Construction of an Immersed Tube Tunnel (ITT) is a specialized means of tunnel construction for water crossings. This construction method has been in use for more than a century; the first use in the US traced back to 1893 on the construction of the Shirley Gut Siphon sewer main in Boston, Massachusetts, followed in 1910 by the Michigan Central Railway Tunnel under the Detroit River. While conceptually straightforward, ITT design and construction is complex and requires a high level of expertise. This construction technique has recently garnered a higher level of interest from owners, designers and contractors as a viable and preferred option on many high-profile projects, such as the Fehmarn Belt Fixed Link, the Hong Kong-Zhuhai-Macau Fixed Link, and the Parallel Thimble Shoal Project.
Unlike traditional tunnels, ITT’s are ideal for short or limited depth crossings such as rivers, channels and harbor basins. Constructing this type of tunnel is also beneficial in that it can dramatically reduce construction costs, project risks and environmental impact for water crossings. This is due to several factors, such as a significant reduction of the approach to the tunnel portal since the tunnel is positioned on or just below the body-of-water bed; an optimized cross-sectional area of the tunnel when compared to a bored tunnel alternative; its resiliency in unfavorable ground conditions such as the soft alluvial deposits or high seismic regions; and the repetitive nature of casting/fabrication and installation that allows for construction efficiencies. In addition, there will be minimal impact on the navigation channel and the construction time can be significantly reduced.
With many recent large scale ITT projects breaking ground, ITT’s are poised to become a more widely employed technique for delivering tunnels for water-body crossings in the years to come.
Location, Location, Location
Geographically, these tunnels are most suitable in low-lying land, where bridges are likely to interfere with the navigation channels, or when use of a traditional bored tunnel would be an expensive alternative due to difficult geology or required depth.
While the majority of existing ITT’s are found in Northern Europe, particularly in the Netherlands, Belgium, and Denmark, their use in Asia (China and Korea) is rapidly gaining prominence. Currently, a major design-build ITT is under construction to create a link between Hong Kong and mainland China – the Hong Kong-Zhuhai-Macau Fixed Link. This landmark structure is considered China’s most important infrastructure project – both technically and politically – and will include the world’s longest immersed tunnel for road traffic. Following close behind will be the Fehmarnbelt Tunnel linking Denmark and Germany. When complete, Fehmarnbelt will be the longest ITT in the world at 17.6 km.
Mastering the Complexity of ITT’s
The complex construction and installation procedure for an ITT involves the construction of a casting basin; batching of segments; design of an appropriate ballast system; logistics of floating, trimming and winching elements out of the casting basin; digging a trench as a base for the tunnel; sequentially floating pre-cast/pre-fabricated concrete or steel sections over the dredged area; sinking each section precisely into the dredged area; and connecting the segments to form a continuous tunnel.
Only a select few engineering firms worldwide have mastered this challenging engineering practice. With its tunneling pedigree and unmatched experience with most major ITT projects worldwide, COWI, a leading international consulting firm, is the undisputed ITT designer of choice. COWI’s North American arm, COWI Tunnel is a specialist tunneling firm that has been at the forefront of ITT advancements in the US.
As part of the Fort Point Channel section of the Transitway Project in Boston for the Massachusetts Bay Transportation Authority (MBTA), COWI was the first North American company to design a concrete ITT that was to be constructed in the US. COWI conducted a feasibility study for tunneling options, assisted the client in selecting the preferred option (ITT), and provided design and construction engineering services for the ITT portion of the project.
As the first Immersed Tube Tunnel of its kind in the US, the project was an overwhelming success. Nonetheless, it was not without challenges. This included a short 2-hour floatation window due to tidal variation that affected tight clearance concerns for navigating the segments under the existing Northern Avenue Bridge; mitigating the risk of contaminated soil condition on the project; and integrating updated Fire and Life Safety code requirements post final design stage.
COWI also produced and optimized the ITT design and worked collaboratively with the contractor during construction support. This significantly reduced the potential channel closure time of 1½ to 2 years that would have typically been required with a more traditional construction approach.
The Parallel Thimble Shoal Project – An International Collaboration of COWI Capabilities
Fast forward 10 years and COWI worked on the technical requirements for the preliminary design of the Design-Build Chesapeake Bay Bridge and Tunnel (CBBT) Commission’s Parallel Thimble Shoal Project; the fixed link involving bridges and tunnels that will provide the only direct link between Virginia’s Eastern Shore and South Hampton Roads.
The Parallel Thimble Shoal Project is a 20 mile long vehicular crossing of the lower Chesapeake Bay consisting of a series of four lane, low-level trestles and bridges, segmented and connected by two parallel, approximately one-mile long, two-lane tunnels. To provide for the tunnel portals, man-made islands will be designed and constructed, each approximately 5¼-acres in size, positioned at each end of the two tunnels.
As a sub to the Design Manager Jacobs, COWI provided Design-Build bridging documents for the 5,800 ft long immersed tube tunnel, as well as a bored tunnel alternative. The COWI North America team has mobilized internationally from COWI’s Bridge, Tunnel and Marine (BTM) group in Denmark for ITT expertise, as well as resources from COWI Marine North America for marine engineering expertise on the two artificial islands and fishing pier.
Building the World’s Deepest ITT
Worldwide, COWI has been involved in some of the most prestigious and challenging ITT projects. Some of these projects include the aforementioned Hong Kong-Zhuhai-Macau fixed link, the Fehmarnbelt Tunnel, the Busan-Geoje immersed tunnel, the Oresund Tunnel, and the Marieholm immersed tunnel.
The Busan-Geoje fixed link in South Korea includes the world’s deepest immersed tunnel built in open waters for road traffic, and represents a major leap forward in ITT technology. The fixed link reduces travel times between Busan and Geoje Island from 3½ hours to just 40 minutes.
COWI served as lead consultant on the detailed design of both the immersed tunnel and bridge segments of the project.
With its lowest foundation point at 48 m below sea level, and the challenging depth and long swell waves, installation was particularly challenging. Additionally, the location of the fixed link on the Korean south coast, with its vulnerability to the open sea, exposes it to extreme weather conditions such as typhoons and huge swell waves. These waves create a fluctuating pressure around the immersed tunnel during passage, leading to a maximum hydrostatic design pressure of 65 m water depth.
The area, also prone to seismic activity due to the nearby fault zone, created challenges in the design of the tunnel element joints. In order to resolve these complexities, COWI utilized advanced techniques for soil improvement below the immersed tunnel to new depths. A combination of soil replacement, sand compaction piles and cement deep mixing helps stabilize the soil down to 65 m below sea level.
Looking ahead, the tunnel specialists at COWI TUNNEL expect an increase in demand for ITT design capabilities from clients worldwide as Immersed Tube Tunnels are a cost-effective and risk-mitigated alternative to traditional tunneling under water-body crossings. Additionally, as the design of steel tunnels continue to decline, concrete will be the preferred material of choice to minimize costs and ensure 100 year service life.
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