Page images
PDF
EPUB
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][subsumed][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

amount of work which was performed by these dreages, and the expense of the process:—

Year ending

Amount expended.

All these dredges had governors, which regulated the speed to about 28 strokes per minute in ordinary working stuff. The average pressure in boilers was about 34 lb. per square inch. In general, 14 buckets were discharged per minute. The speed of the buckets on the frames of dredges Nos. 1, 2, 3, and 4, was 48 feet 5 inches per minute, and that on No. 5, 49 feet 8 inches per minute. They consumed from 15 to 18 lb. of coal per horseThe following is a statement of the power per hour. Tabular View of the Dredging of the Wear at Sunderland in 1842–46.

[blocks in formation]

...

December 25, 1841.... 24, 1842 23, 1843.... 21, 1844.

L. 8. d. 11,841 18 2 13,612 11 3 9,742 7 6 10,659 3 8

L. &. d. 218,110 0 1 1 313,810 0 0 101 294,440 0 0 8 317,660 0 0 8

....

[blocks in formation]

Cross

to 1816

Hence the average cost per ton on five years' work—

For raising and depositing at sea

[blocks in formation]

Mr Murray gives the above tabular view of the dredging of the Wear at Sunderland, which is also an interesting record of the quantity and cost of material raised by a dredging-machine; but this view is not given by way of comparison with the preceding, as there is little analogy between the cases. The contracted state of the Clyde, the frequent interruptions to which the work was subject by the constant passage of vessels, and the expense of removing and depositing the stuff, necessarily increased the cost of executing the work in that situation.

In river-dredging two systems are pursued; one plan consists in excavating a series of longitudinal furrows parallel to the axis of the stream, the other in dredging cross furrows from side to side of the river. It is found that inequalities are left between the longitudinal furrows, when that system is practised, which do not occur to the same extent in side or cross-dredging; and the writer has invadredging, riably found cross-dredging to leave the most uniform bottom. To explain the difference between the two systems of dredging, it may be stated, that in either case the dredge is moored from the head and stern by chains about 250 fathoms in length. These chains in improved dredges are wound round windlasses worked by the engine, so that the vessel can be moved ahead or astern, by simply throwing them into or out of gear. In longitudinal dredging, the vessel is worked forward by the head chain, while the buckets are at the same time performing the excavation; so that a longitudinal trench is made in the bottom of the river. When the dredge has proceeded a certain length, it is stopped and permitted to drop down and commence a new longitudinal furrow parallel to the former one. In cross-dredging, on the other hand, the vessel is supplied with two additional moorings, one at either side, and these

=0·943 = 1.243 = 3.863

chains are, like the head and stern chains, wound round barrels wrought by the engine. In commencing to work by cross-dredging, we may suppose the vessel to be at one side of the channel to be excavated. The bucket-frame is set in motion, but instead of the dredge being drawn forward by the head chain, she is drawn to the opposite side of the river by the side chain, and having reached the extent of her work in that direction, she is then drawn a few feet forward by the head chain; and the bucket-frame being yet in motion, the vessel is hauled back again by the opposite side chains to the side from whence she started. By means of this transverse motion of the dredge, a series of cross furrows is made; she takes out the whole excavation from side to side as she goes on, and leaves no protuberances such as are found to exist between the furrows of longitudinal dredging, even where it is executed with great care. The two systems will be best explained by reference to the annexed cut (fig. 5), where AB repre

B

f

D

C Fig. 5.

sents the head and stern moorings, and DC the side moorings; the arc ef represents the course of the vessel in crossdredging; while in longitudinal dredging, as already ex

Rivers. plained, she is drawn forward towards A, and again dropped down to commence a new longitudinal furrow.

