314 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[OcTOBEB, 



BLACK FRIARS AND WESTMINSTER BRIDGES. 



Two of the metropolitan bridges over the Thames — those of 

 Westminster and Hlackfriars — are giving way, and threaten de- 

 struction, in consei)iience of the sinking of some of tlie piers. 



The ruin of these bridges will inevitably take place if some 

 immediate and effective means be not employed to prevent it; and 

 the consequence will be, a loss to the public of niillicns, a great 

 inconvenience for many years to the thoroughfare between both 

 sides of the Thames, and to the navigation in tlie river. Let us 

 inquire what is the cause of that threatening disaster? The pro- 

 bability of such a result was foretold, and debated long before the 

 removal of the old London Bridge. In 17G6, Snieaton gave an 

 opinion, that if the fall at London Bridge was reduced, the naviga- 

 tion above the bridge would be injured by a reduction in the depth 

 of water; and that tlie transverse section of the bed of the river 

 would be altered, and in many places lowered, in consequence of 

 the increased current of the water. Time has shown, that in this 

 his opinion was correct, but not so in that; liecause the navigation 

 has been much improved by the removal of the old London Bridge, 

 which impeded the uniform flow of the water in the river, and 

 dammed it up, causing a fall under the bridge, dangerous to the 

 passage of boats and barges, except at the time of high water. 

 This damming up of the water had for result to keep the current 

 nearly null on tiie bed of the river above the London Bridge, 

 because the greater the transverse section of the water, the smaller 

 in proportion will be the velocity required to transmit a given 

 <|uantity of water in a given time. This reduction of tlie velocity, 

 in place of excavating tlie bed of the river, allowed it rather to 

 fill up by the accumulation of mud or fine sand drifted from the 

 higher part of the river. But as soon as the obstacle to the water 

 was removed by the removal of the old bridge, then, as was anti- 

 cipated by all parties (I believe), the current on the bottom of the 

 river was increased, the mud and fine sand were removed down the 

 river, and during freshets the larger sand and gravel were also re- 

 moved; and as the piers of the bridges diverted the current, espe- 

 cially at low water, into particular channels, leaving other parts op- 

 posite the piers dry, or with very little water and current thereon, 

 tlie deejier these particular channels became the stronger the cur- 

 rent would at all times thereafter be; and thus tlie transverse section 

 of the river's bed has become deformed, by being successively and 

 continually deepened between the piers. But this deepening 

 between the piers caused an incline plane to be formed, from the 

 piers towards the middle of the arch, or the centre of the excava- 

 tion formed by the current; and the gravel (of which the bed of 

 the river is chiefly composed in the London district) would 

 naturally roll or slide down into the cavity of the channel, and be 

 carried away with the current. This work going on during a 

 succession of years, has, of course, reduced the level of the ground 

 around the piers, and under some of them; the consequence is too 

 apparent to be doubted, and if not speedily remedied, may be 

 deplorable. 



That Westminster Bridge, under the circumstance I have endea- 

 voured to explain, should first show symptoms of dislocation, is 

 what might naturally be expected, because the piers are founded on 

 caissons only, without any piles to sustain them, whereas the piers 

 of Blackfriars Bridge are founded also on caissons; but the bed of 

 the river under these caissons is piled, perhaps not very deep; but 

 tlie ground being piled, whether deeply or otherwise, should, and 

 has, resisted during a longer time to the action of the curi-ents, as 

 above described. If any other proof than common sense and 

 reflection were wanting for the accuracy of these deductions, it 

 will be found in the circumstance, that during the construction of 

 M'estminster Bridge the bargemen had imprudently been allowed 

 to remo\'e gravel from near to one of the piers, so that the pier 

 near to which the excavation was permitted, sunk as soon as the 

 centres of the arches were removed, the 25th July, 1747, which 

 accident canseil the pier and two of the arches to be taken down 

 and rebuilt, thereby preventing the bridge being opened for the 

 jiiiblic till the year 1750. 



Blackfriars Bridge was not finished till 1770, being 20 years 

 later than that of Westminster, so that even the benefit of the 

 piles under the caissons does not appear to have enabled it to 

 withstand the mining action of the currents on a gravel bottom. 

 Neither will the Waterloo, the Southwark, or the new London 

 Bridges be exempt from this casualty, if the foundations of the 

 ])iers be not imbedded deep enough, beyond its influence, or that 

 means be not taken to pre^■ent the further progress of the action 

 of the current on the bed of the river. 



