The extra length of steel pipes necessitates a
greater extent of trench being excavated at one time, which
must be well timbered to prevent the sides falling in On the
other hand, cast iron pipes are more liable to fracture by
heavy stones being thrown upon them by the waves, but this is a
contingency which does not frequently occur in practice.
According to Trautwine, the cast iron for pipes to resist sea
water should be close-grained, hard, white metal. In such metal
the small quantity of contained carbon is chemically combined
with the iron, but in the darker or mottled metals it is
mechanically combined, and such iron soon becomes soft, like
plumbago, under the influence of sea water. Hard white iron has
been proved to resist sea water for forty years without
deterioration, whether it is continually under water or
alternately wet and dry.
Several types of sea outfalls are shown in Figs. 25 to 31.[1]
In the example shown in Fig. 25 a solid rock bed occurred a
short distance below the sand, which was excavated so as to
allow the outfall to be constructed on the rock. Anchor bolts
with clevis heads were fixed into the rock, and then, after a
portion of the concrete was laid, iron bands, passing around
the cast iron pipes, were fastened to the anchors. This
construction would not be suitable below low-water mark.
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