82 



NA TURE 



[May- id, 1887 



duct-approach. The clear headway under the centre of 

 the bridge is 152 feet at high water, and the highest point 

 of the bridge is 360 feet above the same datum. 



Each of the main piers includes four columns of 

 masonry founded on the rock or boulder-clay. Above 

 low water the cylindrical piers are of the strongest flat 

 bedded Arbroath stone set in cement and faced with 

 Aberdeen granite. The. height of these monoliths is 

 36 feet, and the diameter 55 feet at bottom and 49 feet at 

 top, and they each contain forty-eight steel bolts 2| inches 

 in diameter and 24 feet long to hold down the super- 

 structure. 



Below low water the piers differ somewhat in character, 

 according to the local conditions. On the Fife side, one 

 of the piers was built with the aid of a half-tide dam, and 

 the other with a full-tide dam. The rock was blasted 

 into steps, diamond drills and other rock-drills being used. 

 Even this comparatively simple work was not executed 

 without considerable trouble, as the sloping rock bottom 

 was covered with a closely-compacted mass of boulders 

 and rubbish, through which the water flowed into the 

 dam in almost unmanageable quantity. After many 

 months' work the water was sufficiently excluded by the 

 use of cement-bags, and liquid grout poured in by divers 

 under water, and other expedients, and the concrete 

 foundation and masonry were proceeded with. 



At Inch Garvie, the two northernmost piers were 

 founded like the preceding, but the two others presented 

 greater difficulties, owing to the depth of water, and had 

 to be dealt with in a different way. Several designs were 

 prepared for these foundations, but it was finally decided, 

 and, as experience proved, wisely, to put them in by what 

 is known as the pneumatic or compressed air process. 

 The conditions of the problem were a sloping, very irre- 

 gular, and fissured rock bottom, in an exposed seaway, 

 and with a depth at high water of 72 feet. Anything 

 of the nature of a water-tight cofferdam, such as was 

 used at the shallow piers, was out of question, and the 

 plan adopted was as follows : — 



Two wrought-iron caissons, which might be likened to 

 large tubs or buckets, 70 feet in diameter and 50 to 60 

 feet high, were built on launching-ways on the sloping 

 southern foreshore of the Forth. The bottom of each 

 caisson was set up 7 feet above the cutting edge, and so 

 constituted a chamber 70 feet in diameter and 7 feet high, 

 capable of being filled at the proper time with compressed 

 air to enable men to work as in a diving-bell below the 

 water of the Forth. The caisson, weighing about 470 

 tons, was launched, and then taken to a berth alongside 

 the Queensferry jetty, where a certain amount of con- 

 crete, brickwork, and staging was added, bringing the 

 weight up to 2640 tons. At Inch Garvie a very strong 

 and costly iron staging had previously been erected, 

 alongside which the caisson was finally moored in correct 

 position for sinking. Whilst the work described was 

 proceeding, divers and labourers were engaged in making 

 a level bed for the caisson to sit on. The 16-feet slope 

 in the rock bottom was levelled up by bags filled with 

 sand or concrete. As soon as the weight of caisson and 

 filling reached 3270 tons, the caisson rested on the sand- 

 bags and floated no more. The high ledge of rock upon 

 which the northern edge of the caisson rested was blasted 

 away, holes being driven, by rock-drills and otherwise, 

 under the cutting edge, and about 6 inches beyond for the 

 charges. After the men had gained a little experience in 

 this work, no difficulty was found in under-cutting the 

 hard whinstone rock to allow the edge of the caisson to 

 sink, and, of course, there was still less difficulty in re- 

 moving the sand-bags temporarily used to form a level 

 bed. The interior rock was excavated as easily as on dry 

 land, the whole of the 70-feet diameter by 7-feet high 

 chamber being thoroughly lighted by electricity. Access 

 was obtamed through a vertical tube with an air-lock at 

 the top, and many visitors ventured to pass through this 



lock into the lighted chamber below, where the pressure 

 at times was as high as 35 lbs. per square inch. Probably 

 the most astonished visitors were some salmon, who, 

 attracted by the commotion in the water caused by the 

 escape of compressed air under the edge of the caisson, 

 found themselves in the electric lighted chamber. When 

 in the chamber the only notice of this escape of large 

 volumes of air was the sudden pervadence of a dense fog, 

 but outside a huge wave of aerated water would rise above 

 the level of the sea, and a general effect prevail of some- 

 thing terrible going on below. No doubt the salmon 

 thought they had come to a cascade turned upside down, 

 and, following their instinct of heading up it, met their 

 fate. 



Another astonished visitor was a gentleman who took a 

 flat-sided spirit-flask with him into the caisson, and 

 emptied it when down below. Of course the bottle was 

 filled with compressed air, which exploded when passing 

 through the air-lock into the normal atmospheric pressure, 

 the pressure in the bottle being 33 lbs. per square inch. 

 The Garvie piers, notwithstanding the novelties involved 

 in sinking through whinstone rock, at a depth of 72 feet 

 below the waves of the Forth, were completed without 

 misadventure, in less than the contract time. The first of 

 the deep Garvie caissons was launched on March 30, 

 1885, and both piers were finished to sea-level or above 

 by the end of the year. 



At Queensferry all four piers were founded on caissons 

 identical in principle with those used for the deep Garvie 

 piers. The deepest was 89 feet below high water, and 

 weighed 20,000 tons ; the shallowest of the four was 

 •71 feet high, the diameter in all cases, as at Garvie, being 

 70 feet at the base. Some differences in detail occurred 

 in these caissons as compared with Garvie, owing to the 

 differences of the conditions. Thus, instead of a sloping 

 surface of rock the bed of the Forth was of soft mud to a 

 considerable depth, through which the caissons had to be 

 sunk into the hard boulder-clay. Double skins were 

 provided for the caissons, between which concrete could 

 be filled in to varying heights if necessary, so that greater 

 weight might be applied to the cutting edge where the 

 mud was hard than soft. This annular wall of concrete 

 also gave great strength to resist the hydrostatic pressure 

 outside the caisson, for it must be understood that the 

 water was excluded both below and above the working 

 chamber. 



The process of sinking was as follows : — The caisson 

 being seated on the soft mud, which, of course, practically 

 filled the working chamber, air was blown in, and a few- 

 men descended the shaft or tube of access to the working 

 chamber in order to clear away the mud. This was done 

 by diluting it to the necessary extent by water brought 

 down a pipe under pressure, and by blowing it out in this 

 liquid state through another pipe by means of the pressure 

 of air in the chamber. It was found that the mud sealed 

 the caisson so that a pressure of air considerably in excess 

 of that of the water outside could be kept up, and it was 

 unnecessary to vary the pressure according to the height 

 of the tide. In working through this soft mud both in- 

 telligence and courage were called for on the part of the 

 men, and it is a pleasure and duty for me to say that the 

 Italians and Belgians engaged on the work were never 

 found wanting in those qualifications. There was always 

 a chance of the caisson sinking suddenly or irregularly, 

 and imprisoning some of the men ; and, indeed, on one 

 occasion a few men were buried up to their chins in the 

 mud, and on another the caisson gave a sudden drop of 

 7 feet. Happily no serious accident happened, although 

 I confess that I felt a little apprehensive myself, as I was 

 familiar with the details of an accident with a similar 

 caisson sunk in the bed of the Neva, at St. Petersburg, in 

 1876. In that case the wet mud rose rapidly in the 

 working chamber when the caisson sank suddenly 18 

 inches one day, and of the twenty-eight men in the 



