In recent years, casualties caused by the impact of ballistic ejecta from sudden phreatic eruptions have drawn much attention, as observed with Mt. Ontake in September 2014 and Mt. Moto-shirane in January 2018. Hence, improvement of evacuation facilities (shelters) that protects against ballistic ejecta is expected as a forthcoming volcanic disaster prevention initiative. In many cases, the utilized evacuation facilities are outfitted by strengthening existing facilities such as mountain huts. Therefore, it is necessary to understand the baseline impact resistance against ballistic ejecta of the existing mountain huts. In the case of Japanese wooden buildings, Japanese-style rooms with tatami (Japanese-style thick straw mats) are often used. In this study, we focused on the impact resistance of tatami used for flooring. We conducted tests which simulated the impact of ballistic ejecta on various types of tatami, in order to assess the penetration limit of tatami. Three types of bodies of tatami (tatamidoko) were prepared: straw tatamidoko, non-straw tatamidoko type III, and straw sandwich tatamidoko. The projectile was simulated ballistic ejecta with a diameter of 128mm and a mass of 2.66kg, made using a vitrified grinding wheel. This object was launched at a speed of 22 to 69m/s using a pneumatic impact test apparatus. From the impact test, non-straw tatamidoko type III did not prevent penetration, even at an impact energy of 0.63kJ. Therefore, if non-straw tatamidoko type III was to be used in a mountain hut, it cannot be expected to protect against ballistic ejecta. On the other hand, the minimum energy of penetration of straw tatamidoko and straw sandwich tatamidoko were 4.9 and 4.1kJ, respectively, and they had sufficient impact-resistance against ballistic ejecta compared to the mountain hut roof. Thus, it was shown that the downstairs of straw tatamidoko and straw sandwich tatamidoko can be designated as “a safer place in the building”.
