By NAMI SUGIURA/ Staff Writer
February 27, 2023 at 07:00 JST
The five-story Iidamaru tower at Kumamoto Castle, destroyed partially by a series of powerful earthquakes, is supported by a limited number of stones in the right corner in this image, earning the stone wall the nickname of a “miracle” structure. This photo was taken on April 22, 2016, in Kumamoto’s Chuo Ward. (Provided by Kumamoto city government)
KUMAMOTO--Left devastated and in ruins by a spate of strong earthquakes seven years ago, iconic Kumamoto Castle is rising from the ashes.
The temblors caused serious damage to the structures that make up one of Japan’s most traditional fortresses, in addition to the succession of roof tiles and walls that collapsed.
Currently, a project is under way to rebuild damaged stone walls there, boosted by traditional craftsmanship and the latest technology.
The project is helping the castle again attract sightseers to the main tourist destination in this central Kumamoto city.
Especially noteworthy among all facilities being reconstructed is the five-story Iidamaru tower, which is marked by its stone walls' breathtaking height of 15 meters.
The stone base of the building on the southern edge of the castle grounds was destroyed in the disaster. However, a row of rocks in a corner survived to support the tower. The “miraculous” development drew much attention.
The tower was demolished before the work started in October to rebuild the broken stone walls that directly supported the structure. The tower will be reconstructed after the stone walls are fixed.
Fallen rocks will be returned to their original positions during the process of fixing stone walls that are designated important cultural properties.
The locations of “tsuki-ishi” stones that fell from the walls were recorded. Where to return them is determined by comparing images taken before their collapse with the shapes of the fallen stones.
Repairing procedures of damaged walls are being essentially done in the same manner as that of artisans during the Edo Period (1603-1867), whereas heavy machinery is utilized when it comes to piling up stones.
“We are scrambling to make facilities here appear as similar as possible to their pre-tremor designs,” said Tsubasa Tanimori, 42, a foreman from boulder seller Nakaseki Co. in Osaka, who is engaged in the rebuilding of Kumamoto Castle.
No fewer than 1,700 tsuki-ishi are expected to be re-erected at the Iidamaru tower alone. The number of stones to be stacked on the entire castle site is estimated to total 100,000.
Kumamoto city estimated the castle’s renovation expenditure at 63.4 billion yen ($482 million) shortly after the 2016 disaster. The forecast reconstruction period has already been extended and the work is expected to continue for the next three decades.
Seemingly endless labor requirements, along with the difficulty in dealing with the culturally significant castle and the need to ensure safety to prevent damage in case of future disasters, are posing a rash of challenges.
To resurrect the stone walls of Kumamoto’s symbol, a researcher, construction experts and stone wall craftsmen are employing not only the latest technology but also traditional skills, to fully renovate the fortress by fiscal 2052.
Characterized by its magnificent donjon and towering keeps, Kumamoto Castle is comprised of as many as 1,000 stone walls arranged in a complex fashion on its grounds. Viewers have long been left spellbound by the castle’s majestic beauty.
Of all the walls, 517 incurred damage, such as collapsing and slipping out of place. Officials from Kumamoto city initially compared pre-disaster photos of stones with ones that fell, entailing much work.
Seeing that, Go Koutaki, 42, a professor specializing in image processing at Kumamoto University, helped improve the efficiency of the process.
Image processing technology can identify certain shapes in photos in an instant. Koutaki applied the method to stone walls and developed a system that can assess how the contours of fallen stones resemble those in pre-disaster images and drawings.
Koutaki tested the system after minimizing errors based on a mathematical model in the hopes of increasing the precision.
As the result, the original locations of 337 stones, or 91 percent, of 370 were successfully decided within an hour or so. Most of the remaining unidentifiable stones were significantly small or broken.
The technology was used in designing the stone walls to be reconstructed at the Iidamaru tower.
Ensuring safety and security is another major challenge for rebuilding the castle's stone walls.
Stone walls have such a high cultural value that they should be restored to their original condition without spoiling their merits.
At the same time, they must be made less vulnerable to collapse in the event of huge quakes, given the large number of daily visitors to Kumamoto Castle.
Tackling the difficult goal are engineers from major general contractor Obayashi Corp.
Obayashi officials studied the structure of the rock walls and looked at smaller “guri-ishi” stones stacked within the walls to support tsuki-ishi stones from behind.
Guri-ishi support each other firmly under normal conditions but slip forward at times if shaken by large temblors. At those times, tsuki-ishi on the surface alike move forward and become unable to return to their original positions, leading to the collapse of walls.
The engineers, who usually deal with such issues as how to stabilize slopes, sought a solution while applying the technique used to increase the stability of foundations.
Despite many failures in trials using a model, they continued until they succeeded in finalizing a method to enhance the earthquake resistance of stone walls.
They relied on a traditional skill to fit stones together as well as a newly devised method of inserting a netted sheet fashioned from stainless steel and resin fibers between guri-ishi to keep them stable.
From an overall perspective, the castle’s reconstruction work has just started.
Koji Mawatari, 40, a Kumamoto city official in charge of the rebuilding, said the reconstruction work needs to be done with an eye to the future.
“This historic site has existed for 400 years,” noted Mawatari. “I believe the results of this repair project will remain in existence for 100 years to 200 years, too. We must do it in a way that people in the next generation will not make fun of it.”
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