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1856 A cast iron bridge was built to replace the stone bridge. One span was designed to swing open to allow river traffic, but the mechanism was never used and was eventually removed. The cast iron spans were below the road deck, making the bridge relatively low and meant that passing traffic on the river had to navigate to line up with the top of the arch or risk striking the bridge.
Not every ship was successful and many collisions occurred. These took their toll on the bridge and an inspection in 1909 showed fractured ribs and missing bolts. After a relatively short life a new bridge was needed.
From around 1908 the bridge also carried the tracks for the local tram system linking Strood and Frindsbury with Rochester, Chatham, Gillingham and Rainham.
The cast iron bridge was reconstructed at a cost of £95,887. The bridge opened for traffic on 14 May 1914 with new features to allow more clearance for the many boats that had to pass under it. The supporting arched trusses were built further apart and above the road deck. Trams continued to use the bridge until the tram system was abandoned in 1930 and superseded by buses.
'THE BUILDING OF THE NEW ROCHESTER BRIDGE.
Last week we promised to give a description of the mode of constructing Rochester bridge, by means of Dr. Potts's ingenious system of pile-driving using atmospheric pressure. Several bridges have been constructed on this principle, Mr. Joseph Cubitt, for the Great Northern Railway, by Mr. Locke over the Thames at Windsor, by Mr. Hemans over the Shannon. The Rochester bridge is being constructed by Mr. William Cubitt. We cannot better describe the atmospheric system, than by the following extract from the evidence given before the select committee of the House of Commons, on the Westminster Bridge:—(July 9, 1850.)
"William Cubitt, Esq., C.E., M.P., examined — What dimensions of cylinders are you now using at the Rochester Bridge ? The cylinders are 7 feet in diameter, and in this case I should probably put down cylinders of 10 feet in diameter, to make the diameter of one cylinder form the width of one pier, so that row of these very large cylinders sunk, to 10, 20, 30, 40 feet, would make a most durable and excellent foundation, and require no coffer-damming at all.
"Will you explain the mode of operation by which you drive down, or procure such enormous pressure to force those large caissons or piles into the earth?— The piles would be this case, I should say, probably 10 feet in diameter, consisting of hard thick metal hollow cylinders, the bottom edges being quite sharp, and they are in lengths of 9 or 10 feet, turned upon the lathe so together perfectly air and water-tight, and bolted together one upon another as they go down. The mode of driving them is to have a tight cap of the same diameter as the top of the cylinder similar to the top of steam-engine cylinder, and when the cylinder is fixed, to drive it through the water and through the guides to the bottom perfectly perpendicular. The air is exhausted from the inside of the cylinder entirely, and the pressure of the atmosphere upon a 10-feet cylinder would be equal to column of water about 25 feet high, which is a great many tons.
"You fill in those cylinders with masonry, will you not? Yes with solid brickwork.
"Can you conceive any more secure mode of laying a foundation than this plan presents ? —I do not know a better. It is exceedingly good, as we have the means of driving them down; and can do it by two methods, both of which my son has used, both at Huntingdon and at Peterborough. The Huntingdon Bridge is built upon round piles; circular cylinders, one row forming a foundation in the river for one pier, and another row forming foundation for another pier, making two piers, and the bridge is at work as railway bridge. At Peterborough they are now completing another bridge in a more difficult situation, and not so good a foundation as the Huntingdon one, and my son proposed, instead of putting down circular cylinders, which did not fill up the whole space of the pier, to make square cylinders; 6 feet square, and drive them close together, so that they have a large 6-feet cylinder sent down to the bottom, filled with brickwork, which makes a solid brick pier, encased in cast iron.
"Chairman — You would not in that case use the word cylinder ? — No; this is a hollow prism; I have no term for the hollow square.
"Sir C. Burrell — Would the square cylinders be as strong as the circular ones?— Quite. They are quite as strong, because they are filled with brickwork ; they are strong enough to resist the pressure of the atmosphere when there is a vacuum inside them.
"How you fill them ? — By going down in them a ladder.
"Mr. Peto — This mode of construction would admit of making the base of the piers any size that you might require, would it not ?—Yes, whatever you pleased.
