In this blog post, we’ll examine the process of building an undersea tunnel and the challenges encountered during construction—such as ground conditions, water pressure, groundwater, and earthquakes—and explore the potential of undersea tunnels as a future solution that transcends the limitations of existing ones.
Is it possible to travel from Incheon to Shanghai without taking a plane? Or could one take a train trip to the United States? While there might be sections accessible by land if the Korean Peninsula were reunified, such scenarios are currently impossible. So, is it possible to cross the ocean via a route other than by ship or air? Undersea tunnels are what turn such seemingly impossible feats into reality. However, the distance from South Korea to the western United States is approximately 10,000 km, and the average depth of the world’s oceans is about 3.7 km. How can we overcome such vast distances and immense water pressure? Human progress and advancements in science and technology have turned what at first glance seemed impossible into reality. Let’s take a step-by-step look at the process of constructing undersea tunnels—which turn the impossible into reality—and the obstacles that must be overcome along the way.
Essentially, an undersea tunnel is a tunnel excavated through the seabed and constructed beneath it. Before excavating the tunnel, it is necessary to first assess the ground conditions at the planned excavation site. Generally, the types and characteristics of the rocks and geological materials that make up the Earth vary by region, so it is crucial to conduct a detailed survey of the geology and rock conditions along the excavation route. To analyze the ground, boreholes are drilled to collect rock and soil samples from deep underground, and the characteristics of each layer are then analyzed. However, conducting such surveys on the seafloor of the open ocean, where water depths reach several kilometers, is no easy task. Consequently, non-destructive exploration techniques—such as those using sound waves and other geophysical survey methods—have recently been employed to determine geological structures without direct access to the ground. This allows for the analysis of the physical properties of rocks and strata, and the structure and geological conditions of the ground can be determined by observing how sound waves are reflected or refracted. However, as seafloor geological conditions become more complex or the survey depth increases, the uncertainty in interpreting the survey results may grow; therefore, during actual construction, both geophysical surveys and core drilling are used in conjunction to verify the characteristics of the ground. Once the geological structure and rock properties of the excavation section are determined through ground investigations, the next step is the actual excavation. As excavation proceeds incrementally, it is essential to continuously verify whether the actual ground conditions match the anticipated geological conditions.
Once the ground investigation is complete, full-scale tunnel excavation begins. The methods for excavating the ground to construct a tunnel can be broadly categorized into blasting and mechanical excavation. While some tunnels, such as the Han River Submerged Tunnel, were constructed using blasting, underwater tunnels require the selection of an excavation method suited to the ground conditions to manage risks such as ground collapse and large-scale water inflow that may occur during the excavation process. In particular, given the water pressure and geological conditions beneath the sea, mechanical excavation is one of the most important construction methods. Recently, large excavation machines called TBMs (Tunnel Boring Machines) have been widely used. A TBM is designed so that the circular cutter head at the front excavates the ground, while equipment at the rear transports or processes the excavated soil, allowing excavation and subsequent operations to proceed continuously. Since excavation speed varies significantly depending on the type of ground, the size of the equipment, and construction conditions, it is difficult to compare the speeds of blasting and TBMs using simple numerical values; however, TBMs have the advantage of allowing excavation, spoil removal, and tunnel support work to proceed continuously. A representative example of a subsea tunnel utilizing a TBM is the Eurotunnel connecting France and the United Kingdom.
Now, let’s examine the potential hazards that can arise during tunnel excavation. One of the greatest hazards is water. At first glance, one might think of seawater, but in undersea tunnels, particular attention must be paid to high-pressure water—a mixture of groundwater and seawater—that flows into the tunnel through cracks or voids in the seabed. Therefore, it is essential to investigate in advance the form and extent of water present within the rock. If cracks or fault zones with a high likelihood of water inflow are discovered ahead of the tunnel face, the process involves first carrying out reinforcement measures—such as grouting—and waterproofing, followed by resuming excavation. This is the only way to prevent accidents where large volumes of water suddenly flood into the excavated space. During the construction of Japan’s Seikan Tunnel, several incidents of large-scale water inflow occurred. In particular, an incident in 1976 involved a massive influx of water into the tunnel, requiring extensive restoration work over a long period. As such, controlling water in undersea tunnels is a critical challenge directly linked to construction safety.