In some cases, however, the bottom is found to be too hard to be dredged until it has been to some extent loosened and broken up. Thus at Newry, Mr Rennie, after blasting the bottom in a depth of from 6 to 8 feet at lowwater, then removed the material by dredging, at an expense of from 4s. to 5s. per cubic yard. The same process was adopted by Messrs Stevenson at the bar of the Erne at Ballyshannon, where, in a situation exposed to a heavy sea, large quantities of boulder stones were blasted, and afterwards raised by a dredger worked by hand, at a cost of about 10s. 6d. per cubic yard. But the most extensive application of blasting, preparatory to dredging, of which the writer is aware, was that on the works for improving the Severn, by Sir William Cubitt, of which an interesting and instructive account is given by Mr George Edwards, in a paper addressed to the Institution of Civil Engineers, from which the following particulars are taken:1

"It appears that a succession of marl beds, varying from 100 yards to half a mile in length, were found in the channel of the Severn, which proved too hard for being dredged, the whole quantity that could be raised being only 50 or 60 tons per day; while the machinery of the dredges employed was constantly giving way. Attempts were first made to drive iron rods into the marl bed, and to break it up; a second attempt was made to loosen it by dragging across its surface an instrument like a strong plough. But these plans proving unsuccessful, it was determined to blast the whole surface to be operated on. The marl was very dense, its weight being 146 lb. per cubic foot; and it was determined to drill perpendicular bores, 6 feet apart, to the depth of 2 feet below the level of the bottom to be dredged out. The bores were made in the following manner, from floating rafts moored in the river:-Pipes of-inch wrought-iron, 3 inches diameter, were driven a few inches into the marl. Through these pipes holes were bored, first with a 14-inch jumper, and then with an auger. The holes were bored 2 feet below the proposed bottom of the dredging, as it was expected that each shot would dislocate or break in pieces a mass of marl of a conical form, of which the bore-hole would be the centre and its bottom the apex; so that the adjoining shots would leave between them a pyramidal piece of marl, where the powder would have produced little or no effect. By carrying the shot-holes lower than the intended dredging, the apex only of this pyramid was left to be removed; and in practice this was found to form but a small impediment. Fig. 6 is a section, and

Water Level

Fig. 6.

fig. 7 a plan of the bore-holes; the inner dotted circles represent the diameters of the broken spaces at the level of the bottom of dredging. The cartridges were formed

[blocks in formation]

inches, which were secured by ropes to the rafts. Mr Edwards says that not one in a hundred shots missed fire, and these shots were generally saved by the following singular expedient:-The pointed end of an iron bar, 4-inch diameter, was made red-hot, and being put quickly through the water, and driven through the tamping as rapidly as possible, was in nine cases out of ten sufficiently hot to ignite the gunpowder and fire the shot. "The cost of each shot is calculated as follows:Use of material........................................... Labour.....

Pitched bag for charge..

3 lb. of powder at 54d....

15 feet of patent fuse at ths of a penny.. Pitch, tallow, twine, coals, &c......

Cost per shot

£0 1 0 033 0 0 3

1 41

0 0

0 0 41

.£0 7 0

Each shot loosened and prepared for dredging about 4 cubic yards; so that the cost for blasting was 1s. 9d. per yard. The cost of dredging the material, after it had been thus prepared, was 2s. 3d. ; making the whole charge for removing the marl 4s. per cubic yard."

4. Excavation.

But there are cases where the bottom cannot be advan

tageously operated on by any of the means we have mentioned, and where it is necessary to have recourse to other appliances for its removal, such as the diving-bell or divinghelmet, and coffer-dams. The diving-bell has, in conjunc- By diving tion with dredging, been much used on the Clyde, and Mr bell. Bald gives the following account of the operation as conducted on that river :

"Between Erskine Ferry and the New Shot Isle the bed of the Clyde, for a distance of 2000 yards, was greatly encumbered with stones and stone boulders, which were highly injurious to vessels if they grounded there; and frequently large ships, in being tugged through this part of the river-channel, had their copper bottoms injured when they touched the rocky channel-bed. In deepening and clearing this part of the river, two diving-bells were employed, and one, and sometimes two, steam-dredgers. The clearing and deepening of this channel was exceedingly severe on the machinery and working-gear of the steam-dredgers; the speed of the engines was therefore governed by the nature of the material in the bottom; and although the iron-work frequently gave way, yet spare links and buckets being always ready to replace those

[graphic]

"Account of Blasting on the Severn," by George Edwards, C.E. (Trans. of Institution of Civil Engineers, vol. iv., p. 361). 8 Clay weighs about 109 lb., and sandstone about 155 lb. per cubic foot.

