357 



BRIDGE. 



BRIDGE. 



3S< 



ascertain the real character of the formations upon which any great 

 structure is about to be erected ; for the indications obtained by means 

 of trial piles are very likely to prove deceptive, because the resistance 

 to their descent may frequently be so great in an upper stratum lying 

 upon a soft under stratum, as to lead to very false conclusions on the 

 :ubject of the ultimate resistance to a vertical load. 



If the mtural foundations should however have been studied care- 

 fully, in the manner above recommended, they must be treated according 

 to their peculiar characters, as follows : The rocks and other resisting 

 strata must be carefully levelled, without destroying the asperities on 

 their surfaces which might be likely to increase the friction upon the 

 bed. Any great inequalities of surface may be overcome by the appli- 

 cation of a bed of concrete ; but whenever this material is used, it 

 must be composed of the best description of hydraulic lime or cement, 

 and prepared with great care, so as to avoid anything like unequal 

 settlements. When the natural foundations are composed of the sands, 

 loams, and such heterogeneous materials, it becomes necessary to adopt 

 precautions for the purpose of preventing any tendency to undermining; 

 and with this intention, the position about to be covered must be sur- 

 rounded with an inclosure of wood or iron piling ; and, in rapid rivers 

 especially, the level of the new foundations must be carried down so 

 far as to obviate any danger of their being laid bare, by the removal of 

 the subsoil, by the waters likely to find their way under them. The load 

 must be distributed evenly over surfaces of this description, either by 

 means of a timber platform, a coating of concrete, or a bed of large 

 stonec. 



When the natural foundations consist of compressible strata, such as 

 peat, soft mud, or other analogous alluvial deposits, of a considerable 

 thickness, it becomes necessary to resort to the use of piles of wood or 

 of iron ; and when, as in the majority of the river-beds of India, there 

 is a great depth of running sand, it may become necessary even to sink 

 wells, which are subsequently to be filled in with concrete. When 

 wooden piles are used, care must be observed in the selection of the 

 kind of wood employed ; for beech, the material most usually adopted 

 on the Continent, decays rapidly when exposed to alternat ons of wet- 

 ness and dryness; elm resists more satisfactorily in situations so 

 exposed ; whilst oak and fir, especially if free from sap in the com- 

 mencement, will last longer than either of the above woods. There is 

 another danger to which wood piles are exposed, namely, the attacks 

 of the boring worm ; and as it appears that the process of creosoting 

 renders wood so distasteful to those creatures that they will not touch 

 it when so prepared, it would seem to be essential to subject all piles 

 to that process. It is said that the boring insects exercise their ravages 

 more particularly upon piles driven into the strata able to furnish them 

 with the carbonate of lime requisite for their growth ; but, unfor- 

 tunately, they are nearly as destructive to piles driven into silicious 

 sands, and the only places which seem to enjoy immunity from their 

 attacks are those situated near the outlets of sewers or drains containing 

 much sulphuretted hydrogen. 



Piles are called, according to the positions they occupy in a founda- 

 tion, 1, Gauge ptltt, or those driven for the purpose of denning the 

 limits of the ground, or of guiding the position of the rest of the 

 work ; 2, Sheet or dote pilet, which form a continuous inclosure, the 

 term close piles being used when whole baulks are employed, and sheet 

 piles when timber of smaller scantling is employed. These various 

 descriptions of piles are united by whales, or horizontal ties, by cross 

 braces, and occasionally by struts. Their entry into the ground is 

 facilitated by cutting the ends into a pyramidal form, and by fastening 

 either oast or wrought iron thoti upon the point ; the heads of the piles 

 are hooped with wrought iron, in order to prevent their being split in 

 driving. Occasionally however piles are driven with a broad flat end, 

 as in foundations upon a great thickness of compressible mud ; and 

 Peyronnet considered that this system was advantageous when the 

 middle of the piles waa situated at nearly the mean level of the water. 

 It is important, when piles are thus driven with the small end down- 

 wards, to begin by the outer ones, and then proceed inwards. 



The weight to be brought upon the top of a pile is usually calculated 

 upon the supposition that it would derive no support from the ground 

 into which it is driven ; and Rondelet has given a series of tables in 

 which he states the safe load to be placed on piles of certain heights 

 and scantlings, calculated upon the principle of their being able to sup- 

 port those loads in the open air. It may briefly be stated that, provided 

 the height of the pile should not exceed 16 times the scantling (or 

 diameter), it may safely be loaded with from 430 to 500 Ibs. per 

 superficial inch of the sectional area. 