Water inflow through channels and cracks can be reduced or blocked using grouting methods. Grouting is a method that reinforces the ground and blocks the movement of water by injecting reinforcing materials into cracks or voids in the ground. Generally, cement-based materials or chemical grouts are injected at high pressure through boreholes. The injected materials fill cracks and voids within the ground to reinforce it or block the pathways through which water flows. The materials used for grouting include cement-based materials as well as various other grouts selected based on soil conditions. While cement-based materials can provide relatively high strength, they may require time to cure; chemical grouts, such as urethane-based ones, can utilize their properties of rapidly expanding or curing upon reaction with water. Therefore, in actual field applications, the appropriate materials and construction methods are selected by comprehensively considering factors such as rock type, crack size, groundwater volume and pressure, required strength, and construction conditions.
No matter how well waterproofing is performed, cracks may develop over time due to the aging of the waterproofing structure or the effects of unexpected ground deformation and vibrations, leading to water leakage. Consequently, when excavating and constructing a tunnel, drainage facilities must be considered alongside waterproofing. Drainage facilities are installed to collect water infiltrating from the ground along a specific path and safely discharge it. In undersea tunnels, where water entering the tunnel must be continuously managed, equipment such as drainage channels, collection facilities, and pumps is essential. In particular, depending on the tunnel’s location and drainage method, pumping equipment may be necessary to discharge water to the outside. Additionally, separate spaces must be allocated inside the tunnel for drainage, maintenance, and emergency response.
Another variable is undersea faults and seismic activity. For example, if a long-distance tunnel were to be excavated from Jeju Island toward the Pacific Ocean, the possibility of passing through areas with active seismic activity and fault zones must be considered. No matter how robust the ground support materials used, it is impossible to completely eliminate the impact a tunnel will suffer during an earthquake. Therefore, when constructing a subsea tunnel, active faults and areas with high seismic risk should be avoided as much as possible; if passage through such areas is unavoidable, the tunnel must be designed to withstand ground displacement and the forces acting on the structure. One approach currently being researched to address these limitations is the underwater tunnel. The concept of a submerged tunnel involves installing the tunnel structure underwater rather than boring through the seabed; it is being studied as an alternative to avoid sections where deep excavation into the seabed is difficult or where specific seabed geological conditions exist. Submerged tunnels must be designed to maintain a stable position underwater by appropriately balancing the structure’s weight and buoyancy. If the structure’s weight exceeds its buoyancy, it can be supported using pillars or a support structure; if buoyancy is greater, cables or mooring systems can be used to anchor the structure and prevent it from floating away. Concepts have also been proposed to design the structure to achieve an appropriate balance between weight and buoyancy, thereby minimizing the need for mooring systems or separate support structures.
However, there are still many risk factors associated with undersea tunnels that need to be addressed. To construct a safe undersea tunnel, a comprehensive analysis of various external factors—such as waves, currents, hydrostatic pressure, structural vibrations, earthquakes, tsunamis, and ship collisions—is required. In particular, advanced analytical techniques utilizing fluid dynamics and structural mechanics are necessary to precisely predict and control the motion of structures floating or moored underwater. To respond to external impacts such as ship collisions or strong ocean currents, technologies that continuously monitor the position and movement of the undersea tunnel and actively control them when necessary could also be considered. In this context, approaches such as combining thrusters—which generate thrust to adjust the structure’s position or attitude—with mooring systems could be subjects of research. For example, in the event of an earthquake triggering a tsunami, technology is needed to analyze undersea seismic activity and sea-level changes in real time, calculate the expected waves and the structure’s movement, and control the mooring system and safety systems accordingly. However, such active position control methods are not yet considered commercial technologies applied to typical undersea tunnels and remain research challenges to be addressed in the future.
Nevertheless, undersea tunnels—which could address the shortcomings of existing undersea tunnels and pave the way for the future of maritime transportation—have not yet been completed and put into operation as actual transportation infrastructure. Since constructing long-distance tunnels between countries via undersea tunnels involves not only technical challenges but also enormous construction and maintenance costs, as well as geological and seismic risks, some people may still consider traveling around the world via such tunnels a distant prospect. In fact, plans for an undersea tunnel connecting South Korea and China have been discussed on several occasions in the past, but they have not progressed to the concrete construction phase due to issues such as economic feasibility and project viability. However, countries around the world continue to research various technologies to enhance the safety of undersea tunnels and enable long-distance undersea connections, and underwater tunnels are also being studied as one of the future technologies capable of overcoming the limitations of existing undersea tunnels. For underwater tunnels to be practically utilized as a means of transportation across the oceans, numerous challenges must be overcome, including structural safety, construction technology, maintenance, costs, and disaster response. Therefore, while traveling around the world through the ocean won’t become a reality immediately, considering that humanity has solved countless technical problems that once seemed impossible, it is entirely plausible that a new form of maritime route will emerge someday.