"It also presents this advantage, the piles being once driven, that it would not be necessary to disturb the earth by withdrawing the piles with a dam around them ? — Nothing of the kind. At Rochester I should state that I am making piers — there being a heavy tide there, and subject to great rise and fall — with two rows to each pier, two rows of 9-feet piles, and in this case I think one row of 10-feet piles would be ample, or two rows of six feet.
"Will you state to the Committee what saving you suppose you effect at Rochester Bridge by this mode of construction over the old mode of coffer-damming ?—I have a contract going on for making two piers and two abutments, for as small, indeed I think smaller, amount of money than it would cost to make coffer-dams in the same places.
“In what proportion is the amount less ? —Perhaps £2,000 or £3,000 less; the whole contract for the foundation and filling them up will not exceed £25,000 in that large and deep river; coffer-dams for the same work would have cost a great deal more than that. When you say £3,000 or £4,000 more, do you mean for each ? —The coffer-dams for the work to build the Bridge in the usual way would have cost several thousands more than the whole work costs according to this plan.
“Mr. Peto — Do you mean, then, that the cost of the abutments and piers, completely and perfectly executed, will be less at Rochester Bridge by £3,000 in the aggregate than the coffer-dams would have cost? —Yes; and I think a larger sum than that difference in a tideway like that, where the old bridge formerly stood. Driving coffer-dams is a very difficult business, without reckoning the constant expense of pumping them out for months or years during the execution of the work. And what I would strongly press upon the attention of the Committee is, that the two bridges, the one at Huntingdon, over the Ouse, which is finished and at work, and the other at Peterborough, and immediately going to work, over the Nene, are both built; and the last one, at Peterborough, was built under very difficult circumstances, for when they were driving the first, after having bored the ground, they found it pretty well under the first square tube for the foundations, and they worked till they could go no further ; it was not deep enough by several feet ; they then pumped out water and excavated the earth from the inside, and they went down and examined, and they found a stone-floor at the bottom of hard rock, which was some 3 feet and upwards in thickness. Now there the making of the coffer-dam would have been very difficult. They then bored a hole through the stone and tried how thick it was, and afterwards quarried a stone in the inside ; and they made a square hole, rather more than six feet square, and then drove the square 6-feet pile through it till they got the proper depth, and they then filled it full with brickwork; then another was driven alongside of it, and another close alongside that, till they had made a pier of 40 feet long and 6 feet thick; and there was not a coffer-dam used.
“How long do you anticipate the erection of Rochester Bridge will take ?—I suppose, being a heavy tideway, from about eighteen months to two years.
“Charles Fox, Esq., (of the firm of Fox, Henderson and Co.) examined —'Will you describe to the Committee the mode of construction ’— Perhaps the simplest mode of describing it is to say, that instead of using the old-fashioned wooden coffer-dam, which was always a temporary work, we make use of cylinders of iron, which are in themselves coffer-dams, and which remain permanently as a portion of the structure. We adopt various modes of getting them down, but the more general one is this; we have a large receiver of wrought iron, very much like a cylindrical high-pressure boiler, and from that receiver we exhaust the atmosphere, and when we get the cylinder put into its place, just carefully lowered down on to the bed the river, surrounded by temporary frames of timber, so as to be sure that it shall be kept in vertical position, we put a cap on to the top, having an elastic pipe from the top cap to the exhausted receiver, and we, at the proper time, open the communication between the two, and the pressure of the atmosphere the surface of the water in the river produces such a rush to fill up the tube, so as to get rid of any vacuous space, that it carries on a constant state of excavation under the bottom edge of the cylinder, from the pressure of the atmosphere on the top. The atmosphere takes care to push down the pile, aided by its own weight, so as to take up any little space that may have been excavated. When this mode was first spoken of, it was treated with great deal of ridicule, and people naturally said, Why, if the pressure of the atmosphere will push the pile down, when the pile is down it will not carry more than a weight equivalent to the pressure of the atmosphere ;’ and a very practical man raised that objection; not a very scientific man, but a man of very great experience; and I said to him, ‘Now you are quite wrong, for the principle is, that it acts as a sort of excavating process; it is quite true that the pressure of the atmosphere on the top is useful, it gets over any little friction on the sides of the tube, so as to enable it to follow into the excavated space, and without that principle we could not push the cylinder down at all.’ To prove this, we took a 6-feet cylinder, and calculated what the pressure of the atmosphere upon that cylinder would be, and taking the whole pressure of the atmosphere, it amounted to about 30 tons. I had 30 tons of iron rails placed on the top of the cylinder, and the only result was, that it pushed it down about three-quarters of an inch into the gravel, and brought it to bearing; but it did no more.