Rivers. which broke, there was little interruption to the continuous working of the dredgers. When the dredgers had cleared away the material which covered the boulders in the bottom of the channel, the diving-bell boats were worked over the ground so cleared, removing all the larger boulders; and when that part of the channel had been cleared of them, the dredgers went again over the same bottom, removing all the lighter material from the heads of the lower boulders, preparatory to the bells commencing again; and these operations were continued until the necessary depth was attained.

"The buckets of the steam-dredgers, in working along the bottom, always slipped over the head of the large boulders, which the diving-bells alone could lift and remove. Some of these masses of trap or whinstone were 4 and 5 tons in weight, and from their rounded forms and smooth surfaces, it was evident that they had been brought from some distance. Some of them were of sandstone, but they were more angular than the trap boulders. Quantities of these boulders, lifted from the bed of the channel, might be seen lying along the sides of the river; and many of them had since been split and broken up by gunpowder for repairing the river dykes. The tops of some of the large stone boulders lifted from the bed of the channel were found grooved to a depth of about an inch or more, by the ship's keels having been rubbing over them; and metallic particles were distinctly to be seen upon their surface. In removing these stone boulders from the bed of the channel, the diving-bell men found numerous fragments of copper and iron which had been torn off the ship's bottoms and keels by the large stones; but latterly this had not been the case, as great progress had been made in the removal of the boulders, and the deepening of the channel."

Rivers.

Large isolated masses of stone have also been removed en masse from many rivers by fixing louises in them, and raising them by floatation. On the Tay this was done to some extent, and one boulder of 50 tons was raised from the river by that means. Where a large area and considerable depth of solid rock has to be removed, coffer-dams are doubtless the best means of executing the work; but the chief difficulty in employing dams in the narrow channels of rivers is the obstruction which they necessarily present to the passage of floods and also to shipping. It is therefore a matter of high importance to reduce their bulk to the smallest possible limits. With this view the writer' By coffer designed a coffer-dam for the works of the River Ribble, dams. which consisted of two rows of iron rods, 3 feet apart, jumped into the rocky bottom, and supporting two linings of planking, the intermediate space being filled with clay, and the whole structure being stayed from the inside, so as to present no obstruction beyond the outer line of the dam. Three dams of this construction were formed in the Ribble; and by means of them, a bed of rock, 300 yards in length, and of a maximum depth of 13 feet 6 inches, was successfully excavated. The maximum depth of water at high-water against the dam was 16 feet, but in very high floods of the river the whole dam was sometimes completely submerged; but on the water subsiding, it was found that the iron rods, on which alone its stability depended, although only jumped 15 inches into the rock, were not drawn from their fixtures. As this construction of dam completely overcomes the difficulty of fixtures in a hard bottom, where piles cannot be driven, and offers very little obstruction to the navigation; and moreover, as it has been successfully used on a large scale, and seems to fulfil all the conditions demanded in such a situation, it may be perhaps considered generally applicable to situations where

[graphic][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][merged small][merged small][merged small][merged small][merged small]

1 Description of a Coffer-Dam adapted to a Hard Bottom," by David Stevensor C.E. (Trans. of Inst. of Civil Engineers, vol. iii., p. 377).

[blocks in formation]

and curved walls.

