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-- construction continued --
 Secrets
of the Construction of the Qin Wall
As the wall inched across the Chinese wilderness,
its builders were forced to rely upon local materials. Much of
the Qin wall was built with dry-laid native stone, but where stone
was scarce, engineers built the wall from layers of compacted earth.
The tamped-earth process began with a simple wooden frame. Workers
filled the frame with loose earth, which was then tamped into a
compact layer 4 inches thick. The process was repeated layer
upon layer, and the wall slowly rose 4 inches at a time. 
Secrets of the Construction of the Han Wall
In the arid Gobi Desert, the poor quality of the
sandy soil and the lack of brick and stone, forced Han builders
to resort to an ancient and painstaking method of wall construction.
First, they laid a bed of red willow reeds and twigs at the bottom
of a wooden frame, then they filled the frame with a mixture of
water and fine gravel, which was tamped solid. When the mixture
had thoroughly dried, the wooden frame was removed, leaving behind
a solid slab of tamped earth, strengthened by the willow reeds just
as modern concrete is reinforced by steel rods.
The Han also added beacon towers to the Wall, spaced 15 to 30
miles apart. Columns of smoke were used to warn defenders of
an attack. One smoke column meant an outpost was being threatened
by a force of fewer than 500 troops; two columns meant an attacking
force of fewer than 3,000, and so on. The Han found the beacon
system relayed messages faster than a rider on a horse.
Continued...........
 
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