When the piles are driven into soft ground of considerable depth, it is 

 customary to leave off driving when a certain resistance to their progress 

 is attained. This resistance is made proportionate to the weight of the 

 ram, and to the height through which it falls ; but it must be observed, 

 that there is danger in the use of any description of machinery by 

 which frequent heavy blows are struck upon the pile-heads ; for in such 

 cases, their heads may either be set on fire, or knocked up into a 

 apongy mass, or the piles may be split. Moreover, as there is a ten- 

 dency, when frequent heavy blows are struck upon the piles, to produce 

 a state of vibration in the ground around them, which may seriously 

 interfere with their descent, it would appear to be preferable to employ 

 for pile-driving only such machinery as is capable of striking heavy 

 blows at tolerably distant intervals. Of course, however, this rule 



must be considered to be exposed to many exceptions, and great judg- 

 ment will be required in the choice of the mode of pile-driving to be 

 employed in any particular case. The very valuable machinery intro- 

 duced by Mr. Nasmyth has, for instance, failed as signally in some 

 cases as it has succeeded in others. 



Some very ingenious systems of sinking what may be termed hollow 

 cylindrical piles of large diameter (which are subsequently filled in 

 with concrete in the same manner as the brick wells before alluded to, 

 as being used in India), have lately been introduced in England. The 

 first serious attempt of this nature was made by Dr. Potts, who 

 caused the cylinders to descend by pneumatic pressure, and upon this 

 system founded very successfully some of the bridges on the Rich- 

 mond, and the Chester and Holyheacl lines of railway. Great improve- 

 ments upon this system have been made by Mr. Hughes, who directed 

 the works of the Rochester bridge, under Sir William Cubitt ; and 

 more recently still, the continental engineers have applied upon a great 

 scale the method of sinking cylinders by the use of pneumatic 

 machinery. A description of the methods respectively adopted may be 

 found in a small brochure published by Mr. Hughes ; and in vol. xvi. 

 of the ' Annales des Travaux publics de la Belgiques,' for 1857-58. 



The observations above made with respect to pile-driving were made 

 upon the supposition that the piles were to be placed in such positions 

 as to allow of their being driven in the open air; but if they should be 

 entirely under the water-line of the river to be built in, it may be 

 necessary to construct a coffer-dam, or a temporary enclosure, for the 

 purpose of pumping out the water, and allowing the workmen to 

 operate freely in the dry. The principles upon which coffer-dams are 

 formed, are discussed under that head [COPPER-DAM] ; but it is necessary 

 here to remark, that their use is not now considered to be so indispen- 

 sable as it was formerly. When the extreme limit of the scour to which 

 a river's bed is exposed has been ascertained, and a solid substratum is 

 known to exist below that limit, it is found to be sufficient, especially 

 in the cases of bridges of large span, to surround the intended positions 

 of the piers by an enclosure of close piling, and then to dredge out the 

 alluvial matter, or silt, upon the top of the resisting substratum, and 

 to fill in the space thus cleared with good hydraulic concrete, nearly 

 up to the water-line. Nearly all the large bridges lately erected upon 

 the continent have been founded in this manner ; and though tha 

 majority of the older members of the engineering profession in 

 England still adhere to the more expensive system of coffer-dam 

 foundations, there appears to be no substantial reason for the non- 

 adoption of the continental system in our own country. In fact, it has 

 been applied by Messrs. Vignolles, Page, &c., hitherto with success. 

 The important conditions to be aimed at in forming such foundations 

 are, firstly, that they should be protected from any possible danger of 

 undermining; and then, that the concrete filled in between the sheet 

 piling should set rapidly, and permanently. So much indeed depends 

 upon the composition of the concrete used in all descriptions of founda- 

 tions, that it is advisable to devote a separate article to its con- 

 sideration. [CONCRETE.] 



In the old Westminster Bridge, Labeleye introduced a system of 

 foundations, which perhaps may still be safely applied in some posi- 

 tions, but which has proved to be signally unfit for the beds of rivers 

 exposed to great scouring actions. He used, in this, large caissons, or 

 boxes, of the extreme size of the intended piers ; and he caused them 

 to be sunk upon a bed supposed to have been prepared by dredging, to 

 receive them : the masonry of the piers was then continued upon the 

 floor thus formed. Unfortunately the natural bed of the Thames at 

 the place where Labeleye tried his experiment was of a very variable 

 character, and the unequal resistances it has offered to the weights 

 brought upon the caissons have allowed the piers and arches to move 

 in so dangerous a manner that the bridge must be pulled down within 

 about a century from its construction. De Cessart applied the same 

 system to some of the bridges over the Loire ; and Smeaton continued 

 its use in our own country, in the foundations of Ramsgate pier, for 

 instance ; but at the present day, engineers have almost unanimously 

 condemned a mode of construction known to be exposed to very 

 serious and irreparable accidents. 



Floating, turning, lifting, and pontoon bridges being merely local 

 modifications of this class of constructions, are briefly noticed under 

 their respective heads. 



We add a table of the form, material, and size of the principal 

 bridges, aqueducts, and viaducts, of which the construction and pro- 

 portions have been obtained : 



The column marked 1, indicates the total length between abutments ; 2, the 

 No. of arches ; 3, the form of arch ; 4, the chord of principal span ; nnd 5, 

 the vcr. oin. of principal span. 



The capital letters in the first column indicate the material : B. brick ; I. iron ; 

 W.I. wrought iron ; C.I. cast iron ; M. marble j S. stone j T. timber. 



