Kansai International Airport (KIX) is the primary international airport serving the city of Osaka in Japan. The airport opened on September 4th, 1994 to relieve increasing congestion at the existing Osaka Itami Airport (ITM). Although Kansai is located approximately 37 kilometers (22 miles) farther from the city, it handles a majority of the city's international traffic, while Itami now serves as the region's primary domestic airport. However, geological factors are increasingly putting Kansai Airport at risk of sinking, an unusual scenario related to how the airport was built.
Why KIX was Built
Discussions regarding the construction of a second airport in Osaka began in the 1960s. At the time, operations at Itami Airport were severely limited due to its proximity to the city. Noise from arriving and departing aircraft could be heard from the city center, resulting in a nighttime curfew that was imposed, which limited the hours that the airport could operate.
In an ideal world, one way to counteract limited airport operating hours would be to expand the airport so that it could serve more flights. Unfortunately, this was not an option for Itami Airport. Due to the airport's location, it was surrounded by residential development on all sides. Thus, there was no way to properly expand it such that its congestion issues could be resolved. With air traffic volumes continuing to increase in Japan — both on domestic and international fronts — Osaka needed a newer and larger airport to properly handle its growing demand.
Built on an Artificial Island
Construction on the new Kansai International Airport began in 1987. Due to space limitations, the airport was designed to be built entirely offshore on an artificial island measuring 4 kilometers long and 1.3 kilometers wide (2.5 miles long and 0.8 miles wide). The airport would be situated in Osaka Bay, which would offer passengers easy access to the city while providing ample space to accommodate busy airport operations.
However, the proposed location in Osaka Bay posed an issue. Specifically, the seabed in the Bay was composed of Holocene and Pleistocene clay. This form of Holocene clay, found abundantly in Osaka Bay, is a highly compressible, water-saturated soil that poses major engineering challenges. This complicates the airport's construction, as the Holocene clay would not be able to support the load of an airport, and the island would inevitably sink. What's more, this layer of Holocene clay extended as deep as 20 meters (65 feet) in some parts, making it difficult to remove.
Rather than working around the Holocene clay, engineers decided to work with it. Engineers decided to use a technique called "preloading," where extra weight would be temporarily placed on the island during its construction to accelerate the clay's settlement before airport structures were built. This way, by the time the airport structures were constructed, the clay would have already compressed, eliminating the risk of the island sinking. Sand drains were inserted vertically into the clay to expedite this settlement process by creating escape routes for pore water pressure.
Making the Airport Earthquake-Proof
Given Japan's susceptibility to disastrous events like earthquakes and tsunamis, Kansai Airport's offshore location made it especially vulnerable. This was a primary consideration in the airport’s construction. The seawall surrounding the airport was completed in 1989, two years after construction began, and was properly reinforced to withstand the effects of earthquakes and tsunamis. Specifically, rock quarried from nearby mountains and 48,000 tetrapods were used in the fortification. Tetrapods are concrete blocks that are designed to dissipate wave energy and the effects of erosion, making them perfect for airport seawalls. In total, it took approximately ten million man-hours to complete the layers of earth used both outside and inside the sea wall.
Kansai Airport's engineers also chose to use flexible asphalt in its construction. This type of pavement is built in layers, which allows it to flex and bend. In the context of KIX airport, the usage of flexible asphalt would enable it to absorb the shifts in the ground during an earthquake without cracking.
In 1990, a 3.7-kilometer (2.3-mile) bridge was completed to connect the airport to the mainland. This bridge was also designed to withstand the impact of seismic events. Bridges are normally built with such events in mind, with designs that help to distribute seismic energy throughout the structure. This thereby limits the effects of cracking on the bridge. The usage of these anti-seismic construction methods has proven effective, as seen in the 1995 Kobe earthquake, where Kansai Airport sustained only minor damage.
The Airport Opened in 1994
When construction was complete in 1994, Kansai Airport was touted as one of the most unique airports in the world. The measures and techniques used in Kansai Airport's construction were adapted and improved for other similar airport projects. Hong Kong’s Chek Lap Kok Airport is another prime example of a successful airport construction project built entirely offshore on artificial islands.
Today, Kansai straddles two artificial islands, the first of which covers around 510 hectares (1250 acres), and the second of which covers 545 hectares (1350 acres). In total, the airport covers a total of 1,055 hectares (2,600 acres). The completion of Kansai Airport's islands increased the total area of Osaka Prefecture so much that it was no longer the smallest prefecture in Japan. Today, Kagawa Prefecture is Japan's smallest.
The airport consists of two terminals. Terminal 1 was designed by architects Renzo Piano and Noriaki Okabe. The terminal building itself is an architectural feat, spanning 1.7 kilometers (1.1 miles), making it the longest straight-line airport terminal in the world. Its roof is designed to be shaped like an airfoil, which helps to promote air circulation throughout the building. This is the primary terminal for Kansai and, given its length, houses an Automated Guideway Transit (AGT) train system to transport passengers throughout.
