The tunnels of the Tatsumi waterworks, constructed in the early Edo period, wereexcavated through masses of stratified solid rock without using supports.The surface of the rock inside the tunnel was covered over with a protective layer of plastic, metamorphosed rock. The top inner surface, however, had suffered from weathering in the course of years, while the flowing water had washed out the weaker part of the rock under the surface of the water. Beside, the tunnels were made so near to the surface of the earth that roots of the trees wedged into rockjoint, gradually and steadily cleaving them.
In addition, the great Kansei earthquake of 1799 had caused the waterworks a lot of damage, So, in Tempo period (1834), in order to secure more capious flow of water, tunnels were built to take the place of open channels that were more liable to damage by tumbling masses ofearth and rocks down the bench-terraced slopes.
Since the tunnels have received further damage from natural disasters, it was pointed out that they were.in urgent need of repair in many ways.
Four tunnelled routes leading to the Kijiintake were built about 360 years ago. No documents have yet been found to show by and when the outermost tunnel, still in use today, was reconstructed.
Presumably it has to do with repairing upon the damage done by the aforementioned earthquake. Trial calculation by terrestrial magnetism estimates the date of itsconstruction at around the beginning of the nineteenth century, just after the earthquake.
The tunnels excavated some 360 years ago speak volumes for the then new techniques of tunnelling in the early, period.
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