Field Survey of Concrete Materials of Pillboxes Remaining at the Former Aoto Army Airfield in Minamikyushu City
As part of a collaborative research project with Minamikyushu City, the Restoration Technology Laboratory at the Center for Conservation Science, Tokyo National Research Institute for Cultural Properties (TOBUNKEN) is conducting research aimed at preserving concrete structures from WW2 that remain in the city. Among these, two pillboxes that remain at the former Aoto Airfield have few construction records, leaving many unknowns, such as who built them, when, for what purpose, what materials were used, and where the materials were sourced.
In September 2025, we conducted a field survey with Mr. SAKAMOTO Kota (Minamikyushu City Cultural Properties Division), Mr. YAMAKI Satoshi (Chiran Peace Museum), and Mr. TAGUCHI Kiminori (Kanagawa Prefectural Museum of Natural History), to shed light on the procurement of concrete materials for the pillboxes. Our investigation so far has revealed that the pillbox concrete contains fragments of shells believed to be marine, as well as worn ceramic and glass fragments typically found on beaches. Based on this, it was speculated that the sand used to make the concrete was collected from nearby beaches, and that shells, ceramic fragments, and broken glass had become mixed in during the process. Furthermore, Mr. TAGUCHI’s recent observations revealed that the pillbox concrete contained 1-2 mm black sand, transparent green olivine sand, and multiple pieces of coral. This combination closely resembled the sediments found on the coast around Mt. Kaimon, so this information served as a new clue for surveys of nearby coasts and river mouths.
As a result of the survey, on a beach about 10 km northwest of Mt. Kaimon, gravel with a similar composition to the sand, shells, coral, ceramics, and glass found in the pillbox concrete was confirmed. Furthermore, the location was also found to be convenient for procuring and transporting materials, as a railway line (Ishigaki-Kiire Line, opened in 1917) connected the Aoto Airfield, where the pillbox is located, to the coast, approximately 5 km away.
We are currently summarizing the results of this survey, taking into account the results of previous surveys, and plan to publish a report in the near future.