In this blog post, we explore the potential of underground spaces, which are gaining attention not just as traditional storage areas but as strategies for future urban development and survival.
Underground Space: How Far Can It Go?
During art class, when we used to draw scientific illustrations under the warm spring sun, we would each pore over our work to express our unique ideas, but there was always one theme that captured the hearts of many students. It was about new worlds unfolding beneath the sea, deep underground, and in outer space—worlds far removed from the familiar ground we call home. Ten years later, our imagination is becoming a reality. Now, these unknown spaces are gaining attention not merely as objects of curiosity, but as alternative spaces capable of overcoming the increasingly unfavorable conditions for survival on the surface caused by population growth and resource scarcity. In particular, as research actively identifies the unique characteristics of underground bedrock and explores its applications, the potential for utilizing underground space is becoming the first to materialize.
While the use of underground space itself has a long history—much like the tradition of burying food in the ground for long-term preservation—it is only recently that the depth of excavation has increased and the range of potential applications has expanded significantly, thanks to the accumulation of knowledge and technology. Let’s examine the characteristics of underground space and the process by which rock engineering technology is applied through specific examples.
The History and Current State of Underground Space Utilization
Underground spaces are moving beyond their role as mere storage facilities, with their uses expanding to include residential and industrial applications. In fact, the history of underground space utilization is very long. Ancient humans used caves for food storage and built underground temples for religious ceremonies. For example, the Derinkuyu Underground City in Turkey is a massive underground city built in ancient times that served as a refuge from war and external threats. This city includes residential quarters, food storage, and water storage facilities capable of accommodating up to 20,000 people, making it a prime example of how underground space can be utilized in human life.
Today, the use of such underground spaces is becoming increasingly sophisticated. In Tokyo, not only the subway system but also underground shopping malls and mixed-use cultural facilities are located underground. This is gaining attention as an alternative solution to rising land prices and population density issues in city centers, and the efficient use of underground space is emerging as one of the key strategies for sustainable urban development. Furthermore, Singapore is continuously expanding its underground space and making efforts to relocate infrastructure facilities such as power, telecommunications, and logistics underground. This is regarded as a solution that addresses space constraints while maximizing urban efficiency.
Examples of Underground Space Utilization
Underground spaces can accommodate various facilities, including residential, industrial, and transportation/logistics uses, and the depth at which they are utilized varies depending on their purpose. Residential and cultural facilities are typically built in the shallowest layers, within 50 meters below ground, with Norway’s Jovik Olympic Mountain Hall serving as a prime example. As the world’s largest underground arena, it was constructed nine stories high in preparation for the 1994 Winter Olympics. The decision to build the arena underground was largely driven by the favorable characteristics of underground space, which maintains a consistent temperature year-round. The thermal capacity of the ground is only 1/5 to 1/10 that of the surface, resulting in slower heat transfer. Since structures are largely unaffected by seasonal changes once they are buried just 5 meters below the surface, energy consumption is reduced, leading to lower maintenance costs. However, construction depth is limited because natural lighting and ventilation systems must be installed.
The area designated for the construction of the Yovik Olympic Mountain Hall was a gneiss formation. Gneiss is a type of metamorphic rock formed under high heat and pressure, making it an ideal foundation due to its high strength; however, the concurrent use of precise measurements and simulations was essential. Samples were collected on-site to measure the rock’s inherent physical properties, and computer modeling was used to predict the behavior of the bedrock. Based on this, the blasting points and sequence were determined. Once construction began, pressure sensors were used to monitor the rock behavior around the excavation and ensure that ground subsidence remained within the predicted range. Since the process of drilling through the ground with explosives generates strong shock waves that could affect the stability of nearby structures, the intensity of these shock waves also had to be controlled in real time.
Industrial Applications at 500 Meters Below Ground
Let’s expand the depth to 500 meters below ground. At this depth, various industrial facilities can be constructed for purposes such as wastewater treatment, food storage, and oil storage; the low- and intermediate-level nuclear waste disposal facility currently under construction in the Wolseong area of Gyeongju is one such example. Underground spaces are considered the best option for the semi-permanent isolation of nuclear waste that continuously emits radiation. This is because they are not only safe from external impacts but also effectively block radiation. Structures within bedrock can be supported by the surrounding rock layers, and even when the bedrock shakes during an earthquake, they shake along with the ground, sustaining less damage than above-ground structures that resist movement through inertia. Furthermore, the movement of radioactive materials occurs more slowly underground than in the atmosphere, and since most of them are decelerated or absorbed as they pass through the bedrock, the likelihood of them reaching the surface is very low.
However, unlike the Yovik Olympic Mountain Hall, the strata surrounding the Gyeongju Radioactive Waste Repository have the weakness of being composed of sedimentary rock. Sedimentary rock, formed by layers of minerals transported by water and wind, has many voids between its constituent particles, resulting in low strength. If subjected to repeated external impacts, the particles may rearrange within these voids, posing a risk of subsidence. Various construction methods have been attempted to overcome the inherent weakness of sedimentary rock, and artificial hardening—which involves injecting adhesive material at high pressure along the edges of the cavity—is one of them. The high-pressure injection of bonding material fills the voids within the rock, providing stable support for the upper part of the void. By alternating between blasting and injecting bonding material, it becomes possible to excavate long tunnels even within weak ground.
Expanding Underground Space for the Future
The use of underground space can extend to depths of up to 1,000 meters for purposes such as geothermal energy extraction and high-level nuclear waste disposal. However, increasing vertical depth is only one aspect of this expansion. Looking at several projects currently in the planning stage, there are undersea tunnels designed to connect small islands near the southwestern coast to improve island transportation, as well as larger-scale international undersea tunnels that can reduce passenger and logistics costs and transportation times. Additionally, vacuum tunnel trains—which maximize speed and comfort by minimizing air resistance and rail friction—and lunar exploration outposts within lava tubes designed to withstand the Moon’s extreme environment are also garnering attention. These examples demonstrate the boundless potential for horizontal expansion into the seabed and outer space through the medium of bedrock.
As technologies and ideas that once existed only in our imagination are now being realized, we are no longer confined to imagination but are constantly pushing beyond our limits. We look forward to seeing how new frontiers, including underground spaces, will provide even more opportunities for human life in the future.
