What is the process of constructing an underwater tunnel?

Despite what comic book villains or subterranean creatures might suggest, constructing an underwater tunnel involves more than just a massive machine.

Throughout most of history, we’ve relied on cleverness to dig tunnels. From the earliest cave dwellers creating extra living space to the advanced techniques used by the ancient Greeks to irrigate and drain farmland, the basics of digging, supporting, and progressing were well-established [sources: Lane; Browne].

Underwater tunnels have a long history as well. Around 2180 to 2160 BCE, the Babylonians created one of the earliest known underwater tunnels by diverting the Euphrates River. This 3,000-foot (900-meter) brick-lined and arch-supported tunnel, measuring 12 feet high by 15 feet wide (4 meters by 5 meters), allowed pedestrians and chariots to travel between the royal palace and the temple [sources: Lane; Browne].

For centuries, tunnels were mainly used by miners and medieval sappers, who tunneled beneath castle walls to bring them down (which is why we use the term "undermine"). However, the introduction of canal transport—and later railroads—provided workers with new challenges. The 18th, 19th, and 20th centuries saw a series of progressively harder tunnel projects, made possible by significant advances in surveying and ventilation. Despite these advances, the danger and cost of underwater tunneling kept projects at bay until the mid-1800s [source: Lane].

This brings up an important question: If underwater tunneling involves risks that could literally or financially ruin you, why do it at all? Many city planners agree, using tunnels only when overcrowded bridges are at full capacity. However, bridges present their own set of issues. They disrupt shipping traffic, occupy valuable riverfront space, and block scenic views. From a defense perspective, bridges are vulnerable to airstrikes and could pose serious risks if they collapse [source: Hewett].

In contrast, tunnels can endure tides, currents, and storms better than bridges, span longer distances, and support almost unlimited weight. Additionally, as tunnels lengthen, their cost per unit decreases, whereas the opposite is true for bridges. While tunnels require a larger upfront investment, bridges end up being more expensive in terms of maintenance [sources: Everglades Economics; Hewett].

However, let’s avoid becoming too narrow-minded. There’s no denying that tunnels—whether beneath land or water—face unique security challenges and safety concerns. Fires and accidents are serious threats, which is why rail tunnels include crossover passages to allow trains to switch tracks, along with service tunnels that can serve as emergency escape routes [sources: Chan; JR-Hokkaido; WGBH].

Though they might seem intimidating, underwater tunnels have become so commonplace that we seldom consider the incredible dangers and complex construction methods required to create these modern marvels.

Bridge (Under) Troubled Waters

When diving into any remarkable construction project, several questions swiftly emerge: Which undertaking is the largest, deepest, or most perilous to complete? For underwater tunnels, these questions resist easy answers. With cities and countries continuously launching new ventures, the critical facts are often hidden in the depths of intricate details and the expansive sea.

Take, for instance, the Seikan Tunnel that links Japan's Honshu and Hokkaido islands. This tunnel holds the record for being both the longest and deepest underwater rail tunnel. The planning began after a tragic 1954 typhoon that sank five ferries in the perilous Tsugaru Strait, claiming 1,430 lives [sources: WGBH].

Opened in 1988, the Seikan Tunnel stretches over 3 miles (54 kilometers) and reaches a depth of 787 feet (240 meters). Its 14.5-mile (23.3-kilometer) underwater section, however, is overshadowed by the Channel Tunnel (Chunnel) between the United Kingdom and France. Completed in 1994, the Chunnel’s underwater segment accounts for 24 out of its 31 miles (38.6 of 50 kilometers), but it only dips to 246 feet (75 meters) deep [sources: ASCE; Chan; Wise].

As far as the Turks are concerned, both tunnels pale in comparison to their $3.3-billion Marmaray Tunnel, which opened to the public in 2013. Spanning 8.25 miles (13.2 kilometers) of rail track -- including a 4,600-foot (1,400-meter) section across the Bosporus seafloor -- it links Istanbul's Asian and European sides, marking the first rail tunnel to connect two continents [sources: Sweeney; Wise].

So what sets a sub-mile undersea tunnel apart from the multi-mile Seikan and Channel tunnels? It's a matter of approach: Unlike its predecessors, which drilled and blasted their way through solid rock, the Marmaray Tunnel was constructed piece by piece in a trench along the Bosporus' floor, making it the longest and deepest immersion tunnel ever built. Engineers opted for this method, using preassembled sections joined by durable, flexible, rubber-reinforced steel plates, to better withstand regional seismic activity [sources: JR-Hokkaido; Sweeney; Wise].

For a while, cultural and historical artifacts discovered throughout Istanbul's old city delayed the progress of the Marmaray Tunnel excavation, allowing the Øresund Tunnel, connecting Sweden and Denmark, to remain the largest immersed-tube tunnel ever constructed. Contractors built it from 20 segments, each 577 feet (176 meters) long, which were made from eight smaller, 72-foot (22-meter) sections [sources: Landler; Marmaray Project; PERI GmbH; Sweeney].

With tunnels like Marmaray and Øresund that employ immersion techniques, and bored tunnels like the Chunnel, we've explored quite a bit of tunneling territory. But let's go even deeper into the subject and explore another method of tunneling that has been in use since the early 19th century.

Gigantic Shipworms

The most ancient technique for underwater tunneling that avoids disturbing the water above is called a tunneling shield, a method still in use by engineers today.

Shields offer a solution to a frequent yet challenging issue: how to create a long tunnel through soft earth, especially under water, without causing the leading edge to collapse [sources: Assignment Discovery; Encyclopaedia Britannica; Browne; Hewitt].

