18S0.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



107 



lity, such crossing was unavoidable. The total length of the viaduct was 

 470 feet, and ihe greatest height 105 feet, consisting of six semicircular 

 arches, each 50 feet in span, resting on pillars, whose horizontal section 

 was a regular octagon, 5 ft. 9J in. in the side, giving 14 feet as their 

 diameter. The whole structure was upon a curve of 1,320 feet radius, 

 and at the point wliere it was determined to build, the axis of the river 

 made an angle of 45° with the direction of the tangent to the curve. 

 One of the chief merits of the design was the avoidance of the difficulties 

 and expense of an oblique bridge with spiral courses in addition to ihose 

 of curving ; this was effected by the adoption of that form of pier above- 

 mentioned. These pillars were surmounted by a capital of seven feet in 

 height, the base of which, resting on the pier, was, of course, identical 

 in plan with it ; but in this height of seven leet was corbelled out on four 

 of its faces to the extent of 1 ft. 3 in., changing the regular octagon into 

 another, whose sides were 9 feet, and 3 ft. 7^- in. alternately. Two 

 of the 9 feet sides were paralleled to the direction of the line of rails, 

 and the other two formed the impost orspringing of the arch. The 

 easiest way to have an idea of the form of the soffit of the arches, 

 is by conceiving an ordinary semicircular arch of 50 ft. span and 14 

 ft. length, to have the arch quoins bevelled off to an extent of 2 ft. 6 

 in. ; and to turn this arch a corresponding centre had to be made, being 

 the ordinary laggings for the cylindrical pari, and what were called 

 by the workmen saddles for the conical faces. It will be evident to the 

 practical engineer, that the proper bonding of all this work, and especially 

 the arches, must be a matter of great care. A model, cut out of Caen 

 stone, showing four courses of the aixh, was produced, which clearly 

 showed the alternate arrangement of the course. The arches being turned, 

 and the spandrils filled up, there was a clear width of 14 feet from out- 

 side to outside of the up-stream and down stream faces of the bridge, 

 giving ultimately 11 ft. G in. in the clear between the parapet walls for 

 carrying a single line of rails over; nor, indeed, does it seem possible with 

 any advantage to extend the designso as to carry a double way, for thus 

 the pier would be necessarily extended in diameter, or otherwise the cham- 

 fering of the soffit increased — bolh inadmissible, one from interfermg with 

 the water-way, and the other from the practical difficulty of bonding the 

 work. 



The quarries from whence the stone was obtained were in the imme- 

 diate vicinity of the works. It was of the blue Pennant grit, called by 

 Sir H. De la Beche, in the Government Geological Survey of this district, 

 "The equivalent of the Pennant grit of Ihe British coal measures;" and 

 very truly characterised by him as being admirably adapted for engineer- 

 ing purposes. Its colour closely resembles that of the common building 

 limestone of this neighbourhood. The lime used was the celebrated 

 Aberthan hydraulic limestone, not only in the foundations, but in all pans 

 of the structure. The foundations on ihe north side, including one of the 

 river piers, were on rock or indurated gravel ; but on the south side the 

 abutment, one land and one river pier, had to be sunk to a far greater 

 depth than originally designed. 



From the loftiness and peculiar design of this bridge, it was, during its 

 construction, an object of great interest; and most persons who visited it 

 expressed strong opinions unfavourable to its ultimate stability, most of 

 which objections were very futile. The real difficulty in the construction 

 was found to be the management of thespandril walls on the concave side, 

 so as to gain the true uniform curvature at the stringcourse under the 

 parapets, as on the concave side we had to gather out the courses of the 

 spandrils about four inches, which, from the excellent quality of the 

 stone, we were enabled to do. 



It would seem necessary also to explain the reason for crossing the val- 

 ley, and crossing it at such a height. Such structures seem rather to con- 

 stitute the difficully and expense of obtaining good gradients on cross- 

 country lines, which necessaiily intersect the rivers at elevations more or 

 less considerable than that of a valley line, which, following the leading 

 of one single stream, ought not, unless for cogent reasons, cross it at all. 

