Kansai Airport sinking due to subsidence issues

Kansai International Airport faces ongoing subsidence problems as it sinks into Osaka Bay, prompting urgent measures by Japan to address and prevent further damage.

Kansai International Airport in Japan has been recognised as the world’s best airport for luggage handling in 2024, maintaining a flawless record of no lost luggage for over a decade. It has also won international accolades for its architectural design, efficiency, and staff. However, this acclaimed airport is now "sinking."

Built on an artificial island in Osaka Bay, Kansai International Airport is one of Japan's largest construction and engineering projects. 

The airport consists of two islands, with one covering 3,187 acres and the other spanning 6,593.75 acres. It was constructed to alleviate congestion at the nearby Osaka International Airport and first opened its doors on September 4, 1994.

Since then, the airport has increasingly served both domestic and international flights, becoming a vital aviation hub. 

In 2024, approximately 30.6 million passengers passed through its terminals, travelling to 91 cities across 25 countries. 

While these figures are impressive, it is hard to ignore the fact that the airport has sunk deeper into the soft clay beneath Osaka Bay than engineers had initially predicted and continuing to sink every year.

Several factors contribute to the gradual sinking of the airport. These include the weight of the land and infrastructure, as well as the method used to construct the island by dredging millions of tonnes of earth and stone and dumping it onto the seabed. 

This process did not allow the soft sediment to compact sufficiently before construction began, making the island prone to settling over time.

A further major cause is the loose clay and sediment layer beneath the airport, which sinks under weight. As a result, the airport has been subsiding faster than engineers originally calculated.

According to a report from Kansai Airport's service provider, the surface level of the airport’s first island is now 3.84 metres lower than when it first opened in 1994. Since the commencement of landfill construction, the airport has experienced an average subsidence of 13.66 metres.The latest data released in December 2024 indicated that the average subsidence measured at 17 points on the artificial island was only 6 cm. However, the situation on the second island is slightly worse, with the ground surface having sunk by 17.47 metres since construction began. The average subsidence across 54 measurement points on this island in 2024 was 21 cm.

These islands were built on a 20-metre thick layer of clay and sediment that functions like a sponge. Despite the installation of 2.2 million vertical drainage pipes to solidify the clay and limit shrinkage, the weight of the landfill, including more than 200 million cubic metres of debris, has caused the clay to compress more than anticipated.

To address this issue, a seawall costing over US$150 million was constructed around the islands. Nonetheless, engineers still predict that, in the next 30 years, parts of the airport may fall below sea level.

In September 2018, Typhoon Jebi, the most powerful typhoon to strike Japan in 25 years, caused US$13 billion in damages and claimed 21 lives across the Pacific. The typhoon led to the temporary closure of Kansai Airport due to flooding.

This event highlighted significant design flaws, as critical infrastructure, including the disaster response centre and power stations essential for supplying electricity to the facilities, were located underground and completely flooded. Approximately 5,000 people were stranded at the airport without power for over 24 hours.

According to a statement on the service provider's website, the airport's subsidence rate is currently decreasing, and efforts are being made to monitor the airport's foundations while collecting data on the subsidence. Academic experts have also been consulted for advice.

Hiro Ishikawa, Emer Prof of Urban Planning and Policy at Meiji University, commented that the subsidence of the island remains at an acceptable level and is considered a valuable learning experience for similar man-made island projects.

Ishikawa told This Week in Asia, “When we needed to build a second airport for Osaka, we had to decide to build offshore because there were few viable options. Land space on the mainland was very limited for large construction projects, and residents in nearby areas might be affected by noise and night flights.”

Kansai Airport has provided valuable lessons for the development of other airports, including Chubu Centrair International Airport, which was also built on an artificial island off the coast of Nagoya.

Chubu Centrair, which opened in 2005 and is smaller than Kansai, covering only around 2,937 acres, was ranked the best regional airport in the world for the 11th consecutive year in 2024 by the Skytrax industry organisation. It has reported fewer subsidence issues compared to Kansai.

Ishikawa explained the engineering principles behind Kansai Airport: “We knew from the start that the airport would subside, and we considered the various impacts in the design. However, there were still mistakes, such as placing emergency facilities in underground areas prone to flooding.”

Currently, the airport's subsidence rate is less than 10 cm per year, but the situation is gradually improving and manageable. Subsidence in urban areas is not a new phenomenon and occurs worldwide, such as in Venice and Manhattan, due to the weight of buildings.

At present, the airport is undertaking a major $609 million renovation project aimed at upgrading facilities and increasing operational capacity, despite the ongoing challenge of sinking deeper into Osaka Bay.

(2025/06/21-00:00)