Autobiography of Sir John Rennie, F.R.S., Past President of the Institute of Civil Engineers / Comprising the history of his professional life, together with reminiscences dating from the commencement of the century to the present time.

by Sir John Rennie

In my professional career I consider that I have executed the following works:

I. London Bridge. This was designed by my father, as far as the general outline and proportions, but he did not live long enough to design any details, such as the depth of the arch-stones and those of the inverted arches between the main arches, or the adjustment of them, so that the whole might be placed in a perfect state of equilibrium, not only as regards the individual arches, but also with each other; neither was the width of the foundations of the piers and abutments given, nor the extent of piling necessary, the cornice and parapets, stairs, pilasters of the piers and abutments, the construction of the cofferdams and centres; the specification as to what materials should be used, and how they were to be put together; the approaches to the bridge on both sides, or how they were to be designed and put together; all these had to be worked out and executed by myself. It is true that my brother George gave me his advice when I required it, but still I was the sole engineer, and the whole responsibility rested with myself. The execution of these works was rendered much more difficult than intended by my father, for at his death the site was that of the old bridge. But the Committee of the Corporation of London insisted that the new bridge should be built immediately above the old one, the latter to be left standing during the construction of the new bridge. I was therefore obliged to build it in the deep hole above the old bridge, which was from 25 to 30 feet below the level of low-water mark of spring tides.

II. The completion of the great works of Sheerness Dockyard. These, as I have said, had been wholly designed by my father upon an entirely original and novel plan of hollow walls, which he first carried into effect at Great Grimsby Docks, in the year 1786. These walls, though composed of a mass of materials of the same weight as ordinary dock walls, were distributed over a wider area, and pressed less heavily upon that surface in proportion to their extent, and therefore the soft, sandy foundation upon which they were built was able to bear them without yielding; the increased friction also produced by the increased surface of their base enabled them to withstand with greater effect the lateral pressure of the earth behind them; thus a double object was gained, namely, security against both vertical and lateral pressure.

When my father died, on the 4th of October 1821, the northern half of the new dockyard, including the sea wall, the great basin, the three large dry docks at the west end, and the mast ponds and locks, had been nearly completed; so that it only remained to fix iron gates for the dry docks and those of the mast and boat ponds, which had been already designed and ordered, and were put into their places under my direction. This portion of the dockyard, although comprising the most extensive and costly part, was not the most difficult. The most arduous task still remained, namely, the construction of the northern portion. Here was the greatest depth of water, varying from 25 to 30 feet at low water of spring tides, the worst foundation, and the situation was much exposed to northerly and easterly winds. These obstacles were felt so strongly by my father, that he originally contemplated carrying out the works by means of the diving bell; but as so much experience had already been obtained by the employment of cofferdams in similar constructions, where they had been very successful, it became a question for my serious consideration whether it would not be better to use cofferdams for the northern portion of the dockyard, instead of employing the diving bell, which would necessarily require much more time. After consulting with the enterprising contractors, Messrs. Jolliffe, Banks, and Nicholson, who had completed the works already made, and Mr. John Thomas, the experienced resident engineer, we came to the unanimous conclusion that it was perfectly practicable to construct the remainder of the works by means of cofferdams; and although it would be rather more expensive, nevertheless they could be done much better and far more speedily than by the diving bell; and, indeed, they told me that my father had expressed the same opinion before he died; and that there was little doubt but that if he had lived he would have recommended cofferdams instead of the diving bell. I consulted my brother George upon the subject, and he was of the same opinion. We resolved to recommend that the remainder of the works should be completed by cofferdams, and the Admiralty approved of our recommendation. Messrs. Jolliffe, Banks, and Nicholson therefore undertook the contract for these works at the sum of 845,000l., and gave ample security; and they were most successfully finished for the sum of 854,000l. in round numbers, or at about 9000l. beyond the contract price, our estimate being nearly 900,000l.; so that they were actually completed for about 45,000l. below our estimate, and fully three years sooner than they would have been if the diving bell had been used. Of course the real merit of these works is due to my father; but I claim some credit for having successfully carried them into effect, for if any failure had taken place—and there was very great difficulty and risk—I should have been blamed for it, and probably been ruined at the outset of my career, as the whole responsibility rested with me; my brother never went near them.

III. I finished the Chatham dry docks, commenced by my father, at the cost of 100,000l. In these there was nothing remarkable; after those of Sheerness they were much less difficult, although of a somewhat similar kind.

IV. The next great work was the finishing of the great breakwater in Plymouth Sound. The chief merit I claim for this is in adding the benching or berm on the outside, at the base of the sea slope, which breaks the sea before it reaches the slope and prevents it from acting injuriously upon it. I also claim a certain portion of the credit for arranging and executing the paving of the upper surface, and the dovetailed masonry of the two ends of the breakwater.

V. The design and execution of the Royal William Victualling Establishment, at Stonehouse, near Devonport, I claim entirely as my own, with the exception of the machinery, for which my brother George is entitled to an equal share of credit with myself. This establishment, including the cost of the land, amounted, I believe, to between 600,000l. and 700,000l.

VI. The great basin, two building slips for first-rates, mast slip, and the river wall in front, at the Royal Dockyard at Woolwich, costing 340,000l.

VII. In company with Mr. Joseph Whidby, Mr. Walker, and Captain Fullerton, of the Trinity House, I made a report for removing the bar, by means of dredging, at the entrance of Portsmouth harbour, upon which there was only 13 feet at low water of spring tides, which we estimated at 55,000l.; and it is singular that this important work was never carried into effect until many years afterwards, when it proved to be completely successful as far as it went. The bar was lowered 5 or 6 feet, and it might be lowered 8 or 10 feet more, so as to enable the largest class of vessels to enter and depart at low water of spring tides, which would be of the greatest possible advantage to the public service; and although the Admiralty have not carried the dredging far enough, still there is now 18 feet at low water of spring tides, which enables the largest class of vessels to pass the bar at half tide, instead of only at high water as before. This fully proves the value and correctness of our joint report; it only now requires that our recommendation should be carried further, and there can be little doubt that it will be successful. This great national harbour will be rendered accessible at low water, and it ought to be, particularly after the enormous sums that have been expended upon it, for unless the depth over the bar is increased all improvements will be comparatively valueless. Mr. Murray and myself wrote a joint report to the Admiralty, recommending that, in order to assist the dredging operations over the bar, a sluice should be erected across the entrance to Langston harbour, with the gates or doors of the sluice pointing inwards, so that at high water they might be shut, and all the water, or so much of it as might be required, should be sent through Portsmouth harbour at ebb tide, to assist in scouring down the bar. Of course, in order to render these works effective, it would be necessary to enlarge the connecting channel between Portsmouth and Langston harbour, so that all the Langston tidal water should flow out through Portsmouth during the time of ebb.

The Admiralty up to the present time have not adopted this report. They must, however, in order to preserve the requisite depth over Portsmouth bar, do either the one or the other, or both; that is to say, they must increase the dredging operations, or send more tidal water over it, and the latter can only be obtained from Langston; as this harbour is of little commercial value, supposing that any partial silting up should take place, the depth could be restored by dredging; but if both the dredging of Portsmouth bar and the additional quantity of tidal water from Langston harbour should be resorted to, the bar might be kept down to the depth required, and Langston would not be injured. If these two operations are skilfully conducted, so as mutually to assist each other, the result will be successful, and this success is the more necessary, in consequence of the quantity of land which is now being reclaimed from Portsmouth harbour for the new works.