Caisson (engineering)

“ At any instant firm rock may cease, silt or sand or an underground stream may make its appearance and the helpless workmen find a ready grave. In work where there is even the slightest expectation of such a contingency the air-lock, with its artificial pressure to hold back the soft earth and moisture is brought into use. In another chapter we shall see how the caisson is operated. Suffice it to say now that the necessity of “working under the air,” brings no comfort to any one. It vastly hinders and complicates the work of construction, and adds greatly to the expense. ”

“ The caisson is furnished with an air chamber to give it flotation, which is adjusted by ballast according to the depth of water. The advantages of a caisson, as compared with a pair of gates, are that the gate recesses, gate floor, hollow quoins and arrangements for working in the side walls are dispensed with, so that the lock can be made shorter, and the work at each head is rendered less complicated. ”

“ Evidently the caissons must be filled with compressed air to equalize the external pressure, which is constantly increasing as ever deeper water is reached; they must also have an opening connecting with the surface; and to admit of passing from the ordinary atmosphere to the denser one, there must be an air-lock. Before this bridge was built, the air-lock had always been placed at the top of the entrance shaft, where, as the caisson sank and the shaft was lengthened, it had to be constantly moved up. Eads placed it in the air-chamber of the caisson itself, where it never had to be moved ”

“ Being, however, simple in construction, taking up little space, and requiring no chamber or machinery for moving it, this form of caisson is generally used for closing the entrance to a graving dock, where it remains for several days in place during the execution of repairs to a vessel in the dock. A ship caisson only requires the admission of sufficient water to sink it when in position across the entrance to a graving dock ”

“ Being built up of plates cut to the proper size and shape, it is a very simple matter to rivet on additional tiers of plates as the caisson is lowered deeper and deeper into the bed of the river. The lower length is made with a cutting edge to penetrate the ground; the exterior is made without any projection larger than the rivet heads, and the interior is strengthened with T-irons or double L-irons at the joints, and strong cross-bracing to resist the pressure of the water. ”

“ Air is supplied by powerful pumps and is forced into the working chamber. The pressure of the air of course increases constantly as the caisson descends; it must always be sufficient to overbalance the weight of the water and thus prevent the water from entering the chamber.
Descent to the caisson is made through a tube, generally of wrought iron, and having, at a suitable point, an air-lock, which is substantially an enlargement of the tube, forming a chamber, and of sufficient size to accommodate a number of men. ”

“ In working on a sloping face, whether of rock, silt, or clay, it is good practice to keep the caisson tilted very slightly to the lower side, which has the effect of forcing it up, but this should not amount to more than the natural batter of the outside shell. A few inches one way or the other is of no consequence in so large a caisson, as the pier raised upon it could easily be centred as much as that to either side, but it is just as easy and much better to keep it right from the first. ”

“ When an open shaft is to be dug, pig iron is loaded on the caisson to force it down, but in most work the object is merely to sink a concrete column down to rock and so the caisson is filled with concrete above the deck. Sections are added to the caisson as it sinks into the ground, and these sections are filled with concrete. This method of building the column facilitates the work of laying the concrete, and at the same time provides the weight necessary to overcome the buoyancy of the caisson [118] and the skin friction on the side walls of the excavation. ”

“ A bottomless caisson, serving as a sort of diving-bell, in which men can work when compressed air is introduced to keep out the water in proportion to the depth below the water-level, which is gradually carried down to an adequately firm foundation by excavating at the bottom of the caisson, and building up a quay-wall or pier out of water on the top of its roof as it descends. An example of a caisson with a water-tight bottom is furnished by the quays erected alongside the Seine at Rouen, where open-timber caissons were sunk on to bearing-piles down to a depth of 93⁄4 ft. ”

“ This portion is 70 ft. in diameter, and above it is the taper part, which is 24 ft. high in all cases, and terminates with a diameter of 60 ft. at top.
The lower, or so-called cutting edge of the caisson, is stiffened by a stout double angle and by a broad steel belt, 18 in. wide, and 1 in. in thickness.
At a height of 7 ft. above the cutting edge an air-tight floor is formed which extended over the whole area up to the outer skin or shell. From this floor upwards, started the inner shell of the caisson, about 56 ft. in diameter ”

“ The jacks and blocking supporting the caisson are shown in Fig. 4, Plate LXIV. As soon as the cutting edge entered the rock, which was drilled about 6 in. outside of the neat lines, the space surrounding the caisson was back-filled with clay and muck to steady it and provide skin friction. As the friction increased, the walls were filled with concrete, and as the caisson slowly settled, it was checked and guided by blocking. The cutting edge finally came to rest 31 ft. below mean high water, the sinking having been accomplished in about seven weeks, at an average rate of 0.50 ft. per day. ”

“ But the boy's bent towards science was not to be denied. Between 1887, when "Practical Electricity" first appeared, and 1908, when its author died, his book went through eleven editions. It was then committed to the charge of Professor Mather, whose name appears as joint author on the title-page of the later editions. "Electrical Engineering," from the pen of Harold H. Simmons, first appeared in volume form, in 1908, and afterwards serially. Revised and enlarged by Alfred H. Avery, it has recently been issued as a serial. ”

“ To hold the caissons with some amount of security when once they would take the ground, and give them a good bearing all round the edge, it had been decided to place a large number of sandbags upon the rock, and bring these up to the same level and somewhat above even as the highest point of rock the cutting edge of the caisson would be likely to touch. ”

“ The plates gave way, and a great rent, right across the lower side of the caisson, some 25 ft. to 30 ft. long, was the result. After discussing many plans suggested for getting out of this difficulty, it was resolved to construct a sheath or barrel of 12-in. timber balks all round the caisson, to bolt as many of these balks as could be got at firmly through the iron skin, and to make the best joint that could be got either by puddle clay or cement grouting, or a combination of both. ”