To understand how a shield functions, imagine a coffee tin with no lid, but a sharpened bottom featuring several large holes. Grasp the open end and push it, bottom-first, into soft earth, observing how the soil squeezes out through the holes. On a larger scale, several workers (known as 'muckers' and 'sandhogs') would be positioned inside the 'can' to remove the soil as the shield moves forward. Hydraulic jacks would slowly push the shield ahead, while a team behind it installs metal support rings, which are then lined with concrete or masonry [sources: Assignment Discovery]; Encyclopaedia Britannica; Browne].

To prevent water from seeping through tunnel walls, compressed air is sometimes applied to the front of the tunnel or shield. Workers, who can only endure such conditions for brief periods, must pass through multiple airlocks and take steps to avoid pressure-related illnesses [sources: Hewitt; Port Authority].

Shields remain in use, particularly when laying down utility pipes or water and sewage lines. While labor-intensive, their cost is a fraction of the massive tunnel boring machines (TBMs) [sources: Assignment Discovery; Encyclopaedia Britannica; WGBH].

A TBM is a towering machine capable of cutting through solid rock. At its forefront spins a cutting head, a large wheel outfitted with rock-crushing disks and a system of scoops to lift and transfer crushed rock to an outbound conveyor. Following the cutting head is an erector, a rotating system that builds the tunnel’s lining as the TBM progresses. In some large-scale projects, such as the Chunnel, separate TBMs are launched from opposite ends, drilling toward each other while employing advanced surveying to stay on track [sources: Assignment Discovery; Coleman et al.; WGBH].

Drilling through solid rock creates a mostly self-supporting tunnel, and TBMs advance rapidly and persistently (some Chunnel machines could bore 250 feet, or 76 meters, per day). However, TBMs are more prone to breakdowns than a second-hand Jaguar, and they struggle with worn, sheared, or highly fractured rock, which means they aren't as fast as some claim [sources: WGBH; WGBH].

Fortunately, TBMs and shields are not the only options available for tunneling.

Letting It Sink In

Building a steel-and-masonry support structure while simultaneously digging through soft earth or solid rock is a challenging task, but trying to hold back the sea underwater is beyond even the capabilities of Moses. Thankfully, American engineer W.J. Wilgus made this feat possible with his invention of the sunken- or immersed-tube tunnel (ITT) [source: Lane].

Unlike traditional tunnels bored through rock or soil, ITTs are constructed on-site using massive prefabricated sections the size of football fields. Wilgus first used this method when building the Detroit River railroad tunnel (1906-10), connecting Detroit, Michigan, to Windsor, Ontario. Since then, it has become the preferred method for building vehicle tunnels, with over 100 such tunnels completed in the 20th century alone [sources: Lane; Extreme Engineering; Marmaray Project].

To construct each tunnel segment, workers assemble 30,000 tons of steel and concrete—equivalent to the weight of a 10-story apartment building—within a large mold. The concrete is then allowed to cure for nearly a month. These molds form the tunnel's floor, walls, and ceiling and are sealed at the ends to maintain watertightness as they are transported by sea. Immersion pontoons, large ships resembling a combination of a gantry crane and a pontoon boat, handle the transportation [sources: Lane; Extreme Engineering; Marmaray Project].

Once the pre-dug sea trench is prepared, each tunnel segment is submerged to a level that allows it to sink. A crane carefully lowers the section into place, with divers guiding it precisely to its designated GPS coordinates. As the new section connects with the previous one, a large rubber piece at its end expands to form a tight seal. The bulkhead seals are then removed, and the remaining water is pumped out. After the tunnel is fully constructed, it is covered with backfill and potentially protected with rock armor [sources: Lane; Extreme Engineering; Marmaray Project].

Immersed-tube construction can reach deeper than other tunneling methods, as it does not rely on compressed air to keep the water at bay. This allows crews to work longer hours under more comfortable conditions. Additionally, an ITT can take on various shapes, unlike a bored tunnel, which follows the specific design of its shield or TBM. However, since ITTs are only used for the underwater sections of tunnels, they still require other methods to bore the land-based entrance and exit points [sources: Lane; Marmaray Project; WGBH]. In underwater tunneling, variety is key, much like in life.

The Longest Underwater Tunnel

The competition for the title of the longest underwater tunnel has been intense, but Norway is determined to settle the matter once and for all with the Norwegian Coastal Highway Tunnel [source: TunnelBuilder].

This massive vehicular tunnel is projected to be completed in the 2030s and is set to revolutionize underwater tunneling standards. With Norway’s rugged coastline and deep fjords presenting connectivity challenges, the Coastal Highway Tunnel will replace ferry services and bridge physical gaps, providing a continuous route along the western coast of the country.

Planned to stretch over 27 kilometers, this tunnel will break existing records by descending beneath the Norwegian Sea to a remarkable depth of about 390 meters below sea level, standing as the ultimate achievement in the field of underwater tunnel engineering.

The development of this immense structure involved unprecedented creativity. In contrast to conventional bored or immersed-tube tunnels, Norwegian engineers are introducing submerged floating tunnels, an innovative first of its kind in tunnel construction.

The floating tunnels are composed of tube segments that remain suspended underwater, supported by pontoons and securely anchored to the seabed. This novel technique offers an inventive solution to the obstacles of deep-sea tunneling, bypassing the challenge of drilling through dense subsea bedrock while enduring the immense pressure and harsh environmental conditions of the Norwegian Sea.

Should everything proceed as anticipated, the Coastal Highway Tunnel could ultimately become the longest tunnel in the world. Its completion would herald a new era for Norway and the field of civil engineering, standing as a quiet and resilient monument beneath the sea—a submerged triumph over the turbulent ocean.