REPORT ON THE PRESENT STATE OF THE NEW LONDON BRIDGE. BY. MR. CHRIS TOPHER DAVY, ARCHITECT.

(To the Editor of the Mechanics' Magazine.)

Farnival's Inn, Feb. 27, 1832. Dear Sir,-Having on the 20th inst. minutely surveyed along with you the present state of the New London Bridge, I now proceed, agreeably to your request, to state the result of that survey for the information of the public.

A very considerable degree of subsidence is always to be looked for in structures of this magnitude; and wherever such subsidence possesses one uniform bearing, there is little, if any thing, to be feared from it. In laying out the working plans of the New London Bridge, it was inferred, from the soundings that were previously made, and from what had taken place in the case of the Southwark and Waterloo Bridges, built over the same river, and at no great distance from the site of the present, that the centre-arch and pier might settle 9 inches, and each of the others 8 inches; and Sir John Rennie states, that the actual settlement falls short of this calculation (Report to the Corporation of London, Nov. 17, 1831). It cannot be said, however, that the bridge has yet reached its actual bearing; neither is the real state of matters correctly represented, by speaking of the subsidence as merely vertical.

From the survey which we made, it appears clearly that a settlement has taken place in three different directions. The first is the perpendicular settlement of the bearing piers into the bed of the river. The second is a longitudinal settlement of the superstructure from south to north. The third is what may be called a transverse settlement of the entire structure from west to east, that is, sideways, in the direction of the stream.

The effect which these different subsiding tendencies has had on the form and appearance of the bridge, will be found accurately represented in the accompanying drawings. Figs. 1 to 4, represent the present elevation of the cutwaters of the different piers, beginning from the Southwark side. The dotted line, C, is a per fect horizontal line, drawn from the corner of the platband, or string-course, D, of the first bearing-pier on the City side, through all the others, in order to show how the piers have sunk, compared one mother. The lines AB, drawn

The pier and cutwater, Fig. 1, has evidently been the first of the series to settle downwards; and this can only be accounted for from the lesser density of the soil on the Southwark side of the river. The south bank of the Thames was originally at this place a vast marsh or swamp, while on the City side, the ground was firm, and rose somewhat abruptly from the river. In 1823, the whole of the river bed at this part was bored under the directions of Mr. Telford, by order of the Bridge-House Committee; and the borings then made, showed that the rubbish and loose soil on the Southwark side, reached considerably below the level to which the tops of the bearing piles of Fig. 1 have been driven. It has been stated, indeed, by the engineer, the assistant engineers, and contractors, that " every one of the piers was founded on very hard solid clay;" nay, that a considerable thickness of it was removed to get to the depth at which the foundations were laid; (Report of Messrs. Telford and Walker, October 17, 1831), the excavations, too, were made within coffer dams, so as scarcely to admit of any mistake about the nature of the ground; but if the soil on the south bank were not of inferior density--something far short of "very hard and solid," how is the greater sinking on that side to be explained? I am not aware of any other explanation that can possibly be given.

The piers, Fig. 2 and 3, being those on which the centre arch is raised, incline towards each other; and this would seem to arise from the current being much stronger, and of much greater depth in the centre of the river than towards the sides, and from a consequent leaning to the side of least support. Sir John Rennie states, that the piers are

protected from the action of the stream by" 3 complete rows of sheeting piles, 15 inches square, and driven 18 feet into the ground (in addition to the bearing piles) and extending round them for a distance of 30 feet;" and, moreover, that "the bed of the river for the same distance, is 3 to 4 feet above the platforms, and 2 feet 8 inches more above the heads of the bearing piles," (Report, November 17, 1831.) But, on the other hand, Messrs. Telford and Walker have reported that from their "soundings in the middle of the centre arch, the bed of the river at that place is now about two feet six inches under the level of the top of the platforms." (Report, October 17, 1831.) We are constrained, therefore, to conclude, that since the bearing and protecting piles were driven in, the bed of the centre of the river, must have been scooped out to the depth of some 6 or 7 feet; and this would indicate a violence in the scouring operation of the stream

which could not fail, in spite of the pro tecting piles, to affect very materially the solidity of the foundations, laid bare to its action. It seems probable that this result has been very much accelerated by the circumstance of the greater part of the piles, which supported the centering of the middle archi, having been subsequently withdrawn, and the soil having been thus necessarily broken up and loosened to a great depth. It is deserv ing of remark, that in the only other bridge over the Thames in which this plan of raising the centerings was exclusively followed - namely, that of Westminster a similar failure took place; and that, although the piles were not wholly withdrawn, but sawn off under water. The centerings of Blackfriars and Waterloo bridges were supported on the offsets or footings of the piers. In the case of the Southwark bridge, centerings were partly supported on the offsets of the piers, and partly on piles

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driven into the bed of the river-a combination of means called for by the great span of the arches; but the piles were at such a distance from the piers, that the subsequent extraction of them did little if any harm. In no circumstances, however can the withdrawing of piles from such places, and the disturbance of the soil consequent upon it, be considered either safe or prudent; and, least of all, where there is a deep and impetuous current, such as that through the centre arch of the New London Bridge to contend with.

