Geotechnical investigation of the Naval dry dock at the Mietsu Naval Facility World Heritage Site, Japan

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The Mietsu Naval Facility was established as a Western-style military facility by the Saga Clan in 1858. A geotechnical investigation of the Mietsu Naval Dock (MND) at the Mietsu Naval Facility World Heritage Site was performed using portable cone penetration tests, unconfined compression tests with suction measurement, and standard oedometer tests. The pile length of the wooden frame structure was determined using pulse waves. The slope of the western wall consists of four layers, and straw soil bags were arranged for slope stabilization. The slope of the western wall during construction and under use was analyzed based on total and effective stresses, respectively, and was found to be stable. The wooden structure of the eastern wall consists of four stages and has a complex structure of pine wood with intact bark. The second and third-stage piles have anchorage structures with several horizontal elements linked to the first and second-stage piles. The number of the piles in the third-stage indicates that it plays a central structural role. Even though ship were repaired at the bottom of the dock, there is no evidence of ground improvement to prevent damage from ship loads. Because the amount of decrease in effective overburden pressure corresponded to the amount of excavated soil with the load of the Denryu-maru, which was repaired in this dry dock, it is possible that ground improvement for entry and repair of the ship was not needed. The results of this study are useful for the maintenance, management, and restoration of the MND remains.