Page:Forth Bridge (1890).djvu/27

Rh necessary to lay the bottom dry, and the diving bell was never made use of. The north piers on Inchgarvie and the south piers on Fife are founded on the solid whinstone rock.

Table III., given below, shows the time occupied in building the foundations of the four circular piers on Fife, the depth to which the foundations were carried, and other particulars.



On the South Queensferry shore the foundations of piers 3, 4, and 5 were first started, as they were all dry at low water. These three piers are founded on the freestone rock prevalent in ridges all along this shore. Tidal work had to be resorted to without any protection by cofferdam or caisson, and all that was required was to remove the soft or rotten portions of the rock and cut flat steps into any sloping faces met with. The hollows were then levelled up with concrete, and upon this a concrete base was laid enclosed by planking. When this had properly set the granite masonry was at once begun. The concrete base was 61 ft. long by 31 ft. wide, and was the same for all viaduct piers from 3 to 9. The thickness of this concrete foundation varied according to position, from 4 ft. in the shore piers to 11 ft. in No. 9 pier. Piers 2 and 1, as also the abutment, were situated on the hillside. The first was founded on the freestone rock; but the other two were placed on a stiff blue clay of sufficient solidity. Pier 6 required the construction of a cofferdam, the bottom being rock with a thin layer of gravel and clay. Owing to this the cofferdam could not be carried to more than half-tide, the difficulty being to get the piles to hold. A great deal of trouble was experienced with this foundation through the want of a tight joint at bottom; but it was, of course, only a matter of patience and perseverance. The foundation of pier 7 also caused some trouble owing to the same difficulty at bottom, and a half-tide cofferdam had to be used; but in the foundations of piers 8 and 9 the layer of clay at bottom was of sufficient thickness to allow good piling to be done, and whole-tide cofferdams could be constructed. Pier 7 is founded on the rock, but piers 8 and 9 are founded on the hard clay.

The south cantilever pier required a cofferdam of abnormally large proportions and of great strength, for the base upon which this great masonry pier had to be raised was not less than 115 ft. long by 60 ft. wide. It was at first divided into halves by a double row of piles with puddle clay filling; but, after a good portion of the concrete base had been laid all round the side of the dam, this partition was cut down, the piles being sawn off flush with the bottom and the concrete base carried right across. In this pier the boulder clay at bottom was found nearly dry and extremely tough to handle, and it gave an indication of the ground into which the large caissons would have to be sunk.

Table IV., given below, shows the levels at which the foundations were started of all the viaduct piers from 1 to 9, of the south cantilever pier, the north cantilever pier, and the abutment, as also the nature of the ground under foundations.

Figs. 23 to 28 show views of the cantilever end pier and the viaduct piers, with the principal dimensions. The cutwaters with a coping of dressed granite reach to the same height above high water as the circular piers, namely 18 ft., and up to this level all the piers from 3 to 9 were built with a hearting of concrete; above this level the granite was backed by masonry of Arbroath rubble in cement carried up to the top. The hearting in piers 1 and 2 was of concrete up to the level at which the viaduct girders were erected—namely, at about 47 ft. above high water, from this level to top they were, like the other viaduct piers, built with a hearting of Arbroath rubble.

To facilitate the building of these piers, staging was erected alongside and connected with the jetty, and cranes were set up to lift the granite blocks from the jetty adjoining and deposit them in their places. All materials were brought along the jetty, where also the mortar was prepared and the concrete mixed. Two courses of granite were generally laid, and the hearting of concrete filled in. The mortar used consisted of one part cement and two parts sand.

The concrete consisted of 27 cubic feet, or 1 cubic yard of broken whinstone, $5 1/2$ cubic feet of cement, and $5 1/2$ cubic feet of sand, mixed dry by hand and then charged into a concrete mixer where the water was added. From the mixer it was delivered into barrows and tipped where required.

All the south viaduct piers from 3 to 9, and the south cantilever end pier, were built up to a level of 18 ft. above high water, and further work upon them had to be delayed until the girders had been erected and rivetted up, and were ready for lifting. The dates relating to the building of the piers, the raising of the girders and the quantities of materials used are given in a summary on another page.

Piers 1 and 2 were carried up to level 47 ft. above high water, and the girders built at that level. The south abutment was raised to 119 ft. above high water, and the girder erected at that level on staging reaching close up to No. 1 pier.



It has already been mentioned in the description of the Inchgarvie staging, that places were prepared for the reception of the two south caissons presently to be brought across from the Queensferry jetty.

In order to obtain at an early date a correct idea of the contour of the rock upon the sites to be occupied by the two south piers on Inchgarvie, a circular raft was constructed of timber balks decked for about 10 ft. round the outer circumference, with 3 in. planking. The raft is shown in Fig. 31. It was made sufficiently strong to resist the action of the waves in an ordinary breeze of wind, or to bear, if necessary, the strain of being beached on shore in case stormy weather should set in. It was a little under 70 ft. in diameter, and the surface of the planked space was 6 in. above water. Large mooring blocks were laid at some distance on three sides, the fourth being attached by cable chains to some of the iron columns of the staging. Upon the raft four crab winches were placed, by means of which either of the mooring chains could 36 hauled in or slackened. The raft could thus be easily placed in any desired position within tolerably wide limits. A central staff was fixed upon the raft, and about a foot from the outer edge a ring of gas-pipe was laid down upon which the grooved wheels of a carriage ran true to the centre of the raft. A drum was placed upon the carriage, somewhat overhanging the edge of the raft, and upon this drum the sounding line was coiled. This consisted of a fine steel wire and a long weight weighing about 60 lb., with a point at bottom, feet and inches being marked upon the wire by copper tags attached.

Two theodolites—one in the centre line of the piers longitudinally on the iron staging, the other on a masonry pier at right angles to the centre line, sat in line with the centres of the two south piers—were stationed to cheek the position of the centre of the raft every few minutes, while alterations in the tide level were observed on two tide gauges, and recorded. Owing to the heavy current running at this point both during ebb and flood the raft could not be held in position with any degree of accuracy, nor could the sounding line be kept plumb in spite of the heavy weight attached. Soundings were therefore only taken during slack water—that is, for about an hour before and an hour after high water or low water.

After the actual contour of the rock, on and within the two 70-ft. circles, had been taken, the raft was shifted about in various directions and further soundings recorded, the exact position of