Deposits were formed for the reception of the excavated material, which constitute filtering basins enclosed within vaults formed by the solid materials previously removed. Where it was not possible to discharge direct into their depots by the long conductor, barges received the mud and carried it to a convenient destination.
System of Excavator adopted on the Ghent
and Terneuzen Canal.
The floating excavators were placed on two hulls carrying an iron framework, on which the staging supporting the bucket wheel was mounted. The engines and boiler were installed in one of the hulls, and in the other was placed the pump and engine for driving it. The upper level of the conductor was 78 inches below the bucket wheel. The conductor, 100 feet in length, was of the section corresponding to that of the buckets, 17¾ inches in diameter. It was supported by three cables attached to a staging, resting on the boat and secured to the bucket-wheel frame. The slope was 1 in 400, which allowed the material to be deposited at a level 22 feet 3 inches above that of the water. These excavators performed excellent duty; they could be easily transported from place to place, and were not affected by changes in the water level.
The position of the depots often involved the necessity of transporting the dredged material distances of 1200 or 1500 feet from the excavator. In such cases supplementary conductors were added. These were open, and were laid on the ground with a slope of 1 in 1000. Not unfrequently large blocks of old masonry, which formed the revetment of the sides of the canal, were raised by the excavator. These were generally carried down with the rest of the material, but occasionally they stopped, choking the channel, and requiring hand labour to remove them.
When this mode of transport could not be adopted, barges were employed to receive the dredged material and remove it to convenient points of discharge. These boats were built of iron, with double sides; they were 82 feet long and 15 feet 8 inches wide. Barges of similar dimensions were employed in the formation of earthworks under water, which were required at various parts of the canal. In these boats, holes 12 inches in diameter were placed 13 feet apart, iron tubes connecting the inner and outer shells. These holes were closed by means of valves while the boat was being loaded, and they were opened when it was brought over the place where it was desired to discharge.
Excavator on the Ghent and Terneuzen Canal.
One of the most remarkable and successful dredgers of the present day is employed on the Montreal harbour and ship channel improvements, and is known as the Canadian dredger. This machine, instead of being like the ordinary St. Lawrence dredgers, attended by a tug and scows, has an internal mud-hopper, and is self-propelling, thus being in fact dredger, tug, and scows combined, and requiring a proportionately large hull. In a recent comparison of this dredger with one employed at Otago, it was stated that the Otago dredger cuts to 35 feet deep, as do those of the St. Lawrence, but the latter have buckets a third larger, and arranged so as to be very nearly twice as effective. The Otago dredger is reported to have raised at the rate of 400 tons an hour, while filling her hopper, but the improved St. Lawrence dredgers easily fill their scows at the rate of 750 tons per hour, or nearly double the working rate of the “largest dredger in the world.” For the hourly capacity for consecutive hours, something must be deducted for time lost in going to dump or to change scows, and in the case of the St. Lawrence dredgers this reduces the hourly rate to about 650 tons, still, leaving them, however, better than the best rate of the Otago dredger.