“ In quicksand the material is so fluid that it is unsafe to open the ordinary doors of the caisson, and they are then provided with shutters which are raised one at a time, to permit of operating on a very small section of the head of the tunnel. ”

“ MODE OF FIXING UNDER BEDPLATES ON PIERS.
In these two caissons the tendency to go down hill manifested itself strongly, and powerful timber struts and hardwood wedges covered with steel plates had to be employed to keep the caisson in its position, or to force it back when it had moved. These were set on prepared faces on the rock, as shown in the sketch, Fig. 52, and their action upon the caisson will be readily understood.
The actual blasting of the rock under the cutting edge does not appear to have injured the steel edge in any way ”

“ It is not enough that he should be able to compute the stresses on the various members of a bridge y and even to produce a creditable design; he must know how to erect it without killing his men and without ruining the contractor. If he is a mechanical engineer, he must have that practical knowledge of workshop methods that will enable him to design an engine or machine that will not only work when made, but which can be constructed at a reasonable cost. ”

“ Not the dream of flying but the need for quantity and speed in production to take care of the wants of a modern civilization compelled the invention of the internal combustion engine. Before it appeared in the realm of mechanics, experimenters were applying in the construction of flying models the knowledge supplied by Cayley in 1796, who made an instrument of whalebone, corks, and feathers, which by the action of two screws of quill feathers, rotating in opposite directions, would rise to the ceiling ”

“ It is worked partly by steam and partly by electricity on the overhead system, the current being derived from a generating station three-quarters of a mile away, where three hydraulic turbines, fed by an adjoining waterfall, operate the dynamo. Although the railway is out of the way and on a small scale, the attractions of the Causeway and the surrounding district result in a respectable passenger traffic of over a hundred thousand per annum. ”

“ The running-gear is the fundamental part of the machine, and its whole make-up should be thoroughly understood. The builds of trucks are so multifarious that no specified directions can be given respecting accidents happening to them. There is, therefore, the greater need for an engineer’s familiarizing himself with the make-up of his running-gear, so that, when an accident happens, he will know exactly what to do. Disraeli said: “There is nothing so likely to happen as the unexpected.” This applies very aptly to railroad engineering. ”

“ Robert Stephenson—for he it is whom we suppose to be speaking to his friend on this gigantic engineering enterprise—became satisfied by reflection that the principles involved in constructing an immense tubular beam were but a development of those commonly in use; and Sir William Fairbairn was entrusted with the duty of experimenting as to the strength of tubes, the directors of the Railway Company voting a sum of money for the purpose. ”

“ If he asks, he is told that each machine is driven by a properly protected electric motor directly coupled to its starting gear. The printing machines themselves are of various kinds, according to the nature of the work they have to perform, most of them, however, being adapted to the production of the high-class illustrations which characterize the publications of the House. In 1896 rotary machinery was introduced, and at the present time the majority of the magazines are printed on these machines. ”

“ High speed gives some advantage. A judicious use of cushion helps condensation somewhat also, because when any gas like steam or air is compressed, it gives off heat, and this heat in the cushion will keep up the temperature of the cylinder. This cannot be carried very far, however, for the back pressure of cushion will reduce the energy of the engine movement. ”

“ He also recommends that they should be fixed into guides or chairs of iron supported on props placed at distances in no case exceeding three feet, and that they should be connected with a clamp-joint so as to preserve the whole strength of the material. It is not a little singular that this description, given about forty years ago, may, to use engineering phraseology, be not inaptly called a “specification of the permanent way” of our best railways at the present day. ”

“ Cost of Caisson Work.—The following attempt to get at the cost of caisson work is based largely upon information obtained from Mr. John M. Ewen, John M. Ewen Co., Engineers and Builders, Chicago, Ill. Mr. Ewen says:
"My experience has taught me that it is almost impossible to determine any definite data of cost for this work. This is due to the fact that no two caisson jobs will average the same cost, notwithstanding the fact that the cost of material used and the labor conditions are exactly the same. This condition [Pg 167] is due to the great variety in texture of the soil gone through. ”

“ Complete, these machines are not unlike a flying fish.
In England, Sir George Cayley, as early as 1810 studied and wrote largely on the subject of dirigibles but, though the English call him the "father of British aeronautics," his work seems to have been rather theoretical than practical. He did indeed demonstrate mathematically that no lifting power existed that would support the cumbrous steam-engine of that date, and tried to solve this dilemma by devising a gas engine, and an explosive engine. ”

“ But from the time when George Stephenson was appointed fireman, and more particularly afterwards as engineman, he applied himself so assiduously and so successfully to the study of the engine and its gearing—taking the machine to pieces in his leisure hours for the purpose of cleaning and understanding its various parts—that he soon acquired a thorough practical knowledge of its construction and mode of working, and very rarely needed to call the engineer of the colliery to his aid. ”

“ Although skilled workmen were in course of gradual training in a few of the larger manufacturing towns, they did not, at the date of Stephenson’s patent, exist in any considerable numbers, nor was there then any class of mechanics capable of constructing springs of sufficient strength and elasticity to support locomotive engines of ten tons weight. ”

“ When a mere child, and employed as a herd-boy, he amused himself making model engines in clay, and, as he grew older, never lost an opportunity to learn the construction and management of machinery. After having been employed at Newburn and Callerton, where he first became “engine-man,” he began to study with greater interest than ever the various steam-engines which were then in use; and both the Newcomen engine and the Watt pumping-engine were soon thoroughly understood by him. ”