Fig. 9.

which is constantly shifting its course never remains sufficiently long in one position to form for itself a properly defined bed, but is in fact always in a transition state; the sand which is worn from the concave being thrown to the convex side of the stream, while some portion of the floating materials, carried to and fro during this process of perpetual change, is often deposited, and forms shoals in the middle of the fairway. A river left in this state of nature cannot possibly attain the maximum depth due to the natural scour of the tidal currents, as their power is expended in abrading and removing the sand-banks through which the stream flows, and not, as it ought to be, in deepening and scouring its bed. In such cases what is wanted is to secure a permanent channel, by guiding the first of the flood and the last of the ebb tide by means of walls, so that the strength of the currents may constantly operate on the same line of channel. In this way, it is obvious that not only will the advantage of a permanent navigable track be obtained, but the constant action of the currents of flood and ebb tide flowing in the same channel, will secure a much greater permanent depth than they could possibly do if permitted to wander at random through the estuary, sometimes operating in the same channel, and at other times directly opposed to each other.

RIVER

RIVER

Fig. 10.

ComparaQuestions have been raised as to the comparative advantive ad- tages of straight and curved walls for directing a channel. vantages of It is believed that. in most cases, the direction of such walls straight is necessarily determined, not by any abstract consideration as to the superiority of straight or curved walls, but chiefly by the relative positions of the points between which the stream is to be conducted, and the outline and geological formation of the shores and banks of the estuary that intervene between those points. The consideration of such matters may render it expedient, according to the special circumstances of the locality, to adopt walls having concave, straight, or convex outlines, as shown in figs. 10, 11, and 12. Viewed as a purely abstract question, it may, we think, be safely affirmed, that a stream is most likely to follow a permanent course when directed by a concave wall, as

RIVER

Fig. 11.

Fig. 12.

shown in fig. 10, in which the axis of the stream is represented by the dotted line. Dr Young observes that the centrifugal force in curved channels has a tendency to draw the greater portion of the water to the concave side, and thus the greatest scouring power, and consequently the greatest depth of the stream, will be found upon that side. In a channel directed by straight walls (fig. 11), the current has no such decided bias for either wall, and is consequently easily thrown across from side to side. A wall, on the other hand, having a convex outline, as shown in fig. 12, is (especially if the radius of curvature be small) still less suitable as a guide, as the line of wall diverges from the direction of the axis of the current. These remarks are not hypothetical, as the writer has found that their correctness has been verified by cases in actual practice. There is doubtless some disadvantage in the deep water being on one side of the channel, as more particularly shown in the cross section, fig. 13. It would be more convenient for naviga

RIVER

Fig. 13.

tion were the deep water in the centre; but it is found that the current invariably adheres to one or other of the walls, and it is better that the channel should keep constantly to one wall, than that it should alternate from side to side, as is more apt to be the case in absolutely straight channels.

The direction and extent of river-walls must, however, be carefully considered by the engineer with reference to existing circumstances, and every case must be judged per se. But we think it will be found safe, in executing such works, to adhere as closely as possible to the following general rules :

First, The channel through open estuaries should, in all cases where funds will admit of it, be guided by double walls. In cases, however, where the estuary is bounded by a hard beach, presenting a favourable line of direction, a single wall may occasionally be found sufficient. All curves which it may be necessary to introduce should be of as large a radius as possible, and should, if practicable, be tangential to each other, or to the straight parts of the line with which they are connected.

Second, The walls should not be raised to a higher level above the low-water line than is absolutely necessary for the purpose of conducting the early and late currents of the

[blocks in formation]

not prevent the tide at high-water from flowing on either side of them and filling the estuary.

Third, River-walls should, during their erection, be pushed forward with vigour, and not in a desultory, timid manner; the effect of such a course being to increase the depth of water in which the wall has to be made, and the amount of stone required for its construction.

Fourth, It will be found that such walls as we have been describing will be most advantageously formed of rough rubble stones, backed with clay and gravel, in the manner shown in fig. 15.

Fig. 15.

