August I, 1878] 



NATURE 



;6i 



THE TAY BRIDGE 



THE new bridge across the Tay, at Dundee, recently 

 completed according to the plans of Mr. Bouch, 

 M.I.C.E.jby Hopkins, Gilkes, and Co., of Middlesborough, 

 is the longest structure of its kind in the world. What 

 renders it more remarkable than its enormous size even 

 is the originality in its conception and mode of execution, 

 the result of which has been a structure of great stability 

 and comparatively small cost. Popular accounts of the 

 work have appeared in various publications during its 

 progress and since its completion ; but there are various 

 scientific aspects of the undertaking which have not been 

 adequately described. The present article will refer to such 

 technical details as may be of value to those who study 

 the application of scientific principles to industrial ends. 



The new methods employed in the construction of the 

 Tay Bridge cannot any longer be regarded as experi- 

 ments. The severe tests imposed by the Government 

 Inspector and by a heavy traffic uninterrupted during 

 several months removed all uncertainty as to their success 

 and make them of greater value. We may now therefore 

 speak of the details of the structure, so far as these are 

 based on scientific principles, with confidence, seeing that 

 the trial of the bridge may now be regarded as con- 

 clusive. 



The bridge consists of 85 spans varying in length from 27 

 to 245 feet. Beginning on the south side there are first 3 

 spans of 67 feet, then 2 of 88, then 10 of 130, then 13 of 

 145, 13 of 245, I of 162, II of 130, 25 of 69, I of 170, and 

 6 of 27 feet. The direction of the approaches on both 

 sides made it necessary to introduce a curve of 1,320 feet 

 radius at each end of the bridge ; that on the north side 

 extending over 88°, and that on the south over 16° 30', the 

 central part of the bridge, 7,960 feet long, being straight. 

 Of the thirteen 245 feet spans, 7 are level, and placed so 

 high that they offer a clear water-way of 88 feet, at high 

 water. From these northwards, a slope of i in 'j'}, brings 

 the structure to the required height to join the land line, 

 while southward an incline of i in 365 serves the same 

 purpose. In order to gain as much clear height as 

 possible in the channel used for navigation, the roadway 

 is laid on the bottom of these large spans, the trains 

 running between the girders, while on the other parts of 

 the bridge, with the exception of the 1 70-feet span on the 

 north side, the sleepers are fastened on the top of the 

 main girders. 



Nature of the River and its Bottom. 



The Tay is a tidal river about two miles wide at the site 

 of the bridge. The rise and fall of the tide in ordinary 

 spring tides amounts to about 15 feet, but local cir- 

 cumstances affect this height to a great extent, the 

 difference between the observed and the computed 

 heights sometimes amounting to 3 feet. The strength 

 of current is about five knots per hour. Fig. i shows the 

 •depth of the water and the nature of the river bottom. 

 The greatest depth is found between piers 15 and 20, but 

 a bank a short distance above the bridge closes up what 

 otherwise would be the best channel for navigation. The 

 rocks on both sides consist of trap. At the south side 

 they rise abruptly to a height of 53 feet, the base being 

 washed by the flood-tide. The river bed to 100 yards 

 out from the south shore is composed of gravel overlaying 

 blue clay with beds of peat and large layers of decayed 

 hazel nuts which at one time grew in abundance near the 

 site of the bridge. Further out the gravel gets coarser, 

 and boulders are found on the river bottom. The trap 

 rock disappears at pier 5, and its place is taken by red 

 sandstone with a rather worn surface. At pier 20 the 

 surface soil consists of sand, and continues to do so to 

 within a few yards of the north side of the river. The 

 sandstone has a northerly dip increasing at pier 15, and 

 from this point northward to pier 78 rock is not any 



longer found at a depth which makes it available for 

 foundation. 



The old river-bed, consisting of gravel and clay, con- 

 tinues, however, almost level along the whole line of the 

 bridge. The sand overlaying it is very sharp and pure 

 and contains a large number of shells. Underneath the 

 gravel is a mixture of sand and clay. From pier 70 

 northward layers of peat and decayed wood are often 

 found in the sand. 



Pla7i for the Foundations. 



The following plan was followed for the foundations : — 

 From pier No. i to No. 14 the rock had to be reached, 

 the weight per square foot of surface amounting to eight 

 tons. Piers No. 15 to No. 19 consist of a caisson sunk 

 only a few feet into the bottom, and then piled, each pile 

 having to carry a weight of twenty-five tons. Piers No. 20 

 to No. 80 are founded on the gravel layer, the pressure 

 per square foot being about two tons. Thence to the 

 north shore the piers rest again on rock. 



General Description of Piers. 

 Before describing the modus operandi adopted in build- 

 ing the bridge it will be well to give a short description of 

 the piers. The first three commencing from the south side 

 have a rectangular section. At the base they are 14 feet 6 

 inches by 7 feet 6 inches, and at the top 1 1 feet 6 inches by 

 4 feet 6 inches. The clay is excavated at low water, and 

 the rock levelled. A concrete foundation is put in, and on 

 these the piers are erected solid in brick and cement. 

 On the top they have a cope of stone 15 inches in thick- 

 ness, and on this rest the bed plates and girders. For 

 piers No. 4 to No. 14 two cylinders having at the base a 

 diameter of 9 feet 6 inches are placed at a distance of 12 

 feet, centre to centre, and connected by a web wall 2 feet 

 6 inches thick. Under water they consist of brick tubes 

 surrounded by a cast-iron casing i inch thick, the centre 

 of each cylinder and the common base being filled 

 up with concrete. Above water the diameter is 8 feet 

 6 inches below and 6 feet 6 inches on the top, and they 

 are finished off with a course of stone in the same manner 

 as the first three. From piers Nos. 15 to 58 the work 

 above high water consists of iron columns varying in 

 diameter from 12 to 20 inches, and having a thickness 

 of metal of i inch. These are braced together by hori- 

 zontal and diagonal bracings and filled with cement to 

 prevent interior corrosion. In the under water work they 

 differ widely according to the nature of the river bottom 

 and the weight they have to sustain. For those from 



15 to 19 a malleable iron caisson 10 feet high, oval in 

 shape with a major axis of 23 feet, and a minor axis of 



16 feet is provided with a lining of brick- work 9 

 inches thick and sunk a few feet into the river 

 bottom. Forty piles are then driven inside as far as a 

 one-ton ram falling from a height of 10 feet will drive 

 them. The depth to which they penetrate varies from 

 15 to 20 feet. These piles are cut off 3 feet above the 

 ground, and the caisson is filled up level with concrete. 

 On top of this concrete a hollow brick pier, hexagonal in 

 shape, is placed, and the space inside up to low water is 

 also filled with concrete. From low to high water the 

 concrete is replaced by solid brickwork, and a course of 

 stone brings the height of these piers to 5 feet above 

 high water. Piers Nos. 20 to 27 are constructed by join- 

 ing two 15-feet diameter cyhnders above low water, after 

 sinking them through the sand to the gravel bed. They 

 are then carried up in the same manner as described for 

 15 to 19. The fourteen piers supporting the thirteen 

 large spans have a 31 -feet malleable iron cylinder with a 

 lining of brickwork 14 inches thick, and are sunk to the 

 gravel layer, and of such height that after reaching 

 the solid bottom their top is slightly projecting above the 

 river bottom. They are then filled in with concrete, 

 and a brick block corresponding in shape with those 

 used for the other piers, but of g-citer dimensions. 



