Before more particularly describing the engineering illustration we give this week, taken from the London Architect, as showing a new and very clever method of building bridge piers, it may not be unintsresting to many of our readers to refer to one or two former contrivances which have been employed for that purpose. Dr. TJre, in his " Dictionary of Arts," men tions what is considered to have been the first application of sinking cylinders through sand and water (quicksand). He says that a mining shaft formed of a series of large sheet-iron cylinders riveted together was sunk to a great depth through the bed of the river Loire, near Languin. The seams of coal in this district of France lie under a stratum of quicksand, from 18 to 20 meters thick—equal to about 58 to 68 feet English—and they had been found to be inaccessible by all the ordinary modes of mining previously practiced. The difficulty of reaching them had been thought so entirely insurmountable that every portion of the great coal basin, which extends under these alluvial deposits, though well-tnown for centuries, had remained untouched. To endeavor, by the usual workings to penetrate through these semi - fluid quicksands, which communicate with the water of the Loire, was, in fact, nothing less than to try and sink a shaft in that river, or to drain the river itself. This difficulty, how ever, was successfully grappled with by M. Triger, an able civil engineer. By means of the sheet-iron cylinders we have mentioned, he contrived with the aid of force-pumps to keep his workmen immersed in compressed air of sufficient density to force back and out of the bottom of the cylinder all the water which was there, and thus enatled the men to excavate the sand, gravel, and stones to such a depth that when the cylinder was sank to a water-tight stratum, the compressed air was no longer necessary. An air-tight cham-her at the top of the cylinder had a man-hole door in its cover and another in its floor; when the men had entered this chamber the upper door was closed, and compressed air from the cylinder was then admitted by means of a stop-cock. As soon as there was an equilibrium of pressure established between the chamber and cylinder, the man-hole door into the cylinder was opened and the men descended to their work. Here they had to work in air at a pressure of about three atmospheres, i.. , equal to a pressure of, say, 44 lbs. per square inch. While the compressed air thus drives the water of th e quicksand out of the shaft, it is said to infuse at the same time such energy into the miners that they can easily excavate double the work, without fatigue, which they could perform in the open air. Upon many of them the first sensations are painful, especially npon the ears and eyes; but they rapidly get accustomed to the bracing element. It is even said that old asthmatic men here become effective workmen, deaf persons recover their hearing, while others are sensitive to the slightest whisper. Much annoyance was at first experienced by the rapid combustion of the candles, but this was obviated by the substitution of flax for cotton wicks. The contrary principle to this of sinking cylinders was proposed by Mr. Potts, a medical gentleman of great inventive ability. His system was adopted in sinking the piers of the Black Potts Bridge, which crosses the Thames near Richmond. Each cylinder was lowered into the river in its proper vertical position, and then loaded sufficiently to make it sink when the greatest vacuum was obtained. The vacuum was produced by means of suction pumps, and then the external pressure of the atmosphere forced the mud, sand, or gravel and water from the bottom of the cylinder up inside of it, thus allowing the cylinder to descend as much. as the displacement of I the material at its base in the bed of the river would allow with the force of its own weight and load. The material thus forced up into the cylinder was scooped, or dredged, out as much as possible, the operation of creating a vacuum being again and again repeated until the cylinder was Bunk to the supposed proper depth. It has been said that some of the cylinders sunk when the weight of the bridge and proving load came on them. This fault, however, cannot be charged to the mode of jinking, for in that case the cylinders could not have been sunk deep enough, or they were imperfectly filled in. At the same time, if the water had been forced or kept out by means of compressed air, there would naturally have been far greater facility for seeing and insuring a good and secnre foundation. The new cast-iron arched bridge over the Medway at Eochester is one of the first bridges built upon cast-iron piles sunk deep into the bed of a river by means of compressed air, used to keep out the water while the workmen were employed in excavating the material inside the piles, and allowing them to sink by means of their own weight and the load placed on them. This bridge is built near the site of the celebrated old bridge at Eochester, and consists of three spans (one an opening span). Each pier is formed oi 14 cast-iron cylinders placed in a double row and sunk through the bed of the river into the hard chalk. All these cylinders were sunk by means of compressed air, to keep out the water while the men were at work in them, in a very similar manner to the method adopted by the French engineer, M. Triger, for sinking his shaft. Mr. John Hughes, Civil Engineer, was the first to adopt this mode of keeping tha watar out of piles while be-ing sunk to form piers in the beds of rivers, and great praise is due