The Reclaimed Island of Kansai International Airport

Land Subsidence on the Reclaimed Island of Kansai International Airport

Kansai International Airport is a construction built on an artificial island above the sea in Osaka, Japan, which opened in 1994. Surveys have found that this airport has faced land subsidence issues since its construction. The land subsidence that has occurred is greater than expected, approximately 12 meters since its construction. Kansai Airport was built on two reclaimed islands, Island I and Island II, which were built on soft clay soil containing a lot of water, requiring various engineering techniques to stabilize the island’s foundation and prevent land subsidence during construction.

Reclamation begins with the installation of a 1.5-meter-thick sand blanket on the seabed to create an effective horizontal drainage system that allows water in the soil to easily escape. Next is the installation of vertical sand drains (VSDs), which serve to accelerate the consolidation of soft Holocene soil, especially in the Ma13 layer, which is approximately 15-24 meters thick. Drain pipes with a diameter of 40 cm will be installed in a dense pattern, namely 2.5 meters × 2.5 meters. The number of drains is enormous, with more than one million drains installed on Island I and more than 1.2 million on Island II. The working principle is to accelerate the discharge of pore water from the clay soil so that significant subsidence can occur more quickly at the beginning of construction. With this installation, the soil becomes more compact and less soft. If drains are not installed, Holocene soil is estimated to take a very long time to consolidate, but with the installation of drains, the time can be shortened.

After the drain was properly installed, the backfilling process began. The backfill material consisted of large amounts of soil and rock with large grain sizes, reaching a diameter of 300 mm. The height of the backfill reached 36–43 meters from the seabed. The total volume of material used is also enormous, approximately 180 million cubic meters for Island I and 250 million cubic meters for Island II. According to Funk (2013), the weight of this fill creates enormous pressure on the soil layers beneath it, not only on the Holocene layer but also on the Pleistocene layer deep below. The combination of very soft soil conditions and this heavy load became the main source of land subsidence that continued during construction and even to this day. To understand how the soil beneath Kansai Airport moved after construction, engineers installed numerous measuring devices in the ground. According to Funk (2013), these devices were installed at a depth of up to 350 meters and consisted of devices to measure the extent of land subsidence, devices to measure water pressure in the ground, and devices to measure load weight. From the installed devices, it was found that the ground on Island I subsided by about 10-15 meters, while the ground on Island II subsided by about 11-16 meters. These figures for ground subsidence turned out to be much greater than initially estimated during construction.

Then, in this study, Mesri & Funk (2015) attempted to calculate why the ground could sink so much. The study used a computer program called ILLICON. This program can calculate how water in the soil flows out when the soil is compressed by deposits. The results found that it was not only the soft soil at the top that experienced subsidence, but also the deeper soil layers, such as the Pleistocene layer. This deeper layer contains a lot of fine sand and silt, making it very difficult for water to flow out. Its permeability is very low, so the soil takes a very long time to compact. Therefore, the subsidence continues. These studies show that the Kansai reclamation islands continue to experience subsidence to this day. Initially, the elevation of the islands had to be maintained above +4 meters to be safe from ocean waves. However, Island I had already subsided beyond the safe limit before 2012.

According to Funk (2013), several points on the island could reach sea level around 2056–2067. This means that subsidence will continue for a very long time. From these studies, it can be concluded that Kansai Airport is an extraordinary project built on very difficult terrain. Even though many vertical sand drains have been installed to accelerate soil compaction, the very thick soil layer that is difficult to drain water from continues to cause the island to slowly sink in the long term. Therefore, regular maintenance and elevation of the embankment is necessary to ensure its continued safe use.

References:

Yesudian, A. N., Dawson, R. J. (2021). Global analysis of sea level rise risk to airports. Climate Risk Management, 31, 1-10.

Mesri, G., Funk, J. R. (2015). Settlement of the Kansai International Airport Islands. Journal of Geotechnical and Geoenvironmental Engineering, 141(2), 04014102.

Funk, J. R. (2013). Settlement of the Kansai International Airport Islands (Doctoral dissertation, University of Illinois at Urbana-Champaign).