On the Tees, at Stockton and Middlesbrough, the cost of dredging sand, gravel, and occasionally boulders, including the conveyance of deposits out to sea, a distance of about 12 miles, is 4·96d. per cubic yard or about 2½d. per ton. This amount includes everything except interest on capital expended on dredging plant.
On the Birmingham Canal., when there has been any slipping of the sides, or a discharge into it of water laden with silt and detritus from cuttings and high lands, the material, if soft, costs 5d. to 9d. per ton to dredge; and if hard, from 10d. to 14d. per ton. With a “spoon dredger” the cost is about 8d. per ton, and with a grab dredger it is about 5d. where the circumstances are favourable.[301] Where hard material has to be dealt with, the water is taken out of the canal, and the material is excavated by pick and shovel. On narrow canals dredging costs more, owing to the necessity of having a narrow beam, to enable the dredger to enter the canal. The beam is, however, sometimes increased when the machine is working by attaching baulks of timber or iron pontoons to the sides, to prevent its capsizing.[302]
The dredging machines that were chiefly employed on the Danube regulation works were, on an average, from 25 to 30 H.P., and had one inclined arm, which could be depressed to work in a depth of 22 feet of water or more. They were high enough to load direct into the waggons, by means either of a distributing table or an elevating endless chain bucket. The dimensions of the machine, which was found to be very economical, were:
| ft. | in. | |
|---|---|---|
| Length of boat | 88 | 7 |
| Breadth ” | 19 | 8 |
| Height ” | 7 | 9 |
| Draught of water | 3 | 11 |
The working steam pressure was six atmospheres, and the power consisted of a vertical engine of 15¾ inches cylinders and 357⁄16 inches stroke; the main shaft was 71⁄16 inches in diameter, and the ratio of the pinion to the driving-wheel was 1 to 7. The buckets were of steel, having a capacity of 8·75 cubic feet. The links of the chain were 31½ inches long, 1¾ inch by 3½ inches for those to which the buckets were attached, and 15⁄16 inch by 3½ inches for the others. These machines were employed in several different ways on the Danube works. They load direct into waggons, running upon a side track, either by means of a transporting apparatus or of an elevating wheel and buckets. The transporting apparatus was attached to the dredge, and consisted of a girder about 46 feet long, guiding and carrying an endless band formed of steel plates mounted on chains, which were driven by wheels at each end of the girder. The buckets of the dredging machine discharged their contents upon this band, to which a forward motion was imparted by an independent six-horse power engine, and the forward movement thus given discharged the ballast in the waggons alongside. The whole of this system rested at one end on the deck of the drag, and at the other on trestles, secured in a small auxiliary boat fastened alongside the machine. It was afterwards considered that a useful alteration might be made in the means of transferring the ballast, and with this object a large wheel, fitted with buckets, was mounted on the dredge, and driven by an independent engine. The wheel was of wrought iron, 19 feet 8 inches in diameter, and furnished with buckets which received the ballast from those of the dredging machine, and, after raising, discharged it into an open channel, whence it fell into the waggons. The buckets of this wheel were fixed to the periphery, and were so arranged as to discharge automatically into the channel. It was found that this mode of loading produced excellent results, but the full capacity of the dredgers could not be developed, both on account of the loss of time incurred, and because the material dredged was not always easily transferred into the waggons. A large quantity of the material excavated was also loaded into barges and taken by them to suitable points of discharge.
The amount of work performed by the dredging machines depended greatly on the means available of removing the earth excavated, and to do this with regularity, and without loss of time, was one of the most difficult portions of the work of excavation.
During 1870 and 1871 the dredging machines loaded almost exclusively into the waggons by means of the endless bands already described. Two of them were worked exclusively in this manner; other two began to load into boats in 1872, and the following year this method was entirely adopted with them, and their production was remarkably large. Another machine loaded the waggons by means of the large wheel. The dredging machines employed on the first and third sections of the works, and which also loaded into boats, gave remarkable results.
The Condreux excavating machine consists essentially of a carriage carried upon three lines of rails. A lateral projecting arm carries an endless chain with buckets, passing around a wheel at the lower end of the arm. This chain is driven by a 20 horse-power engine, mounted on the frame of the carriage, and the whole machine is caused to traverse on the rails by means of a small four-horse locomotive. The buckets, which become filled in succession in traversing the face of the slope, being excavated, are of steel plate or of wrought iron mounted with steel edges. The buckets are mounted on two pitched chains, which, in rising, pass over a loose pulley placed at the level of the road, and serve as a support to the loaded buckets. This arrangement largely reduces the friction, and prevents excessive torsion of the chain. The loaded buckets are discharged automatically, by means of flap openings in their bottoms, and their contents fall either into the waggons alongside, or into inclined conducting channels. These machines run alongside, and at the top of, the excavations they make, and the earth which they raise can be either deposited alongside so as to form a continuous embankment, or be loaded into waggons.
On the Mersey Dock Estate, which extends over a total water area of 520 acres, the dredgers used up to 1875 were of the ladder type, five of them having double, and one single ladders. A double set of hopper barges was attached to each dredger. The barges were 50 feet long by 20 feet beam, and contained 82 cubic yards. The expense of towing the barges out to the Seacombe Narrows, where they deposited their silt, rendered the operations costly, and in 1874 a steam hopper barge was brought into use, 144 feet long, 23 feet beam, 11 feet 9 inches depth of hold, and with a hopper capacity of 285 cubic yards. In 1876 two other hopper barges of the same size were brought into use. Subsequently, larger barges, with a hopper capacity of 414 cubic yards, were introduced. These have been found much more economical than the old system.
FOOTNOTES
CHAPTER XXXIII