364 



NATURE 



{August I, 1878 



rapid changes, the operation was sometimes interrupted 

 and frequently endangered by sudden gales, it becoming 

 necessary to take the piers away from the place where it 

 was intended to lower them, and as they could not be 

 placed back in the same position they had to be taken to 

 Dundee Harbour for shelter. The weight of these piers 

 was about 100 tons, and the carrying barges were 60 feet 

 long by 12 feet wide, and 6 feet deep. If the weather 

 continued favourable, the work of lowering took place the 

 same tide. For this purpose the rams were pumped up 

 12" while the weight was on the steel pins resting on top 

 of the girders. The connection between the rams and 

 the links was then made, whereupon a few additional 

 strokes relieved the lower pins of the weight, so that they 

 could be withdrawn and inserted in the next hole. By 

 allowing the water to leave the rams, the pier would of 

 course be lowered till the weight came again on the lower 

 pins, after which matters were in the same position as at 

 the beginning, and the operation could be repeated, and 

 the pier lowered foot by foot. 



No great attention was paid to the position of the pier 

 till it neared the river bottom. Then, however, an observer 

 with a theodolite placed on shore or on an adjacent pier, 

 directed the men at the capstans placed on the barges to 

 draw them to such a position as to have the pier exactly 

 in line, while double steel wires, of which the length 

 had been carefully determined, gave the distance from 

 the pier previously placed in position. As soon as the 

 position was in every respect satisfactory the lowering 

 was proceeded with quickly, and it became in this manner 

 possible to put a pier very nearly in its right place. 

 Especially in the curve this work required great atten- 

 tion and care, but it has admirably succeeded. After 

 disconnecting the links which joined the lifting girders to 

 the piers the rising tide would float these with the barges 

 over the top, leaving the cylinder standing on the river 

 bottom. Fig. 2 shows one of these cylinders moored in 

 the river and the general arrangements for floating and 

 lowering. Another of the piers is ready to 5' above high 

 water, and others are in progress. One has the pneumatic 

 apparatus placed on it. 



In most cases the ground was sufficiently level, in 

 others divers had first to clear the bottom of boulders, 

 and in three cases even this was not found effectual, 

 and the settling of the pier on one side caused it to 

 upset. The fallen mass had to be lifted out of the road 

 and broken up as it would have been too costly to set it 

 on end again. Special struts to prevent the possibility of 

 a recurrence of such disasters were then introduced. 

 Two iron tubes of seven and eight inches diameter 

 respectively slid into each other by means of a stuffing- 

 box like the parts of a telescope. The upper end of the 

 top part was fastened to the top of the pier and connected 

 with a pump, the lower part had a mushroom-like base, 

 and when this part was pulled away from the pier under 

 an angle and then water pumped in it took a firm hold in 

 the ground. Four of these struts applied to each pier 

 held it perfectly in position as long as the water pressure 

 was not withdrawn from them. In the subsequent 

 operation of sinking the pier the cocks giving egress to 

 the water were left open, and as long as the motion of 

 the pier was a vertical one the water would escape 

 from all four struts in a gentle flow. As soon, however, 

 as the pier went out of the perpendicular the strut on that 

 side would eject a stronger stream of water, and the man 

 in attendance had only to shut the cock pertaining to that 

 strut to make it answer its purpose of supporting the pier 

 on the side to which it was inclined to fall. This appa- 

 ratus fully answered the purpose for which it was 

 constructed. 



Sinking by the Pneumatic Process. 



In order to sink the piers through the layer of clay over- 

 lying the rock the pneumatic process was employed. Tem- 



porary castings were added to the top of the pier bringing 

 the latter above the highest water, and a malleable iron 

 air-bell, having the usual air-lock for the passage of the 

 workmen, and smaller locks for the passage of excavating 

 material to the outside or concrete to the inside was placed 

 on the top (Fig. 3). A six horse-power steam-engine, con- 

 nected with an air-pump, supplied the air by the pressure of 

 which the bottom of the pier was kept dry. The breathing 

 of the men and the burning of candles required, of course, 

 a more Hberal supply than would have been necessary 

 for the first purpose only. Instead of allowing this surplus 

 to escape at the bottom of the pier it was used to drive a 

 pneumatic hoist lifting the excavated materials up to the 

 air-lock. 



As the excavations proceeded, the pier would sink 

 down till it reached the rock, and this movement was 

 facilitated by the bottom of the pier being slightly conical. 

 The rock was then levelled down by pick-axes and chisels 

 so as to give a firm bearing, and concrete material intro- 

 duced filUng up the bottom chamber and the shafts which 

 had been left in the brick-work for the passage of the 

 men. When this work had proceeded to above low-water, 

 the temporary castings and air-bells were removed and 

 the pier built up between low and high water in the usual 

 manner from a barge. 



This process, though slow and costly, worked on the 

 whole satisfactorily, so far as the piers were founded upon 

 rock. It was carried through without many accidents, 

 though one occurred in which six men unfortunately lost 

 their lives. On a stormy day in August, 1873, a barge 

 loaded with coals, moored near one of the piers in process 

 of sinking, probably knocked against the structure and 

 must have damaged the iron — though it was not observed 

 at the time — to such an extent that at the next high tide, 

 when the air pressure was greatest, one of the top plates 

 was blown out, the water rushing in from both top and 

 bottom, and of course making escape an impossibility. 



Piled Piers. 



The oval caissons for piers Nos. 16 to 19, were built, and 

 floated out in precisely the same manner, and then 

 excavated under water by a diver (the excavated sub- 

 stance being hoisted up from a barge) and sunk 2 feet 

 into the ground. A staging constructed of angle iron 

 was then floated out and put over this caisson, and was 

 used as a support for the pile driver. The piles had a 

 length of 60 feet. The depth at the highest tide was 

 45 feet, and it was therefore possible to drive them 15, 

 feet into the ground, before any means of lengthening 

 them for the purpose of driving, had to be adopted. 

 Divers were engaged to cut them under water, leaving 

 3 feet standing in the caisson. Concrete lowered in large 

 buckets, which only opened when getting to the bottom, 

 surrounded and covered the heads of the piles, the height 

 of the concrete being 7 feet above these heads. During 

 all these operations the caisson extended temporarily to 

 above high water, so that the diver could continue his 

 operations during the whole of the tide, notwithstanding 

 the strong current, and the danger of washing the essential 

 parts of the concrete away was considerably lessened. 

 An inspection, undertaken for the purpose of ascertaining 

 how far the latter had been successful, gave very satis- 

 factory results, the whole mass of the concrete being 

 found perfectly hard after a few days. The disconnection 

 of the terftporary caisson from the lower part, and floating 

 away in the same manner as that by which it had been 

 brought out was a work of little difficulty. 



The floating out and fixing of the brick pier which was 

 put on top of this concrete foundation will be described 

 more fully with the operations necessary to complete the 

 large piers. These are certainly the most interestmg ones 

 on account of their magnitude, although this very size 

 may have been the means of avoiding particular difficulties 

 which stood in the way of the work connected with the 



