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They're japanese construction breaker barrels. In the 1990s Japan was under construction so that they can fund development of the country, so much so that almost 10% of the entire country was employed by some construction firm (compare to near 10% retail employment in the USA, and 4.5% construction in the entire USA in 2016). They are low and close together because when there are earthquakes the sand in the barrels bangs together and dampens the effects of the ever common earthquakes on nearby structures by transfering kinetic energy into the sand.