to him for the thorough and practical way in whiSh this system was carried out, in sinking seventy cylinders to a great depth in the bed of a strong tidal river like the Med-way. The bed of the river was found to consist of strata of soft clay, sand, and gravel over the chalk, which was reached at a depth of 44 feet below average water line. Each cylinder was like an immense diving bell, always having its top out of the water, no matter at what depth the bottom was. They are formed of cast-iron pipes, 9 feet long and 7 feet diameter, with internal flanges, so that the external faces are free of any projections that would interfere with their free descent through the bed of the river. The accesr to and from the inside of the pile, while being sunk, was through two air-locks or chambers, made of cast iron, passing through the cover-plate bolted on the top length of the pipe forming the pile. The tops of these locks had openings 2 feet in diameter, and flap-doors which, when closed, allowed them to be filled with the compressed air from the cylinders. From each air-lock there was a vertical door opening into the air-chamber, which, when closed, was also air-tight, so that when the workmen had to pass in or out, or to take out the excavated material, they could do so without decreasing the pressure of the air very much. In coming out, they entered, through one of the vertical doors, into one or the other of the air-locks, and when this door was closed, the pressure of the air was reduced to atmospheric pressure by means of a small cock, opened to the atmosphere. As soon as there was an equilibrium of pressure, the top door was opened and the men came out. The operation of entering the pile or cylinder was the reverse of coming out. The only loss of the compressed air from the cylinder at each operation was the amount contained in the small air-lock. Within the cylinder were two small cranes to lift the full buckets and lower the empty ones, which were worked by a two-handled windlass. As each pile was sunk 9 feet, the air-chamber was disconnected and a fresh length of pipe bolted on, and the air-chamber bolted on top of this. At each joint a floor or staging was fixed, with openings to allow of the ascent and descent of the workmen and the full and empty buckets, etc. These cast-iron pipes form part of the permanent structure of the piers, and when they were sunk to their proper depth they were filled in with concrete and brickwork. The method of working was by setting the air-pumps in motion, having the top door of one of the air-locks and the bottom one of the opposite air-lock closed. The pumps were of such a size that in about five minutes 15 feet head of water was forced out through the bottom of the piles; and while the pumping continued the workmen passed through the airlocks to their various stations. The engineering illustration which we give this week shows a more economical method of building piers in the beds of rivers, or under water. It shows a caisson or diving-bell, designed by Messrs. Burmeister and Wain, and adopted by them in building the piers of the new bridge in Copenhagen. The principal economy consists in having the caisson, or cylinder, of less cubic capacity than the finished pile of the piers, and in being able to take it away as each pile was built. When the excavation was made deep enough for a firm foundation, the building of the pile was commenced, and as it increased in hight the caisson was lifted accordingly until the pile was above water-line, when the caisson was removed to the required position of the next pile, and so on, until the two piers, each formed of two piles, were completed. This plan of lifting the caisson avoided leaving the whole of the piles of the piers encased in ironwork, as in the piers of Rochester and many other bridges. This caisson was made of wrought-iron, 18 feet diameter at the lower part by 8 feet high, and ' above this to the air-chamber out of the water it was only 10 feet diameter. Just above the 18-feet diameter chamber there were two annular rings, or chambers—one to contain iron ballast, A, and the lower one water ballast, B, so that in sinking the caisson the water chamber was filled with water for weight in addition to the iron ballast in the annular chamber above. When they had excavated to the solid strata, a bed of concrete 3 to 4 feet thick was formed, and on this the remainder of the pile was built with granite facing filled in with brickwork. As the building of the pile proceeded, the caisson was lifted by means of the suspension chains, C, con-1 nected with staging overhead, and by pumping air into the I annular air-chamber, B, to displace the water. The finished j piles are about 18 feet diameter at their bases, and 16 feet diameter at their tops, by 30 feet high. The whole of the work below water line was done in the 18 feet by 8 feet chamber at the bottom of the caisson. Between the time of lowering it to the bed of the river and the completion of the first pile to water line was only twenty-eight days, and then the apparatus was moved into position for the next pile. In lowering it for the second pil, it unfortunately got upset, and caused so much delay that it took thirty-six days to complete this pile. The third pile, was, however finished in sixteen days, and the fourth in seventeen days. The air-chamber and locks on top of the caisson were very similar to those used for sinking the piles of Eochester Bridge.
This article was originally published with the title "Caissons for Pier-Building" in Scientific American 21, 6, 81-82 (August 1869)