Underground spaces are gaining attention as an alternative solution to population growth and resource scarcity. Utilized since ancient times, underground spaces have expanded with technological advances to include a variety of uses, including housing, industry, and energy, and are playing an important role in future projects such as nuclear waste storage and high-speed transportation.

As the warmth of spring arrived, it was time for science imagination drawings, and while each student struggled to express his or her unique ideas, there was a theme that captured the hearts of many. A decade later, our imaginations are becoming a reality, and the uncharted spaces are becoming an object of curiosity. Now, uncharted spaces are more than just objects of curiosity, they are being recognized as alternative spaces that can overcome the increasingly unfavorable conditions for survival on Earth due to population growth and resource scarcity. In particular, researchers are actively identifying the unique properties of underground rocks and exploring their applications, and the possibility of utilizing underground space is the first to be visualized.
The use of underground space itself has a long history, such as the tradition of burying food in the ground to preserve it for a long time, but it is only recently that the depth of excavation has increased and the possible fields of utilization have expanded significantly as knowledge and technology have accumulated. Let’s take a look at the characteristics of underground spaces and the application of geotechnical engineering through specific examples.

History and current uses of underground spaces

Underground spaces have expanded beyond their role as storage to include residential and industrial uses. In fact, the use of underground space has a very long history. Ancient humans utilized caves to store food, or built underground temples for religious ceremonies. For example, the Derinkuyu Underground City in Turkey is a huge underground city built in ancient times that served as a refuge from war or external threats. It includes housing, food storage, water storage, and more for up to 20,000 people, making it a prime example of how underground space can be utilized in human life.
Today, this use of underground space is becoming more sophisticated. Tokyo has not only subways, but also underground shopping malls and complex cultural facilities. This is being seen as an alternative to addressing rising land prices and overcrowding in urban centers, and the efficient use of underground space is emerging as one of the key strategies for sustainable urban development. Singapore is also continuously expanding its underground space to move infrastructure facilities such as power, telecommunications, and logistics underground. This is seen as a way to address space limitations while maximizing the city’s efficiency.

Use cases for underground space

Underground spaces can be used for a variety of purposes, including residential, industrial, and transportation, and the range of depths used depends on the purpose. Residential and cultural facilities are built in the shallowest levels, within 50 meters, such as the Jovik Olympic Mountain Hall in Norway. The world’s largest underground stadium, it was built nine stories high for the 1994 Winter Olympics. The decision to build the stadium underground was largely driven by the properties of underground spaces that favor maintaining a constant temperature year-round. The heat capacity of the ground is one-fifth to one-tenth that of the surface, which means that heat transfer is slow. At just five meters below the surface, the space is largely unaffected by seasonal changes, resulting in lower energy consumption and lower maintenance costs. However, the depth of construction is limited because natural lighting and ventilation systems must be installed.
The area where the Jovik Olympic Mountain Hole was to be built was a gneiss field. Gneiss is a type of metamorphic rock created by high heat and pressure, making it an ideal base with high strength, but it required a combination of precise measurements and simulations. Samples were taken in the field to measure the rock’s unique properties, and computer modeling was used to predict its behavior. This determined the location and sequence of blasts, and once construction began, pressure sensors monitored the behavior of the rock around the cavity and the ground settlement to ensure that it was within the predicted range. The intensity of the shock waves also had to be controlled in real time, as the process of drilling into the ground with gunpowder generates strong shock waves that can affect the stability of neighboring facilities.

Industrial applications at 500 meters underground

Let’s extend the depth to 500 meters underground. Within this depth, various industrial facilities for sewage treatment, food storage, oil storage, etc. can be built, including the low- and intermediate-level nuclear waste disposal facility being built in the Wolseong area of Gyeongju. Underground spaces are considered to be the best alternative for semi-permanent isolation of nuclear waste that continuously emits radiation. Not only are they safe from external impacts, but they also provide effective radiation protection. Structures in the ground can be supported by the surrounding rock layers, and even when the rock shakes during an earthquake, they move with the ground and suffer less damage than above-ground structures that resist by inertia. In addition, the movement of radioactive materials in the ground is slower than in the atmosphere, and most of them are slowed down or adsorbed while traveling through the rock, making it very unlikely that they will reach the surface.
However, the strata around the Gyeongju site, unlike the Jovik Olympic Mountain Hole, are sedimentary rocks. Sedimentary rocks, formed by the accumulation of minerals transported by water and wind, have low strength due to the many voids between their constituent particles, and repeated external impacts can cause the particles to rearrange themselves in the voids and cause subsidence. Various methods have been tried to overcome the inherent fragility of sedimentary rocks, including artificial curing, which involves injecting an adhesive material at high pressure along the edges of the cavity. The high-pressure injected adhesive fills in the gaps in the rock to stabilize the top of the cavity, and the alternation of blasting and injecting the adhesive makes it possible to drill long tunnels through soft ground.

Expanding underground for the future

The use of underground space could extend to within 1000 meters of the surface for geothermal energy extraction and high-level nuclear waste disposal. But increasing vertical depth is only one aspect. Several projects are currently in the planning stages, including a submarine tunnel to connect small islands in the Southwest Sea to improve island transportation, and a larger international submarine tunnel to reduce passenger and logistics costs and transit times. Other high-profile projects include vacuum tunnel trains that can maximize speed and comfort by minimizing air resistance and rail friction, and lunar outposts in lava tubes to overcome the extreme conditions of the moon. These examples illustrate the endless possibilities for horizontal expansion through rock to the seafloor and outer space.
As technologies and ideas that once existed only in our imagination are being realized, we are no longer just imagining, but constantly pushing the limits. We look forward to seeing how many more opportunities these new areas, including the underground, will provide for human life.