 The consideration of the section of the river made it clear that no other 

 alternative remained but this lofty and curved viaduct, intersecting the 

 stream at the angle of 45°, 



The paper was accompanied by a model of the river piers and cutwaters, 

 with the centering and its supports, at a scale of one twenty-fourth, con- 

 structed under the author's direction by Mr, Keenan, and also by a 

 diagram map, at two inches to the mile, showing the general features of 

 the valley of the Taff — and another map, at six chains to the inch, showing 

 the immediate locality of the viaduct, and the natural difficulties of the 

 ground, with the added difficulty of carrying a line of rails through that 

 district, from the great pre-occupation of the surface by the canal and its 

 feeders, and the mineral tram-roads — and also a diagram section of the 

 gradients of the line of railway, with a large isometrical drawing of two 

 of the arches, showing by part section the arrangement of the spandril 

 walls, the mode of clu.oing thein over as designed, and as carried out in 

 the construction, with the form of the soffit, the capital, and pillar, 



"An Account of the Construction of t tie Midland Great-Western Rail- 

 way of Ireland, over a Tract of Boys, in the Counties of Mealh and West- 

 meat/t. By GiiORGE W. He.mans, Engineer-in-Chief, 



The railway from Dublin to Mullingar was projected, from motives of 

 interest and policy, to follow the line, and occupy the banks, of the Royal 

 Canal, The canal banks afforded some facilities for the construction of a 

 railway, but it soon became evident that there were also disadvantages in 



following them too closely. The earthworks in constructing the canal 

 had been very heavy in character, with some of the deep cuttings thiough 

 rock ; and to relieve them »s much as possible, the canal h.id been laid 

 out to follow every sinuosity of the ground which offertd a favourable 

 level. 'Ihr railway, as far as Mullingar, was also laid out along nearly 

 the whole of these siduusities; and there being great anxiety to open at 

 least a portion of it at the earliest period, it was at ouce, on the passin"- of 

 the bill, put into a contractor's hands for one-half the distance (as far as 

 Enfield), and rapidly constructed on the canal banks. During the progress 

 of these works, it was found to be desirable to avoid constructing the 

 remainder of the line on a continued system of curves, which, although 

 no longer, by well-inforaied engineers, considered a source of danger, are 

 decidedly olijectionable, as offering a resistance to the trains, causing 

 greater friction, wear and tear, consumption of fuel, and loss of time 

 besides lengthening the distance. In considering the plans fur the second 

 division of the line, between Enfield and Mullingar, the canal bank, wliicli 

 is a continued series of curves, was clearly to he avoided ; but another 



difficulty presented itself on the straight line — the chord to these curves 



it would have to traverse a long line of bogs, which, on careful examina- 

 tion with the boring-rod, proved to be from twenty five to as much as 

 seventy feet deep. Some of them were swell bogs of the softest pulpy 

 nature, having gradually risen to a higher level than the surrounding 

 country, and holding much water in suspension. After an extended 

 examination of the subject, particularly in reference to drainage, it was at 

 length apparent that one of the causes of the excess of water, and conse- 

 quent want of solidity in these bogs, was the position of the canal 

 embankment, traversing the edges of them for a great distance, and 

 completely intercepting all drainage from them along the general fall of 

 the country towards the river Deal. The following general plan was then 

 at ouce resolved upon : — First, immediately to open full and sufficient uev7 

 outlets for the escape of suspended water from ihe whole area; next, to 

 form a system of drains all along and across the intended line ; and finally, 

 as a fixed principle, not to attempt either to excavate or embank the line, 

 but to lay the rails on the nalur.il level of the high bogs, trusting to drain- 

 age only to reduce the parts that were too high. With toleralile confidence 

 in this plan, a Deviation Bill was passed through Parliament, and the 

 straight line, traversing about eight miles of deep bog, was immediately 

 commenced. An old wooden shoot, nine indies square, which was the 

 sole outlet for the drainage of a district of about 1,500 square acres of wet 

 bog, was the most ineffective point of the existing drainage, and was, 

 Iheiefoie, the first to demand improvement. The banks and bottom of 

 the canal at the place consist of clay artificially superposed on the cut 

 away bog, lying on fine gravel of a very loose, treacherous description, 

 being of a mixed sandy and marly nature. Having lesolved on intro- 

 ducing a tunnel culvert, three feet diameter, under tlie canal at this spot, 

 and that its invert should be six feel lower than the existing shoot, it 

 became a matter of anxious consideration how to do this, in such bad 

 ground, without interfering with Ihe navigation of the canal, or running 

 the risk of bursting a leak in the bottom. The canal level at this stage is 

 twenty miles long, without a lock, and a breach would have been a serious 

 affair. 



Mr. Hemans here described very minutely the details of the execution 

 of this very difficult work, whicli was altogether very successful, which 

 secured the command for drainage of nearly four miles of the line of rail- 

 way. The description of this important operation was further illustrated 

 by reference to several drawings prepared for the purpose. 



While the foregoing work was in progress, a sum of about 1,000/. was 

 being expended in the sinking a length of some miles of a river, and under- 

 pinning a culvert, ten feet wide, leading out of the next district of bogs. 



This underpinning and building a new invert, at a level four feet below 

 the old one, was also a work requiring great caution. The weight of the 

 embankment and the canal overhead was very great; and here also a 

 breach would have caused extensive damages. As soon as these outlets 

 were ready, the drains in the bog were opened. 



iMr, Hemans next proceeded to enter into a very clear explanation of 

 the plan of operation pursued in the drainage of the surface of the bogs 

 destined to receive the upper works of the railroad. He then described 

 the nature of the soling finally decided upon and adopted, iMving given an 

 account of the results of experiments OQ the several descripliuus of soling 

 which had been tested. 



The construction of the upper works of the railroad were minutely 

 detailed, and explanatory drawings were exhit)ited. 



The mode of operation adopted in conveying this line of railway over 

 Ihe bogs of most unpromising aspect was eminently succi ssful ; and as the 

 details of the works were so very different from the very expensive process 

 generally adopted, and sometimes with but little success, the account was 

 particular!) interesting to the engineering world. 



Mr. Hemans having made some observations on the cost of maintaining 

 railways constructed through bogs, aad also on a paper of great interest 

 by the Messrs. Mullens, published in the second volume of ihe Transac- 

 tions of the Institute of Civil Engineers of Ireland, concluded by reading 

 a detailed estimate of the cost of these works, which clearly showed Ihe 

 possibility of constructing a double line of railway O'Cr deep bogs, when 

 Healed as described by hiui, at a cost not exceeding 0,0UU/. per mile, 

 including all expenses. 