The longitudinal settlement or leaning of the superior part of the structure, from south to north, that is, from the Southwark to the City side, is a fact at seeming variance with the southward inclination of the foundations; but it may be explained in this way. The theory of equilibrium, as applied to elliptical arches, requires that an immense weight should rest upon the haunches-the line of the extrados at that part forming an asymptote with the springing. In prac tice this condition is considered to be fulfilled either by loading the spandrils with rubble, or by the more approved method followed, in this instance, of building hance-walls upon a considerable portion of the extrados. The necessary effect, however, of the great sinking which has taken place in the first pier on the Southwark side (fig. 1) must have been to draw off this required weight from the haunches, and to make it lean backwards and longitudinally, instead of downwards. Nor can it seem surprising that the backward pressure of so vast a body of materials, combined with the natural thrust of the arch, should communicate its influence, more or less, to all the arches and piers throughout the bridge. I think I may, with great safety, venture to assert, that if the roadway of the bridge were taken up, and the extrados of the first arch on the Southwark side examined, the hance-walls would be found in a very fractured state. External symptoms of such a result are even already extremely apparent. Several of the voussoirs at the shoulders of the arch-the very part which in the case we have supposed would be first affectedhave fallen out of their places, and given to the curve of the intrados the tortuous appearance represented in fig. 6. It may be proper to observe, that this distortion

is not readily perceptible when viewing the bridge from the water, and that it is only to be seen properly from the plying stairs on the Southwark side.

The extent of what I have called the transverse settlement-that from west to east-is represented in fig. 5, which is a section through the centre of the first arch, on the Southwark side. The line, CC, marks the varying height of the first course of stones above the water; at the west or upper end, it is 8 inches, and at the east, or lower end, only two inches. The dotted line A, is a correct perpendicular, showing by comparison with B, the present face of the structure, how much it has fallen from its proper vertical position.

It appears, that from the first the east side of the bridge was considered to require greater support than the other; first, because of its greater proximity to the rapids at the old bridge; and, secondly, because the clay stratum was known to be several feet lower at the east than the west side. It was accordingly deemed expedient to cut off and leave in, two rows of the coffer dam piling, in front of each cutwater on the east side, while the dam piles on the west side were withdrawn. But this precaution has been apparently of little avail; neither was it of a nature to counteract any tendency to depression, arising from insufficiency or irregularity in the foundations, or a want of equi librium in the superstructure. As this transverse settlement is uniform throughout the bridge, and corresponds closely with the degree of dip in the clay stratum, it is difficult to resist the conclusion, that there is a connection of dependence between them; though in what particular way the deviation from the hori zontal line has been produced, mnst ever remain matter of conjecture. It may probably have arisen from the length of the bearing piles not having been graduated according to the dip of the

strata.

The resident engineers and sub-contractors, say, that this transverse settlement of the bridge has been caused by the first platform having been “ laid with a small fall to take the water towards

This is the view of the case taken by our correspondent, Mr. Deakin, in an ingenious communication, which we shall afterwards lay before our readers,-Ed. M. M.

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the pumps, which were fixed at the lower or east end of the first pier, and that the other piers having been set out from the first one have the same inequality;" and they add, that "the stones, after having been dressed at the Isle of Dogs, came up in pieces ready to be set, and so perfectly squared, that the variation from a level in the platform was carried up to the top of the bridge." But of this explanation, Messrs. Telford and Walker very truly observe, "it implies a difficulty of execution; which, to say the least, it is not easy to account for." "The points of the piers, the faces of the walls, and the courses of the stone work through the piers, are, with little exception, in straight lines, battering or overhanging, or sloping at the places where the irregularities are, but all nearly corresponding; that is, if the courses of stone in the piers are out of level, the ends of the piers and spandril walls to the top of the arch are nearly proportionally out of perpendicular, but still sufficiently straight, and it is not easy to suppose them to have been built so. We do not think the statement of the stones being so square, as that the work, when once begun on an inclination, naturally inclined to slope and batter to the end, without the superintendents or workmen being aware of it sufficient to account for the irregularities to the present extent." (Report, Oct. 17, 1831."

Fortunately, the abutments at both ends of the bridge have undergone little or no variation; and should they remain firm, and the different settlements not extend much farther, no fears need be enter tained for the permanent stability of the structure. They will have marred its beauty, but that will be all. Were the City abutment, however, ever to fail, the fabric has such a general leaning towards that side, that it would infallibly tumble into the river.

The parapet of the bridge was not set till a considerable time after the centres were struck, and till after the settlements before bescribed must have made considerable progress. But it exhibits, notwithstanding, very evident traces of their disturbing influence; a circumstance from which it may be inferred that the movement has by no means yet reached its termination. The stones appeared to us, in a great many instances, parted to a degree, which no imperfection of work

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manship, or any shrinking of the mortar from the winter's frost, could possibly account for.

It is deserving of observation, that all the imperfections which have been noticed are imperfections of execution merely, and do not in the least detract from the superiority of the elliptical principle of construction. Had it been the principle that was in fault, it would have been at or near the key-stone, that we should have found failures taking place; but though we carefully examined the voussoirs and line of intrados of all the arches, we found the upper part or crown invariably firm and sound. I am, Dear Sir, Yours truly,

CHRISTOPHER DAVY,

HIGHWAY COTTAGES.

Sir, Your correspondent T., in No. 448, has recalled to my recollection a letter which attracted my notice some years ago in the Evening Mail; in which the writer suggested the idea of substituting cottages for milestones along our great public roads.

The subject has often engaged my serious thoughts while employed in road surveying; and although I am afraid there are obstacles almost insurmountable to the realization of the plan, yet the contemplation of the beauty it would give to the roads, and the comfort and security it would impart to the traveller, is so truly delightful, that perhaps you will indulge a new correspondent (though one of your oldest subscribers) in a few remarks upon it.

I do not see the necessity of departing from the present measure of distance (miles); nor do I think the cottages should be more than one mile apart upon the turnpike roads (to which class of thoroughfares, I apprehend, the plan must be confined), as but few of them require less than the labour of a man for every mile during the winter months, to keep them in tolerable repair; and during the summer, his spare time might be advantageously employed in making little improvements upon the sides and footpaths, &c. &c.; indeed, I have rea son to know that turnpike trusts generally, would "find their account" in ex