There is no direction in which the triumph of man over the material forces with which he has to deal may be studied with greater advantage, even by the casual reader and the unscientific observer, than in that of the making of railways, the deepening or widening of rivers, the construction of artificial waterways, and the maintenance of ports and harbours. Each of these operations involves the employment of machinery and appliances that were quite unknown to our forefathers. The modern processes of excavation of the soil, in order to form or deepen the bed of a waterway, or of the construction of an embankment, in order that a railway or a canal may be carried above the level of the surrounding country, are now so familiar and commonplace, that we are accustomed to think but little of the slow and laborious steps whereby the means of carrying them out have been evolved from the necessities, the experience, and the scientific acquirements of modern times. Had the engineers of the present day been limited to the rude and imperfect appliances that they had alone at command a hundred years ago, such works as the making of the Suez, the Panama, and the Manchester canals; the deepening of the harbours that are now scattered up and down our extensive coast-line; and the adaptation to the requirements of modern shipping of the navigable rivers that have done so much for our maritime supremacy would have been all but impossible.
Let us take only a single instance, by way of illustration, and it shall be one that is very familiar, and easily capable of verification. On the works of the Manchester Ship Canal, in a length of only 35½miles, and over a comparatively level country, 45 million cubic yards of excavation are necessary. More than one-half of this vast quantity had been accomplished up to the end of 1889, by the employment, of 95 steam navvies or dredgers, 180 cranes, 160 locomotives, using 205 miles of temporary railway, 5500 waggons, and 220 portable and other steam engines, the number of employés being about 4000. To have executed this amount of work by any other system would have involved, perhaps, twenty times the amount of labour and more than twenty times the amount of time, if we are to judge by the accounts that have been handed down to us from ancient records as to the period over which the great works of antiquity extended.
It is not, however, in the mere work of excavation, that economy and progress have been effected. Another notable economy has resulted from the employment of large hopper barges, whereby 1000 tons or more of the dredged or excavated material may at once be removed to sea. The economy resulting from this source is stated to have enabled a saving of 40,000l. per annum to be effected in the works connected with the port of Dublin, without which economy the improvements actually carried out there would have been impossible.[299] In larger spheres of operations the economy must, of course, have been correspondingly greater.
Ralph Dodd appears to have contrived a machine to be worked by men, by means of levers, for excavating canals, which was tried in the year 1792, in the deep cutting at Dawley, near Hayes, on the Grand Junction Canal. Carne’s machine, for the same purpose, but worked by a horse at length, appears to have been used in 1794, in the deep cutting near Cofton Hacket, on the Worcester and Birmingham Canal. In the ‘Monthly Magazine’ (vol. ii., page 594) we have the following account of the operation of E. Haskew’s patent excavator:—
“This machine takes the soil from the bottom of the canal at 40 feet deep with equal facility as at six feet from the surface. One of them is at work upon the Gloucester and Berkeley Canal; by the assistance of two men only it removes 1400 loaded barrows from the bottom of the canal, to the distance of 40 feet, in twelve hours, and is so contrived as to take up the loaded barrows, leave them at the top, bring down the empty ones in regular rotation, and leave them at the bottom; it can be moved along the canal to the distance of 26 yards in ten minutes by the two men that work it.” In October 1793 Joseph Sparrow took out a patent for a machine, consisting of a box, with its bottom opening on hinges, suspended by a sort of universal gib or crane, the whole moving upon wheels, which he strongly recommended for elevating and discharging the soil dug out of the canal.
Among the most considerable deep cuttings in England up to the end of the last century were those at Ashton, on the Lancaster; Tring, on the Grand Junction; Coston Hacket, on the Worcester and Birmingham; Burbage, on the Kennet and Avon; Littleborough, on the Rochdale; and Smethwick, on the old Birmingham canals.
As the development of the processes of excavation and embankment forms one of the fullest chapters in the history of both civil and mechanical engineering, we shall not here presume to enter upon it at any length. A history of dredgers would be almost as serious an undertaking as a history of steamboats or locomotive engines. Their actual number is legion; but the dredgers that are now used on a large scale are comparatively few. Of course, everything depends upon the amount of work to be done, its locality, and other surrounding circumstances; but for operations on a scale of magnitude, few dredgers appear to have a better reputation than that which bears the name of Couvreux.
In the case of the improvement works undertaken on the Belgian Ship Canal, the Couvreux excavator removed, in 1875, 218,400 cubic yards of material in 166 days, being at the rate of 1316 cubic yards per day. Notwithstanding this, hand labour was very largely employed upon the work, a large quantity of water having to be dealt with at a depth of about 10 feet.
The earth excavated was carried to spoil, and in many cases was employed to form dykes enclosing large areas, which served as receptacles for the semi-liquid material excavated by the dredging machines with the long conductors; the Couvreux excavator used had already done service on the Danube regulation works. The material with which it had to deal in this case was, however, of a more difficult nature, being a fine sand, charged with water, and very adherent. The length of track laid for the excavator was about three miles along the side of the old canal, which had been previously lowered to the level of the water. The floating dredgers employed were 88 feet 7 inches long, 19 feet 8 inches wide, and 7 feet 9 inches deep; the arm was 39 feet 4 inches long, and passed through the hull. The form of the buckets was the same as that used at the Vienna regulation works, but the staging was higher, the axis of the driving-wheel of the bucket-chain being 26 feet 3 inches above the water-level. This increased elevation was necessary on account of the different methods employed for transporting the dredged material. To a large extent the same method of transport that was adopted on the Suez Canal was repeated in the case of the Belgian Canal Works. The conductor used allowed the sand and mud excavated to be delivered at a point 140 feet and 150 feet from the dredge, and at a height of 13 feet from the water-line. The excavated materials fell into the concave conductor 6 feet below the point of their discharge, and on falling they encountered the action of a stream of water which was constantly pumped along the conductor, and by which they were converted into semi-liquid mud. The slope of the conductors was generally 1 in 2000; it was supported by cables attached to a staging connected with the framing of the dredge, and the base of which rests on the deck of the vessel. The conductor is counterbalanced by a platform, on which was placed the portable engine and pump used for lifting the water into the conductor. This platform was suspended to the dredge in the same manner as the conductor itself. The general arrangement is shown on the engraving at [p. 453]. The supply and the maximum incline depend on the facility of disintegrating the ground, and on the quantity of water contained in the mixture. The proportions generally used were three parts of water to one of sand.
When the excavators met with compact clay which disintegrates slowly, or not at all, under the action of the water, the fragments raised were carried along in the current running through the conductor, but, of course, at a slower rate than the sand. Stones even of large size were also easily dealt with in the same manner. These materials were, however, only occasionally met with, the ground being chiefly composed of the fine sand, already referred to, mixed with a little clay, which was easily reduced to the required consistency.