530 



BRIDGE. 



PLATE 

 XCV1J. 



Practice pile in the front of the dam ; one through the mid- 

 dle leading frame, and the other one foot below low 

 water common spring-tides, at which place the low- 

 est leading beam on the outer row of piles was Hxed. 

 The two rows of piles were kept together at the 

 tops by means of a piece of strong timber being 

 gained on the side of each main pile on the inside of 

 the dam, and being spiked down on the outside lead- 

 ing beam. 



This description, with the representation in Plate 

 XC VII. will, we trust, convey a distinct notion of the 

 progress of this difficult operation. The descrip- 

 tion may, no doubt, to some appear minute, but those 

 engaged or interested in similar works will not think 

 'it more than sufficient ; and we consider it a fortu- 

 nate circumstance, to be enabled to furnish so mi- 

 nute and faithful an account of it. 



OF PIERS. 



Of I'ier, The piers of most of the ancient bridges in Eng- 

 land were made of great thickness, in proportion to 

 the span of the arches which they sustained. In Lon- 

 don bridge some of them are larger than the original 

 openings of the arches. 



These piers consisted of small rubble stone laid in 

 lime-mortar, surrounded merely by a thin casing of 

 squared stones. It is probable that the Roman 

 bridges, whose piers were said to be equal in thick- 

 ness to one half the span of the arches, were con- 

 structed in the same manner. But these piers oc- 

 cupying so great a proportion of the breadth of the 

 river, either raise a head, and tear up the bed, or 

 cause a very great widening of the banks. In order 

 to avoid, or as much as possible to lessen these in- 

 conveniences, the piers have been reduced in thick- 

 ness, and constructed wholly with squared stones, 

 each course being of equal height quite through the 

 body of the pier. In Bewdly, Conon, Dunkeld, and 

 J-/t>vat bridges, Mr Telford has made them from 

 about a seventh to less than an eighth part of the 

 span ; and at Neuilly, Mr Perronet has given only 

 one-ninth part of the span to his piers. All these 

 measures are taken at the springing of the arches. 



In discussing the principles of bridges, it has 

 been demonstrated, that the thickness ought to be 

 regulated by the span and rise of the arches, com- 

 bined with the height of the piers ; we have there- 

 fore cited the before-mentioned examples, merely as 

 being some of the narrowest already practised with 

 success on a large scale, but upon which engineers 

 ought in future to improve. 



Their In determining to what height piers ought to be 



height. carried,- great attention should be paid, not only to 

 the highest points to which, from the best evidence, 

 the water appears to have risen, but also to the na- 

 ture of the country where the bridge is situated. If, 

 in the course of the upper part of the river, there 

 are large lakes, or it the adjacent country is very 

 flat, there .can be no risk of the flood rising to a very 

 great height j but if there are no lakes in the course 

 of the river, and if the bridge is placed -where the 

 adjacent country on each side is above its level, then 

 the floods being thus confined, may possibly rise to a 

 still greater height than has been formerly noticed. 



For example, in the rivers Ness and Lochy in Scot- Pructii 

 land, which in their upper parts are connected with v v~ 

 large lakes, the floods never rise more than four feet 

 above the level of low %vater ; but in the river Clyde, 

 where there are no lakes, the floods have lately, near 

 to Glasgow, risen 20 feet ; and the river Severn, in 

 England, since it has been embanked in the low 

 lands in Montgomeryshire, has frequently, in some 

 parts of Shropshire, risen 15 feet. 



Unless the bed of the river consists of rock, or is 

 completely protected by inverted arches or well-con- 

 structed causeways, it is advisable to raise the piers 

 as near to the line of ordinary floods as due regard to 

 the arches and^access will admit of; and at all events 

 to avoid finishing them under the line of ordinary low 

 water. 



The dimensions of the piers having been deter. Their 

 mined, the next step is to consider their shape. Tht shape, 

 portion of pier which supports the arch is usually of 

 an oblong form, with its sides right-lined and paral- 

 lel ; under low water, the pier increases in breadth 

 downwards to the foundation, at the rate of from 

 one inch in the Pont Royal to nine inches in the 

 bridge of Neuilly, for every foot in height, and the 

 platform extending from two to six inches beyond 

 the masonry. The rate of this increase of breadth 

 must be in some measure regulated by the nature of 

 the foundations, and the proportions which the body 

 of the pier twars to the span of the arches. In Neuilly, 

 the thickness of the pier being at the springing of the 

 arches only one ninth of the span, it became neces- 

 sary to spread the base, in order to embrace a con- 

 siderable extent of ground ; but as in practice it is not 

 easy to get the workmen to make the back part of 

 the beds of the outside stones equally perfect with 

 that which is near the face, these large offsets 

 throw too much of the weight upon the imperfect 

 workmanship of the beds, and should therefore be 

 avoided. An increase at the rate of three inches for 

 every foot in height, appears preferable, and has been 

 adopted ,in several of the British bridges. Large 

 offsets certainly afford an opportunity of supporting 

 the centres very advantageously ; but this may be 

 sufficiently obtained by having them three or four 

 inches in the stone-work, and making the wooden 

 platform to project considerably more around the 

 pier. The shape of the points of the piers are, 

 1st, Acute-angled; 2d, Right-angled; 3d, Semi- 

 circular ; and 4-th, Having two segments of a circle 

 intersecting each other. The 3d and 4th seem 

 the preferable modes. These projecting points usual- 

 ly diminish from the line of each side of the piers, 

 though we have seen them formed upon a narrower 

 base than the breadth- of the pier, leaving a square 

 projection on each side ; but these projections ob- 

 struct the current, and cause a head which frequent- 

 ly injures the foundations. In the case of St Maxence, 

 Perronet has departed from the right-lined figure, 

 and also divided the body of the pier longitudinally 

 into two parts, leaving an opening between them 

 equal to their thickness ; but as every water conduit 

 should have its sections equal, and its course as di- 

 rect as possible, this mode seems very objectionable ; 

 it likewise diminishes the strength of the pier : and 

 as he has made an inverted arch under the opening!, 



