These latter are usually
constructed, for these works, of untrimmed wood and mud, with thatched
roof. There were thus constructed at Portao de Ferro a few kilometers of
roads, then some houses for the engineers and special workmen, barracks
for 200 laborers, stores, kitchens, etc., a forge, and a shop with a lathe
and a saw run by a wheel at the side. It was afterward necessary to repair
the old lateral canal which had been dug out of the rock in the times of
the Royal Extraction, but which had been torn open for a considerable
length. This necessitated the erection of tight walls of dry stone, grass,
and mud, for a length of 200 meters, and with thicknesses of from 6 to 10
meters.
In order to divert the water into this canal, it was necessary to raise
its level 5 meters. The dam, then, had to support a strong pressure, and
it could not be built upon sand. It therefore became necessary to build a
temporary dam and to turn the river into a plank flume, so as to make it
possible to dig at the location of the permanent dam in order to reach a
solid bottom at a depth of nearly 4 meters. The permanent dam thus had a
total height of 10 meters, with a thickness of 15 at the base and 7 at the
top.
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