We may now inquire what are the means hitherto employed, or 

 suggested to remedy the work of destruction now going on, already 

 so effective on two bridges not yet one hundred years of age, while 

 the age of similar structures elsewhere are meted by thousands of 

 years! — the old London Bridge, with all its defects, had endured 

 nine hundred years. 



In regard to ^V'estnlinster Bridge, which had no piling under 

 the caisson, and under which the action of the current had formed 

 a deep channel in the bed of the river, under several of its arches, 

 on a level with, or lower than the bottom of, the caissons. It was 

 determined to construct cofferdams round the several piers thus 

 undermined, and then to drive piles all round the foundations, 

 with the intention, no doubt, to keep the foundation from slipping, 

 and to keep the soil or gravel from going from under the pier. 

 Hut the remedy thus applied unfortunately only tended to increase 

 the evil, inasmiicli that, during the driving of the piles, the con- 

 cussion given by the driving, as well as the grapplings and anchors 

 used fur this purpose in that part of the river, would facilitate and 

 urge the already too prone disposition of the soil or gravel to fall 

 into the depth of the channel, and be carried off by the current. 

 And every idle thus driven, besides causing the removal of a large 

 quantity of gravel from under and around the piers within the 

 sphere of its action, would also lessen the passage for the water, 

 and consequently increase the velocity of the current, which 

 would be thus [irogressively increased; and its action in the work 

 of undermining the pier would also go on in a proportional pro- 

 gression, till the cofferdam could be completed, which is not the 

 work of a day, of a week, or of a month, whereas the action of 

 the current is incessant and loses no time, but will increase in 

 energy as the pile-driving proceeds; and if, when the cofferdam is 

 closed, putting the foundation of the pier in apparent security, the 

 current then being contracted into a more narrow space, will, as if 

 in retaliation, act with so much the more energy in deepening the 

 channel, and thus tend the more to undermine the piers, and even 

 the very piles which were driven for to baffle its efforts. That 

 this is the result, has been fully shown, and will at all times, under 

 similar circumstances, be so, is evident from numerous examples 

 which could be cited, if it were necessary; but those who will not 

 believe what they see, cannot be expected to take for granted 

 what they are told. AV'hat I have here endeavoured to explain 

 has, I believe, taken place at Westminster Bridge; during the 

 time the piling was being proceeded with, the piers under that 

 operation sunk so considerably as to endanger the falling of some 

 of the arches. It was then decided to apply centerings to prop up 

 the arches, — thus one expedient invariably leads to another. Let 

 us examine for an instant the consequence which may be expected 

 from this new expedient. I have already ventured to affirm that, 

 after all the Jiiling they have applied, and if as many more were 

 added, it would not give stability to the piers, but, by contracting 

 the water-way, would render the destruction of the bridge, if pos- 

 sible, more certain. If, then, under that dilemma, being certain 

 the piers will continue to descend, while the arches are by centres 

 retained at their present elevation and position, is it not evident 

 that the pier will be separated from the arch, and thus the bridge 

 would become completely dislocated, never uguln to be re-united 

 until the pier be secure, and until the arch be rebuilt.-' 



What is now going on at Blackfriars Bridge? They are titing 

 and tatting about the channel, under the bridge, and about the 

 foundations of the piers, and after many weeks delay in searching 

 out what might ft once have been inferred, it has been discovered 

 that the foundations of the piers are degraded, and the bridge 

 consequently in a dangerous state ; all this can be discovered by a 

 superficial observer walking along the bridge, without either a 

 diving-bell or a sounding-pole; the crushings and fractures, and 

 variations of levels — taking place on the parapet from day to day, 

 with a fearful rapidity — seem to announce the downfall of the 

 bridge as near at hand, while nothing appears to be doing to pre- 

 vent so deplorable a catastrophe, which, in case of the event taking 

 place, will be anything but creditable to the country, and more 

 especially to our engineering community. 



It is in contemplation, say they, to stop the thoroughfare on the 

 bridge, and to erect centres under the arches, as at Westminster 

 Bridge ; this will certainly be an interesting feature at the great 

 Exhibition, to exhibit to the world, that after building costly, and 

 what might well be deemed efficient bridges, that from unjustifiable 

 parsimony, or neglect, we allow these bridges prematurely to perish ! 

 I have already said, and endeavoured to illustrate, what will be the 

 result of piling and centering — it will aggravate the evil, and 

 accelerate the destruction of the bridges to which it may be ap- 

 plied. — What then is to be done ? 



