A Treatise On The Principles And Practice Of Harbour Engineering

HARBOUR ENGINEERING. 236 another, and to the suction nozzle. Many of the earlier experimental forms were far from successful in their attempts to remove plastic material. The blades become clogged, and a very small proportion of solid matter forind its way into the discharge pipe. Substantial improvements have, however, been effected of late years, and a modern suction cutter dredger is quite capable of dealing with the most adhesive and tenacious materials. Roek-eutting ’involves dredging appliances of a different type—those allied to the pick or hand-drill. A long, heavy cylinder of steel, fitted with a hard cutting-point, is raised, and allowed to fall by its own weight upon the surface of the rock, which it splinters and pulvérisés. The hardest rock yields to this treatment, and the blows are repeated until the fragments are reduced to the size of ordinary ballast ready for removal by a bucket or grab. The Bucket Dredg'er is to be found either in the form of a continuons band of buckets, called the ladder dredger, or of a single bucket, worked at the end of a long arm or lever, and called the dipper dredger. The first of these stands foremost in importance. The principle on which it is constructed is that of an endless chain connecting a series of buckets, which revolve continuously around two pivots, or tumblers, at different levels. The buckets excavate material at the lower tumbler, and discharge it into a shoot while passing over the upper tumbler. Dredgers of the ladder type present two varieties : those in which the ladders are centrally situated, and those in which the ladders are set at each side of the dredger. The bucket dredger can remove sand, clay, shingle, and niarl, with equal facility, and it can even deal with the softer kinds of rock. In harder varieties of rock it follows in the wake of biasting operations, or of a rock-cutter. It will lift boulders of a moderate size. A dredger at Bristol, on one occasion, raised a boulder weighing 2j tons without the least damage to the bucket. Most dredgers working in glacial clay have had some experience of boulder- lifting. The Dipper Dredg’er, with a single fixed bucket at the end of a long lever arm, is almost exclusively an American type. It is used mainly on river beds and channels where the working depth is not very great; for sea work in deeper and more exposed water, the ladder dredger shows to better advantage. Mounted on a barge, and working either from one end or through a well-hole in the centre, the lever makes a curved upward cut, and the con- tents of the bucket, after slewing, are dropped into a seow or hopper ranged alongside. The Grab consists of two or more curved plates, or jaws, capable of opening and closing in response to suitable mechanism. It is worked, to a very large extent, with the aid of gravity. Suspended by a chain or chains from the head of a crane jib, the bucket is allowed to fall freely by its own weight, with open jaws, until it buries itself in the ground. The jaws are then brought together, and the inclosed mass of earth is lifted. The economical scope of grab dredgers is limited to confined situations where other forms of dredger are unworkable.