PORT SERVICES AS REGARDS SHIPPING 4;
For the removal of hard or stiff material, such as ballast,
clay and gravel, as well as soft mud and silt, the Bucket
Dredger is generally employed in this country. It consists
of an endless chain of buckets revolving in an elliptical
path round two axes or tumblers, situated one at the top
and the other at the bottom of an inclined “ladder” or
frame, excavating material when in the lowest position
and depositing it into a shoot on passing over the upper
tumbler. The shoot directs the material into a hopper,
or receptacle, either in the dredger itself or in an attendant
hopper. In either case, the material is carried out to sea
and deposited in some suitable spot. The counterpart
of the Bucket Dredger in America is the Dipper Dredger,
which consists of a huge bucket at the end of a long arm or
lever. The cut is made in a single sweep.
Sand is more effectively and economically dealt with
by the Suction Pump, which sucks the sand while in a state
of suspension and discharges it into the hopper, where it
settles and the surplus water passes over the hopper sides.
The difficulty is to reduce the flow in the hopper to such
an extent as to enable the sand to settle in reasonable time.
Pumping will, of course, raise mud and silt in the same
way, but these materials are so light and flocculent in
suspension that they do not settle at all and are washed
overboard again. Clay can also be dealt with, provided
it be disintegrated in the first instance, so as to be capable
of passing up the suction tube; this can be effected by the
use of rotary cutters, and dredgers of this class are
designated Suction Cutter Dredgers.
Rock is generally treated in the first instance by the
Rock-breaker—a long, heavy ram, or chisel, with a hard
conical head which, being made to fall repeatedly on the
surface, breaks it up into fragments, which can easily be
removed by a bucket dredger.
Wrecks. The clearance of the channel from wrecks
and other obstacles is a function of a harbour authority.
It is necessary to provide special appliances for this class