Terminal 2 is used entirely by low-cost airlines — namely Peach, Spring Airlines, and Jeju Air. This terminal has a less sophisticated design than Terminal 1. While Kansai is not specifically described as a passenger hub for any major airline, it is a major cargo hub for FedEx Express, which uses KIX as a North-Pacific hub.
Operationally, Kansai Airport is the 30th-busiest airport in Asia and the third-busiest in Japan. In 2019, it served more than 31 million passengers with nonstop links as far as Dubai, Toronto, Paris, and Sydney. The airport has received numerous accolades, including Skytrax awards for Best Airport Staff in Asia, World's Best Airport Staff, and World's Best Airport for Baggage Delivery.
KIX is Sinking
A primary concern for Kansai Airport — one that was taken into consideration during its construction — was that the land it was built on would sink gradually. This is because over time, the weight of the material used for its construction would further compress the clay in the seabed. Although the airport continues to experience gradual settlement, the rate and extent of sinking remain within the parameters anticipated by engineers. While the settlement of the Holocene clay layer ended during the airport’s construction, the current settlement under Kansai Airport’s land is taking place in the Pleistocene clay layer. Unlike the Holocene clay layer — which received soil modifications — the Pleistocene clay layer is much thicker, meaning it compresses slowly and naturally over time. This also explains why the island’s sink rate has decreased significantly since the airport opened, falling from 50 centimeters (20 inches) per year in 1994 to 6 centimeters (2 inches) per year in 2024.
Since the 1990s, engineers have adjusted the airport terminal to counteract uneven sinking. Because the clay layers compress at different depths depending on location, parts of the airport grounds settled at varying rates. This uneven settlement risked structural warping in the terminal. To resolve this, iron plates were installed beneath the hydraulic jacks to support the terminal’s columns, allowing them to be gradually raised in stages to maintain stability.
The islands on which Kansai International Airport was built had supporting walls of sufficient height, which were predicted at the time to be capable of preventing "once-in-50-years stochastic waves." However, on September 4, 2018, Typhoon Jebi struck with a force far beyond expectations. Jebi was the most powerful storm to hit Japan in 25 years and, with sustained winds of approximately 210 km/h (130 mph), it pushed a record-breaking storm surge into Osaka Bay. Seawater breached the island, inundating both runways and submerging the basement levels of Terminal 1. Operations were completely suspended for three days, and over 8,000 passengers and staff were stranded as the only bridge to the mainland was damaged by a drifting oil tanker. Flights were diverted to nearby airports, and international air traffic in western Japan ground to a halt. Engineers responded to this catastrophe by elevating seawalls by an additional 1.2 meters (4 feet) and increasing flood resilience with watertight bulkheads and submersible pumps.
Kansai Airport Today
To combat the uneven subsidence that poses threats to structural integrity, Kansai's engineers conduct regular checks using a high-tech array of instruments. Tiltmeters, strain gauges, and GPS monitoring stations continuously feed data to an operations center. A network of 900 adjustable hydraulic jacks supports the terminal. Each one can be fine-tuned to re-level the structure. While this system is remarkably effective, it is costly and labor-intensive, requiring manual calibration every three years.
While the decreased land sink rate is undoubtedly encouraging news, the continued risk of sinking leaves many wondering how long Kansai Airport will last before it sinks to an irrecoverable height. Airport officials say that there is no official estimated life calculated for Kansai Airport. However, engineers predict that by 2056, some sections of the two artificial islands will sink by four more meters (13 feet), placing sections of the airport at sea level. Although as of August 2025, the airport has sunk 13 meters (43 feet), most of the sinking has occurred due to ground improvement. The height of the seawalls was raised to account for future settlement, and other safety measures have also been implemented.
From its initial construction to the costly corrective measures required due to accelerated sinking, Kansai Airport has come to be regarded as one of the most expensive civil engineering projects in modern history. Completing Kansai Airport cost tens of billions of dollars when taking into account land reclamation, two runways, terminals, and their facilities.
A Lesson for Others
Regardless of the natural risks confronting it, Kansai International Airport remains an engineering marvel. The airport's ambitious offshore construction provided critical lessons for global airport engineering. Other airports on reclaimed land, such as South Korea’s Incheon, Dubai's Al-Maktoum, and Doha's Hamad International, studied Kansai's strategies to reinforce foundations and prevent subsidence. However, few of these airports face the same geotechnical challenges posed by Holocene clay.
While the circumstances surrounding its construction should be appreciated, its future remains uncertain, especially considering issues like climate change. Without proper intervention, key infrastructure could be impacted severely, potentially left unusable, and jeopardize the airport’s future. Yet Kansai Airport, which was completed under unprecedented large-scale construction, is designed to handle airport functionality challenges by predicting settlements after its opening. By raising the height of the embankments and implementing measures to address uneven settlement, authorities have been able to maintain the airport's facilities.
While the airport continues to be a key economic engine for the region, large sums of money continue to be invested to sustain the airport. Kansai Airport is unlikely to sink indefinitely, but ongoing measures will likely be required to preserve its operations amid changing climate conditions.
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