Airports on Water

River deltas are difficult places for map makers. The river builds them up, the sea wears them down; their outlines are always changing. The changes in China's Pearl River delta, however, are more dramatic than these natural fluctuations. An island six kilometres long and with a total area of 1248 hectares is being created there. And the civil engineers are as interested in performance as in speed and size. This is a bit of the delta that they want to endure.

The new island of Chek Lap Kok, the site of Hong Kong's new airport, The giant dumper trucks rumbling across it will have finished their job and the airport was built at a similarly breakneck pace.

As Chek Lap Kok rises, however, another new Asian island is sinking back into the sea. This is a 520-hectare island built in Osaka Bay, Japan, that serves as the platform for the new Kansai Airport. Chek Lap Kok was built in a different way, and thus hopes to avoid the same sinking fate.

The usual way to reclaim land is to pile sand rock on to the seabed. When the seabed oozes with mud, this is rather like placing a textbook on a wet sponge: the weight squeezes the water out, causing both water and sponge to settle lower. The settlement is rarely even: different parts sink at different ratees. So buildings, pipes, roads and so on tend to buckle and crack. You can engineer around these problems, or you can engineer them out. Kansai took the first approach; Check Lap Kok is taking the second.


Kansai Airport, Japan

The difference are both political and geological. Kansai was supposed to be built just one kilometre offshore, where the seabed is quite sold. Fishermen protested, and the site was shifted a further five kilometres. That put it in deeper water (around 20 meters of soft alluvial silt and mud deposits. Worse, below it was a not-very-firm glacial deposit hundreds of metres thick.

The Kansai builders recognised that settlement was inevitable. Sand was driven into the seabed to strengthen it before the landfill was pilled on top, in an attempt to slow the process; but this has not been as effective as had been hoped. To cope with settlement, Kansai's giant terminal is supported on 900 pillars. Each of them can be individually jacked up allowing wedges to be added underneath. That is meant to keep the building level. But it could be a tricky task.