The piling is left in permanently if it is not
wanted again for use.
Sometimes these bottoms are too soft to be treated in this manner; in
that case boxes or caissons are formed, loaded with stone and sunk into
place with pig iron until the weight they are to carry is approximated.
When settled, the weights are removed and building begins.
Foundations on shifting sand are met with in banks of streams, which
swell and become rapids as each winter breaks up. This kind is most
troublesome and dangerous to rest upon if not properly treated.
Retaining walls are frequently built season after season, and as
regularly become undermined by the scouring of the water. Regular
docking with piles and timbers is resorted to, but it is so expensive
for small works that it is not often tried.
Foundations are formed often with rock well planted out; and again
success has attended the use of bags of sand where rough rock was not
convenient or too expensive.
In such cases it is well to try a mattress foundation, which may be
formed of brushwood and small saplings with butts from 1/2 inch to
21/2 inches in diameter, compressed into bundles from 8 to 12 inches
diameter, and from 12 to 16 feet long, and well tied with ropes every
four feet.
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