It was found by Mr Park, under whose immediate directions, as local engineer, the walls on the River Ribble, which are about 12 miles in length, were constructed, that their foundations, with few exceptions, did not sink more than a few feet below the sand. He found that it was advantageous to mix clay in the internal core of the wall; and after the materials were deposited, it was necessary from time to time, in certain places, to add additional stones to make up slips, before attempting to pitch the top or the face of the slope. Walls somewhat similar have also been largely introduced on the Clyde by Mr Walker.

6. Scouring.

The removal of hard portions of the bed of a river by dredging or coffer-dams, and the direction of the channel by low walls, are operations which are in themselves improvements; but they further operate beneficially in causing the currents to scour the softer parts of the river's bed, so that it sometimes happens that by dredging a few hundred yards of hard material from a river's bed, or erecting a short wall, thousands of tons of soft materials are scoured away by the action of the current. In all river improvements this is an effect which should be fully taken into consideration by the engineer, especially in forming estimates; and its importance will be apparent on inspecting the section of the River Lune (Plate II.) By dredging the upper shoals of that river, which are marked in hatched lines in the section, the whole lower part of the river was deepened by the natural scour, without entailing any expense in its removal. To facilitate this scour, a species of harrow has sometimes been applied,

which is drawn to and fro by a tug-steamer across the bank to be removed. This system was extensively employed by Captain Denham in opening the Victoria Channel at the Mersey; it was also employed by Messrs Stevenson at the Tay; but it is obvious that it can only be advantageously used where there is deep water in the immediate neighbourhood of the bank to be removed, in which the sand and mud disturbed by the harrow, and carried off by the current, may be deposited. The process of scouring has, in some situations, to the knowledge of the writer, continued in operation for many years after the completion of the original work, the low-water level of the river continuing gradually to sink; and as this process goes on, it sometimes happens that hard portions of the bottom originally covered become gradually exposed. Such obstructions are, in fact hard portions of the bed brought to light, in consequence of the improvement of the river, and must not be mistaken for accumulations due to ill-regulated currents. It is necessary, however, that such hard portions should be removed as soon as they appear, otherwise they disturb the currents and occasion shoals. Whenever the depth due to the currents acting in their improved direction has been reached, such obstructions will cease to present themselves.2

The effect of works executed according to the principles indicated is,-First, to fix the navigable track in a defined course; second, to deepen the bed of the river; third, to reduce the slope, and lower the low-water level; and, fourth, to increase the duration of tidal influence and the quantity of tidal water in the river. The benefits to navigation are threefold:-First, greater depth of water; second, a properly defined channel; and, third, a greater length of time during which, in consequence of the presence of the tide, the river is navigable.

The writer has, before leaving this part of the subject, to state that the works specified are believed to be those most generally applicable. All of them may not be applicable in every case; and there may be special cases which render it expedient to adopt works of a somewhat different, and, in some respects, apparently antagonistic character,-such, for example, as the contraction of channels by means of quay-walls.

[graphic]
[blocks in formation]

1 Mr Rendel, in his address as president of the Institution of Civil Engineers in 1852, says,-" At the present moment changes are taking place in the Thames and most of the principal rivers, which afford invaluable opportunity for observations on the effects rivers can produce by their own action, and also on what is done by the passage of steam-vessels in keeping the lighter silt constantly in motion."

2 Admiral Beechey, in his Observations on the Tides of the River Severn, mentions a fact which it is proper to record. He says, "While upon the subject of the low-water line, it may here be remarked, that the inverse of the ordinary effect of the spring-tide occurs in the river above Lidney. From Lidney downwards to the sea, the low-water at springs follows the general rule of being lower at such times than at the neaps; but above Lidney the reverse takes place, the low-water at the springs being higher than at the neaps. This, no doubt, is occasioned by the tide at springs throwing more water into the river than can escape before the return of the following tide."

VCL XVI.

K

« EelmineJätka »