid-build contracting practices. However, some projects such as the Whittier Access (Anton Anderson Memorial) Tunnel used design-build as the contracting mechanism quite successfully. The Whittier Access Project Tunnel Segment is the conversion of a 2.5-mile single-track, hard-rock rail tunnel to a dual use rail/highway tunnel, forming part of the highway connecting the port of Whittier to the Seward Highway and the commercial heart of Alaska. Although the tunnel is not along an interstate highway, it is the longest road tunnel in North America. Its dual function is accomplished with a precast concrete roadway surface with integral rails and a sophisticated traffic control system to both control the single-lane highway traffic flow direction and enable safe operation of train traffic.

This landmark project boasts several other firsts. It is the first combined-use rail/highway tunnel in North America, the first tunnel to use a ventilation system that combines jet and portal fans in the U.S. and the first tunnel designed to operate in temperatures down to -40°F, winds up to 150 mph and portal buildings able to withstand avalanches.

Challenges on the project included designing a surface that would accommodate both automobiles and trains, rock excavation enlargement of the tunnel constructed in 1941-42, design and construction of the portal structures able to withstand avalanches and “A-frame” to split snow slides, a unique two-tiered drainage system, and systems to assure all vehicular traffic has cleared the tunnel prior to allowing opposite direction traffic and/or train traffic.

There have been several major interstate highway tunnel construction projects over the past 50 years in the U.S. In addition to interstate highway tunnel projects, several other tunnels have been built along other non-interstate roadways using similar construction methods and contracting practices. Many of the tunnels were required to carry traffic across waterways or through mountains. Others were constructed to provide grade separation with intersecting transportation routes, replacement of old and decrepit viaducts or to reclaim surface space at grade. Significant strides have been made to make this type of facility both economical and safe. It is expected that these trends will continue for a long time.

About The Author: Abramson is executive vice president and eastern regional manager for Hatch Mott MacDonald, Millburn, N.J.

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