“Chairman — Was that upon a cylinder of 6 feet in diameter? —Yes; we then took off the 30 tons of iron rails and put on the cap and opened the communication with the exhausted receiver, and the cylinder immediately descended into the solid gravel 6 feet 6 inches by one impulse.
"Having descended only three-quarters of an inch before? Only three-quarters an inch; it just pressed it in a little into the ground with the dead pressure of 30 tons. We then removed the cap, and put on the top of the pile 100 tons of rails; but we could get no depression, except some three-quarters of an inch, which was done by the little compression that you would have from the weight of the edge of the cylinder the gravel. That is the general mode of sinking these cast-iron cylinders. But it will obvious to the Committee, in the event of our meeting with, say, the trunk of an old tree, or a very large stone, we could not proceed any further, and we have had to devise many means of getting over any difficulty of that kind. In the case of the bridge at Nene, we have had to go through not only a layer or two of gravel, but through 2 feet 6 inches of solid rock, and that rock not lying in a horizontal position, has offered difficulties which, under other circumstances, would be very expensive to overcome. To enable us to get through any unforeseen matter, it is necessary to get into the cylinder and excavate any material that may be within it, and cut through the obstruction and to do that we have devised a means by which we convert the cylinder virtually into a diving bell; that is to say, we fix a cap on the top of the cylinder, and the air-pumps are constructed so that they are, when required, compressing pumps, and we can pump just enough air into the cylinder to make it counterbalance the pressure of the column of water without, by which means keep the work perfectly dry, and the men can get at it just as well if they were working in this room.
“What is the greatest depth to which you have driven a single cylinder ?—I think the greatest depth to which we have driven a single cylinder is about 19 feet; but one has been driven in the Goodwin Sands 65 feet, by the same process.
"The Committee understand that the cylinders are not single, but are piled one upon each other to the required depth ?—Exactly so ; they are generally used in 9-feet lengths; the piles for the bridge at Rochester are of two diameters, they are 6 feet and 7 feet, and they are cast in lengths of 9 feet, with flanges at the top and bottom, which are accurately turned and fitted together, so that they drop on to one another; there is a projection.
"The external water will be found to be effectually excluded by such a mode of junction ?—Perfectly; we never have drop through them; they require nothing more than a single coat of paint, and when use one of the eastings we clean the flange carefully and give it one coat of good red lead paint, and put another down upon it, which is prepared the same way; they never leak a drop.
“Do you recommend as a general principle the adoption of a cylinder or of a square form:—Generally a cylinder, for several reasons. “Will you state the reasons ?—In the first place, because it is the cheapest form to construct the preparation of the casting itself; and in the second place, because it better capable of bearing pressure, and therefore can be cast with a much less quantity of material in it; the object in a foundation being to get the largest bearing surface at the least possible cost; in the third place, because we have found in practice that it is difficult to sink square caissons close together, because, having a very small space between them, one having been sunk, it is very apt to make it difficult to sink an adjoining one, have no ground between them to work upon.
“The Committee understand likewise that there is round each cylinder a girdle of timber, which is necessary in order to keep the cylinder in its perpendicular position ?—Yes; I have made use of piles upon which temporary frames are fixed, and put two rows of what we call wallings, forming a square space, in which the cylindrical pile is placed and driven by means of the pressure on the cap of the cylinder.
"Are the Committee to understand that the surface, or the bed of the river, is in the first instance level, in order to receive the cylinder? - Not at all; we deal with it as we find it.
"You use no mechanical means, except the experiment to which you have adverted, of 30 tons and 100 tons of actual weight; you have recourse rather to physical means of exhausting the air and admitting the pressure of the atmosphere?—Yes, because it is so much cheaper. It is a serious job to put 30 tons on to a pile, whereas a simple cast iron cap, as I have before described, put on to the top, is so exceedingly easy.'